Thermomechanical and transition properties of dimyristoylphosphatidylcholine/cholesterol bilayers

Abstract

Mixtures of dimyristoylphosphatidylcholine (DMPC) and cholesterol (Chol) have been used to examine the effects of cholesterol on the chain crystallization transitions and thermomechanical properties in phospholipid bilayer membranes. The mechanical properties.sbd.elastic moduli and level of tension at membrane rupture.sbd.were derived from micropipet pressurization of giant single-walled vesicles. Also, the micropipet method allowed temperature-dependent area transitions to be measured at constant membrane tension. X-ray diffraction measurements were made on selected lipid/cholesterol mixtures. Wide-angle patterns and electron density profiles were used to measure bilayer thickness as an indication of chain tilt and fluidity. Vesicle area versus temperature plots showed that the main acyl chain crystallization transition of DMPC broadened and shifted to higher temperatgures. Both above and below the broad transition, the elastic area compressibility modulus, K, was greatly increased with cholesterol addition. The value for the 1:1 DMPC/Chol complex was found to be .apprx. 700 dyn/cm, comparable to that for DMPC in the L.beta.'' phase. However, for all concentrations above 12.5 mol% (which was weakly solid), vesicle bilayers behaved as surface liquids with no surface shear rigidity even at temperatures well below the DMPC phase transition. Area changes over the broadened trnasitions were reduced by cholesterol and disappeared with the addition of 50 mol% to leave the thermal area expansivity at 1.3 .times. 10-3/.degree. C. These area changes are consistent with separate formation of a 1:1 DMPC/Chol complex that does not condense plus residual free lipid and lipid loosely associated with the 1:1 complex that freezes normally. X-ray diffraction measurements indicated that at low cholesterol concentrations (12.5 mol%) lipid chain tilt still existed and that for 50 mol% Chol the bilayer remained in a liquid phase to temperatures as low as 10.degree. C.