CHANGES IN PHOSPHATIDYLCHOLINE LIPOSOMES CAUSED BY A MIXTURE OF TRITON X-100 AND SODIUM DODECYL-SULFATE

The mechanisms governing the interaction of equimolecular mixtures of Triton X-100 (T-X-100) and sodium dodecyl sulfate (SDS) with phosphati dylcholine liposomes were investigated. Permeability alterations were determined as a change in 5(6)-carboxyfluorescein released from the in terior of vesicles and bilayer solubilization as a decrease in the sta tic light-scattered by liposome suspensions. At subsolubilizing level, a maximum bilayer/water partitioning of surfactant mixture was reache d at 30% CF release, which correlated with the increased presence of S DS in the bilayers. However, transition stages between 70% CF release and 100% light-scattering corresponded to the increased presence of T- X-100 in these structures. These findings may be correlated with the r educed deleterious effects caused by this mixture in different tissues versus pure SDS, given that the presence of T-X-100 may modulate the level of SDS partitioning in the human stratum corneum. At subsolubili zing level, the mixture showed higher affinity with bilayers than thos e reported for single components, whereas at solubilizing level this a ffinity was slightly lower and higher than those reported for T-X-100 and SDS respectively. A direct relationship was established in the ini tial interaction steps between the growth of vesicles, the leakage of entrapped CF and the effective molar ratio of surfactant to phospholip id in bilayers (R(e)). This dependence was also detected during solubi lization, where the decrease in the vesicle size and in the scattered light of the system depended on the R(e) parameter and hence on the bi layer composition. The fact that the free surfactant concentration at subsolubilizing and solubilizing levels showed respectively lower and similar values than the critical micelle concentration (c.m.c.) of the surfactant mixture indicates that permeability alterations and solubi lization were determined respectively by the action of surfactant mono mer and by the formation of mixed micelles. This finding supports the generally admitted assumption, for single surfactants, that the concen tration of free surfactant must reach the c.m.c. for solubilization to occur and highlights the influence of the negative synergism of this surfactant mixture on the free surfactant concentration needed to satu rate or solubilize liposomes.