Shielding effects of the D-Phe aromatic ring in the 1H NMR spectrum of gramicidine S.

Abstract

Analysis of the side-chain coupling constants for the D-Phe residue of gramicidin S shows that the rotamer predicted for the minimum-energy structure is the predominant one. The aromatic ring of D-Phe shields the Pro Cδ hydrogens, and the calculated shielding closely matches that found in the 1H NMR spectrum. As the temperature is raised, the rotamer population changes and the shielding is reduced accordingly. The shielding is absent in the saturated analogue of gramicidin S in which the aromatic rings were hydrogenated without changing the conformation of the rest of the molecule. © 1978.