VESICLE TO MICELLE PHASE-TRANSITIONS INVOLVED IN THE INTERACTION OF SODIUM CHOLATE WITH PHOSPHATIDYLCHOLINE LIPOSOMES

The interaction of sodium cholate surfactant with phosphatidylcholine unilamellar liposomes was investigated. Alterations in bilayer permeab ility were detected as a change in 5(6)-carboxyfluorescein (CF) releas ed from the interior of vesicles to the bulk aqueous medium and bilaye r solubilization as a decrease in the static light-scattered by liposo me suspensions. The value corresponding to the effective surfactant to phospholipid molar ratio parameter (Re) for each interaction step inv estigated progressively increased as the surfactant concentration rose , this rise being more pronounced during bilayer solubilization. At th e subsolubilizing level, a maximum bilayer/water surfactant partitioni ng (K) was reached at 30% of CF release, correlated with the saturatio n of the outer vesicle leaflet, followed by an abrupt decrease in K up to the complete saturation of these bilayer structures (Re-SAT). At t he solubilizing level, the K parameter also drastically increased up t o the complete bilayer solubilization (Re-SOL). The fact that the free surfactant concentration at subsolubilizing and solubilizing levels s howed values respectively lower than and similar to its critical micel le concentration (CMC) indicates that permeability alterations and bil ayer solubilization were determined respectively by the action of surf actant monomer and by the formation of mixed micelles. (C) 1997 Elsevi er Science B.V.