Effects of cholesterol on the lamellar and the inverted hexagonal phases of dielaidoylphosphatidylethanolamine

Effects of cholesterol on the lamellar and the inverted hexagonal (H-II) phases of dielaidoylphosphatidylethanolamine (DEPE) were studied by means of not only differential scanning calorimetry (DSC) but also simultaneous X-ray diffraction and DSC (XDDSC). XDDSC shows that structural changes are related to thermotropic events of the mixtures. Addition of cholesterol to DEPE induces to broaden the transition from the lamellar gel (L(beta)) to lamellar liquid-crystalline (L(alpha)) phase. In fact, in the broad transition region, a coexistence of two lamellar X-ray diffraction peaks of the L(beta) and the L(alpha) phases take place. In samples containing above 30 mol% cholesterol, no peak at the L(beta)-L(alpha) phase transition was observed in the DSC thermogram On the other hand, cholesterol causes biphasic effects on the L(alpha)-H-II phase transition: At low cholesterol concentrations below 20 mol%, the incorporation of cholesterol reduces the transition temperature and at high cholesterol concentrations above 30 mol%, the transition temperature increases by addition of cholesterol. Based upon the results of X-ray diffraction, the thermal expansion coefficients of lattice spacings, i.e., the temperature dependence of lattice spacings, were calculated in each phase. Addition of cholesterol reduces the thermal expansion coefficients of the lamellar phases and, in contrast, increases that of the H-II phase. From the above results it is suggested that cholesterol in cell membranes works in keeping the bilayer membrane nature notwithstanding the change of external conditions.