Differential scanning calorimetric and Fourier transform infrared spectroscopic studies of the effects of cholesterol on the thermotropic phase behavior and organization of a homologous series of linear saturated phosphatidylserine bilayer membranes

We have examined the effects of cholesterol on the thermotropic phase behav ior and organization of aqueous dispersions of a homologous series of linea r disaturated phosphatidylserines by high-sensitivity differential scanning calorimetry and Fourier transform infrared spectroscopy. We find that the incorporation of increasing quantities of cholesterol progressively reduces the temperature, enthalpy, and cooperativity of the gel-to-liquid-crystall ine phase transition of the host phosphatidylserine bilayer, such that a co operative chain-melting phase transition is completely or almost completely abolished at 50 mol % cholesterol, in contrast to the results of previous studies. We are also unable to detect the presence of a separate anhydrous cholesterol or cholesterol monohydrate phase in our binary mixtures, again in contrast to previous reports. We further show that the magnitude of the reduction in the phase transition temperature induced by cholesterol additi on is independent of the hydrocarbon chain length of the phosphatidylserine studied. This result contrasts with our previous results with phosphatidyl choline bilayers, where we found that cholesterol increases or decreases th e phase transition temperature in a chain length-dependent manner (1993. Bi ochemistry, 32:516-522), but is in agreement with our previous results for phosphatidylethanolamine bilayers, where no hydrocarbon chain length-depend ent effects were observed (1999. Biochim. Biophys. Acta, 1416.119-234). How ever, the reduction in the phase transition temperature by cholesterol is o f greater magnitude in phosphatidylethanolamine as compared to phosphatidyl serine bilayers. We also show that the addition of cholesterol facilitates the formation of the lamellar crystalline phase in phosphatidylserine bilay ers, as it does in phosphatidylethanolamine bilayers, whereas the formation of such phases in phosphatidylcholine bilayers is inhibited by the presenc e of cholesterol. We ascribe the limited miscibility of cholesterol in phos phatidylserine bilayers reported previously to a fractional crystallization of the cholesterol and phospholipid phases during the removal of organic s olvent from the binary mixture before the hydration of the sample. In gener al, the results of our studies to date indicate that the magnitude of the e ffect of cholesterol on the thermotropic phase behavior of the host phospho lipid bilayer, and its miscibility in phospholipid dispersions generally, d epend on the strength of the attractive interactions between the polar head groups and the hydrocarbon chains of the phospholipid molecule, and not on the charge of the polar headgroups per se.