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.