Chem. Pharm. Bull. 53(9) 1140—1146 (2005)

fonate (CHAPS) is a zwitterionic detergent that was specifically designed for membrane biochemistry. It is known to be a mild, nondenaturing detergent and is a derivative of natural bile salts. Its properties are of interest in several biochemical applications. These include the perturbation of membrane structures, membrane protein solubilization and functional reconstitution, and preparation of lipid vesicles with controlled size. The mechanism of vesicle destruction has been widely studied in connection with the reconstitution of membrane proteins after purification in detergent solutions. It has been pointed out that several parameters are important for optimal reconstitution efficiency, i.e., the type of detergent, detergent/phospholipid ratio, detergent/protein ratio, and salt concentration. Therefore it is important to clarify the effects of detergent incorporated in the membrane phase on membrane properties. In the past decades much attention has been paid to the interaction of detergents with phospholipid membranes. Octylglucoside, Triton X100, and bile acid derivatives were most widely studied. Although CHAPS has been widely used in membrane biology, little is known of the details of the interaction of CHAPS and vesicles. In a previous study in our laboratory on the solubilization of small unilamellar vesicles (SUV) by CHAPS, it was shown that small vesicles containing large amounts of CHAPS can fuse to increase their sizes during the solubilization process. Furthermore, when CHAPS was removed from mixed CHAPS/egg yolk phosphatidylcholine (EggPC) micelles, large vesicles formed by slow step-by-step dilution, but small vesicles formed by fast one-step dilution. The results demonstrated that the formation of large vesicles with the detergent removal method was related to the size growth of vesicles containing large amounts of detergent. In the present study, we report the solubilization process of large unilamellar vesicles (LUV) by CHAPS. The physicochemical properties including turbidity, apparent particle size, and Cl permeability were invesitigated. The partition behavior of CHAPS between the membrane phase and water phase was clarified. Additionally, electron spin resonance (ESR) spectroscopy of nitroxide derivatives of stearic acid at the 5and 16-positions of the alkyl chain were used to determine the motional profiles at two main regions of the bilayers, near the polar head group and near the end of the hydrophobic chain, respectively.