Conformational changes of phospholipid headgroups induced by a cationic integral membrane peptide as seen by deuterium magnetic resonance

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Abstract
Deuterium nuclear magnetic resonance (2H NMR) was used to study the interaction of a cationic amphiphilic peptide with pure DMPC membranes and with mixed bilayers of dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylserine (DMPS). The choline and serine headgroups were selectively deuteriated at the .alpha. and .beta. positions. The amphiphilic peptide, with 20 leucine residues in the hydrophobic core and two cationic hydrophilic lysine residues at each end, spanned the lipid bilayer. Although 2H NMR experiments using DMPC with perdeuteriated fatty acyl chains showed that the average order parameter of the hydrophobic region was not significantly modified by the incorporation of the amphiphilic peptide, for either DMPC or DMPC/DMPS (5:1) bilayers, large perturbations of the quadrupolar splittings of the choline and serine headgroups were observed. The results obtained with the DMPC headgroup suggest that the incorporation of the cationic peptide in both DMPC and DMPC/DMPS (5:1) bilayers leads to a structural perturbation directly related to the net charge on the membrane surface. The magnitude of the observed effect seems to be similar to those observed previously with other cationic molecules [Seelig, J., MacDonald, P. M., and Scherer, P. G. (1987) Biochemistry 26, 7535-7541]. Two of the three quadrupolar splittings of the PS headgroup exhibited large variations in the presence of the amphiphilic peptide, while the third one remained unchanged. Our data have led us to propose a model describing the influence of membrane surface charges on headgroup conformation. In this model, the surface charge is represented as a uniform charge distribution. The electric field due to the charges produces a torque which rotates the polar headgroups. This simple model is shown to be capable of accounting quantitatively for the dependence of headgroup quadrupolar splittings on a variety and range of membrane-associated charges.

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