Fluorescent probe studies of mixed micelles of phospholipids and bile salts. Effect of cholesterol incorporation.

The binding of the fluorescent alkylamines, N-(2-aminoethyl)-5-dimethyl-amino-1-naphthalene sulfonamide, N-(5-aminopentyl)-5-dimethylamino-1-naphthalene sulfonamide (dansyl cadaverine) and N-(10-aminodecyl)-5-dimethylamino-1-napthalene sulfonamide with phospholipid and phospholipid-deoxycholate micelles, has been shown to increase with the length of the alkyl spacer chain. The probes bind more effectively to micelles containing unsaturated phospholipids and do not interact strongly with bile salt solutions at low concentrations. Cholesterol incorporation into mixed micelles results in a quenching of probe fluorescence due to displacement of probe molecules. The enhanced rigidity of the mixed micelles on solubilizing cholesterol is established by a decrease in pyrene excimer fluorescence and by the less effective perturbation of the micellar structure by 1-anilino-8-naphthalene sulfonate. The anionic probe 1-anilino-8-naphthalene sulfonate is also displaced from the mixed micelles when cholesterol is incorporated, suggesting a dominant role for packing and hydrophobic effects in binding both positively and negatively charged probes.