VESICLE-MICELLE STRUCTURAL TRANSITION OF PHOSPHATIDYLCHOLINE BILAYERSAND TRITON X-100

The structural transition stages induced by the interaction of the non -ionic surfactant Triton X-100 on phosphatidylcholine unilamellar vesi cles were studied by means of static and dynamic light-scattering, tra nsmission-electron-microscopy (t.e.m.) and permeability changes. A lin ear correlation was observed between the effective surfactant/lipid mo lar ratios (Re) ('three-stage' model proposed for the vesicle solubili zation) and the surfactant concentration throughout the process. Howev er, this correlation was not noted for the partition coefficients of t he surfactant between the bilayer and the aqueous medium (K). Thus a s harp initial K increase was observed until a maximum value was achieve d for permeability alterations of 50%, (initial step of bilayer satura tion). Further surfactant additions resulted in a fall in the K values until 100% of bilayer permeability. Additional amounts of surfactant led to an increase in K until bilayer solubilization. Hence, a prefere ntial incorporation of surfactant molecules into liposomes governs the initial interaction steps, leading to the initial stage of bilayer sa turation with a free surfactant concentration that was lower than its critical micelle concentration (c.m.c.). Additional amounts of surfact ant increased the free surfactant until the c.m.c. was reached, after which solubilization started to occur. Thus the initial step of bilaye r saturation was achieved for a smaller surfactant concentration than that for the Re-sat, although this concentration was the minimum neede d for solubilization to start. Large unilamellar vesicles began to for m as the surfactant exceeded 15 mol% (50% bilayer permeability), the m aximum vesicle growth being attained for 22 mol% (400 nm). Thereafter, static light-scattering started to decrease gradually, this fall bein g more pronounced after 40 mol%. The t.e.m. picture for 40 mol% (Re-sa t) showed unilamellar vesicles, although with traces of smaller struct ures. From 50 mol% the size distribution curves began to show a bimoda l distribution. The t.e.m. pictures for 50-64 mol% revealed tubular st ructures, together with open bilayer fragments. Thereafter, increasing amounts of surfactant (65-69 mol%) led to planar multilayered structu res which gradually tended to form concentric and helicoidal conformat ions. The scattered intensity decreased to a low constant value at mor e than 71-72 mol%. However, the surfactant concentration for the Re-so l (72.6 mol%) still presented traces of aggregated structures, albeit with monomodal size-distribution curves (particle size of 50 nm). This vesicle size corresponded to the liposome solubilization via mixed-mi celle formation.