Stereochemical studies on cyclic peptides. Part X. Conformational analysis of hydrogen bonded cyclic pentapeptides

Conformational aspects of 4 leads to 1 hydrogen bonded cyclic pentapeptides are considered in this paper from the point of view of "contact criteria" and potential energy calculations. Three types of such hydrogen bonded conformations, designated A1, A2 and B, are possible, involving some amount of strain on the bond angles. The energy of hydrogen bonded cyclopentaglycyl is somewhat less than that of the five-fold symmetrical conformation. The stereochemical feasibility of introducing L- and D-alanyl resudues in these structures has also been studied and the possible types for different sequences of alanyl residues have been determined. The results are discussed further in the light of the limited data available from crystal structure and nuclear magnetic resonance studies on cyclic pentapeptides.