N‐methylleucine gramicidin‐S and (di‐N‐methylleucine) gramicidin‐S conformations with cis L ‐Orn‐ L ‐N‐MeLeu peptide bonds

Abstract Conformations containing all trans peptide bonds have previously been proposed for N‐methylleucine gramicidin‐S and (di‐N‐methylleucine) gramicidin‐S based on an evaluation of proton nuclear magnetic resonance parameters in a series of solvents. These gramicidin‐S derivatives exhibit full biological activity despite the fact that the proposed solution conformations differ in backbone topology and relative orientation of the Phe and Orn side chains compared to gramicidin‐S. The present authors discuss conformations for N‐methylleucine gramicidin‐S and (di‐N‐methylleucine) gramicidin‐S which incorporate cis peptide bonds at L‐Orn‐L‐N‐MeLeu, where the gramicidin‐S backbone is essentially retained, and the relative orientation of the Pro, Orn, Val, and Phe side chains correspond to those observed for gramicidin‐S. A novel hydrogen‐bond arrangement involving one carbonyl group interacting with two peptide protons (1 ←4 and 1 ←5 types) is proposed to stabilize the backbone conformation in the gramicidin‐S derivatives. A recent report on the cyclic heptapeptide antibiotic, Ilamycin B1, shows the presence of cis peptide bonds at N‐CH3 amino acids, as well as the novel hydrogen‐bond arrangement presented above.