Comparison between the dynamics of lipid/gramicidin A systems in the lamellar and hexagonal phases:: a solid-state 13C NMR study

We have investigated the effect of gramicidin A on the dynamics of two model membranes: dimyristoylphosphatidylcholine (DMPC) in the lamellar phase at a lipid-to-peptide molar ratio of 10:1 and dioleoylphosphatidylcholine (DOPC) in the hexagonal H-II phase at a lipid-to-peptide molar ratio of 5:1. Natural abundance C-13 nuclear magnetic resonance (NMR) spectroscopy was used in combination with magic angle spinning to increase the spectral resolution, therefore allowing the different regions of the lipid bilayers to be investigated from the same spectra. P-31 NMR was also used to detect and confirm the formation of the DOPC H-II phase in the presence of gramicidin A. In order to examine the effect of gramicidin A on both the fast and slow motions of DMPC and DOPC, the H-1 spin-lattice relaxation times in the laboratory frame (HT1) as well as the H-1 spin-lattice relaxation times in the rotating frame (HT1p) were calculated for each resolved protonated lipid resonance in the C-13 spectra. For both DMPC and DOPC, we found that the presence of gramicidin A does not significantly affect the fast motions of the lipid acyl chains but increases slightly the fast motions of the polar head group. However, the HT1p are significantly decreased, this effect being more pronounced for DOPC most likely due to a decrease in the rate of the lipid lateral diffusion. (C) 1998 Elsevier Science B.V. All rights reserved.