Solubilization of liposomes by sodium dodecyl sulfate: new mechanism based on the direct formation of mixed micelles.

The vesicle-to-micelle structural transitions that occurred in the interaction of sodium dodecyl sulfate with phosphatidylcholine vesicles were studied at the equilibrium by means of dynamic light scattering (at different scattering angles) and freeze-fracture electron microscopy techniques. The incorporation of surfactant monomers in the bilayers resulted in an initial contraction of the mixed vesicles formed up to their saturation (size reduction of about 10%). Then, a progressive relaxation of these structures (growth from 170 to 225 nm) and a simultaneous formation of mixed micelles (particles of about 6 nm) occurred. Hence, in this interval "relaxed mixed vesicles" and mixed micelles coexisted in different proportions without formation of intermediate complex aggregates (bimodal size distribution curves). Freeze-fracture electron microscopy showed a direct formation of mixed micelles within the bilayer and their subsequent separation from the vesicle surface without formation of complex intermediate aggregates. This simple process progressed up to the complete vesicle solubilization. Copyright 1999 Academic Press.