Important definitions in Protein Crystallography

Crystal:
- A regular repeat of molecules, usually with some sort of internal rotational symmetry. Protein crystals usually contain about 40-60% solvent by weight and are thus fragile and sensitive to drying out.

Unit cell:
- The smallest repeating unit that can generate the hole crystal with only translation operations.

Lattice:
- The regular spacing defined by lengths and angles of the origins to the individual unit cells.

Mosaicity:
- An angular measure of the degree of long-range order of the unit cells within a crystal. Lower mosaicity indicates better ordered crystals and hence better X-ray diffraction

Asymmetric unit:
- The smallest unit which can be rotated and translated to generate one unit cell using only the symmetry operators allowed by the crystallographic symmetry. The asymmetric unit may be one molecule or one subunit of a multimeric protein, or it can also be several molecules in the asymmetric unit.

Symmetry:
- Description of how the asymmetric units are arranged in the unit cell by rotation and translation operators. The symmetry operators are expressed in coordinate frame of the lattice and thus may not be orthogonal and are not in Cartesian coordinates (Å). The translations are expressed as fractions of a unit cell axial length. Application of these operators to the molecular coordinates must also be in the frame of fractional coordinates.

Non-crystallographic symmetry:
- Symmetry operations (rotation and translation) which relate monomers within the asymmetric unit.

Scattering
- Scattering of X-rays is due to electrons, hence heavier atoms with more electrons scatter more strongly.

Diffraction:
- Scattering of X-rays (in this case) from a protein crystal. It depends on the "long- range" order in the crystal. More disorder means poorer diffraction especially at higher resolution.

Resolution:
- Minimum inter-planar spacing of the real lattice for the corresponding reciprocal lattice point (reflection) that is being measured. It is directly related to the optical definition in which it is the minimum distance two objects can be apart and still be seen as two separate objects. Resolution is normally quoted in Ångstroms (Å).

Adopted from http://adelie.biochem.queensu.ca/~rlc/work/teaching/definitions.shtml