STEREOCHEMICAL STUDIES ON CYCLIC PEPTIDES

Abstract

Conformational analysis of cyclic pentapeptides having two intra‐ring 3 1 hydrogen bonds has been carried out. It is found that the structure can easily be formed with trans planar peptide units without causing significant angular strain at the α‐carbon atoms. Four different types of conformations designated Types I‐IV are possible for the backbone structure. Details of these four types of conformations and also the accommodating possibility of these types for all‐glycyl and all‐alanyl residues are presented. Three of the four types have relatively low energies for glycyl residues whereas the other one has a slightly higher energy. When alanyl residues are introduced at the five α‐carbon atoms, the types that are energetically favourable depend upon the sequence of isomers. Energy calculations have also been carried out for the combinations of glycyl, L‐ and D‐alanyl residues. The theoretical results are compared with available experimental observations both from solution and solid state studies.