Cholesterol-multilipid interactions in bilayers

To extend our knowledge of model membrane systems based upon one lipid component, multi-lamellar bilayers were made of cholesterol with two phospholipids in equimolar ratio, and the enthalpy change delta H of the main phase transition of the temary mixture was measured by differential scanning calorimetry (DSC) as a function of increasing cholesterol concentration c. The lipids were saturated phosphatidylcholines CnPC of acyl chain length n, and as the n of the two lipids became more different (from C13PC/C14PC to C14PC/C15PC to C14PC/C18PC to C14PC/C19PC) distinct breaks in the delta H versus c plots were observed. These mixtures displayed only one broad DSC endotherm. Mixtures of an unsaturated lipid C18: 1PC (dioleoyl) with C16PC or with C18PC showed two peaks, with each peak being associated with its parent lipid. However, the delta H versus c plots for each of these peaks showed an initial independence of cholesterol concentration followed by a dependence on cholesterol concentration. These results indicate that, in lipid mixtures, the type of interaction of cholesterol with each lipid component depends on the concentration of cholesterol present.