Dynamic properties of gramicidin A in phospholipid membranes

Abstract

The flexibility of the trypton side chains of gramicidin A and the rotational diffusion of the peptide in methanolic solution and in three membrane systems were studied with deuterium nuclear magnetic resonance (NMR). Gramicidin A was selectively deuteriated at the aromatic ring systems of its four tryptophan side chains. In methanolic solution, the tryptophan residues remained immobile and served as a probe for the overall rotation of the peptide. The experimentally determined rotational correlation time of .tau.c = 0.6 .times. 10-9 s was consistent with the formation of gramicidin A dimers. For gramicidin A incorporated into bilayer membranes, quite different resuls were obtained depending on the chemical and physical nature of the lipids employed. When mixed with 1-palmitoyl-sn-glycero-3-phosphocholine (LPPC) at a stoichiometric lipid:peptide ratio of 4:1, gramicidin A induced the formation of stable bilayer membranes in which the lipids were highly fluid. In contrast, the gramicidin A molecules of this membrane remained completely static over a large temperature interval, suggesting strong protein-protein interactions. The peptide molecules appeared to form a rigid two-dimensional lattice in which the interstitial speces were filled with fluidlike lipids. When gramicidin A was incorporated into bilayers of 1,2-dioleoyl-sn-glycerol-3-phosphocholine or 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) above the lipid phase transition, the deuterium NMR spectra were motionally narrowed, indicating large-amplitude rotatinal fluctuations. From the measurement of the quadrupole echo relaxation tme, a rotational correlation time of 2 .times. 10-7 s was estimated, leading to membrane viscosity of 1-2 P if the rotational unit was assumed to be a gramicidin A dimer. For DMPC in the gel state, we observed an immobilization of the peptide molecules. The tryptophan side chains of the immobilized gramicidin A in both the DMPC membrane inthe gel state and the stoichiometric gramicidin A-LPPC membrane were found to execute rapid fluctuations of small angular amplitude with correlation times .tau.c < 10-8 s.