Multiple solution conformations and internal rotations of the decapeptide gramicidin S

The conformations of every C alpha H-C beta H2 moiety of the peptide gramicidin S are reported. Internal rotation occurs, but distinct preferences for one side chain rotamer, greater than 80%, are found for the D-phenylalanine and ornithine residues. Leucine and valine exhibit more extensive averaging while proline is shown to be at least 90% in the Ramachandran B conformation. The data are consistent with the coexistence of many tertiary conformations of gramicidin S; the statistical weights of the twelve major tertiary conformations consistent with the rotamer populations are reported. The relative statistical weights of the tertiary conformers depend upon temperature and solvent. A comparison of the conclusions from this publication and conformations derived by energy minimization procedures is made. Partial agreement was found, but the calculations have not yet predicted the wealth of coexisting tertiary conformations nor accounted for the subtle effects of solvent. It is proposed that a more complete picture of the conformational dynamics of gramicidin S and other peptides will result from calculations which use as a basis the extensive data reported here.