Orientational polarisability of lipid membrane surfaces

Here we present a fluorescence method based on the Stokes shift of the voltage-sensitive dye di-8-ANEPPS to quantify the orientational polarisability of lipid membrane surfaces, i.e. the polarisability due to molecular reorientation. Di-8-ANEPPS is already an established probe of membrane dipole potential. Its use, therefore, as a probe of both the dipole potential and orientational polarisability allows a direct comparison of these two properties in an identical region of the lipid bilayer. We applied the new technique on phosphatidylcholine vesicles to study the effects of different degrees of hydrocarbon saturation and of the incorporation of cholesterol and some of its oxidized derivatives. We found that lipids with unsaturated chains had a lower orientational polarisability than those with saturated chains. This could be explained by a reduction in membrane dipole potential as a result of a decrease in lipid packing density. Cholesterol derivatives were found to either increase or decrease the orientational polarisability depending on their molecular structure. The varying effects could be explained by antagonistic effects of the dipole potential and membrane order, which are both changed to varying degrees by the cholesterol derivatives and which lead to increases and decreases in orientational polarisability, respectively.