Yk. Shin et al., THERMODYNAMICS AND DYNAMICS OF PHOSPHATIDYLCHOLINE-CHOLESTEROL MIXED-MODEL MEMBRANES IN THE LIQUID-CRYSTALLINE STATE - EFFECTS OF WATER, Biophysical journal, 65(3), 1993, pp. 1283-1294
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.