CONFORMATION OF POLYPEPTIDE CHAINS CONTAINING BOTH L‐AND D‐RESIDUES:

Abstract

Polypeptides with alternating L- and D-amino acid residues can take up stereochemically satisfactory coaxial double-helical structures, both antiparallel and parallel, which are stabilized by systematic interchain NH...O H-bonds. Semiempirical energy calculations over allowed regions of conformational space have yielded the characteristics of these double-helices. There are 4 possible types of antiparallel double-helices - A3, A4, A5 and A6, with n, the number of LD peptide units per turn, around 2.8, 3.6, 4.5 and 5.5, respectively, while for the parallel double-helices there are 2 types, P3 and P4, having similar helical parameters as in A3 and A4. The H-bonding scheme restricts the pitch in all the models to the narrow range of 10.0-11.5 .ANG.. All these helices have large central cores whose radii increase proportionately with n. In this respect, A3 and A4 are suitable models for the structure of gramicidin A. In terms of their relative energies, antiparallel double-helices are marginally more stable than those with parallel strands. The results indicate that the energy differences among the members in the antiparallel family are not significant and provide an explanation for the polymorphism reported for poly(.gamma.-benzyl-LD-glutamate).