ASSEMBLY PROPERTIES OF TRITON X-100 PHOSPHATIDYLCHOLINE AGGREGATES DURING LIPOSOME SOLUBILIZATION/

The assembly properties of the nonionic surfactant Triton X-100 and ph osphatidylcholine (PC) aggregates during the overall solubilization pr ocess of PC liposome were investigated. Permeability alterations were detected as a change in 5(6)-carboxyfluorescein (CF) released from the interior of vesicles and bilayer solubilization as a decrease in the static light scattered by liposome suspensions. A direct dependence wa s established between the bilayer/aqueous phase surfactant partition c oefficients (K), the growth of vesicles and the leakage of entrapped C F in the initial interaction steps (surfactant to phospholipid molar r atio Re up to 0.2). These changes may be related to the increasing pre sence of surfactant molecules in the outer monolayer of vesicles. In t he Re range 0.2-0.35 the coexistence of a low vesicle growth with a co nstant increase of CF release may be correlated with the decrease in K (increased rate of flip-flop of surfactant molecules). Furthermore, i n the Re range between 0.64 and 2.0 (lytic levels) almost a linear dep endence was detected between the composition of these aggregates (Re) and the decrease in both the surfactant-PC aggregate size and the stat ic light scattered by the system. This dependence was not observed in the last solubilization steps (Re range 2.0-2.60) possibly due to the increased formation of mixed micelles in this interval. The fact that the free Triton X-100 concentration at sublytic and lytic levels showe d respectively lower and similar values than its critical micelle conc entration confirms that permeability alterations and solubilization we re determined respectively by the action of surfactant monomer and by the formation of mixed micelles.