COMPRESSIBILITY AND DENSITY OF LIPID BILAYERS COMPOSED OF POLYUNSATURATED PHOSPHOLIPIDS AND CHOLESTEROL

Solvent-free bilayer lipid membranes (BLM) and unilamellar liposomes c omposed of mixed chain phosphatidylcholines, containing 1-3 double bon ds in the hydrocarbon chains in position sn-2, have been studied by va rious physico-chemical techniques. The application of the electrostric tion method allowed us to determine the elasticity modulus (E-perpendi cular to) perpendicular to the membrane plane. Furthermore, ultrasound velocimetry and densitometry enabled us to determine the coefficient of adiabatic compressibility (beta). The elasticity modulus E-perpendi cular to, the modulus of volume compressibility K similar to 1/beta an d the partial specific volume of the lipids decreased with increasing degree of unsaturation, while adiabatic compressibility increased with increasing degree of unsaturation. These findings are consistent with a decrease in the ordering of the acyl chains of the phospholipids wi th an increasing number of double bonds. Cholesterol considerably infl uences the mechanical properties of BLMs containing phospholipids with polyunsaturated fatty acids. E-perpendicular to was found to be at a maximum at 66 mol% cholesterol. Analysis of a three phase model of cho lesterol-phospholipid mixtures showed that the non-monotonous dependen ce of E-perpendicular to(c) is owing to the inhomogeneous lateral dist ribution of cholesterol in the lipid bilayer. Changes in E-perpendicul ar to, of BLM composed of 18:0, 18:2-PC (SLPC), were more pronounced a nd were observed already at relatively low cholesterol concentrations, while E-perpendicular to for BLM of either 18:0, 18.3-PC (SLnPC) were practically independent of cholesterol up to 50 mol% of cholesterol. This is most likely to be due to different interaction of cholesterol with the respective phospholipids depending considerably on the degree of polyunsaturation of fatty acids. (C) 1998 Elsevier Science B.V. Al l rights reserved.