Conformation of gramicidin A channel in phospholipid vesicles: a 13C and 19F nuclear magnetic resonance study.

Abstract

Conformation of the channel-forming polypeptide antibiotic gramicidin A in phosphatidylcholine vesicles was determined by using 13C and 19F NMR spectroscopy. Models previously proposed for conformation of the dimer channel differ in the surface localization of the NH2 and COOH termini. Specific 13C and 19F nuclei were incorporated at the NH2, and COOH termini of gramicidin and 13C and 19F chemical shifts and spin lattice relaxation time measurements were used to determine accessibility of these labels to 3 paramagnetic NMR probes, 2 in aqueous solution and 1 attached to a phosphatidylcholine fatty acid chain. The COOH terminus of gramicidin in the channel is apparently located near the membrane surface, and the NH2 terminus is buried deep within the lipid bilayer. These findings strongly favor an NH2-terminal to NH2-terminal helical dimer as the major conformation for the gramicidin channel in phosphatidylcholine vesicles.