Stereochemistry of protected ornithine side chains in gramicidin S derivatives and their resistance to N-methylation

Abstract

Derivatives of gramicidin S (GS) and its mono- and di-d-cyclohexylalanine (d-Cha) analogs possessing various protecting groups on Orn side chains were prepared. ¹H NMR spectra of the unsymmetrically protected analogs [Orn(X)²,Orn(X′)²′,d-Cha⁴]GS were similar to the composites of the spectra of the symmetrical derivatives [Orn(X)²,2′,d-Cha⁴,4′]GS and [Orn(X′)²,2′]GS, revealing the proximity of the protecting groups of N^δH of Orn residues at the 2 and 2^′ positions to the side chains of d-Phe (or d-Cha) residues at the 4 and 4^′ positions, respectively. The results indicated the presence of H-bonds between the N^δH of Orn and the carbonyl of d-Phe residues in the i → i + 2 sense and not in i → i – 3, which was also supported by the ROESY analysis. The substantially strong H-bonds can explain the observed resistance of the urethane NH of the Orn side chains in the GS derivatives to the N-methylation with CH³I–Ag²O in DMF.