Stereochemical studies on cyclic peptides. IV. Conformational analysis of cyclopentapeptides

Abstract

A study of the cyclopentapeptides has been made from the conformational point of view, using cyclopentaalanyl and cyclopentaglycyl as model systems. Structures with five-fold symmetry have been examined using stereochemical criteria and energy considerations. Nonplanarity in the peptide units has been introduced to find out the minimum energy conformation. The conformation (3°, 114°, 64°, 134°) corresponding to (ω, τ, π, φ) has a minimum energy of 7.5 kcal/mole/residue, for the cyclopenta-L-alanyl. This conformation is completely free of short contacts. Mixed L and D residues are not as energetically favourable as an all L or all D conformation. For cyclopentaglycyl, the conformation having a minimum energy of 8.6 kcalj mole I residue has the parameters co=3°, τ−109.6°, π−103°, and φ−101°. Cyclopentaalanyl formed by linking three and two peptide units, with a hydrogen bond (N-H...O) of the 4-→1 type in the three units, lias also been studied, and the results show that conformations of comparable stability to that of symmetrical structure can be obtained in this case also.