Mechanism of mixed liposome solubilization in the presence of sodium dodecyl sulfate

Structural transformations of vesicles to micelles that take place during the interaction of sodium dodecyl sulfate (SDS) with individual and mixed vesicles of phosphatidyl choline (PC) and phosphatidic acid (PA), have been studied by monitoring changes in optical density and in the concentration of free SDS monomer in the respective systems. Incorporation of the surfactant monomers (SDS) in the bilayers was found to result in an initial increase in concentration of the mixed vesicles up to its saturation. Subsequently a progressive relaxation of these structures together with a simultaneous formation of mixed micelles was found to occur. The breakup of bilayer and the formation of mixed micelles were completely dependent on the structure of the individual phospholipid. The solubilization of the anionic phosphatidic acid vesicle was very fast in the presence of SDS, due to its simple structure and its compatibility with the SDS molecule. But solubilization of the zwitter ionic phosphatidyl choline vesicle was very complicated in the presence of SDS, possibly due to its complex structure and the zwitterionic nature. On the other hand, solubilization of the mixed vesicle (mixed liposome) and formation of mixed micelle was found to be relatively easier. This may be attributed to one of the phospholipid component preferentially coming out first through interaction with SDS, thereby making the overall system unstable and enhancing the micellization processes.