Abstract
Animal cell membranes pose conceptual problems related to the physical chemistry of liquids. An avenue to the solution of some of these problems has been opened by the discovery of liquid-liquid immiscibility in synthetic membranes composed of cholesterol and phospholipids. This discovery has led to the development of a thermodynamic model involving condensed complexes. In this model, the phospholipids with longer fatty-acid chains react reversibly with cholesterol to form complexes. The complexes themselves can have a repulsive interaction with other phospholipids, leading to immiscibility. A striking example of this effect is revealed in the phase diagrams of ternary mixtures of cholesterol, a saturated phosphatidylcholine (or sphingomyelin), and an unsaturated phosphatidylcholine. As found by a number of investigators, all binary pairs are miscible in bilayers, whereas the ternary mixture can form two liquid phases. The model of condensed complexes accounts for this effect. Condensed complexes also have a major effect on the chemical activity of cholesterol and on the ordering of phospholipid acyl chains both in the presence and absence of phase separations. Model calculations of phospholipid order parameters account for several features of the deuterium NMR spectra of labeled phospholipid molecules in bilayer mixtures with cholesterol.