Surface properties of cholesterol-containing membranes detected by Prodan fluorescence.

The fluorescent membrane probe 6-propionyl-2-dimethylaminonaphthalene (Prodan) displays a high sensitivity to thepolarity and packing properties of lipid membrane. Contrary to 6-lauroyl-2-dimethylaminonaphthalene (Laurdan), Prodancan also monitor the properties of the membrane surface, i.e., the polar-head pretransition. In bilayers composed ofcoexisting gel and liquid^crystalline phases, Prodan shows a preferential partitioning in the latter, so that the detectedmembrane properties mainly belong to fluid domains. In the presence of cholesterol, the packing properties of the gel phasephospholipids are modified in such a way that Prodan can penetrate and label the membrane. Although Prodan labeling ofthe gel phase is a function of cholesterol concentration, 3 mol percent cholesterol is sufficient for a 60% Prodan labeling withrespect to the maximum labeling reached at 15 mol percent cholesterol. We present steady-state and dynamical fluorescencemeasurements of Prodan in bilayers in the presence of cholesterol. Our results fit the liquid-ordered/liquid-disordered phasemodel for cholesterol-containing membranes and show that the presence of cholesterol, in addition to modification to thephase state of the hydrophobic portion of the bilayer, strongly affects the packing and the polarity of the membranehydrophobic^hydrophilic interface.