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.