Shin Y K, Freed J H
Baker Laboratory of Chemistry, Cornell University, Ithaca, New York 14853.
Biophys J. 1989 Dec;56(6):1093-100. doi: 10.1016/S0006-3495(89)82757-2.
It is shown that good estimates of the activity of cholesterol in phosphatidylcholine-cholesterol mixed model membranes are obtained by examining the orientational order parameter S of cholestane spin probe (CSL) that is obtained from electron spin resonance by spectral simulation. By introducing thermodynamic stability conditions of liquid mixtures, the variation of activity (or S) as a function of cholesterol mole fraction is utilized to predict the concentration at which the phase separation occurs. These results for DMPC and cholesterol binary mixtures agree very well with those of Tempo-partitioning experiments. The comparison of activity coefficients and the phase boundary in DMPC/cholesterol mixtures with those of POPC/cholesterol mixtures suggests that acyl chain unsaturation leads to poorer mixing of cholesterol in phosphatidylcholine model membranes at higher temperatures (i.e., greater than 35 degrees C). In ternary solutions of DMPC, POPC, and cholesterol, it is found that cholesterol shows less deviation from ideality than in either of the two binary mixtures, and this implies that the phase separation occurs at higher cholesterol concentration than in either of the two binary mixtures. The present analysis suggests that there may not be a critical point in DMPC/cholesterol mixtures, even though phase separation does occur.
结果表明，通过电子自旋共振光谱模拟获得胆甾烷自旋探针（CSL）的取向序参数S，可以对磷脂酰胆碱 - 胆固醇混合模型膜中胆固醇的活性进行良好估计。通过引入液体混合物的热力学稳定性条件，利用活性（或S）随胆固醇摩尔分数的变化来预测相分离发生时的浓度。这些关于二肉豆蔻酰磷脂酰胆碱（DMPC）和胆固醇二元混合物的结果与Tempo分配实验的结果非常吻合。将DMPC/胆固醇混合物中的活度系数和相边界与1-棕榈酰-2-油酰磷脂酰胆碱（POPC）/胆固醇混合物的活度系数和相边界进行比较表明，在较高温度（即高于35摄氏度）下，酰基链不饱和度会导致胆固醇在磷脂酰胆碱模型膜中的混合性变差。在DMPC、POPC和胆固醇的三元溶液中，发现胆固醇与理想状态的偏差比两种二元混合物中的任何一种都小，这意味着相分离发生时的胆固醇浓度比两种二元混合物中的任何一种都高。目前的分析表明，即使确实发生了相分离，DMPC/胆固醇混合物中可能也不存在临界点。
