Empirical calculations on cyclic dipeptides. Conformations of serine, threonine and histidine residues.

Empirical conformational energy calculations were carried out for the dipeptides cyclo-(L-Ser-L-His) and cyclo-(L-Thr-L-His). Various DKP structures have been investigated to study side chain conformations through the DKP ring deformations, and prevalent conformers of monosubstituted and disubstituted dipeptides are discussed. The most stable conformations occur when both side chains are folded over a planar DKP ring, the imidazole ring being in the epsilon-tautomeric form. Theoretical results are consistent with those obtained from n.m.r. and crystallographic studies.