THERMODYNAMICS AND DYNAMICS OF PHOSPHATIDYLCHOLINE-CHOLESTEROL MIXED-MODEL MEMBRANES IN THE LIQUID-CRYSTALLINE STATE - EFFECTS OF WATER

A method for obtaining the thermodynamic activity of each membrane com ponent in phosphatidylcholine (PC)/cholesterol mixtures, that is based upon ESR spin labeling is examined. The thermodynamic activity coeffi cients, gamma(PC) and gamma(chol), for the PC and cholesterol, respect ively, are obtained from the measured orientational order parameters, S(PC) and S(chol), as a function of cholesterol content for a spin-lab eled PC and the sterol-type cholestane spin probe (CSL), respectively, and the effects of water concentration are also considered. At water content of 24 weight%, the thermodynamics of DMPC/cholesterol/water mi xtures in the liquid-crystalline state may be treated as a two-compone nt solution ignoring the water, but at lower water content the role of water is important, especially at lower cholesterol concentrations. A t lower water content (17 wt%), gamma(chol) decreases with increasing cholesterol content which implies aggregation. However, at higher wate r content (24 wt%), gamma(chol) is found initially to increase as a fu nction of cholesterol content before decreasing at higher cholesterol content. This implies a favorable accommodation for the cholesterol in the membrane at high water and low cholesterol content. Good thermody namic consistency according to the Gibbs-Duhem equation was obtained f or gamma(PC) and gamma(chol) at 24 wt% water. The availability of gamm a(chol) (and gamma(PC)) as a function of cholesterol concentration per mits the estimate of the boundary for phase separation. The rotational diffusion coefficients of the labeled PC and of CSL were also obtaine d from the ESR spectra. A previously proposed universal relation for t he perpendicular component of the rotational diffusion tensor, R(perpe ndicular-to), for CSL in PC/cholesterol mixtures (i.e., R(perpendicula r-to) = R(perpendicular-to)0 exp(-AS2(chol)2/RT)) is confirmed. A chan ge in composition of cholesterol or of water for DMPC/cholesterol/wate r mixtures affects R(perpendicular-to) only through the dependence of S(chol) on the composition. In particular, the amount of water affects the membrane fluidity, monitored by R(perpendicular-to) for CSL, sole ly by the structural changes it induces in the membrane for the compos itions studied. Rotational diffusion for the labeled PC is found to be more complex, most likely due to the combined action of the internal modes of motion of the flexible chain and of the overall molecular reo rientation.