We have so far studied the effect of cholesterol on the bilayer phase behavior of a homologous series of symmetric linear phosphatidylcholines (CnPCs) with different chain lengths (n = 14-18) by means of high-sensitivity differential scanning calorimetry and fluorescence spectroscopy using 6-propionyl-2-(N,N-dimethylamino)naphthalene (Prodan) as a membrane probe. The temperature?cholesterol composition (Xch) phase diagrams constructed for these binary bilayers indicated several important characteristic of the phase behavior of them. First, the phase diagrams are similar to those for the liquid/solid phase equilibria of general binary mixtures that exhibit incongruent melting behavior, suggesting that cholesterol and CnPC molecules are essentially immiscible within the binary bilayer. Second, a characteristic point like a peritectic point appears on the phase diagram, suggesting that cholesterol has a propensity to form a molecular cluster with its surrounding CnPC molecules. The Xch-dependent phase behavior indicated by the phase diagrams, especially cholesterol-induced lateral phase separation, was successfully explained by introducing the so-called superlattice view. In addition, this view allowed us to consider the relationship between the extent of the cholesterol effect and the hydrocarbon-chain length of the host CnPC molecules.
Keywords:cholesterol, incongruent melting, lateral phase separation, lipid raft, phase diagram, phospholipid bilayer.
Publication Date: 2017-07-25