Interaction of the antimicrobial peptide gramicidin S with dimyristoyl-phosphatidylcholine bilayer membranes: a densitometry and sound velocimetry study

We determined changes in the volume and adiabatic compressibility of large multi- and unilamellar vesicles composed of dimyristoylphosphatidylcholine containing various concentrations of the antimicrobial peptide gramicidin S (GS) by applying densitometry and sound velocimetry. Gramicidin S incorpor ation was found to progressively decrease the phase transition temperature of DMPC vesicles as well as to decrease the degree of cooperativity of the main phase transition and to increase the volume compressibility of the ves icles. GS probably enhanced thermal fluctuations at the region of main phas e transition and provide more freedom of rotational movement for the phosph olipid hydrocarbon chains. The ability of GS to increase the membrane compr essibility and to decrease the phase transition temperature is evidence for regions of distorted membrane structure around incorporated gramicidin S m olecules. At relatively high GS concentration (10 mol%), more significant c hanges of specific volume and compressibility appear. This might suggest ch anges in the integrity of the lipid bilayer upon interaction with high conc entrations of GS. (C) 2001 Elsevier Science B.V. All rights reserved.