Compressibility and density of lipid bilayers composed of polyunsaturated phospholipids and cholesterol

Solvent-free bilayer lipid membranes (BLM) and unilamellar liposomes composed of mixed chain phosphatidylcholines, containing 1-3 double bonds in the hydrocarbon chains in position sn-2, have been studied by various physico-chemical techniques. The application of the electrostriction method allowed us to determine the elasticity modulus (E-perpendicular 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-perpendicular to, the modulus of volume compressibility K similar to 1/beta and 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 with an increasing number of double bonds. Cholesterol considerably influences 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 cholesterol-phospholipid mixtures showed that the non-monotonous dependence of E-perpendicular to(c) is owing to the inhomogeneous lateral distribution of cholesterol in the lipid bilayer. Changes in E-perpendicular to, of BLM composed of 18:0, 18:2-PC (SLPC), were more pronounced and 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. All rights reserved.