Characterization of swollen lamellar phase of dimyristoylphosphatidylcholine-gramicidin A mixed membranes by DSC, SAXS, and densimetry.

For the fully hydrated multilamellar stack of dimyristoylphosphatidylcholine (DMPC) fluid membranes containing hydrophobic peptide gramicidin A (GrA), the membrane thickness and the bilayer-bilayer separation (i.e., water layer thickness) were determined by measuring small-angle X-ray scattering and the density of aqueous suspensions of DMPC-GrA mixtures. When the molar ratio of GrA to DMPC was 0.04, the membrane thickness decreased by 2-3 A by the incorporation of GrA molecules into DMPC bilayers, whereas the water layer thickness increased by 3-4 A. As the cause of the increment of water layer thickness, two possibilities were considered; (1) attractive van der Waals force acting between the bilayer membranes weakened by the decrease of membrane thickness, and (2) repulsive undulation force enhanced by the incorporation of GrA which may stabilize the gauche conformers of the lipid acyl chains. Copyright 1998 Elsevier Science B.V. All rights reserved.