DPH IS LOCALIZED IN 2 DISTINCT POPULATIONS IN LIPID VESICLES

The fluorescence lifetime and anisotropy of DPH molecules in vesicles of lipids is commonly interpreted in terms of intrinsic structural het erogeneities of the bilayer. However, these heterogeneities are not re vealed in experiments using other probe molecules. Here, we present an alternative analysis in terms of two distinct populations of DPH mole cules in the bilayer structure. The results indicate that the heteroge neities seen by DPH reflect the variations of the packing density of t he lipids in a direction perpendicular to the bilayer surface. The two populations are created by a competition between the interactions bet ween the DPH molecules with the lipid chains and their tendency to acc ess the regions of free volume in the middle of the bilayer.