Table 2. Selection criteria for tetrapeptides.

Selection parameter for residues	Criterion
Position in structure	Not a C-terminus or an N-terminus; not adjacent to a chain break
Amino-acid type	Must be canonical
Angles, dihedrals, improper dihedrals	Must be determinable
Atoms	All must be present; all B factors > 02; all occupancies = 1.0
Covalently bound atoms	Only canonical bonds and no other bonds, not even disulfide bonds
Anything outside own molecule	Not within 2.5 of O atom in central peptide plane