Octyl glucoside-mediated solubilization and reconstitution of liposomes: Structural and kinetic aspects

Structural and kinetic aspects of the solubilization of phosphatidylcholine (PC) liposomes induced by the nonionic surfactant octyl glucoside (OG) and the reconstitution of PC vesicles by dilution of OG/PC mixed micellar syst ems were studied using dynamic light scattering and freeze-fracture electro n microscopy. To this end, three regions were delimited in the equilibrium phase diagram. of this interaction: a "vesicular" region formed only by mix ed vesicles, a "coexistence" region, in which fragmented vesicles and mixed micelles coexisted, and a "micellar" region formed by only mixed micelles. A simple mechanism of liposomes solubilization is proposed based on the fo llowing points: (a) Up to saturation of vesicles by OG, a direct formation of mixed micelles within the bilayer occurred. (b) The progressive separati on of the formed micelles from the liposome. surface led to the complete so lubilization of vesicles. (c) This separation would take place without form ation of complex intermediate aggregates in equilibrated systems with the o nly presence of mixed micelles and fragmented vesicles. The systems placed in the micellar and vesicular regions were more stable with time than those placed in the coexistence region in which a growth of the fragmented vesic les and a reduction in their proportion were detected. The dilution (one st ep fast dilution) of OG/PC micellar solutions placed on the micellar phase boundary (corresponding to the effective surfactant to PC molar ratio for l iposome solubilization, Re-SOL) led to the formation of vesicles, in which the higher the total concentration of OG and PC in the initial system, the higher the size of the reconstituted vesicles. The kinetics of solubilizati on and reconstitution processes showed that the "induction time" (time need ed for the processes to start) for the formation mixed micelles was higher that for the reconstitution of mixed vesicles, indicating that the reconsti tution process was faster than that of solubilization.