Protein imperfections: separating intrinsic from extrinsic variation of torsion angles.

The article focuses on the variation of the values of dihedral angles in proteins. The variation is divided into two categories by analyzing distributions in a database of structures determined at a resolution of 1.8 Å or better. Each dihedral angle of any given side chain in a protein has its own specific equilibrium value which is determined by the details of the packing of the rest of the protein around the side chain. In order to determine the distribution of side-chain dihedral angles X-ray crystallography is beset by errors in the experimental. Thermal motion, static disorder and irregularities of the crystal lattice further increase the width of the distribution.