Mechanisms of Solubilization of Mixed Liposomes: Preferential Dissolution of Liposome Components

The mechanism of vesicle-to-micelle transformation due to the interactions with sodium dodecylsulfate (SDS) has been studied by monitoring changes in the optical density, surface tension, SDS monomer concentrations, pyrene fluorescence, and mass spectrometry. Two inflection points appeared on the optical density as well as effective hydrodynamic diameter curves. Based on the results of the surface tension and SDS monomer measurements, the first inflection point is attributed to the saturation of bilayers by SDS monomers and the onset of liposome solubilization processes. The second inflection point corresponds to the onset of complete disruption of bilayers and the critical micelle concentration of the mixed systems. The fluorescence results show the core of the mixed micelles to be more hydrophobic than that of the SDS micelles, suggesting that liposome solubilization is a micellization process. From the individual phospholipid liposome solubilization studies, it was found that phosphatidic acid (PA) molecules are more susceptible toward SDS. Interestingly, it has been detected from the mass spectra that the disruption of liposome bilayers is a preferential dissolution process. It is proposed that, at low SDS concentration, PA molecules preferentially exit first from the mixed liposome bilayers (1:1 PA/phosphatidylcholine), causing liposome solubilization. In contrast, at high SDS concentrations, the breakdown of liposome takes place instantaneously.