2007 Proton-Coupled Hole Transfer in X-Irradiated Doped Crystalline Cytosine.H2O Radiation-Induced Defects in Sucrose Single Crystals, Revisited: a Combined Electron Magnetic Resonance and Density Functional Theory Study The energy dependence of lithium formate EPR dosimeters for clinical electron beams An International Intercomparison on "Self-Calibrated" Alanine EPR Dosimeters Response of Chronic Hypoxic Cells to Low Dose-Rate Irradiation 2006 EPR Dosimetry of Radiotherapy Photon Beams in Inhomogeneous Media using Alanine Films Single Crystals of L-O-Serine Phosphate X-Irradiated at Low Temperatures: EPR, ENDOR, EIE and DFT studies Electron Transfer in Amino Acid:Nucleic Acid Base Complexes: EPR, ENDOR and DFT Study of X-Irradiated N-Formylglycine:Cytosine Complex Crystals Q-band EPR and ENDOR of low temperature X-irradiated beta-D-fructose single crystals LET effects following neutron irradiation of lithium formate EPR dosimeters. Study on the EPR/dosimetric properties of some substituted alanines. 2005 Dosimetry of ion beams using lithium formate EPR dosimeters. Dose measurements in heterogeneous media using alanine film dosimetry. Alpha-thymidine at 105 K. Disodium cytidine-5'-monophosphate 6.07-hydrate methanol 0.27 solvate. Formates and dithionates: Sensitive EPR-dosimeter materials for radiation therapy. An experimental investigation of the electron energy dependence of the EPR alanine dosimetry system. 2004 Cytosinium N-benzoylglycinate monohydrate, a redetermination at 105 K. Electron paramagnetic resonance (EPR) dosimetry using lithium formate in radiotherapy. Comparison with thermoluminescence (TL) dosimetry using lithium fluoride rods. N-acetyl-L-alanine N-formylglycine The Solid State Radiation Chemistry of Simple Amino Acids, Revisited Radiation-induced radicals in lithium formate monohydrate (LiHCO2.H2O). EPR and ENDOR studies of X-irradiated crystal and polycrystalline samples Allyl Type Radical Formation in X-Irradiated Glutarimide Crystals Studied by ENDOR and ENDOR-Induced EPR ENDOR assisted study of the stable EPR spectrum of X-irradiated alpha-L-sorbose single crystals: MLCFA and simulation decomposition analyses Experimental and theoretical electron magnetic resonance study of radiation-induced radicals in alpha-L-sorbose single crystals Estimation of X-ray beam quality by EPR spectroscopy. EPR Study of Effects of Light Illumination on Free Radicals Induced in L-Alanine by Ionizing Radiation. The photon energy dependence of the Alanine/EPR dosimetry system, an experimental investigation 2003 Hole Transfer and Differential Radical Recombination in X-Irradiated Doped Crystalline DNA Model Systems.. EPR Dosimetric Properties of Formates. EPR and ENDOR Study of Crystalline Cytosine•HCl Doped with 5-Methylcytosine. Radiation Induced Radical Formation and Hole Transfer. An investigation of the photon energy dependence of the EPR alanine dosimetry system Radical Formation in Pyrimidine Bases Following X-, Proton- and Alpha-Irradiation Hole Transfer in Crystals of Cytosine Monohydrate: an EPR Study Identification of the Hydroxyl Radical and other Reactive Oxygen Species in Human Neutrofil Granulocytes Exposed to a Fragment of the Amyloid Beta Peptide. Alanine Radicals, Part 4: Relative Amounts of Radical Species in Alanine Dosimeters Following Exposure to 6-19 MeV Electrons and 10 kV-15 MV Photons Alanine Radicals, Part 3: Properties of the Components Contributing to the EPR Spectrum of X-irradiated Alanine Dosimeters 2002 EPR and ENDOR Studies of Single Crystals of 2-Oxazolidinone X-Irradiated at 295 K Alanine Radicals, Part 2: The Composite Polycrystalline Alanine EPR Spectrum Studied by ENDOR, Thermal Annealing and Spectrum Simulations. Radiation Dose Measurements with Alanine/Agarose Gel and Thin Alanine Films around a 192Ir Brachytherapy Source, using ESR Spectroscopy EPR Dosimetric Properties of 2-Methylalanine: EPR, ENDOR and FT-EPR Investigations ENDOR Study of 14N Hyperfine and Quadrupole Couplings of N2D4·+ Formed in Deuterated Li(N2H5)SO4 Single Crystal 2001 ENDOR Study of the Chlorinated Thiocytosine Radical in a Crystal Matrix 2000 ENDOR Study of a Thiocytosine Oxidation Product in Cytosine Monohydrate Crystals Doped with 2-Thiocytosine, X-irradiated at 15 K. Experimental and Theoretical Investigation of the Mechanism of Radiation Induced Radical Formation in Hydrogen Bonded Co-Crystals of 1-Methyl-Cytosine and 5-Fluoro-Uracil. Weakly Coupled Proton Interactions in the Malonic Acid Radical: Single Crystal ENDOR Analysis and EPR Simulation at Microwave Saturation. EPR and ENDOR Studies of Deuteron Hyperfine and Quadrupole Coupling in .CD(COOD)2: Experimental and Theoretical Estimates of Electric Field Gradients from an a-Carbon. The Effects of Aliphatic (n-nonane), Naphtenic (1,2,4-trimethylcyclohexane) and Aromatic (1,2,4-trimethylbenzene) Hydrocarbons on Respiratory Burst in Human Neutrophil Granulocytes. A Fluorescence and Electron Paramagnetic Resonance (EPR) Spectroscopy Study. Automatic Spin-Hamiltonian Diagonalization for Electronic Doublet Coupled to Anisotropic Nuclear Spins in One- and Two-Dimensional Electron-Spin-Echo Simulations. Structure of N2H4.+ formed in X-irradiated Li(N2H5)SO4 single crystals. Free Radical Formation in X-Irradiated Crystals of 2'-Deoxycytidine Hydrochloride. Electron Magnetic Resonance Studies at 10 K. 1999 Effects of the Solvent 1,2,4-Trimethylcyclohexane on the Respiratory Burst in Human Neutrophil Granulocytes. A Chemiluminescense and Electron Paramagnetic Resonance (ESR) Study. Comments on "Effects of Ionizing Radiation on Crystalline Cytosine Monohydrate" High Resolution EPR and Quantum Effects on CH3, CH2D, CHD2 and CD3 Radicals under Argon-Matrix Isolation Conditions. ESR Studies of the Effect of Zeolite Structures on Motional Dynamics of NO2. Measurement of Dose Rate at the Interface of Cell Culture Medium and Glass Dishes by means of ESR Dosimetry using Thin Films of Alanine. 1998 EPR and ENDOR Studies of Single Crystals of a-Glycine X-Irradiated at 295 K. EPR and ENDOR Studies of X-Irradiated Single Crystals of Deoxycytidine 5'-Monophosphate at 10 K. Radiation-Induced Solid State Radical Formation in b-Glycerophosphate by EPR and ENDOR Spectroscopy at 77 K and Room Temperature: Formation of an Allylic Type Radical. Theory of ESEEM in Isotropic Tunneling Methyl Rotor Systems. Radiation Damage to DNA Base Pairs. II. Paramagnetic Resonance Studies of 1-Methyluracil . 9-Ethyladenine Complex Crystals X- irradiated at 10 K Electron Paramagnetic Resonance and Electron Nuclear Double Resonance Studies of X-Irradiated Crystals of Cytosine Hydrochloride. Part I: Free Radical Formation at 10 K following High Radiation Doses ESR and ENDOR Study of X-Irradiated Deoxyadenosine: Proton Transfer Behavior of Primary Ionic Radicals A Simple method for Estimating Dose Uncertainty in ESR/Alanine Dosimetry 1997 The Effect of Isotopic Substitution on the Electron Spin Dynamics of the Radical in X-Irradiated Methyl Malonic Acid Powder. Intrinsic Potentials and Activation Energies. Alanine Radicals: Structure Determination by EPR and ENDOR of Single Crystals X-Irradiated at 295 K The EPR Spectrum of the General =C-CX3 Quantum Rotor ESR, ENDOR, and FSE Studies of Hippuric Acid Single Crystals X-irradiated at 295 K: A Reinvestigation ENDOR analysis of X-irradiated l-Alanine and NaHC2O4 × H2O. Simulation of microwave power dependence of satellite lines Motional Dynamics of NO2 on Na-ZSM-5: An ESR Investigation The Methyl-Rotor Electron-Spin Dynamics in the Smoluchowski Drift Diffusional Framework. Anders R. Sørnes and Nikolas P. Benetis, The Methyl-Rotor Electron-Spin Dynamics in the Smoluchowski Drift Diffusional Framework J. Magnetic Resonance 125, 52 - 64 (1997). Abstract: The EPR study of the internal motion of the methyl fragment in radicals is simulated using the Smoluchowski drift diffusion model. EPR stochastic Liouville lineshape calculations using this model are presented in this work, allowing to the exploration of the whole span of conditions between the discrete- site-exchange and the stepless-free-diffusion limits and offering a unification of these approximative theories. An accurate value of the correlation time for the isotropic hyperfine interaction is calculated for the full ranges of the parameters describing the system and is compared to those given by the approximative models in the two limiting models, allowing a discussion of their quality. Long and short correlation time limit spectra are reported and interpreted using basic analytic models. For the isotropic hyperfine interaction studied here, no broadening effects are observed for Redfield (strong narrowing) conditions. This suggests absence of lifetime broadening effects in agreement with site-model results. The addition of a cos fi term in the hyperfine interaction is essential for pyramidal radical centres, as is demontrated by simulation of experimental reports in the literature. Download full paper here (pdf-file) Back to Top of Page Einar Sagstuen, Eli O. Hole, Sølvi R. Haugedal, Anders Lund , Omer I. Eid, and Roland Erickson, EPR and ENDOR analysis of X-irradiated l-Alanine and NaHC2O4 × H2O. Simulation of microwave power dependence of satellite lines Nukleonika 42(2), 353 - 372 (1997). Abstract: The microwave power dependency of satellite lines caused by three distant protons, (3H) in X-band EPR spectra of CO2- (3H) in NaHC2O4×H2O, and the SAR, CH3-CH-COOH (3H), in L-alanine was analysed by line shape simulation. The distant protons are hydrogen bonded to the radical in the first case, and the methyl protons of a neighbour alanine molecule in the second case. The radicals were produced by X-irradiation. A simulation programme was developed which includes a phenomenological adaptation of continuous saturation theory. Refined g- and hyperfine data including the interaction with the three neighbour protons of the SAR were obtained by ENDOR and EPR, while previously determined parameters were used in the EPR simulation of the CO2- (3H). Good agreement was obtained between experimental and simulated single crystal spectra over a microwave power region from none to strong saturation. Simulation of the SAR powder spectrum deviated more from the experimental at saturation, probably because of the presence of other radicals. The findings are discussed in the context of EPR dosimetry. Download full paper here (pdf-file) Back to Top of Page Haitao Li, Anders Lund, Mikael Lindgren, Einar Sagstuen, and Hidenori Yahiro, Motional Dynamics of NO2 on Na-ZSM-5: An ESR Investigation Chem. Phys. Lett. 271, 84 - 89 (1997). Abstract: Electron Spin Resonance (ESR) was used to study the motional dynamics of NO2 adsorbed on Na-ZSM-5 with different SiO2/Al2O3 ratios at 3 to 160 K. The temperature dependence of the ESR spectral line shape was quantitatively analyzed with slow-motion ESR theory. The computer analysis with the Heisenberg spin exchange model reveals that the spin exchange of NO2 below 160K increases quickly as the SiO2/Al2O3 ratio increased from 23.8 to 70 and to 200. It is concluded that the difference on the amount of Na+ among the various zeolites is the main reason for the observed variation in the spin exchange rates. Download reprint here (pdf-file) Back to Top of Page Nimir Ali Salih, Audun Sanderud, Einar Sagstuen, Omer I. Eid and Anders Lund, ESR, ENDOR, and FSE Studies of Hippuric Acid Single Crystals X-irradiated at 295 K: A Reinvestigation J. Phys. Chem. A 101, 8214 - 8220(1997). Abstract: Single crystals of the amino acid analogue hippuric acid, Ph-CO-NH-CH2-COOH, have been X-irradiated at 295 and studied using X-band EPR, ENDOR and FSE (EI-EPR) spectroscopy at 295K and at 130K. Two different radical species were observed and characterized. The dominant species is radical R1, Ph-CO-NH-·CH2, supposedly formed by net decarboxylation from a pristine oxidation product. The nitrogen hyperfine- and quadrupolar interactions yield information on the electronic structure in the nitrogen valence orbitals. The second radical species, radical R2, is formed by a net hydrogen addition to the phenyl entity of hippuric acid. As a reduction product it may be formed by protonation of the negatively charged anion of the phenyl group, but the alternative mechanism of direct hydrogen addition to the phenyl ring can not be ruled out. Spectral simulations indicate that radical R1 contributes about 85% of the total EPR spectrum, while the remaining 15% is contributed by radical R2. Download full paper here (pdf-file) Back to Top of Page Anders R. Sørnes and Nikolas P. Benetis, The EPR Spectrum of the General =C-CX3 Quantum Rotor Chemical Physics 226, 151 - 170(1997). Abstract: The quantum mechanical Liouville equation for the general =C-CX3 rotor is employed to obtain low temperature EPR spectra exhibiting E-line splittings arising from the spin-rotational coupling in the hyperfine interaction. Analytic expression for the low temperature EPR spectrum for the general C3-symmetry rotor is presented. Analytic spectra for I=1/2, I=1 and I=3/2 nuclei-containing rotors are discussed in detail. Intensities and positions of first order transition sidebands are given for the general system and explicitly discussed for I=1/2 and I=1 systems, with high accuracy determination of tunneling frequencies and potential twist angles from low temperature experimental data as possible applications. Anomalous A/E mixing effects on the EPR spectrum in the transition region between tunneling and stopped rotor conditions are demonstrated for I=1/2 and I=1 systems and a particular choice of twist angle. Download full paper here (pdf-file) Back to Top of Page Einar Sagstuen, Eli O. Hole, Sølvi R. Haugedal, and William H. Nelson, Alanine Radicals: Structure Determination by EPR and ENDOR of Single Crystals X-Irradiated at 295 K J. Phys. Chem. A 101, 9763 - 9772 (1997). Abstract: Single crystals of the amino acid l-a-alanine have been irradiated at room temperature, and the free radical formation has been studied using X-band and K-band EPR, ENDOR, and EIE spectroscopy in the temperature interval 220 - 295 K. Aided by the results from EIE, as well as ENDOR from selected magnetic field positions, 9 hyperfine coupling tensors were obtained and assigned to three different radicals. Room temperature relaxation behaviors characterized by efficient W1x and W1e and by slow W1n relaxation rates allowed for determination of the signs of the various hyperfine couplings from the ENDOR spectra obtained at room temperature. The temperature dependence of the W1x relaxation is qualitatively discussed. The EPR spectra from alanine are dominated by the well known resonance of the "stable alanine radical", SAR, formed by a net deamination of the protonated alanine anion. Precise hyperfine coupling tensors due to the a-proton coupling, the methyl group coupling, and a dipolar coupling to a methyl group of a neighboring molecule, as well as the g-tensor, are given for this radical. Spectral simulations show that these parameters in a satisfactory manner reproduce all observable features of the resonance from this radical. Radical R2, apparently formed in roughly the same amounts as SAR, exhibits the structure H3N+- .C(CH3)- C(=O)O-. It is formed from alanine by a net H-abstraction from the Ca position. The hyperfine coupling tensor to the freely rotating methyl group was obtained from both X-band and K-band data. K-band spectra obtained at several temperatures between 220 K and 290 K revealed that the amino group is not freely rotating, i.e. the three protons of the amino group are locked in their hydrogen bonds also after radical formation. A significant increase in ENDOR line widths upon increasing temperature made the ENDOR lines due to the amino protons practically non-observable at 295 K. However, the three corresponding hyperfine coupling tensors were easily obtained from K-band ENDOR data at 220 K. The B0 and B2 values for b-coupling to N+-H fragments were determined to be -4.3 and 117.6 MHz, respectively. Due to partly unresolved nitrogen hyperfine interaction leading to larger line widths, the individual EPR lines from radical R2 is of far less intensity as compared to that of the SAR. However, simulations strongly indicate that there is an almost equal relative distribution (60% : 40%) of the two radicals. Two hyperfine coupling tensors were assigned to two conformations of a third minority radical species (radical R3) which tentatively is suggested to be the species H2N- C(CH3)-C(OH)=O. Possible mechanisms for the formation of the three radicals are discussed in light of the basic radiation chemistry of the amino acids. The simultaneous presence of two stable radicals of similar relative amounts in alanine may have consequences for the use of alanine as a radiation dosimeter. Download full paper here (pdf-file) Back to Top of Page Eva S. Bergstrand, Eli O. Hole, and Einar Sagstuen, A Simple method for Estimating Dose Uncertainty in ESR/Alanine Dosimetry Appl. Radiat. Isotopes 49, 845 - 854 (1998). Abstract: ESR/alanine dosimetry is widely used as a reference, transfer, and routine standard dosimetry system. In order to use it as a reliable standard, or in every situation where a high degree of accuracy is needed, the calculation of the absolute uncertainty is required. Even though general guides for calculating dose uncertainties are available, the actual calculation may be quite complicated. In this paper, an analytical expression for the dose uncertainty for the ESR/Alanine dosimetry system is derived and shown to be rather simple to use. The treatment is valid for doses below ca. 1000 Gy. An alternative, and slightly more time-consuming procedure should be followed if the calibration dose uncertainties are comparable to the uncertainty in the signal per mass. This procedure is described, using the so-called "effective variance" method (Orear, J. (1982) Amer. J. Phys. 50, 912). Both methods are supplied with one numerical example. Suggestions on obtaining the calibration dose parameters and their uncertainties are emphasized. Download full paper here (pdf-file) Back to Top of Page William H. Nelson, Einar Sagstuen, Eli O. Hole, and David M. Close, ESR and ENDOR Study of X-Irradiated Deoxyadenosine: Proton Transfer Behavior of Primary Ionic Radicals Radiation Research 149, 75 - 86 (1998). Abstract: Study of deoxyadenosine crystals (anhydrous form) following irradiation at 10 K found four base-centered and one sugar-centered radical. Radical R1, thermally stable to 100K and photobleachable easily with white light, was the product of deprotonation at the amino by the primary radical cation. Radical R2, also thermally stable to ca. 100K, was the product of protonation at N3 of the primary radical anion. Radical R3 was centered in the deoxyribose moiety, and evidently was the result of net hydrogen abstraction from C4'. Radicals R4 and R5 were the C2 and C8 H-addition products with couplings typical of those species. Both R4 and R5 were formed at 10 K and were stable at room temperature. The behavior of R1 in several systems provides additional evidence for significant involvement of the hydrogen bonding environment in controlling the stabilization (or formation) of radicals directly resulting from ionization, as described previously. (Radiat. Res. 131, 272-284, 1992) From comparison of amino-group hydrogen-bonding environments in which radicals with the structure of R1 were stabilized, we conclude that oxygen atoms as proton acceptors are important in permitting the charge and spin separaration necessary for radical stabilization. In particular, oxygens of ROH-like structures seem most efficient by readily permitting a multi-proton shuffle through a mechanism amounting to proton exchange. The collective results show that stabilization of these products is unlikely unless the charge and spin can be separated by at least one intervening molecule. Back to Top of Page Anders R. Sørnes , Nikolas P. Benetis, Roland Erickson, Ahmed S. Mahgoub, Lennart Eberson and Anders Lund The Effect of Isotopic Substitution on the Electron Spin Dynamics of the Radical in X-Irradiated Methyl Malonic Acid Powder. Intrinsic Potentials and Activation Energies. J. Phys. Chem. A 101, 8987 - 8994 (1997). Abstract: The temperature dependent EPR lineshapes from the methyl rotor fragment of X-irradiation induced CH3.C(COOH)2 radical in powder (Methyl Malonic Acid) MMA and its specifically deuterated analogue CD3.C(COOH)2 are studied experimentally for the temperature range 4.8 K to 77 K and 5.0 K to 65 K, respectively. The protonated system is simulated using a quantum inertial dynamical model with a hindering potential and three-site exchange rotation. The deuterated system is simulated using a classical three-site exchange model. The results show that due to the increase in moment of inertia, the tunneling frequency is negligible for the deuterated rotor, resulting in a stopped rotor low temperature spectrum, while being sufficiently large for the protonated system for this to exhibit tunneling. From the low temperature deuterated analogue spectrum, the potential twist angle is estimated to {50+n*60}(+/-)2 degrees, for integer n. The site-exchange activation energy of the deuterium rotor is observed to be 387 K, substantially lower than the hindering potential depth of 618 K. The protonated system exchange rotation rate assumes a linear behavior with 754 K activation energy in the classical region of temperatures above 50 K. In the tunneling region below this temperature it follows a linear relationship with an activation energy of 275 K. Download full paper here (pdf-file) Back to Top of Page Eli O. Hole, Einar Sagstuen, William H. Nelson, and David M. Close, Electron Paramagnetic Resonance and Electron Nuclear Double Resonance Studies of X-Irradiated Crystals of Cytosine Hydrochloride. Part I: Free Radical Formation at 10 K following high radiation doses Radiation Research 149, 109 - 119 (1998). Abstract: Anhydrous single crystals of cytosine hydrochloride (protonated at N3) have been x-irradiated at 10 K, and studied using K-band EPR, ENDOR and FSE spectroscopy. At least seven radicals were present at 10 K following x-irradiation with a dose of ca 150 kGy. Two of these are radical anions; an amino-protonated anion (R1), and the pristine anion (R2) with zero net charge. Three versions of the radical cation were observed, the amino-deprotonated cation (R3), an N3-deprotonated cation (R4), and an N1-deprotonated cation (R5). Finally, two products formed by net H-addition to the cytosine base were observed, a C5 H-addition radical (R6), and a C6 H-addition radical (R7). The crystalline lattice of cytosine hydrochloride is characterized in part by a cytosine base initially protonated at the N3-position, and in part by an extended network of H-bonding involving chlorine anions. Proton transfer properties of pristine anions and cations in this lattice are discussed and suggested to explain the unusual multitude of positions for cation deprotonation as well as for the two conformations of anions observed. The magnetic parameters for the amino-protonated anion R1 agree well with those extracted from previous studies of cytosine derivatives in frozen solutions and various glasses. Download full paper here (pdf-file) Back to Top of Page Einar Sagstuen, Eli O. Hole, William H. Nelson, and David M. Close, Radiation Damage to DNA Base Pairs. II. Paramagnetic Resonance Studies of 1-Methyluracil . 9-Ethyladenine Complex Crystals X- irradiated at 10 K Radiation Research 149, 120 - 127 (1998). Abstract: Single crystals of the co-crystalline complex of 1-methyluracil and 9-ethyladenine were X-irradiated and studied using EPR, ENDOR and FSE spectroscopic techniques at 10 K. Altogether seven radicals were identified, and experimental evidence for at least one more species, as well as a very low population of radical pairs are available. Oxidation and reduction products appear to be stabilized at both base constituents of the pair. Of the uracil moiety, the product formed by net H- abstraction from the methyl group was observed (MUEA1), together with the uracil anion (MUEA2) and the uracil-5-yl radical (MUEA3). From the adenine moiety, the N3-protonated adenine anion (MUEA4) is stabilized. In addition, the adenine cation (MUEA5) as well as traces of the amino- deprotonated cation (MUEA6) was observed, together with the C8-H hydrogen adduct (MUEA7). The presence of oxidation and reduction products in each of the two bases indicates that negligible energy transfer between them takes place. This is a behavior different from that observed in the similar pair of 1-methylthymine 9-methyladenine. There also seems to be minor proton exchange between the two stacks of molecules: interbase protonation-deprotonation channeled through the hydrogen bonding scheme seems to be almost completely suppressed. Download full paper here (pdf-file) Back to Top of Page Anders R. Sørnes and Nikolas P. Benetis, Theory of ESEEM in Isotropic Tunneling Methyl Rotor Systems. Chemical Physics Letters 287, 590 - 596 (1998). Abstract: The first order A- and E-state mixing in tunneling methyl rotors as mechanism for observable Electron Spin-Echo Envelope Modulation (ESEEM) spectra is demonstrated. Explicit formulas for the ESEEM frequencies and intensities are obtained using a secular model. At low temperatures in systems where the hyperfine coupling a is exceeded by the tunneling frequency 3D , the results simplify to the two positive weight components at 3D ± a/4 and the two negative weight components at 6D and a/2 of half intensity. Numerical ESEEM simulations based on the diagonalization of the full spin rotation Hamiltonian support these findings. The generalization to higher spin rotors is pointed out. Download full paper here (pdf-file) Back to Top of Page Terrence J. Baine and Einar Sagstuen, Radiation-Induced Solid State Radical Formation in b-Glycerophosphate by EPR and ENDOR Spectroscopy at 77 K and Room Temperature: Formation of an Allylic Type Radical. Radiation Research 150, 148 - 158, 1998. Abstract: Single crystals of disodium b-glycerophosphate pentahemihydrate (bGP), Na2.CH2OH-CH(OPO3--)-CH2OH.5½H2O, were x-irradiated at 77 K and 280 K. EPR, ENDOR and FSE techniques were used to study the free radical formation in these irradiated crystals in order to characterize possible reaction mechanisms leading to dephosphorylation. 77 K irradiation reveals the presence of four different radicals; two alkoxy radical conformations AR#1 and AR#2,CH2OH-CH(OPO3--)-CH2O. , and two carbon centered hydroxyalkyl radicals, LTR#1 , and LTR#2 with a tentative structure CH2OH-CH(OPO3--)-CHOH. AR#1 and AR#2 are determined to be formed on each of the two independent molecules A and B of the asymmetric unit of bGP, each on the 3-end of the molecule. Irradiation at 280 K reveals the presence of three hydroxyalkyl allyl radicals, R1, R2, and R3, with the common chemical structure CHOH-CH-CHOH . Radicals R1 and R2 are determined to form on Molecules B and A of the asymmetric unit, respectively. The site of radical formation for R3 could not be absolutely ascertained from the available data, and there is also evidence that suggests the possibility that R3 has a hydroxyphospho-allyl radical structure. Possible reaction mechanisms for the formation of both the 77 K radicals and the 280 K radicals are suggested and discussed. The formation of an allyl-type radical in such a small single crystal model system is an interesting and surprising result which may have relevance to the formation of the so-called 3aH radical species commonly observed in solid cytosine nucleosides and nucleotides. It is also an important result since the formation of the hydroxyalkyl allyl radical in solid b-glycerophosphate is at variance with mechanisms and products shown to dominate for this and similar systems in solution radiolysis. Back to Top of Page David M. Close, Eli O. Hole, Einar Sagstuen and William H. Nelson, EPR and ENDOR Studies of X-Irradiated Single Crystals of Deoxycytidine 5'-Monophosphate at 10 K. Journal of Physical Chemistry A 102, 6737 - 6744 (1998). Abstract: The EPR spectra of single crystals of Deoxycytidine 5'-phosphate monohydrate (5'dCMP), x-irradiated at 10 K, exhibit signals from several distinct radical species. Analysis of the ENDOR spectra from two of these radicals indicates that these result from oxidation and reduction of the cytosine base. The reduced species exhibits hyperfine coupling to the C6-H proton, and an additional small exchangeable hyperfine coupling from the N3-H proton. No additional couplings that may be associated with protonation of the amino group have been observed. Since the native molecule is protonated at N3, it appears that reduction of the cytosine base does not result in any further protonation. The oxidized species exhibits hyperfine couplings to C5-H , C1'-H , and two small exchangeable couplings from the C4-NH2 protons. Since no hyperfine coupling to N3-H was observed, the oxidized species is believed to be the N3-H deprotonated cation. At high x-ray dose there is also evidence for both C5 and C6 H-addition radicals at 10K. The fate of these radicals has been studied under controlled warming conditions. Attempts have been made to relate the fate of the low temperature radicals with several radicals that have been detected previously at 77 K and at room temperature. Download full paper here (pdf-file) Back to Top of Page Torbjørn Furre, Eva S. Bergstrand, Eivind Pedersen, Marianne Koritzinsky, Dag R. Olsen, Eli O. Hole and Erik O. Pettersen, Measurement of Dose Rate at the Interface of Cell Culture Medium and Glass Dishes by means of ESR Dosimetry using Thin Films of Alanine. Radiation Research 152, 76 - 82 (1999). Abstract: Background and Purpose: Previous studies on human cervix cancer cells, NHIK 3025, have indicated that cells, when X-irradiated in suspension, appeared to be more radiosensitive than when irradiated attached to glass dishes. This result is, however, depending on dosimetry, which is difficult in the set-up where cells are attached to glass dishes due to backscattering electrons at the glass-liquid interface. Recently developed dosimetry, based on detection of radiation induced stable radicals in alanine, using electron spin resonance-spectroscopy, offer a possibility of more relevant dosimetry at the glass-liquid interface, than the previous estimates of doses based on Fricke dosimetry. Material and Methods: Thin alanine films (10 um and thicker) were used to measure dose at the interface by irradiating the films while they were placed tight to the bottom of dishes and covered with 1 mm of vax simulating the medium above cells. Also Fricke dosimetry was performed, with different depths of Fricke solution in the dishes, to throw into relief the contribution to the dose delivered by backscattering electrons at the glass-liquid interface. Results: A dose rate of 1.9 Gy per minute was measured with a thin layer (0.2-0.3 mm) of Fricke solution in Petri dishes made of glass. However, this estimate seems to be too high, due to a contribution to dose by short-ranged electrons generated in a steel lid 4.5 cm above the dishes. Dosimetry by use of alanine films resulted in dose rates of 1.2 and 0.9 Gy per minute at the interfaces of respectively glass-liquid and plastic-liquid. Hence, there is a significant contribution to dose from backscattering electrons on dishes made of glass. Conclusions: The reason for our previous observation of a difference in radiosensitivity between cells irradiated in suspension as compared to cells irradiated attached to glass, seems to be lack of accurate dosimetry at the glass-liquid interface. Back to Top of Page Haitao Li, Hidenori Yahiro, Kenji Komaguchi, Masaru Shiotani, Einar Sagstuen and Anders Lund, ESR Studies of the Effect of Zeolite Structures on Motional Dynamics of NO2. Microporous and Mesoporous Materials 30, 275 - 281 (1999). Abstract: Electron Spin Resonance (ESR) spectroscopy was used to study the effect of zeolite structure on the motional dynamics of NO2 adsorbed on the zeolites. The temperature-dependent ESR spectral line shapes were quantitatively analysed using slow-motion-ESR theory. It was observed that the motional dynamics of NO2 is strongly dependent on the structure of zeolite (Beta-type, ZSM-5, Mordenite, L-type and Ferrierite zeolites). The following important observations were made: (1). In similar channel structural zeolites, the diffusion rate of NO2 is proportional to the channel size, the orders of the diffusion rates are Beta-type > ZSM-5 > Ferrierite and L-type > Mordenite, (2). The diffusion of NO2 is faster in the multiple-channel structural zeolites (Beta-type, ZSM-5 and Ferrierite) than that in the single-channel zeolites (L-type and Mordenite). Download full paper here (pdf-file) Back to Top of Page Audun Sanderud and Einar Sagstuen EPR and ENDOR Studies of Single Crystals of a-Glycine X-Irradiated at 295 K. Journal of Physical Chemistry B 102, 9353 - 9361 (1998). Abstract: Single crystals of the amino acid a-glycine, H3N+-CH2-COO- were x-irradiated at 280 K and studied at 295 K and at 100 K using EPR, ENDOR and EIE techniques. 5 radicals were detected and characterized. Two of these are well known from previous studies, the oxidation product H3N+-CH-COO- (Radical I) and the reduction product CH2-COO- (Radical II). It is shown that at 295 K the ENDOR and EIE spectra from Radical I are characteristic of efficient W1x cross-relaxation induced by rapid rotation of the amino group. This allows for the determination of the absolute signs of the hyperfine coupling constants of Radical I. The other three radicals are two geometrical conformations of the product H2N-CH-COOH (Radicals III and IV) and a species suggested to be the dimer product H3N+-CH2-CO-CH-COO- (Radical V). Radical III exhibits spectral parameters related to those previously reported by M. Brustolon et al. (J. Phys. Chem. 1997, 101, 4887). However, based on our analysis of ENDOR and EIE spectra from normal as well as from partially deuterated crystals, the structural assignment for Radical III is different from that suggested by these authors. A comparison with a new radical species recently observed and characterized in irradiated crystals of the amino acid l-a-alanine indicates that the two radicals probably exhibit similar structures. A review of the mechanistic aspects of radical formation in glycine is given, and the newly detected species (Radicals III-V) are tentatively fitted into this scheme. Download full paper here (pdf-file) Back to Top of Page Tomoya Yamada, Kenji Komaguchi, Masaru Shiotani, Nikolas P. Benetis and Anders R. Sørnes High Resolution EPR and Quantum Effects on CH3, CH2D, CHD2 and CD3 Radicals under Argon-Matrix Isolation Conditions. Journal of Physical Chemistry A 103, 4823 - 4829 (1999). Abstract: High-resolution EPR spectra of CH3, 13CH3, and even CH2D radicals with their natural abundances have been observed in Ar matrix in the temperature range 4.2 to 40 K. This was achieved by X-ray radiolysis of Ar matrix containing 0.1 mol% CH4. The high-resolution EPR spectra of CD3 and CHD2 radicals were also obtained under similar conditions. At the lower temperatures the EPR lineshapes of these radicals are dominated by anomalous-intensity hyperfine (hf) patterns attributed to quantum effects. Upon increasing temperature the EPR lineshapes turn rapidly to regular appearance, characteristic of fast motion. The application of the Pauli principle in combination to D3 point group symmetry results to interesting exclusion of EPR-transitions for both the alpha-proton- and the alpha-deuteron-rotor spectra. In contrast to the ß-proton methyls >.C-CH3, the hf coupling is anisotropic and no rotation-hindering barrier is present here. The "E" lines of the corresponding isotropic ß-proton methyl rotor [A.R. Sørnes, N.P. Benetis, Chem. Phys. 226 (1998) 151] are absent from their regular positions. The deuteron rotor is giving a peculiar spectrum at the lowest experimental temperature, i.e. an extremely strong central singlet superimposed on a much weaker fast motional spectrum. The quantum effects are attributed to spin-rotation coupling through the anisotropic part of the hf interaction and exchange symmetry of at least two identical fermions or bosons in the studied radicals. The experimental findings are consistent with a three-dimensional, free quantum-rotor motional model. The problem is treated theoretically by using a mixed spin-rotation hamiltonian. Download full paper here (pdf-file) Back to Top of Page Eli O. Hole, Einar Sagstuen, William H. Nelson and David M. Close Free Radical Formation in X-Irradiated Crystals of 2'Deoxycytidine Hydrochloride. Electron Magnetic Studies at 10 K. Radiation Research 153, 823-834 (2000). Abstract: Single crystals of deoxycytidine hydrochloride (CdR HCl) have been irradiated at 10 K to doses of about 150 kGy and studied using 24 GHz (K-band) EPR, ENDOR and FSE spectroscopy. In this system, the cytosine base is protonated at the N3 position. Nine different radicals were characterized and identified. Three of these are ascribed to three versions of the one-electron reduced species, probably differing in their protonation state. Radicals formed by net hydrogen addition to the cytosine C5- and C6-positions were observed at 10 K. The H-abstraction radical at the deoxyribose C1'-position most probably results from initial oxidation of the base. The remaining radical species are all localized to the sugar moiety, representing products formed by net hydrogen abstraction from three of the five available carbons of the deoxyribose sugar. The lack of base-centered oxidation products as well as the structures of the one-electron reduced species are rationalized by considering the specific proton donor-acceptor properties of this crystalline lattice in comparison with other similar systems. Download full paper here (pdf-file) Back to Top of Page David M. Close, Einar Sagstuen, Eli O. Hole and William H. Nelson Comments on "Effects of Ionizing Radiation on Crystalline Cytosine Monohydrate". Journal of Physical Chemistry B 103 , 3049 - 3050 (1999) Abstract: Not available Download full paper here (pdf-file) Back to Top of Page Oddvar Myhre, Tor Arne Vestad, Einar Sagstuen, Halvor Aarnes and Frode Fonnum Effects of the Solvent 1,2,4-Trimethylcyclohexane on Respiratory Burst in Human Neutrophil Granulocytes. A Chemiluminescense and Electron Paramagnetic Resonance (EPR) Study Ann. New York Academy of Sciences 893 , 358 - 361 (1999) Abstract: We investigated the effects of 1,2,4-trimethylcyclohexane (TMCH, 10-50 µM), n-nonane (10-50 µM) and 1,2,4-trimethylbenzene (TMB, 0.05-0.5 mM) on free radical production in vitro in human neutrophil granulocytes and rat brain synaptosomes. Human granulocytes and rat brain synaptosomes were incubated with 2',7'-dichlorofluorescein diacetate prior to exposure of the organic solvents. Reactive oxygen species (ROS) production was subsequently measured by chemoluminescence spectroscopy. The solvent-induced elevation of free radicals in human granulocytes were also measured by EPR spectroscopy after incubation with the spin trap DEPMPO. Mechanisms for ROS production was elucidated by use of enzymatic inhibitors. Low concentrations of TMCH, n-nonane and TMB stimulated respiratory burst (production of O2-) in human granulocytes and ROS production in rat brain synaptosomes. The respiratory burst in response to TMCH was dependent of extracellular calcium. U-73122, a phospholipase C (PLC) inhibitor, and 7,7-dimethyleicosadienoic acid, a phospholipase A2 (PLA2) inhibitor lowered the TMCH-stimulated O2- production in granulocytes. Furthermore, bisindolylmaleimide, a protein kinase C (PKC) inhibitor, and diphenyleneiodonium, a NADPH-oxidase inhibitor, reduced the TMCH-stimulated respiratory burst almost completely. In conclusion, these results indicates that TMCH-activated respiratory burst is dependent of Ca2+-dependent PLC, PLA2 and PKC prior to activation of the NADPH-oxidase. Download full paper here (pdf-file) Back to Top of Page Y. Itagaki, R.M. Kadam, A. Lund, E. Sagstuen and J. Goslar Structure of N2H4.+ formed in X-irradiated Li(N2H5)SO4 single crystals. Physical Chemistry and Chemical Physics 2 , 37 - 42 (2000) Abstract: X-irradiated Li(N2H5)SO4 single crystals were investigated using ESR and ENDOR spectroscopy at several temperatures. The hydrazine radical cation N2H4o+ was selectively produced by irradiation at room temperature. From the analysis of the orientation dependent ENDOR spectra, the 1H-hfc tensors of the cation radical were precisely obtained and the radical structure was supported by theoretical calculations. It is suggested that the cation radical has a planar p* structure D2h (2B2g) in the crystal down to 230K. By using the evaluated 1H-hf tensor the powder ESR line shape was successfully simulated. Concomitant with the radical formation, the N-N bond of the N2H4o+ is suggested to reorient so as to optimize hydrogen bond interactions. 1H-ENDOR line splittings for the N2H4o+ radical was observed at temperatures below 230 K. Apparently this splitting is due to a reversible structural change where one of the -NH2 moieties in N2H4o+ becomes slightly bent out of the molecular plane, whereas the other one remains planar. This deformation evidently arises from interactions between the cation radical, adjacent H2N-NH3+ molecules and the SO4-LiO4 framework. Interacting N2H5+···N2H4o+···N2H5+ molecules along the c-axis are proposed to explain the deformation mechanism. Download full paper here (pdf-file) Back to Top of Page Nikolas P. Benetis and Anders R. Sørnes Automatic Spin-Hamiltonian Diagonalization for Electronic Doublet Coupled to Anisotropic Nuclear Spins in One- and Two-Dimensional Electron-Spin-Echo Simulations Concepts in Magnetic Resonance 12, 410-433 2000. Abstract: We explore the possibility of putting together some earlier theoretical inventions in EPR into an efficient method leading to accurate explicit expressions of echo modulation spectra in one and two dimensions. The resulting "automatic" diagonalization of the high-field spin-hamiltonian gives a fast method for Electron-Spin-Echo Modulation computations. It accounts accurately for the effects of non-axial electron Zeeman and hyperfine coupling tensors to an arbitrary system of nuclei in any relative orientations, including also the secular part of the quadrupole tensors in the computation of the modulation frequencies. Explicit expressions for the time-resolved signals, involving a single geometric parameter, particularly useful in economizing powder simulations of multipulse sequences like hyperfine level correlation spectroscopy (HYSCORE), are presented. Finally, implementation in a complete package including fundamental EPR-line shape, primary-echo, stimulated-echo and HYSCORE simulations, and application to computations in experimentally interesting model systems is demonstrated. Download full paper here (pdf-file) Back to Top of Page Oddvar Myhre, Tor Arne Vestad, Einar Sagstuen, Halvor Aarnes and Frode Fonnum The Effects of Aliphatic (n-nonane), Naphtenic (1,2,4-trimethylcyclohexane) and Aromatic (1,2,4-trimethylbenzene) Hydrocarbons on Respiratory Burst in Human Neutrophil Granulocytes. A Fluorescence and Electron Paramagnetic Resonance (EPR) Spectroscopy Study. Toxicology and Applied Pharmacology 167, 222 - 230 (2000). Abstract: This study investigates the effects of aliphatic (n-heptane, n-nonane), naphtenic (methylcyclohexane, 1,2,4-trimethylcyclohexane (TMCH)) and aromatic (methylbenzene, 1,2,4-trimethylbenzene (TMB)) hydrocarbons on respiratory burst in human granulocytes, measured as 2,7-dichlorofluorescein diacetate-amplified fluorescence and by electron paramagnetic resonance (EPR) spectroscopy. The chemotactic peptide N-formyl-met-leu-phe (fMLP) and phorbol 12-myristate 13-acetate (PMA), a diacylglycerol analogue, were included for purposes of positive control. Fluorescence was elevated in a concentration dependent manner by C9 hydrocarbons. The C7 hydrocarbons did not stimulate respiratory burst in the concentration range examined. The naphtenic hydrocarbon TMCH showed strongest potential for elevation of respiratory burst, and was then selected for mechanistic studies of this free radical formation. In the absence of extracellular Ca2+, fluorescence in response to TMCH and fMLP was reduced by 77% and 90%, respectively. Preincubation of the granulocytes with the protein kinase C inhibitor bisindolylmaleimide reduced the fluorescence stimulated with TMCH, fMLP and PMA by 82%, 56% and 90%, respectively. The phospholipase C inhibitor U73122 lowered the TMCH- and fMLP activated fluorescence by 87% and 76%. In addition, the TMCH- and fMLP induced fluorescence, after the preincubation with the phospholipase D modulator n-butanol, was lowered by 83% and 52%, respectively. The importance of protein kinase C, phospholipase C and phospholipase D for elevation of respiratory burst were also demonstrated by the EPR experiments using the spin trap DEPMPO. Preincubation with the NADPH oxidase inhibitor diphenyleneiodonium, and diethyldithiocarbamate, which inhibits superoxide dismutase, led to an almost complete reduction of fluorescence in response to TMCH, fMLP and PMA. Preincubation with diethyldithiocarbamate led to the elevation of superoxide adducts of DEPMPO. In conclusion, our findings indicate that TMCH activated respiratory burst is dependent on the Ca2+ dependent phospholipase C, phospholipase D and protein kinase C prior to activation of the NADPH oxidase. Download full paper here (pdf-file) Back to Top of Page Einar Sagstuen, Anders Lund, Yoshiteru Itagaki and Jean Maruani Weakly Coupled Proton Interactions in the Malonic Acid Radical: Single Crystal ENDOR Analysis and EPR Simulation at Microwave Saturation. Journal of Physical Chemistry A 104, 6362 - 6371 (2000). Abstract: The a-proton hyperfine coupling observed by electron paramagnetic resonance (EPR) spectroscopy on the radical .CH(COOH)2 in irradiated crystals of malonic acid, CH2(COOH)2, has served as a standard against which hundreds of observations of similar couplings have been held and scaled. The major doublet of the malonic acid radical is accompanied by less intense, 'forbidden' (f) a-proton transitions and 'spin-flip' (s) transitions due to weakly interacting protons. Both s- and f-transition lines exhibit microwave power saturation behaviors different from that of the major doublet. At high microwave power, the prominence of these s and f-lines may be misinterpreted as originating from different radical species. Computer simulations could help distinguish between the different cases, but no computer simulation programs taking into account the microwave power saturation case are commonly available. Based on classical line-shape theory, an algorithm describing the microwave power dependency of an EPR line shape has been developed and implemented in an existing simulation program. To test this new program, malonic acid was chosen due to the simplicity of its EPR spectra. However, sufficiently detailed information about the hyperfine coupling parameters for a satisfactory simulation of the room temperature data (including s- and f-lines) was not available in the literature. Therefore, a detailed EPR / ENDOR study on a single crystal of malonic acid was performed. In addition to the major a-proton coupling, 7 weaker proton interactions have been characterized and partly identified. Simulations under non-saturating conditions excellently reproduce all features of the experimental EPR spectra. Simulations under saturating conditions similarly reproduce the power-dependent EPR spectra and yield information of the relaxation behavior of the radical system, amenable to verification using other spin resonance methods. Download full paper here (pdf-file) Back to Top of Page Audun Sanderud, Einar Sagstuen, Yoshiteru Itagaki and Anders Lund EPR and ENDOR Studies of Deuteron Hyperfine and Quadrupole Coupling in .CD(COOD)2: Experimental and Theoretical Estimates of Electric Field Gradients from an a-Carbon. Journal of Physical Chemistry A 104, 6372 - 6379 (2000). Abstract: In single crystals of malonic acid grown from heavy water, the methylene protons have been partially exchanged with deuterons. Upon x-irradiation at room temperature, the .CD(COOD)2 radical is formed in a relative amount comparable to that of the .CH(COOD)2 radical species. In the present work, EPR and ENDOR analyses of the a-deuteron hyperfine coupling and quadrupolar coupling tensors at room temperature have been made. The hyperfine coupling tensor is, when scaled with the differences in the nuclear g-factor, almost identical to the a-proton coupling of the .CH(COOH)2 radical at room temperature. The quadrupolar coupling tensor was found to be virtually coaxial with the hyperfine coupling tensor. The quadrupolar coupling constant is 149.8 ± 1 kHz and the asymmetry factor h = 0.092 ± 0.020. It is known that at room temperature, the malonic acid radical exhibits thermal motion between two potential energy minima separated by about ± 12°. Assuming that the observed hfc and nqc tensors are the result of thermal averaging between these two conformations of the radical, a simple two-site jump model was used to estimate the rigid-limit tensors. The most significant result obtained was for the nqc tensor, where the calculations resulted in a quadrupolar coupling constant of 160 kHz and an asymmetry factor h = 0.026. These values are fairly close to the nqc parameters for the methylene deuterons in malonic acid at low temperature. The quadrupolar coupling tensor has been theoretically modeled using Slater orbitals and formal electronic populations, as well as electron populations obtained from RHF/CI INDO-type calculations. The simple model to compute the electric field gradient at the a-deuteron caused by the charge distribution at the sp2 hybridized a-carbon was found to be as successful as more advanced methods. Furthermore, density functional theoretical (DFT) calculations for both the malonic acid radical and the native malonic acid molecule have been carried out. Field gradients calculated by the DFT method significantly overestimate of the quadrupolar tensors for both the a-deuteron of the radical and the methylene deuterons for the malonic acid molecule. Calculations using electron populations from the RHF/CI INDO calculations show that contributions to the quadrupolar coupling tensor from electrons and nuclei beyond the nearest neighboring atom of the deuteron are significant. Download full paper here (pdf-file) Back to Top of Page David M. Close, Leif A. Eriksson, Eli O. Hole, Einar Sagstuen and William H. Nelson Experimental and Theoretical Investigation of the Mechanism of Radiation Induced Radical Formation in Hydrogen Bonded Co-Crystals of 1-Methyl-Cytosine and 5-Fluoro-Uracil. Journal of Physical Chemistry B 104, 9343 - 9350 (2000). Abstract: The process of stabilizing radiation damage in a base pair of 1-Methylcytosine:5-Fluorouracil has been investigated. The formation of free radicals in purines and pyrimidines is influenced by the matrix in which they are irradiated. Of particular interest are the systems in which two different nucleic acid bases are complexed together, providing situations which approximate the close proximity of bases in nucleic acid polymers. It is shown from detailed EPR/ENDOR experiments that only the N3 protonated cytosine anion and the N1 deprotonated uracil cation are observed in single crystals of 1-MeC:5-FU, x-irradiated and observed at 10 K. Upon warming one observes a radical formed by net hydrogen addition to C6 on uracil, and an allylic radical on the C4-C5-C6 region of the uracil. No cytosine C5 or C6 H-addition radicals are observed. The implications that free radical damage is transferred from the cytosine moiety to the uracil moiety in 1-MeC:5-FU is discussed. Single point calculations were performed on the optimized geometries at the B3LYP/6-311G(2df,p) level, to obtain accurate energies and spin populations. To obtain electron affinities of the neutral parent molecules, the larger 6-311+G(2df,p) basis set was employed. Results show that indeed the cytosine base will be the preferred site of electron addition, and the uracil base will be the site of electron loss. Further studies have also been conducted on the influence of the hydrogen bonded matrix on these base pairs. It is shown how a proton shuttle in the matrix can be used to effectively move the initial charge away from the sites of initial electron addition or electron loss. This means that after electron gain, it is energetically favorable for the cytosine anion to protonate via the hydrogen bond matrix, and at the same time, for the uracil cation to deprotonate, leaving behind neutral radical sites that are less susceptible to recombination. Download full paper here (pdf-file) Back to Top of Page Janko N. Herak, Kresimir Sankovic, Eli O. Hole and Einar Sagstuen ENDOR Study of a Thiocytosine Oxidation Product in Cytosine Monohydrate Crystals Doped with 2-Thiocytosine, X-irradiated at 15 K. Physical Chemistry and Chemical Physics 2, 4971 - 4975 (2000). Abstract: ENDOR spectroscopy has been used for the analysis of radiation induced electron-loss radicals in thiocytosine. The radicals are induced by X-irradiating single crystals of cytosine monohydrate, substitutionally doped with 2-thiocytosine. X-irradiation and measurements were carried out at about 15 K. The electron-loss radicals are found to be associated with the thiocytosine molecules in a proportion much above the thiocytosine/cytosine ratio in the lattice, the concentration enhancement factor is estimated to be more than 45. The significantly elevated spectroscopic splitting factor (g) for the sulfur-centered radicals in the EPR spectra make these radicals easily discriminated from the spectra of other radicals present in the system. The present study is the first detailed ENDOR study of one of the sulfur-centered radicals selectively formed in an appropriate related crystal lattice as a host. Six different intra- and inter-molecular proton couplings were detected. The respective coupling tensors have been determined, and the couplings assigned to protons in the thiocytosine N1-deprotonated cation radical (S1) itself and in its environment. The structure of radical S1 is analogous to the related cytosine radical in irradiated pure crystals of cytosine monohydrate. However, the two radicals differ in their g values and in the spin density distribution. In the thiocytosine-derived radical gmax(S1) = 2.132, compared to 2.005 for the corresponding radical in cytosine. In radical S1 most of the spin density (~ 65-70%) is located at sulfur, and rather little at the ring atoms: 0.21 at C5 and about 0.14 at N1, compared to spin densities of 0.58 at C5 and 0.30 at N1 in the corresponding radical in cytosine. A concentration enhancement of > 45 suggests long-range hole transfer through the lattice. Assuming that this takes place mainly by electron transfer through vertically overlapping (stacked) bases, preliminary considerations shows that the effective range for the hole transfer is more than 22 base separations, or more than 7 nm. Download full paper here (pdf-file) Back to Top of Page Yoshiteru Itagaki, Audun Sanderud, Einar Sagstuen and Anders Lund ENDOR study of 14N hyperfine and quadrupole couplings of N2D4.+ formed in deuterated Li(N2H5)SO4 single crystals. Journal of Physical Chemistry A 106, 2617 - 2622 (2002). Abstract: X-irradiated Li(N2D5)SO4 single crystals were investigated using EPR and ENDOR spectroscopy. The 14N-ENDOR spectra of the deuterated hydrazine radical cation N2D4·+ were clearly observed at 240 K. 14N hyperfine (hfc) and nuclear quadrupole (nqc) tensors of the N2D4·+ cation were determined from angular variation ENDOR measurements in the three orthogonal planes of the crystal. The hfc tensor obtained for the two equivalent 14N atoms is very close to that reported in a N2H5HC2O4 single crystal and the present results support the previous conclusion that the cation has a planar structure at 240 K. The 14N nqc tensor was estimated using semiempirical methods, and also calculated from the field gradients in the LiHzS crystal as evaluated by Density Functional Theory methods. The general agreement with experimental observations further supported the suggested geometrical structure of the N2D4·+ radical. The 1H and 14N ENDOR enhancements observed are probably due to weakened dipolar interactions between 14N-2D and 1H-2D leading to increased T1n relaxations of the 14N and residual 1H nuclei of the N2D4·+ radical as compared to those of the N2H4·+ radical in the Li(N2H5)SO4 crystals. Download full paper here (pdf-file) Back to Top of Page Janko N. Herak, Kresimir Sankovic, Eli O. Hole and Einar Sagstuen ENDOR Study of the Chlorinated Thiocytosine Radical in a Crystal Matrix Physical Chemistry and Chemical Physics 3, 4926 - 4931 (2001) Abstract: Interaction of protons with the unpaired electron in the s2s*1 three-electron S-Cl bond has been investigated by electron-nuclear-double resonance (ENDOR) spectroscopy. The S-Cl bond in the chlorinated thiocytosine radical, imbedded in the crystal lattice of cytosine hydrochloride, interacts significantly with at least seven protons. Five of these interactions have been analyzed. From the nature of the coupling tensors, it is concluded that three couplings are due to protons being constituents of the chlorine-thiocytosine radical, and the remaining two are due to protons belonging to neighboring cytosine molecules. The nature of the dipolar component of the coupling tensors is not consistent with the location of the entire electron spin density in the S-Cl bond. From isotropic as well as anisotropic hyperfine coupling tensor components, it is concluded that the unpaired electron is somewhat delocalized over the radical, with a major portion being concentrated in the C(2)-S-Cl region but with a significant fraction occupying a p* molecular orbital extending over the cytosine ring atoms. ab initio molecular orbital calculations (DFT) support such kind of spin distribution. The data also suggest that the radical remain protonated at both the N(1) and N(3) positions. Download full paper here (pdf-file) Back to Top of Page Sara Olsson, Einar Sagstuen, Marco Bonora and Anders Lund EPR Dosimetric Properties of 2-Methylalanine: EPR, ENDOR and FT-EPR Investigations Radiation Research 157, 113 - 122 (2002) Abstract: In order to find an EPR dosimeter material sensitive enough for clinical use, the substance 2-methylalanine (2MA) with the chemical structure (CH3)2C(NH3+)COO- has been tested regarding sensitivity to ionizing radiation, dose response, and radical stability over time. At equal and moderate settings of microwave power and modulation amplitude, 2MA was found to be 70% more sensitive than L-a-alanine, which is the most common EPR dosimeter material today. The dose response is linear in at least the entire dose range of interest (0.5-100 Gy), and the time-dependent variations in signal intensity are very small and may easily be corrected for. The energy dependency of stopping power and energy absorption was calculated and found similar to that of alanine. The dependence of signal intensity on microwave power and modulation amplitude was investigated and the optimal settings were found to be 25 mW and 12 gauss, respectively. Single crystals of 2MA were analyzed using ENDOR and ENDOR induced EPR to identify the formed radiation induced radicals. Only one radical, where the amino group is detached from the original molecule, was identified. This radical is obviously dominating, and is apparently the only one relevant for dosimetry purposes. The complete set of coupling parameters for three hyperfine couplings is reported. Power saturation properties and spectral line width are ruled by the relaxation times T1 and T2. To determine the relaxation times of 2MA, pulsed EPR experiments on single crystals were performed. Two different values of T1 were obtained, one in the range 1-3 µs, shown to be of importance for the dosimetry properties, another strongly anisotropic with a value between 10 and 35 µs, that does not seem to affect the saturation behavior. T2 was estimated to be in the order of 200-300 ns. Download full paper here (pdf-file) Back to Top of Page Sara Olsson, Eva S. Bergstrand, Åsa K. Carlsson, Eli O. Hole and Eva Lund Radiation Dose Measurements with Alanine/Agarose Gel and Thin Alanine Films around a 192Ir Brachytherapy Source, using ESR Spectroscopy Physics in Medicine and Biology 47, 1333-1356 (2002) Abstract: Alanine/agarose gel and alanine films in stacks have been used for measurements of absorbed dose around a HDR 192Ir-source in a vaginal cylinder-applicator, with and without a 180 tungsten shield. The gel and the films were analysed by means of ESR spectroscopy and were calibrated against an ion chamber in a 4 MV photon beam to obtain absolute dose values. The gel serves as both dosimeter and phantom material, and the thin (130 µm) films are used to achieve an improved spatial resolution in the dose estimations. Experimental values were compared with Monte Carlo simulations using EGS4 and a non-public photon transport code. Results from the measurements generally agree with the simulations to within 5%, for both the alanine/agarose gel and the alanine films. Download full paper here (pdf-file) Back to Top of Page Mojgan Z. Heydari, Eirik Malinen, Eli O. Hole and Einar Sagstuen Alanine Radicals, Part 2: The Composite Polycrystalline Alanine EPR Spectrum studied by ENDOR, Thermal Annealing and Spectrum Simulations. Journal of Physical Chemistry A 106, 8971 - 8977 (2002). Abstract: Radiation induced free radical formation in the amino acid L-a-alanine has been studied using powder and single crystal X, K- and Q-band EPR spectroscopy, X-band powder ENDOR, thermal annealing and EPR spectrum simulations. The spectra obtained after room temperature irradiations are composite, consisting of resonances from mainly three radicals denoted R1, R2 and R3. R1 is the well known, stable room temperature species formed by deamination from a protonated alanine anion radical. Based on simulations of EPR spectra obtained at X, K,- and Q-bands it appears that the room temperature EPR spectrum consists of about 55% of R1. Upon thermal annealing, the R1 resonance disappears faster than those of the other two components. The R2 species is presumably formed in the oxidative chain of radiation induced events by net H-abstraction from the central alanine carbon atom. Q-band EPR was used to determine the g-tensor of R2. This species contributes about 35% to the resonance recorded at room temperature. Upon thermal annealing this radical decays slower than R1, resulting in the predominance of this species in spectra obtained after prolonged warming at 480 K. Powder ENDOR was used to verify that the dominating species remaining after thermal annealing at this temperature indeed is R2 and not a successor species of either of the room temperature radicals. The R3 species was previously assigned to an N-deprotonated version of R2 being additionally protonated at the carboxyl group. Detailed spectral data for this resonance are missing but a set of parameters based on available data and otherwise estimated using literature values for similar products was constructed. Simulations indicated that 5-10% of the room temperature resonance could be ascribed to R3. R3 is more heat-resistant than the R1 and R2 radicals, and after prolonged annealing at 480K it was estimated that the resulting resonance consisted of about 51% R2 and 43 % R3. The remaining part (about 6%) of the resonance was due to R1. These numbers must, however, be considered as tentative due to the lack of precise spectral data for R3. Download full paper here (pdf-file) Back to Top of Page Eirik Malinen, Mojgan Z. Heydari, Einar Sagstuen and Eli O. Hole Alanine Radicals, Part 3: Properties of the Components Contributing to the EPR Spectrum of X-irradiated Alanine Dosimeters. Radiation Research 159, 23 - 32 (2003) Abstract: The amino acid L-a-alanine has attracted considerable interest for use in radiation dosimetry and has formally been accepted as a secondary standard for high dose and transfer dosimetry. Recent results have shown that the alanine EPR spectrum consists of contributions from three different radicals. A set of benchmark spectra describing the essential spectral features of these three radical components were used for reconstructions of the experimental spectra. In the present work, these basis spectra have been used to investigate the differential effects of variations in radiation doses and microwave power, as well as upon temperature annealing and UV illumination. The results presented here, solely based on relatively low-energy (60 – 80 keV) X-rays, indicate that the three components behave very similarly with respect to radiation doses at room temperature. However, with respect to the thermal annealing/fading behaviour and microwave power saturation properties, the three species behave significantly differently. It is concluded that even if it is now realized that three different radicals contributes to the composite EPR alanine spectrum, this has minor impact on the established protocols for present-day applications (high-dose) of EPR/alanine dosimetry. However, some care should be exercised when e.g. constructing calibration curves, as fading and power saturation behaviour may vary over the dose range in question. New results from UV-illumination experiments suggest a possible procedure for experimental spectral separation of the EPR signals due to the three radicals. Download full paper here (pdf-file) Back to Top of Page Eirik Malinen, Elin A. Hult, Eli O. Hole and Einar Sagstuen Alanine Radicals, Part 4: Relative Amounts of Radical Species in Alanine Dosimeters Following Exposure to 6-19 MeV Electrons and 10 kV-15 MV Photons. Radiation Research 159, 149 - 153(2003) Abstract: The amino acid L-a-alanine can be used for high precision dosimetry over a wide dose range, using EPR spectroscopy for monitoring radical concentrations. It is, however, important to understand the underlying composition of the observed EPR spectrum. In previous work it was shown that the EPR-signal from irradiated alanine consists of at least three different radical species, with the relative importance of each of these being almost independent of absorbed dose. However, it was not known whether the relative importance of each radical is independent of the radiation quality responsible for the EPR signal. In the present work, the relative contributions of the different radical species to the total EPR signal from alanine dosimeters irradiated with 6-19 MeV electrons and 10 kV-15 MV photons to a dose of 10 Gy have been examined. By a spectrum reconstruction procedure using benchmark spectra generated from a simulation procedure, relative amounts of the three different radical species were shown to be virtually independent of these radiation beam qualities. Download full paper here (pdf-file) Back to Top of Page Jannike M. Andersen, Oddvar Myhre, Halvor Aarnes, Tor A. Vestad and Frode Fonnum Identification of the Hydroxyl Radical and other Reactive Oxygen Species in Human Neutrofil Granulocytes Exposed to a Fragment of the Amyloid Beta Peptide. Free Radical Research 37, 269 - 279 (2003) Abstract: A fragment of the amyloid beta peptide, betaA(25-35), was investigated for its effect on production of reactive oxygen species (ROS) in human neutrophil granulocytes. The formation and identification of ROS were examined by using a 2´,7´-dichlorofluorescin (DCF) fluorescence assay, a luminol chemiluminescence assay, electron paramagnetic resonance (EPR) spectroscopy with DEPMPO as a spin trap, and hydroxylation of 4-hydroxybenzoate (4-HBA). The DCF assay showed that betaA(25-35) stimulated formation of ROS in a concentration and time dependent manner. The inverted peptide, betaA(35-25), gave no response. Also luminol-amplified chemiluminescence was stimulated by betaA(25-35). Incubation with diethyldithiocarbamate (a superoxide dismutase inhibitor) and salicylhydroxamate (SHA; a myeloperoxidase inhibitor) reduced the chemiluminescence. This indicates that hypochlorous acid (HOCl) is formed after exposure to betaA(25-35). The EPR spectra indicated a concentration dependent formation of superoxide (O2.-)- and hydroxyl (.OH)- radicals. Hydroxylation of 4-HBA to 3,4-dihydroxybenzoate confirmed production of .OH. This response was attenuated by SHA, indicating involvement of HOCl in formation of .OH. The DCF fluorescence was inhibited with U0126 (an extracellular signal-regulated protein kinase (ERK) inhibitor). Further analysis with western blot confirmed phosphorylation of ERK1/2 after exposure to betaA(25-35). The phospholipase A2 (PLA2) inhibitor 7,7-dimethyl-(5Z,8Z)-eicosadienoic acid, and diphenyleneiodonium, which inhibits the NADPH oxidase, also led to a reduction of the DCF fluorescence. The present findings indicate that betaA(25-35) stimulates the NADPH oxidase by activating the ERK pathway and PLA2. Production of O2.-can lead to HOCl and further formation of .OH, which both have a cytotoxic potential. Download reprint here (pdf-file) Back to Top of Page Eva Stabell Bergstrand , Ken R. Shortt, Carl K. Ross and Eli Olaug Hole The photon energy dependence of the Alanine/EPR dosimetry system, an experimental investigation. Standards and Codes of Practice in Medical Radiation Dosimetry. Proceedings of an International Symposium, IAEA, Vienna; 405-413 (2003). Abstract: Electron paramagnetic resonance (EPR) dosimetry with the amino acid alanine as dosimeter material is widely used for transfer dosimetry, mostly for 60Co gamma-irradiation. The alanine dosimeters are nearly water equivalent and with the efforts of recent years to improve the accuracy for low doses, the alanine/EPR dosimetry system represents a promising candidate for clinical application. 60Co gamma-irradiation is often used for calibrating the alanine dosimetry system to make it traceable for that energy. As the photon energies used in radiation therapy are often different from those of 60Co, it is important to verify if the response of the alanine dosimeters relative to absorbed dose to water varies with photon energy. In the present work the energy dependence of alanine dosimeters (containing polyethylene as a binder) has been investigated experimentally using three photon x-ray beams of 10, 20 and 30 MV, with 60Co gamma-rays (1.25 MeV) as a reference. The doses delivered ranged from 8 to 54 Gy and were determined for all the beams using calorimetry. All irradiations were performed at the National Research Council (NRC) in Ottawa, Canada, whereas all EPR measurements and analysis were performed at the EPR laboratory at the University of Oslo, Norway. Comparing the relative dose response of the three high photon energies to that of 60Co gave a maximum deviation of 1.3 % for the case of 10 MV. Thus, it is concluded that the alanine/EPR may have a very small energy dependence at the lowest energy photon x-ray beam investigated. Download full paper here (pdf-file) Back to Top of Page Ali Hosseini, Anders Lund and Einar Sagstuen EPR and ENDOR Studies of Single Crystals of 2-Oxazolidinone X-Irradiated at 295 K. Physical Chemistry and Chemical Physics 4, 6086 - 6091 (2002). Abstract: When single crystals of the 5-membered heterocyclic ring structure 2-oxazolidinone (C3O2NH5) are irradiated at room temperature, the major radical formed (R1) decays during a period of a few hours, leaving a broad, unstructured EPR spectrum not amenable for analysis. Cooling the crystals to about 140 K immediately after irradiation at room temperature allows analysis of the R1 radical by EPR and ENDOR. The radical is formed by a net H-abstraction from one of the two methylene groups of the molecule. A full EPR and ENDOR analysis of four proton interactions (one alpha-coupling, and three beta-couplings) together with ESR evidence for a small nitrogen hyperfine interaction allowed for a precise identification of R1. The results show that the carbon-centered radical R1 is puckered at the radical center. Using DFT calculations together with the experimental EPR and ENDOR results, the torsion angle of the C1-H bond with respect to the N-C1-C2 plane is estimated to be 13-15 degrees (bending angle theta=7 degrees). The DFT calculations reproduced the carbon-bonded proton hyperfine coupling constants satisfactorily but failed to reproduce the experimental results for the nitrogen and nitrogen-bonded proton hyperfine interactions. Download full paper here (pdf-file) Back to Top of Page Kresimir Sankovic, Eirik Malinen, Z. Medinic, Einar Sagstuen and Janko N. Herak Hole Transfer in Crystals of Cytosine Monohydrate: an EPR Study Physical Chemistry and Chemical Physics 5, 1665 - 1670 (2003). Abstract: Single crystals of cytosine monohydrate were used as a model for studying hole transfer in systems related to nucleic acids. Holes (and excess electrons) were induced by ionizing radiation at 15 K. Thiocytosine, imbedded in the cytosine lattice in variable amounts, served as a hole trap. The holes which are stabilized at thiocytosine give rise to an EPR signal well separated from the signals of other radicals present. The ratio of the amount of thiocytosine cations (i.e. trapped holes) to the amount of the remaining radicals is at least 10 times larger than the stoichiometric thiocytosine/cytosine ratio, which displays significant hole transfer from cytosine to thiocytosine and subsequent trapping by the latter. The analysis of the hole transfer range in the lattice of cytosine monohydrate, composed of parallel sheets of cytosine bases, is rather complex. Unlike DNA, where each base in a pi-stack is flanked by one base on each side, each cytosine base in the present system is surrounded by four other bases in a pi-stacked structure. This results in a higher probability of finding a thiocytosine base in a closer proximity to the initially formed hole. Consequently, it seems that the net hole transfer is not larger than 4 inter-base distances in the present system. Download full paper here (pdf-file) Back to Top of Page Eirik Malinen and Einar Sagstuen Radical Formation in Pyrimidine Bases Following X-, Proton- and Alpha-Irradiation Radiation Research 160, 186 - 197 (2003) Abstract: Single crystals of anhydrous thymine (Ta) and cytosine monohydrate (Cm) were irradiated at room temperature using X-rays, 20 MeV protons and 35 MeV a-particles, and the relative distributions of the various radiation-induced stable radicals in the crystals have been investigated. These two crystal systems were chosen due to systematic differences in their molecular packing and hydrogen-bonding network. The radicals stabilized in these systems have previously been identified and analyzed by several authors. Experimental EPR spectra could thus be reconstructed based on simulated benchmark spectra using a fitting procedure, yielding the relative amounts of the different radical species. It was found that the relative amounts of a given radical species varied with the type of radiation used, with differences being most prominent between a-particles and protons or X-rays. In Ta, an increased production of hydrogen addition- or abstraction radicals was found after exposure to a-particles. These radicals are believed to be formed predominantly from superexcited states, resulting from the higher density of ionizations along the track of the ?-particle. A corresponding reduction in radicals derived from proton transfer reactions was observed in comparison with exposure to protons or X-rays. In Cm, the differences were smaller than for Ta. This is probably due to differences in the arrangement of the crystal lattice between the two systems, with Cm having an extended hydrogen-bonding network promoting fast proton transfers following ionization. Most interestingly, however, was the observation that more than 50% of the radical population in Cm not could be accounted for by known radical species. The population with an unknown origin was represented with a broad singlet, having a linewidth of 1.5 mT and a g-value of 2.0045, in the spectral reconstruction procedure. Download full paper here (pdf-file) Back to Top of Page Eva Stabell Bergstrand, Ken R. Shortt, Carl K. Ross and Eli Olaug Hole An investigation of the photon energy dependence of the EPR alanine dosimetry system Physics in Medicine and Biology 48, 1753 - 1771 (2003) Abstract: The EPR alanine dosimetry system is based on EPR measurements of radicals formed in alanine by ionising radiation. The system has been studied to determine its energy dependence for photons in the 10-30 MV region relative to those of 60Co and to find out if the system would be suitable for dosimetry comparisons. The irradiations were carried out at the National Research Council, Ottawa, Canada and the doses ranged from 8 to 54 Gy. The EPR measurements were performed at the University of Oslo, Norway. The ratio of the slope of the alanine reading vs. dose to water curve for a certain linac photon beam quality and the corresponding slope for a reference 60Co gamma-radiation gives an experimental measure of the relative dose-to-water response of the EPR alanine dosimetry system. For calculating the linear regression coefficients of these alanine reading vs. dose curves, the method of weighted least squares was used. This method is assumed to produce more accurate regression coefficients when applied to EPR dosimetry than the common method of standard least squares. The overall uncertainty on the ratio of slopes was between 0.5 and 0.6% for all 3 linac energies. The relative response for all the linac beams compared to cobalt was less than unity: by about 0.5% for the 20 and 30 MV points but by more than 1% for the 10 MV point. The given standard uncertainties negate concluding that there is any significant internal variation in the measured response as a function of beam quality between the three linac energies. Thus, we calculated the average dose response for all three energies and found that the alanine response is 0.8% (± 0.5%) lower for high energy x-rays than for 60Co gamma-rays. This result indicates a small energy dependence in the alanine response for the high-energy photons relative to 60Co which may be significant. This result is specific to our dosimetry system (alanine with 20% polyethylene binder pressed into a particular shape) including its waterproofing sleeve of PMMA (2 mm thick), however, we expect that this result may apply to other similar detectors. Download full paper here (pdf-file) Back to Top of Page Andre Krivokapic, Eli Olaug Hole and Einar Sagstuen EPR and ENDOR Study of Crystalline Cytosine•HCl Doped with 5-Methylcytosine. Radiation Induced Radical Formation and Hole Transfer. Radiation Research 160, 340 - 354 (2003). Abstract: Radical formation and hole transfer have been investigated in crystals of cytosine.HCl (C.HCl) doped with 0–1.1 mol-% 5-methylcytosine.HCl (5MC.HCl). The doping level was determined by NMR spectroscopy. Crystals and polycrystalline samples were X-irradiated at 295 K, 77 K and 12 K and studied with EPR, ENDOR and FSE spectroscopy at these temperatures. At 295 K the dominant radicals were the so-called 3aH radical, formed in 5MC by a net H-abstraction from the methyl group, and the cytosine C6 H-addition (5-yl) radical. At 12 K five radicals were identified. These were the 3aH radical, cytosine reduction and oxidation products and the cytosine C6 and C5 H-addition (5-yl and 6-yl, respectively) radicals. The spectroscopic parameters for the 3aH radical are very similar to those of a radical previously observed in the crystalline cytosine nucleotides cytidine (CR), 2’deoxycytidine hydrochloride (CdR•HCl), 5’dCMP and 3’CMP as well as in the uracil derivative 2-thiouracil (2-TU). It was shown that amounts of the order of tenths of a per cent 5MC.HCl doped into crystals of C.HCl give rise to a considerable yield of 3aH radicals after exposure to ionizing radiation both at room temperature and at lower temperatures. This supports a previous suggestion that naturally occurring 5-methylated cytosine impurities may be responsible for the formation of 3aH radicals in the crystalline cytosine nucleotides CR, CdR•HCl, 5’dCMP and 3’CMP and suggests that the 3aH radical in these systems is a 5-methylated base-centered radical. The total radical yield in doped C.HCl crystals increased considerably with the doping level, both at low temperatures and at room temperature, implying that the 3aH radical is more stable than the primary cytosine radicals. The relative amounts of the 3aH radical were obtained by using simulated benchmark spectra to reconstruct experimental EPR spectra of doped polycrystalline samples. Evidence is presented suggesting that the enhanced yield of the 3aH radical in doped samples is due to holes originally formed at cytosine bases and transferred to 5-methylcytosine bases in addition to the 3aH radical being less exposed to recombination than other cytosine radicals. Download full paper here (pdf-file) Back to Top of Page B. Ciesielski, K. Schultka, M. Penkowski and E. Sagstuen EPR Study of Effects of Light Illumination on Free Radicals Induced in L-Alanine by Ionizing Radiation. Spectrochimica Acta A 60, 1327 - 1333 (2004) Abstract: Exposure of gamma-irradiated L-alanine samples to sunlight and to light from a regular, luminescent lamp resulted in significant changes in their EPR resonance patterns, both to spectral shapes and intensities. The experimental EPR spectra were numerically decomposed into three components reflecting contributions of three different radicals (R1 - R3) generated by ionizing radiation in alanine. The light exposure caused a decay of the measured EPR signal intensity. For similar light intensities and exposure times the decay was much more pronounced in samples illuminated by sunlight than in samples illuminated by the luminescent lamp. In both cases light-induced decay of R1 radicals was observed. Sunlight illumination resulted in a moderate decay of R2 radicals and in a doubling of the R3 radical population. On the other hand, luminescent light caused a significant increase of R2 radicals and did not change the amount of R3 radicals. A quantitative analysis of the variations of the three radical contributions to the total EPR spectra upon luminescent light exposure suggests a net R1->R2 free radical transformation. These effects of light on the alanine dosimetric signal should be taken into account in dosimetry protocols, assuring protection of alanine dosimeters from extended exposure to light. Download full paper here (pdf-file) Back to Top of Page Tor A. Vestad, Eirik Malinen, Anders Lund, Eli O. Hole and Einar Sagstuen EPR Dosimetric Properties of Formates. Applied Radiation and Isotopes 59, 181 - 188 (2003) Abstract: As a part of a program to develop an EPR dosimeter suited for clinical use (doses in the cGy range), polycrystalline samples of lithium formate monohydrate (HCO2Li•H2O), magnesium formate dihydrate (C2H2O4Mg•2H2O), and calcium formate (C2H2O4•Ca) have been examined. L-alanine was included for comparison and reference. Samples were irradiated using 60Co gamma-rays and 60 - 220 kV X-rays. The dosimeter response was assessed using the peak-to-peak intensity of the first-derivative EPR spectrum. Dose-response curves for 60Co gamma-irradiated samples were constructed, and photon energy, microwave power and modulation amplitude dependencies of the EPR spectra as well as the stability of the irradiation products upon storage (fading characteristics) were investigated. Lithium formate monohydrate is by far the best candidate of the tested formates, suitable for measuring doses down to about 0.1 Gy. Lithium formate monohydrate is more sensitive than alanine by a factor 5.6-6.8 in the tested photon energy range, exhibits no zero-dose signal and shows a linear dose response in the dose range 0.5 to 1000 Gy. The EPR signal is unchanged in shape and intensity after one week following irradiation to a dose of 10 Gy. Various less favorable properties rendered the other formates generally unsuitable, although e.g. calcium formate exhibits some interesting EPR dosimetric properties. Download full paper here (pdf-file) Back to Top of Page Andre Krivokapic and Einar Sagstuen Hole Transfer and Differential Radical Recombination in X-Irradiated Doped Crystalline DNA Model Systems Journal of Physical Chemistry A 107, 9561 - 9566 (2003) Abstract: Radiation induced hole transfer and differential recombination of radicals at room temperature and lower temperatures (77 K and 12 K) have been studied in crystals of cytosine.HCl doped with 5-methylcytosine.HCl (doping level 0.25 – 1.1 mol-%). The main oxidation product stabilized in the doped crystals at room temperature is an allylic radical, called the 3alphaH radical, which is formed in the 5-methylcytosine dopant by net H-abstraction from the methyl group. This radical has previously been observed in various crystalline cytosine nucleosides and nucleotides shown to contain 5-methylated impurities and it is of interest to investigate why this radical is formed in unproportionally large yields. Two effects are important in this respect. Firstly, the 3alphaH radical in the present system is far less prone to recombination than the initially formed cytosine radicals, rendering the relative yield of this radical much greater than expected from the concentration of the dopant in the crystals. Secondly, as 5-methylcytosine has a lower ionization potential than cytosine the 3alphaH radical may in addition be formed by hole transfer from oxidized cytosine to 5-methylcytosine followed by deprotonation at the methyl group. A simple model is presented which isolates the effect of such hole transfer on the relative radical yields from the effect of differential recombination. Based on the experimental data, and according to this model, the 3alphaH radical most probably is formed by fast hole transfer and radical trapping upon irradiation at room temperature. At lower irradiation temperatures the model predicts that the 3alphaH radical is not the dominant radical in crystalline 5-methylcytosine. Download full paper here (pdf-file) Back to Top of Page Eva S. Bergstrand, Hans Bjerke and Eli O. Hole An experimental investigation of the electron energy dependence of the EPR alanine dosimetry system Radiation Measurements 39, 21 - 28 (2005) Abstract: The dose-to-water response of the EPR alanine dosimetry system for some electron beams used in radiotherapy is studied. The EPR alanine dosimetry system is established for high dose dosimetry, usually transfer dosimetry of 60Co gamma-rays. However, for clinical applications, where radiation qualities other than 60Co gamma-rays are used, the behavior of the alanine dosimeter response needs to be further investigated. In the present work, electron beams from a clinical linear accelerator of the type Varian Clinac 2100 were used. Alanine dosimeters were irradiated with doses between 16 and 54 Gy in a plastic sleeve at the depths of dose maximum in a water phantom with electron beams of nominal energies of 6, 9, 12, 16 and 20 MeV (corresponding beam quality indexes R50: 2.4, 3.4, 4.8, 6.5 and 8.2 cm2/g). The alanine dose response for electrons was compared to that of 60Co gamma-rays, which is the common calibration beam quality. For the electron beams, the dose to water was monitored using a plane parallel ion chamber, while for the 60Co gamma-rays, the dose was monitored using a cylindrical chamber that was calibrated with water calorimetry. The obtained alanine dose-to-water responses for electrons relative to 60Co gamma-rays indicated a small deviation from unity, except for the lowest beam energy of 6 MeV where the relative response was unity. An average relative response of about 0.98 ± 0.015 for the electron beam qualities is indicated from the data. Possible explanations for the observed response reduction relative to 60Co gamma-rays are discussed. Download reprint here (pdf-file) Back to Top of Page Eirik Malinen, Tor Arne Vestad, Elin Agathe Hult, Eli Olaug Hole and Einar Sagstuen Estimation of X-ray Beam Quality by EPR spectroscopy Applied Radiation and Isotopes 60, 929-937 (2004) Abstract: A novel dosimetry system using EPR spectroscopy to determine X-ray beam quality is proposed. The composite dosimetry system is a mixture of two polycrystalline substances, where the two materials have different X-ray absorption properties. The composite dosimeters, consisting of polycrystalline lithium formate monohydrate and calcium formate, were prepared as pellets, X-irradiated and analyzed with EPR spectroscopy. Taking the ratio between the EPR intensities of the two substances constituting the dosimeter, a measure of the beam quality was obtained in terms of the equivalent photon energy. The intensity ratio was calculated by a simple EPR spectrum reconstruction method. Plotting the logarithm of the EPR intensity ratio against the logarithm of the equivalent photon energy, a linear relation was obtained. The linear relation may be used as a calibration formula for the estimation of photon energy from the composite EPR spectrum. The composite dosimeters were used to investigate the changes in equivalent photon energy with increasing depths in a Perspex phantom. Using a 220 kV X-ray beam with an equivalent photon energy of about 100 keV, a buildup of scattered photons was detected with increasing depths in the phantom, displayed by a change in the EPR intensity ratio. This could furthermore be related to the equivalent photon energy in the logarithmic calibration curve. It was found that the equivalent photon energy was significantly reduced by about 25% from the surface of the phantom to a depth of 13 cm. The composite dosimetry system may be used to estimate effective beam energy in both standardized and non-standardized situations, with the latter corresponding to beam setups where the use of filters and ion chambers are difficult or not possible. Also, the system can provide means of measuring beam quality in phantoms following X-ray exposure. Download reprint here (pdf-file) Back to Top of Page Eva Lund, Håkan Gustafsson, Marek Danilczuk, Medury D. Sastry, Anders Lund, Tor Arne Vestad, Eirik Malinen, Eli Olaug Hole and Einar Sagstuen Formates and dithionates: Sensitive EPR-dosimeter materials for radiation therapy Applied Radiation and Isotopes 62, 317 - 324 (2005) Abstract: Polycrystalline formates and dithionates are promising materials for EPR dosimetry, as large yields of radiation induced stable radicals are formed with a linear dose response. Rapid spin relaxation rates were detected in many of the substances, indicating that a high microwave power can be applied during EPR acquisition in order to improve sensitivity. Different techniques used to further improve the sensitivity, such as the replacement of 7Li with 6Li or exchange of protons with deuterons in the corresponding crystalline matrices and metal ion doping are discussed. It is concluded that formates and dithionates may be up to 10 times as sensitive as L-alpha-alanine.. Download reprint here (pdf-file) Back to Top of Page Gauthier C. A. M. Vanhaelewyn, Bjørn Jansen, Ewald Pauwels, Einar Sagstuen, Michel Waroquier and Freddy J. Callens Experimental and theoretical electron magnetic resonance study on radiation-induced radicals in alpha-L-sorbose single crystals Journal of Physical Chemistry A 108, 3308 - 3314 (2004) Abstract: alpha-L-Sorbose single crystals were X-irradiated at 295 K (room temperature). A combined Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR-induced EPR (EI-EPR) study at 120 K revealed a realm of radiation-induced free radicals in this sugar system. In the present work, a pair of closely related radicals is focused on, being dominant immediately after irradiation, but unstable with respect to long time storage or upon warming the samples. A Density Functional Theory (DFT) study was carried out considering the complete hyperfine coupling tensors (principal axes, anisotropic and isotropic couplings) in comparison with the observed electron-proton interactions. This combined approach yielded very plausible models for both radicals, which are formed by a net hydrogen-abstraction from the C3-position of the six-membered sorbose ring. It appears that the difference between the two species is linked to the molecular disorder in the sorbose crystal structure. In addition, DFT calculations of the g-tensors were performed for the plausible radical conformations. Download full paper here (pdf-file) Back to Top of Page Gauthier C. A. M. Vanhaelewyn, Bjørn Jansen, Freddy J. Callens and Einar Sagstuen ENDOR assisted study of the stable EPR spectrum of X-irradiated alpha-L-sorbose single crystals: MLCFA and simulation decomposition analyses Radiation Research 162, 96 - 104 (2004) Abstract: Following X-irradiation of single crystals of alpha-L-sorbose at 295 K, previous Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR Induced EPR (EI-EPR) results have indicated the formation of at least 10 different free radicals, and that conceivably each carbon in the pyranose ring is a possible radical centre. The radicals appear mostly to be formed by net H-abstraction reactions followed by standard elimination (e.g. beta-OH elimination) reactions or proton shifts, in turn leading to ring opening and fragmentations. In the present work, EPR spectra were recorded at room temperature with the external magnetic field along each of the three crystallographic axes subsequent to careful annealing at different temperatures using a high temperature cavity. Each of the three sets of spectra was subjected to a Maximum Likelihood Common Factor Analysis (MLCFA) analysis which mainly contributed to a better understanding of the spectral decays. Furthermore, the most stable spectra were simulated by optimization of previous ENDOR and EI-EPR results. The optimized EPR parameters resulted in excellent simulations of the experimental stable sorbose spectra and hence an improved insight of their spectral compositions. Download full paper here (pdf-file) Back to Top of Page Omer I. Eid, Anders Lund and Einar Sagstuen Allyl Type Radical Formation in X-Irradiated Glutarimide Crystals Studied by ENDOR and ENDOR-Induced EPR Physical Chemistry and Chemical Physics 6, 3604 - 3610 (2004) Abstract: X-irradiated glutarimide single crystals were investigated with the purpose to obtain information about possible ring opening processes involving free radicals in this compound, previously only observed to take place in aqueous solutions. In a recent publication, the two radicals I and II that were identified using EPR, ENDOR and ENDOR-Induced EPR (EIE) techniques at 295 K were both of the hydrogen abstraction type, present in a 3:1 relative ratio. In the present work the finding of a third radical species, III, in X-irradiated single crystals of glutarimide measured at 150 K using ENDOR and EIE techniques, is reported. Based on simulations of the EPR spectra at various orientations, the relative abundancy of the three radicals was estimated to be 60%, 25% and 15% for radicals I, II and III, respectively. Radical III is proposed to be formed formally either by scission of a CH2-CH2 bond at C2 or C4 of radical I or II, or by H-atom elimination from C2 and C3 or C3 and C4 of the same radicals. Both propositions are at variance with observations from aqueous solution studies, where the CO-CH2 bond was that susceptible for rupture. The assignments are consistent with an observed radical of the basic type RCH2in the first instance and an allylic type radical in the second, both giving rise to almost equivalent hyperfine interactions to two alpha-type protons. EIE results confirmed that both couplings originate from the same radical species. The g- and hyperfine coupling tensors for all three radicals at 150 K were obtained. Possible mechanisms for the formation of radical III are discussed. Download full paper here (pdf-file) Back to Top of Page Tor Arne Vestad, Håkan Gustafsson, Anders Lund, Eli Olaug Hole and Einar Sagstuen Radiation-induced radicals in lithium formate monohydrate (LiHCO2.H2O). EPR and ENDOR studies of X-irradiated crystal and polycrystalline samples Physical Chemistry and Chemical Physics 6, 3017 - 3022 (2004) Abstract: Single crystals and polycrystalline samples of lithium formate monohydrate (LiHCO2.H2O) were X-irradiated at 295 K and studied using X-band EPR, ENDOR, and ENDOR-induced EPR (EIE) spectroscopy at 200 or 295 K. Two different radical species were observed. The overall dominating species is the .CO2- radical trapped in the crystal matrix at an orientation not very different from that of the parent CO2 fragment in the unirradiated matrix. The g- and 13C hyperfine coupling tensors of the .CO2- radical were determined. The large line width (about 1.5 mT) of the polycrystalline EPR spectrum is due to extensive hyperfine couplings with lithium ions and protons in the environment. Four lithium couplings and four proton couplings associated with the .CO2- radical were measured, and all couplings were assigned to specific matrix nuclei. The spectra yield evidence for a second radical in low relative abundance. One small lithium hyperfine interaction detected was ascribed to this radical. Spectral simulations of the EPR and ENDOR spectra support the conclusions made. Download full paper here (pdf-file) Back to Top of Page Einar Sagstuen, Audun Sanderud and Eli O. Hole The Solid State Radiation Chemistry of Simple Amino Acids, Revisited Radiation Research 162, 112 - 119 (2004). Abstract: The solid state radiation-induced free radical formation in simple amino acids like alpha-glycine (gly) and l-alpha-alanine (ala) has been subject to investigations by EPR spectroscopy since the late 1950’s. The EPR spectra from crystals of gly and ala generally are very complex due to the simultaneous trapping of several free radicals regardless of irradiation and observation temperatures. Untangling these complex spectra is necessary for understanding the mechanisms for the solid state radiation chemistry of amino acids. Recently, radical formation in gly and ala after room temperature irradiation has been reinvestigated in our laboratories using X, K and Q-band EPR and ENDOR spectroscopy, combined with the ENDOR-Induced EPR (EIE) techniques as well as single crystal and powder EPR and ENDOR spectrum simulations. Several new radical products have been detected and characterized, most prominently the gly species H2N-CH-COOH and the ala species H3N+-C(CH3)-COO- and H2N-C(CH3)-COOH. A short description of these radicals will be given, and an overview of the solid state radiation chemistry of the simple amino acids will be presented, based on a review of the literature combined with these recent experimental results. Download reprint here (pdf-file) Back to Top of Page Carl Henrik Görbitz and Einar Sagstuen N-Formylglycine Acta Crystallographica E 60, 841 - 843 (2004). Abstract: The title compound, C3H5NO3, is used as a model system for studies of radiation damage in more complex polypeptides.The crystal structure is only the third of an isolated N-formylamino acid. Essentially planar molecules form hydrogenbonded sheets with three types of intermolecular interactions, including a very rare >N-H ... O hydrogen bond with the OH group of the carboxylic acid function as the acceptor. Download reprint here (pdf-file) Back to Top of Page Carl Henrik Görbitz and Einar Sagstuen N-Acetyl-L-Alanine Acta Crystallographica E 60, 860 - 862 (2004). Abstract: The crystal structure of the title compound, C5H9NO3, has been investigated as part of a search for suitable materials for EPR dosimetry. Download reprint here (pdf-file) Back to Top of Page Tor Arne Vestad, Eirik Malinen, Dag Rune Olsen, Eli Olaug Hole and Einar Sagstuen Electron paramagnetic resonance (EPR) dosimetry using lithium formate in radiotherapy. Comparison with thermoluminescence (TL) dosimetry using lithium fluoride rods Physics in Medicine and Biology 49, 4701 - 4715 (2004) Abstract: Solid-state radiation dosimetry by electron paramagnetic resonance (EPR) spectroscopy and thermoluminescence (TL) was utilised for the determination of absorbed doses in the range of 0.5-2.5 Gy. The dosimeter materials used were lithium formate and lithium fluoride (TLD-100 rods) for EPR dosimetry and TL dosimetry, respectively. 60Co gamma-rays and 4, 6, 10 and 15 MV X-rays were employed. The main objectives were to compare the variation in dosimeter reading of the respective dosimetry systems and to determine the photon energy dependence of the two dosimeter materials. The EPR dosimeter sensitivity was constant over the dose range in question, while the TL sensitivity increased with more than 5 % from 0.5 to 2.5 Gy, thus displaying a supralinear dose response. The average relative standard deviation in the dosimeter reading per dose was 3.0 % and 1.2 % for the EPR and TL procedures, respectively. For EPR dosimeters, the relative standard deviation declined significantly from 4.3 % to 1.1 % over the dose range in question. The dose-to-water energy response for the megavoltage X-ray beams relative to 60Co gamma-rays was in the range of 0.990-0.979 and 0.984-0.962 for lithium formate and lithium fluoride, respectively. The results show that EPR dosimetry with lithium formate provides dose estimates with a precision comparable to that of TL dosimetry (using lithium fluoride) for doses above 2 Gy, and that lithium formate is slightly less dependent on megavoltage photon beam energy than lithium fluoride. Download reprint here (pdf-file) Back to Top of Page Veselka Gancheva, Einar Sagstuen and Nicola D. Yordanov Study on the EPR/Dosimetric Properties of some Substituted Alanines Radiation Physics and Chemistry 75, 329 - 335 (2006) Abstract: Polycrystalline phenyl-alanine and perdeuterated L-a-alanine (L-a-alanine-d4) were studied as potential high-energy radiation sensitive materials (RSM) for solid state/EPR dosimetry. It was found that phenyl-alanine exhibits a linear dose response in the dose region 0.1 - 17 kGy. However, phenyl-alanine is about 10 times less sensitive to gamma-irradiation than standard L-a-alanine irradiated at the same doses. Moreover, the EPR response from phenyl-alanine is unstable and, independent of the absorbed dose, decreases by about 50 % within 20 days after irradiation upon storage at room temperature. Gamma-irradiated polycrystalline perdeuterated L-a-alanine (CD3CD(NH2)COOH) has not previously been studied at room temperature by EPR spectroscopy. The first part of the present analysis was with respect to the structure of the EPR spectrum. By spectrum simulations the presence of at least two radiation induced free radicals; R1 = CH3C•(H)COOH and R2 = H3N+-C• (CH3)COO- , was confirmed very clearly. Both these radicals were suggested previously from EPR and ENDOR studies of standard alanine crystals. The further investigations into the potential use of alanine-d4 as RSM, after choosing optimal EPR spectrometer settings parameters for this purpose, show that it is ca. two times more sensitive than standard L-a-alanine. Download preprint here (pdf-file) Back to Top of Page Carl Henrik Görbitz and Einar Sagstuen Cytosinium N-benzoylglycinate monohydrate, a redetermination at 105 K Acta Crystallographica E60, 1945-1947 (2004) Abstract: The crystal structure of the title complex, C4H6N3O+:C9H8NO3-.H2O, has been redetermined as part of an electron paramagnetic resonance (EPR) study of the effect of radiation on such systems. Protonated cytosine and the deprotonated carboxylate group of the amino acid form a typical eight membered hydrogen-bonded ring motif. Download reprint here (pdf-file) Back to Top of Page Susan Villa Gonzalez, Kristofer Larsen, William H. Nelson, Einar Sagstuen and Carl Henrik Görbitz Disodium cytidine-5'-monophosphate 6.07-hydrate methanol 0.27 solvate Acta Crystallographica E61, m554 - m556 (2005) Abstract: The crystal structure of the title complex, Na2+-C9H12-N3O8P- 6.07H2O 0.27CH4O, in space group P212121 is remarkably similar to the monoclinic P21 structure of sodium cytidine-5'-monophosphate 6.5 hydrate [Borodi et al. (2001). Acta Cryst. E57, m514-m516]. Download reprint here (pdf-file) Back to Top of Page Carl Henrik Görbitz, William H. Nelson and Einar Sagstuen Alpha-thymidine at 105 K Acta Crystallographica E61, o1207-o1209 (2005) Abstract: Two independent nucleosides in the crystal structure of the title compound, C10H14N2O5, form a dimer connected by two interbase hydrogen bonds. The ring puckering modes are envelope C4-endo and halfchair C3-exo-C4-endo, respectively, which are quite uncommon conformations for 2'-deoxyriboses. Download reprint here (pdf-file) Back to Top of Page Eirik Malinen, Einar Waldeland, Eli Olaug Hole and Einar Sagstuen LET effects following neutron irradiation of lithium formate EPR dosimeters Spectrochimica Acta A 53, 861-869 (2006) Abstract: Lithium formate EPR dosimeters were irradiated using 60Co gamma-rays or fast neutrons to doses ranging from 5 to 20 Gy and investigated by electron paramagnetic resonance (EPR) spectroscopy. Using a polynomial fitting procedure in order to accurately analyze peak-to-peak line widths of first derivative EPR spectra, dosimeters irradiated with neutrons had on average (4.4±0.9) % broader EPR resonance lines than gamma-irradiated dosimeters. The increase in line width was slightly asymmetrical. Computer simulated first derivative polycrystalline EPR spectra of a .CO2- radical gave very good reconstructions of experimental dosimeter spectra. The spectrum simulations could then be used as a tool to investigate the line broadening observed following neutron irradiation. It was shown that an increase in the simulated Lorentzian line width could explain both the observed line broadening and the asymmetrical effect. The ratio of the peak-to-peak amplitude of first derivative EPR spectra obtained at two different microwave powers (20 mW and 0.5 mW) was (7.8±1.2) % higher for dosimeters irradiated with neutrons. The dependence of the spectrum amplitude on the microwave power was extensively investigated by fitting observations to an analytical non-linear model incorporating, among others, the spin-lattice (T1) and spin-spin (T2) relaxation times as fitting parameters. Neutron irradiation resulted in a reduction in T2 in comparison with gamme-irradiation, while a smaller difference in T1 was found. The effects observed indicate increased local radical density following irradiation using high LET neutrons as compared to low LET gamma-irradiation. A fingerprint of the LET may thus be found either by an analysis of the line width or taking the ratio of the lithium formate dosimeter reading at two different microwave powers. Lithium formate is therefore a promising material for EPR dosimetry of high LET radiation. Download preprint here (pdf-file) Back to Top of Page Gauthier C. A. M. Vanhaelewyn, Ewald Pauwels, Freddy J. Callens, Michel Waroquier, Einar Sagstuen and Paul F. A. E. Matthys Q-band EPR and ENDOR of low temperature X-irradiated beta-D-fructose single crystals Journal of Physical Chemistry A 110, 2147 - 2156 (2006) Abstract: beta-D-Fructose single crystals were in situ X-irradiated at 80 K and measured using Electron Paramagnetic Resonance (EPR), Electron Nuclear Double Resonance (ENDOR) and ENDOR-Induced EPR (EIE) techniques at Q- band (34 GHz) microwave frequencies. The measurements revealed the presence of at least four carbon centered radicals being stable at 80 K. By means of ENDOR angular variations in the three principal crystallographic planes, six proton hyperfine coupling tensors could be determined and were assigned to four different radicals by the aid of EIE. Two of the radicals exhibit only beta-proton hyperfine couplings and reveal almost identical EIE spectra. For the other two radicals, the major hyperfine splitting originates from a single alpha-proton hyperfine coupling and their EIE spectra were also quite similar. The similarity of the EIE spectra and hyperfine tensors led to the assumption that there are only two essentially different radical models. The radical exhibiting only beta-proton hyperfine couplings was assigned to a C3 centered radical arising from H3 abstraction and the other radical suggested to be an open-ring species with a disrupted C2 – C3 bond and a double C2 – O2 bond. A possible formation mechanism for the latter open-ring radical is presented. By means of cluster Density Functional Theory (DFT) calculations, the structures of the two radicals were determined and a fairly good agreement between the calculated and experimental hyperfine tensors was found. Download reprint here (pdf-file) Back to Top of Page Erik O. Pettersen, Ingunn Bjørhovde, Åste Søvik, Nina F.J. Edin, V. Zachar, Eli O. Hole, Joe A. Sandvik and Peter Ebbesen Response of chronic hypoxic cells to low dose-rate irradiation International Journal of Radiation Biology 83, 331 - 345 (2007) Abstract: Purpose: Compare the sensitivity of human cells in vitro to low dose-rate irradiation in air and in moderate hypoxia (4% O2). Material and methods: Continuous low dose-rate b-irradiation at a dose rate of 0.015 or 0.062 Gy/h was given to human T-47D breast cancer cells by incorporation of [3H] -labelled valine into cellular protein. Acute irradiation at a dose rate of 0.4 Gy/min was performed using [137Cs] g-irradiation. Cells were cultivated in an atmosphere with 4% O2 using an INVIVO2 hypoxia cabinet. Results: When grown in ambient air with continuous irradiation, T-47D cells were able to continue growth for at least 23 weeks at a dose-rate of 0.015 Gy/h with a surviving fraction stabilized at around 60%. When the dose rate was increased to 0.062 Gy/h the cell culture died out after about 23 days (corresponding to about 22 Gy). When grown in an atmosphere with 4% O2 we surprisingly found that the continuously irradiated T-47D cells (0.015 Gy/h) were severely inhibited in their growth, and cell death became extensive after about 3 weeks while un-irradiated cells continued growth seemingly unaffected by this low oxygenation. Peri cellular oxygenation varied between 4% and below 0.1% over an ordinary passage due to diffusion-limitations through the 2 mm deep medium. Online O2-recordings over a whole passage showed that oxygen was more depleted in the irradiated compared to the un-irradiated cultures indicating increased respiration during irradiation. While cells growing attached to the bottom were inhibited and inactivated during irradiation it was found that cells attached high up in the neck region, i.e., having only a shallow layer of medium above them, survived and formed colonies. When cells cultivated in 4% O2 for 7 weeks were irradiated with acute doses of 137Cs g-rays, the radiosensitivity was the same as for cells cultivated in ambient air. Conclusion: Continuous irradiation with 0.015 Gy/h for several weeks results in a stronger inhibition for T-47D cells grown in an atmosphere with 4% as compared to 20% O2. The data indicate that this may be due to increased oxygen consumption resulting in more severe hypoxia in [3H]-incorporating compared to control (un-irradiated) cells. Download reprint here (pdf-file) Back to Top of Page Bjørn Helge Østerås, Eli Olaug Hole,Dag Rune Olsen and Eirik Malinen Dose measurements in heterogeneous media using alanine film dosimetry Radiotherapy and Oncology 76 (Suppl 2), s205 - s206 (2005) Abstract: Heterogeneous media is generally a problem in dosimetry and radiotherapy, as absolute dose standards and dose planning algorithms usually are based on measurements in water. Therefore, it is important to investigate the effects caused by heterogeneities in order to improve the current standards. The free radicals stabilized in polycrystalline alanine following irradiation have been utilized in EPR (Electron Paramagnetic Resonance) dosimetry for almost 40 years. Lately, thin films coated with alanine have been developed for EPR dosimetry. In the current work, alanine films (130 microns thick) have been utilized for dose determinations in heterogeneous media following irradiation with 6 and 15 MV photon beams. Comparisons are made with measurements using a PTW ‘Advanced Markus’ plane-parallel ion chamber, Monte Carlo simulations using the EGSnrc code and calculations performed by the treatment planning system Oncentra Treatment Planning (OTP). In the first part of this study, alanine film measurements of the dose distribution near the interface between dissimilar materials (e.g. PMMA and aluminum) are compared with ion chamber measurements and Monte Carlo simulations. In the second part, the depth-dose characteristics in a cylindrical, heterogeneous phantom were evaluated by all the aforementioned methods. The cylindrical phantom consisted of PMMA and Styrofoam. Quite good agreement (usually within ±3 %) was found between alanine film measurements, ion chamber measurements and Monte Carlo simulations. In the case of the cylindrical phantom, some calculations performed by the treatment planning system OTP deviated significantly from the measurements and Monte Carlo simulations. The reasons for these discrepancies will be discussed. The results show that alanine film dosimetry is a useful method for determining doses in heterogeneous media. Back to Top of Page Einar Waldeland, Eli Olaug Hole, Einar Sagstuen, Bo Stenerlöv, Erik Grusell and Eirik Malinen Dosimetry of ion beams using lithium formate EPR dosimeters Radiotherapy and Oncology 76 (Suppl 2), s122 - s123 (2005) Abstract: The use of accelerated hadrons in radiation therapy has become increasingly popular, largely due to the superior physical properties of heavy charged particles in comparison with photons and electrons. Following the introduction of such techniques in radiotherapy, it is essential to perform detailed analyses of the resulting energy depositions. In this work, dosimetry using Electron Paramagnetic Resonance (EPR) spectroscopy has been employed for dose determinations and further characterizations of hadron beams. Recently, lithium formate has emerged as a promising EPR dosimeter, as high concentrations of stable radicals are formed in this substance following photon irradiation. However, the response of this dosimeter material to ion beam irradiation is not known. In the current study, the EPR response of lithium formate dosimeters following irradiation with 60Co gamma-rays, 6-20 MeV electrons, 180 MeV protons and 630 MeV nitrogen ions has been examined. Ion chamber dosimetry (following water-based dose standards) has accompanied the EPR measurements. The EPR response of dosimeters irradiated with electrons was practically equal to the response following gamma-irradiation. For nitrogen ions, the resulting dosimeter response was reduced by about 30 % in comparison with gamma-rays. For dosimeters irradiated with protons, a similar but much smaller reduction was observed. Most important, dosimeters irradiated with nitrogen ions displayed a significant increase in the line width of the EPR resonance constituting the dosimeter signal. The effects observed are very likely to be caused by the increased ionization density resulting from irradiation with high LET particles (e.g. nitrogen ions) in comparison with irradiation using (low LET) gamma rays and electrons. Although a relatively high level of recombinations following irradiation with nitrogen ions was found, the characteristic changes in the dosimeter signal may be used to correct the dosimeter reading in order to provide more accurate dose estimates. Lithium formate is therefore a good candidate for solid-state EPR dosimetry of ion beams. Back to Top of Page Einar Sagstuen, David M. Close, Randi Vågane, Eli O. Hole and William H. Nelson Electron Transfer in Amino Acid:Nucleic Acid Base Complexes:EPR, ENDOR and DFT Study of X-Irradiated N-Formylglycine:Cytosine Complex Crystals Journal of Physical Chemistry A 110, 8653-8662 (2006) Abstract: Single crystals of the 1:1 complex of the nucleic acid base cytosine and the dipeptide N-formylglycine (C:NFG) have been irradiated at 10 K and 273 K to doses of about 70 kGy and studied at temperatures between 10 K and 293 K using 24 GHz (K-band) and 9.5 GHz (X-band) EPR, ENDOR and EIE spectroscopy. In this complex, the cytosine base is hydrogen bonded at positions N3 and N4 to the carboxylic group of the dipeptide, and the N3 position of cytosine has become protonated by the carboxylic group. At 10 K, two major radicals were characterized and identified. One of these (R1) is ascribed to the decarboxylated N-formylglycine one-electron oxidized species. The other (R2) is the N3-protonated cytosine one-electron reduced species. A third minority species (R3) appears to be a different conformation of the one-electron reduced cytosine radical. Upon warming, the R2 and R3 radicals decay at about 100 K and at 295 K, the only cytosine centered radicals present are the C5- and C6 H-addition radicals (R5, R6). The R1 radical decays at about 150 K and a glycine backbone radical (R4) grows slowly in. Thus, in the complex a complete separation of initial oxidation and reduction events occurs, oxidation localized at the dipeptide moiety whereas reduction occurs at the nucleic acid base moiety. DFT calculations indicate that this separation is driven by large differences in electron affinities and ionization potentials between the two constituents of the complex. Once the initial oxidation and reduction products are trapped, no further spin transfer between the two constituents of the complex takes place. Download reprint here (pdf-file) Back to Top of Page Kjell Tage Øhman, Audun Sanderud, Eli Olaug Hole and Einar Sagstuen Single Crystals of L-O-Serine Phosphate X-Irradiated at Low Temperatures:EPR, ENDOR, EIE, and DFT studies Journal of Physical Chemistry A 110, 9585 - 9596 (2006) Abstract: Single crystals of the phosphorylated amino acid L-O-serine phosphate were X-irradiated and studied at 10 K and at 77 K using EPR, ENDOR, and EIE techniques. Two radicals, R1(10 K) and R1(77 K), were detected and characterized as two different geometrical conformations of the protonated reduction product >CH-C(OH)2. R1(10 K) is only observed after irradiation at 10 K, and upon heating to 40 K, R1(10 K) transforms rapidly and irreversibly into R1(77 K). The transition from R1(10 K) to R1(77 K) strongly increases the isotropic hyperfine coupling of the C-CHß coupling (delta=32 MHz) and the major C-OHß coupling (delta=47 MHz), in sharp contrast to the their much reduced anisotropic hyperfine couplings after the transition. An umbrella-like inversion of the carboxylic acid centre, accompanied with minor geometrical adjustments, explains the changes of these observed isotropic and anisotropic couplings. DFT calculations were done on the reduced and protonated L-O-serine phosphate radical at the B3LYP/6-31+G(d)//B3LYP/6-311+G(2df,p) level of theory in order to support the experimental observations. Within the single molecule partial energy-optimization scheme, several different conformations of the anion radical could be found. Two of these energy-optimization conformations, which were linked together through an inversion of the carboxylic centre, could reproduce the experimental hyperfine couplings from radicals R1(10 K) and R1(77 K). A third radical, radical R2, was observed experimentally at both 10 K and 77 K and was shown to be due to the decarboxylated L-O-serine phosphate oxidation product, a conclusion fully supported from the DFT calculations. Upon warming above 77 K, radicals R1(77 K) and R2 transformed into two of the three previously observed room temperature radicals. No phosphate-centered radicals could be observed at any temperatures, indicating that phosphate-ester bond break does not occur by dissociative electron capture at the phosphate group. Download reprint here (pdf-file) Back to Top of Page Bjørn Helge Østerås, Eli Olaug Hole, Dag Rune Olsen and Eirik Malinen EPR Dosimetry of Radiotherapy Photon Beams in Inhomogeneous Media using Alanine Films Physics in Medicine and Biology 51, 6315-6328 (2006) Abstract: Dosimetry of high energy photon beams in inhomogeneous media may be complicated, as the scatter conditions of secondary charged particles varies. In such situations, small non-perturbing dosimeters are required for accurate dose estimates. In the current work, EPR (Electron Paramagnetic Resonance) dosimetry using alanine films (134 um thick) was utilized for dose measurements in inhomogeneous phantoms irradiated with radiotherapy photon beams. The main phantom material was PMMA, while either Styrofoam or aluminium was introduced as an inhomogeneity. The phantoms were irradiated to a maximum dose of about 30 Gy with 6 or 15 MV photons. The performance of the alanine film dosimeters was investigated and compared to results from ion chamber dosimetry, Monte Carlo simulations and radiotherapy treatment planning calculations. It was found that the alanine film dosimeters had a linear dose response above approximately 5 Gy, while a background signal obscured the response at lower dose levels. The alanine film dose estimates in general yielded results comparable to those from the Monte Carlo simulations and the ion chamber measurements, while the treatment planning calculations exhibited limited applicability. The current work shows that alanine film dosimetry is a suitable method for estimating radiotherapeutical doses and for dose measurements in inhomogeneous media. Download reprint here (pdf-file) V. Gancheva, N.D. Yordanov, F. Callens, G. Vanhaelewyn, J. Raffi, E. Bortolin, S. Onori, E. Malinen, E. Sagstuen, S. Fabisiak and Z. Peimel-Stuglik An international intercomparison on "self-calibrated" alanine EPR dosimeters Radiation Physics and Chemistry (2007, in press) Abstract: The results obtained by six independent Electron Paramagnetic Resonance (EPR) laboratories on the dose response coefficients (Kdr) of self-calibrated solid state EPR dosimeters, containing alanine as a radiation sensitive material and Mn2+/MgO as an internal reference material, are reported. The intercomparison trial was divided in three steps. It started with distribution of dosimeters among the participating EPR laboratories with the purpose of irradiating them with known doses of gamma-rays and to estimate the Kdr. The Percentage Standard Deviation (PSD) of the Kdr obtained at individual labs was in the range of 1.4 - 4.6 %. The inter-laboratory PSD of the Kdr was 8 %, primarily pointing to variations in irradiation procedures and EPR spectrometer settings. Further investigations showed that the main source to the inter-laboratory PSD is differences in the calibrations and settings of EPR acquisition parameters. In order to provide reproducible estimates of the Kdr, low microwave power and modulation amplitude using a combination of sweep time and time constant that gives a distortion-free EPR spectrum should be utilized. In the third step following such a procedure, measuring the same irradiated alanine dosimeter at the respective laboratories gave an inter-laboratory PSD of 3.1 %. In conclusion, EPR dosimetry using self-calibrated alanine dosimeters may be used as a secondary standard, although a careful calibration of the EPR spectrometer must be performed in order to further reduce the uncertainty. Download preprint here (pdf-file) Back to Top of Page Eirik Malinen, Einar Waldeland, Eli Olaug Hole and Einar Sagstuen The energy dependence of lithium formate EPR dosimeters for clinical electron beams Physics in Medicine and Biology 52, 4361 - 4369 (2007) Abstract: The objective of the present study was to investigate the potential of using polycrystalline lithium formate for EPR (Electron Paramagnetic Resonance) dosimetry of clinical electron beams, with main focus on the dose-to-water energy response. Lithium formate dosimeters were irradiated using 60Co gamma-rays and 6-20 MeV electrons in a PMMA phantom to doses in the range of 3-9 Gy. A plane-parallel ion chamber was used for water-based absolute dosimetry. In addition, the electron/photon transport was simulated using the EGSnrc Monte Carlo code. The dosimeter reading was linearly related to the absorbed dose to water for all radiation beams. The standard deviation of single dosimeter readings was 1.3 %. The experimental energy response (the lithium formate dosimeter reading per absorbed dose to water for electrons relative to that for 60Co gamma-rays) was nearly independent of the electron energy and on average 0.99±0.03. The Monte Carlo calculated energy response was on average 0.5 % higher than the experimental energy response, the difference being not significant. Further simulations with water and polystyrene as irradiation media indicated that the energy response of lithium formate dosimeters was nearly independent of the phantom materials. In conclusion, lithium formate EPR dosimetry of clinical electron beams provides precise dose measurements with low dependence on the electron energy. Download reprint here (pdf-file) Back to Top of Page Hendrik De Cooman, Ewald Pauwels, Henk Vrielinck, Anna Dimitrova, Nicola Yordanov, Einar Sagstuen, Michel Waroquier and Freddy Callens Radiation-Induced Defects in Sucrose Single Crystals, Revisited: a Combined Electron Magnetic Resonance and Density Functional Theory Study Spectrochimica Acta Series A (2007, in press) Abstract: The results of an Electron Magnetic Resonance analysis at 110 K of radiation-induced defects in sucrose single crystals X-irradiated at room temperature are presented, yielding a total of nine 1H hyperfine coupling tensors assigned to three different radical species. Comparisons are made with results previously reported in the literature. By means of Electron Paramagnetic Resonance and Electron Nuclear Double Resonance temperature variation scans, most of the discrepancies between the present 110 K study and a previous 295 K study by Sagstuen and co-workers are shown to originate from the temperature dependence of proton relaxation times and hyperfine coupling constants. Finally, radical models previously suggested in the literature are convincingly refuted by means of quantum chemical Density Functional Theory calculations. Download proof here (pdf-file) Back to Top of Page André Krivokapic, Janko N. Herak, and Einar Sagstuen Proton-Coupled Hole Transfer in X-Irradiated Doped Crystalline Cytosine.H2O Journal of Physical Chemistry A (2007, submitted for publication) Abstract: Following exposure to X-irradiation at low temperatures, the main reactions taking place in single crystals of cytosine monohydrate doped with minute amounts of 2-thiocytosine are hole transfer (HT) from the electron-loss centers to the dopant and recombination of oxidation- and reduction products, assumedly by electron transfer. A huge deuterium kinetic isotope effect (KIE) (>102 - 103) at 100 K, together with the kinetic curves obtained and Density Functional Theory (DFT) calculations of equilibrium energy changes, indicates that these reactions proceed through a concerted proton-coupled electron/hole transfer where the proton transfer occurs between hydrogen-bonded cytosine molecules. The temperature dependence of these reaction rates between 10 and 150 K in normal and partially deuterated samples was investigated by monitoring the growth and decay of the various radical species over time using EPR spectroscopy. By assuming a random distribution of the hole donors and acceptors in the crystals, the data are consistent with an exponential distance-dependent rate, giving a distance decay constant beta around 1 Å-1 for the HT, which indicates that a long-range single-step superexchange mechanism mediates the charge transfer. The reactions undergo a transition from a slow, weakly temperature-dependent rate to an Arrhenius-type rate at 40 - 50 K, presumably being activated by excitation of low-frequency intermolecular vibrations that couple to the process. Below this transition temperature, the transfer probability might be dominated by temperature-independent nuclear tunneling. A similar beta-value in both temperature regions suggests that hopping is not activated. 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Last updated Oct. 09, 2007; ES