Lateral distribution of cholesterol in membranes probed by means of a pyrene-labelled cholesterol: effects of acyl chain unsaturation Original Research Article

The lateral distribution of cholesterol in membranes in the fluid state was investigated by studying the variation of the molar absorption coefficient of pyrene-labelled cholesterol (Py-chol) vs. its concentration in vesicles made of phosphatidylcholine, with variable acyl chain unsaturations. Absorption measurements indicated non-ideal mixing of Py-chol in unsaturated lipids, a process mainly controlled by the cholesterol moiety of the probe. Similar abilities of cholesterol and Py-chol in perturbing the phase properties of pure saturated phosphatidylcholine were observed by DSC experiments. Immiscibility of sterols was corroborated by fluorescence polarization measurements, which indicated a weaker ordering effect of cholesterol in unsaturated membranes. The sizes and the quantities of sterol oligomers formed were calculated. A model for the lateral distribution of cholesterol in membranes is proposed and is applied to known cholesterol/phosphatidylcholine phase diagrams. Finally, the results are discussed with regard to recent models of biological membrane organization, (i.e. rafts).