SOLUBILIZATION OF PHOSPHOLIPID-BILAYERS BY C-14 ALKYL BETAINE ANIONICMIXED SURFACTANT SYSTEMS

The mechanisms governing the solubilizing interactions between zwitter ionic/anionic mixed surfactant systems at different molar fractions of the zwitterionic surfactant (X(zwitter)) and unilamellar liposomes we re investigated. Solubilization was detected as a decrease in static l ightscattering of liposome suspensions. Three parameters were regarded as corresponding to the effective surfactant/lipid molar ratios (Re) at which the surfactant system a) saturated the liposomes Re-sat; b) r esulted in 50% solubilization of liposomes Re-50%, and c) led to a com plete solubilization of these structures Re-sol. These parameters corr esponded to the Re at which light scattering stars to decrease, reache s 50% of the original value and shows no further decrease. From these parameters the surfactant partition coefficients for these three steps (K-sat, K-50% and K-sol) were also determined. The mixed systems were formed by N-tetradecyl-N,N-dimethylbetaine (C-14-Bet) and sodium dode cyl sulphate (SDS) in PIPES buffer at pH 7.20. Liposomes were formed b y egg phosphatidylcholine and phosphatidic acid (9:1 molar ratio). Whe n the range of X(zwitter) was about 0.4-0.6 Re and K parameters showed a maximum, whereas the critical micelle concentration (CMC) of these systems exhibited a minimum. Given that the ability of the surfactant systems to solubilize liposomes is inversely related to Re-sol, this c apacity appeared to be directly correlated with the CMC of the systems . The progressive rise of K during the process indicates that an incre asing surfactant partition equilibrium governs the interaction process from the saturation until the solubilization of vesicles, the free su rfactant concentration remaining almost constant with similar values t o the CMC for each mixed system studied.