STRUCTURAL PHASE-TRANSITIONS INVOLVED IN THE INTERACTION OF PHOSPHOLIPID-BILAYERS WITH OCTYL GLUCOSIDE

The transitional stages induced by the interaction of the nonionic sur factant octyl glucoside (OcOse) on phosphatidylcholine liposomes were studied by means of transmission electron microscopy (TEM), light scat tering and permeability changes. A linear correlation was observed bet ween the effective surfactant/lipid molar ratio (Re; three-stage model proposed for liposome solubilization) and the OcOse concentration in the initial and final interaction stages, despite showing almost a con stant value during bilayer saturation. The bilayer/aqueous phase parti tion coefficient (K) decreased in the subsolubilizing interaction step s and increased during solubilization. Thus, whereas a preferential di stribution of surfactant monomers in the aqueous phase with respect to the lipid bilayers took place in the initial interaction steps, a lar ger association of OcOse molecules with these lipids in bilayers occur ed during solubilization. The initial steps of bilayer saturation (50- 70% permeability) were attained for a lower free surfactant (S-W) than that for its critical micellar concentration (cmc). When S-W reached the OcOse cmc, solubilization started to occur (Re-sat). Large unilame llar vesicles began to form as the OcOse exceeded 60 mo1/100 mel, exhi biting for 65 mol/ 100 mol (50% permeability) vesicles of approximatel y 400 nm. TEM pictures for 100% permeability (72 mo1/100 mol) and Re-s at still showed unilamellar vesicles, albeit that the Re-sat TEM pictu re showing traces of smaller structures. Exceeding surfactant amounts led to a decrease in static light scattering; the vesicle-size curve b egan to show a bimodal distribution. The TEM picture showed tubular st ructures together with bilayer fragments. Thereafter, the open structu res were gradually affected by the surfactant and the scattered intens ity gradually decreased to a constant low value.