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 component in phosphatidylcholine (PC)/cholesterol mixtures, that is based upon ESR spin labeling is examined. The thermodynamic activity coefficients, gamma PC and gamma chol, for the PC and cholesterol, respectively, are obtained from the measured orientational order parameters, SPC and S(chol), as a function of cholesterol content for a spin-labeled 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 mixtures in the liquid-crystalline state may be treated as a two-component solution ignoring the water, but at lower water content the role of water is important, especially at lower cholesterol concentrations. At lower water content (17 wt%), gamma chol decreases with increasing cholesterol content which implies aggregation. However, at higher water content (24 wt%), gamma chol is found initially to increase as a function 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 thermodynamic consistency according to the Gibbs-Duhem equation was obtained for gamma PC and gamma chol at 24 wt% water. The availability of gamma chol (and gamma PC) as a function of cholesterol concentration permits the estimate of the boundary for phase separation. The rotational diffusion coefficients of the labeled PC and of CSL were also obtained from the ESR spectra. A previously proposed universal relation for the perpendicular component of the rotational diffusion tensor, R perpendicular, for CSL in PC/cholesterol mixtures (i.e., R perpendicular = R0 perpendicular exp(-AS2chol/RT)) is confirmed. A change in composition of cholesterol or of water for DMPC/cholesterol/water mixtures affects R perpendicular only through the dependence of S(chol) on the composition. In particular, the amount of water affects the membrane fluidity, monitored by R perpendicular for CSL, solely by the structural changes it induces in the membrane for the compositions 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 reorientation.