DPH is localised in two 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 heterogeneities of the bilayer. However, these heterogeneities are not revealed in experiments using other probe molecules. Here, we present an alternative analysis in terms of two distinct populations of DPH molecules in the bilayer structure. The results indicate that the heterogeneities seen by DPH reflect the variations of the packing density of the lipids in a direction perpendicular to the bilayer surface. The two populations are created by a competition between the interactions between the DPH molecules with the lipid chains and their tendency to access the regions of free volume in the middle of the bilayer.