SOLUBILIZATION OF UNILAMELLAR LIPOSOMES BY BETAINE-TYPE ZWITTERIONIC ANIONIC 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 neutral or electrically charged unilamellar liposomes were investigated. The mixed systems wer e formed by N-dodecyl-N,N-dimethylbetaine and sodium dodecyl sulfate i n the presence of piperazine-1,4-bis-(2-ethanesulfonic acid) buffer at pH 7.20. Unilamellar liposomes formed by egg phosphatidylcholine, in some cases together with stearylamine or phosphatidic acid, were used. Solubilization was detected as a decrease in static light-scattering of liposomes. Two parameters were regarded as corresponding to the eff ective surfactant/lipid molar ratios (Re) at which the surfactant syst em (i) saturated the liposomes, Re-sat, and (ii) led to a total solubi lization of liposomes, Re-sol. From these parameters the bilayer/aqueo us medium surfactant partition coefficients for the saturation (K-sat) and complete bilayer solubilization (K-sol) were determined. When X(z witter) was 0.40, The Re and K parameters showed a maximum, whereas th e critical micellar concentration (CMC) of these systems exhibited a m inimum, regardless of the electrical charge of bilayers. Given that th e ability of the surfactant systems to saturate or solubilize liposome s is inversely related to the Re-sat and Re-sol parameters, these capa cities appear to be directly correlated with the CMC of the mixed syst ems. The similarity of both K-sat and K-sol (particularly for X(zwitte r) = 0.2-0.8) suggests that a similar partition equilibrium governs bo th the saturation adn the complete solubilization of bilayers, the fre e surfactant concentration (S-a,S-b), remaining almost constant with s imilar values to the CMC for each mixed system studied.