ROLE OF THE ELECTROSTATIC INTERACTION IN THE SOLUBILIZATION PROCESS OF BIOMIMETIC MEMBRANES BY DETERGENTS

Detergents are amphiphilic molecules capable of solubilizing and separating the components of biological membranes. In particular, efficient solubilization of lipid bilayers depends on several factors, including physicochemical characteristics of detergents and lipids, as well as the bilayer phase. Studies on the mechanism of lipid bilayer solubilization by detergents using model systems are important in the search for more efficient solubilization protocols. In general, these studies are performed with neutral lipids, although biological membranes contain a relevant fraction of anionic lipids. Here we investigate the role of the electrostatic interaction in the process of liposome solubilization. For this purpose, we used liposomes composed of zwitterionic (phosphatidyl choline, PC) and anionic (phosphatidyl glycerol) lipids and neutral (octyl glucoside, OG), cationic (CTAB) and anionic (sodium dodecyl sulfate, SDS) detergents. The solubilization profile of large unilamellar vesicles (LUVs, 100 nm) of PC, PG and PC:PG by the detergents was obtained by turbidity measurements. In parallel, optical microscopy of giant unilamellar vesicles (GUVs, ~50 m) was used to visualize the solubilization process. Subsequently, the calorimetric techniques of ITC (isothermal titration calorimetry) and DSC (differential scanning calorimetry) were employed to determine the enthalpy variation of the processes of micellization and detergent incorporation into bilayers and to evaluate the impact of the presence of detergents on the gel-fluid phase transition of bilayers The turbidity data show that anionic bilayers are more susceptible to solubilization by all detergents, regardless of the charge, corroborating previous data that demonstrate the greater instability of charged bilayers. Observation with optical microscopy revealed that the OG detergent causes an area increase in GUVs with subsequent solubilization, while the ionic detergents cause vesicle burst, due to the insertion of the detergent only in the outer monolayer of the GUVs. The ITC data demonstrated that the incorporation of detergents into bilayers is highly sensitive to the electrostatic interaction. The DSC results also showed that PG vesicles are more susceptible to solubilization.