Cholesterol Effect on the Dipole Potential of Lipid Membranes

The effect of cholesterol removal by methyl-β-cyclodextrin on the dipole potential, ψd, of membrane vesicles composed of natural membrane lipids extracted from the kidney and brain of eight vertebrate species was investigated using the voltage-sensitive fluorescent probe di-8-ANEPPS. Cyclodextrin treatment reduced cholesterol levels by on average 80% and this was associated with an average reduction in ψd of 50 mV. Measurements of the effect of a range of cholesterol derivatives on the ψd of DMPC lipid vesicles showed that the magnitude of the effect correlated with the component of the sterol’s dipole moment perpendicular to the membrane surface. The changes in ψd observed could not be accounted for solely by the electric field originating from the sterols’ dipole moments. Additional factors must arise from sterol-induced changes in lipid packing, which changes the density of dipoles in the membrane, and changes in water penetration into the membrane, which changes the effective dielectric constant of the interfacial region. In DMPC membranes, the cholesterol-induced change in ψd was biphasic, i.e., a maximum in ψd was observed at ∼35–45 mol %, after which ψd started to decrease. We suggest that this could be associated with a maximum in the strength of DMPC-cholesterol intermolecular forces at this composition.