Calorimetric studies of the effects of cholesterol on the phase transition of C(18):C(10) phosphatidylcholine

Abstract

Differential scanning calorimetry (DSC) has been employed to study the effects of cholesterol on the phase transition of C(18):C(10) phosphatidylcholine (C(18):C(10)PC). C(18):C(10)PC is an asymmetric mixed-chain phosphatidylcholine known to form mixed-interdigitated structures below the transition temperature and form partially interdigitated lipid bilayers above the transition. Three types of samples were used. The treated sample is the lipid dispersion that had undergone three freeze-thaw cycles and stored at 4 degrees C for more than 48 h. The untreated sample was made by vortexing the dry lipid in 50 mM KCl, without the above-mentioned pretreatment. The cold-treated sample was prepared by incubating the treated sample at -20 degrees C for 15 d. There is no apparent difference in the DSC curves between the treated and cold-treated samples. The data derived from the treated samples seem to be more reproducible. The DSC curves between the cholesterol/C(18):C(10)PC and cholesterol/symmetric diacylphosphatidylcholine mixtures are different in three aspects: overall appearance, the cholesterol dependence of delta H, and the effect of cholesterol on the maximal transition temperature Tm, the onset temperature To, and the completion temperature Tc. for both the treated and untreated samples, the total enthalpy change delta H of the phase transition of C(18):C(10)PC decreases with increasing cholesterol content, approaching zero at approximately 25 mol%. This level is lower than the total enthalpy changes reported previously for the cholesterol/symmetric diacylphosphatidylcholine mixtures. Both the heating and cooling thermograms show that Tm, To, and Tc decrease with increasing cholesterol content. The decreasing rates of these temperatures with cholesterol are in the neighborhood of -0.24 degree per mol% of cholesterol. This value is greater than those reported previously for cholesterol/symmetric diacylphosphatidylcholine mixtures. The phase transition between interdigitated lipid bilayers appears to be more sensitive to cholesterol than that between noninterdigitated lipid structures. The formation of highly ordered interdigitated lipid bilayers requires stringent structural conditions such as specific chain length differences and high molecular order. Apparently, in the presence of cholesterol, these stringent structural conditions are no longer satisfied. It is likely that cholesterol causes a local disordering effect on the gel phase of C(18):C(10)PC and that as a consequence the physical state of the gel phase changes continuously with the cholesterol content. The implication of the present study is that cholesterol may have a function in preventing lipids from forming highly ordered interdigitated structures in natural membranes.