Stereochemistry of Protected Ornithine Side Chains of Gramicidin S Derivatives:  X-ray Crystal Structure of the Bis-Boc-tetra-N-methyl Derivative of Gramicidin S

An X-ray crystallographic analysis of the bis-Nδ-Boc-tetra-Nα-methyl derivative of gramicidin S, cyclo(−Val−MeOrn(Boc)−Leu−d-MePhe−Pro−)2, was undertaken successfully (R-factor = 0.088). As expected, the main chain adopts an antiparallel pleated β-sheet conformation, but the pleated sheet is slightly twisted, and the sense of twisting is opposite to that found in the reported crystal structures of the gramicidin S-urea complex and the bis-Nδ-(trichloroacetyl) and bis-Nδ-(m-bromobenzoyl) derivatives of gramicidin S. In agreement with the observed resistance toward N-methylation, the urethane NH groups of the protected Orn side chains are hydrogen bonded to the carbonyl groups of the d-Phe residues. However, the side-chain−main-chain hydrogen bonding is in the i → i − 3 mode, although hydrogen bonding in the i → i + 2 mode was deduced from a 1H NMR study of protected gramicidin S derivatives and was actually found in the crystal structures of the diacylated gramicidin S.