NMR Based Study Of The Aggregate Property Of Zwitterionic Detergent CHAPS And Its Interaction With Triton X-100

Nuclear magnetic resonance (NMR) spectroscopy plays an important role in the study of detergents. It is a powerful spectroscopic method for structure analysis and molecular interactions. In this dissertation, the concentration-dependent aggregate behaviors of CHAPS, the effects of temperature and NaCl upon the micellar property of CHAPS, and the aggregate property of the mixture of CHAPS and nonionic detergent Triton X-100 in aqueous solutions have been investigated by NMR.(1) CHAPS (3[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate) is a zwitterionic detergent, which has been extensively used in various biological fields. We have found that there are two critical micelle concentrations (cmc) available for CHAPS, the first one corresponds to the widely accepted normal critical micelle concentration (cmc), the second one is unexpected and indicates that there is another type of micelle at higher CHAPS concentrations. Further analysis using 1D selective NOESY with spin-diffusion quenching has revealed that CHAPS may form a two-layer spherical structure of micelle at the concentration above the second cmc, with the aliphatic groups of CHAPS molecules in the inner layer interacting with the steroid groups in the outer layer. The dependence of CHAPS micelle size on concentration explains why some proteins are soluble and stable only at concentrations above the second cmc.(2) The effects of temperature and NaCl on the micellization of CHAPS was investigated. We have found that the two apparent cmc values of CHAPS decrease with the increase of temperature, as well as the NaCl concentration. The thermodynamic parameters derived from the temperature-dependent cmc values show that the micellization process is spontaneous and exothermic, and the van der Waals interaction is likely to be the main factor for the micellization of CHAPS. The micellar hydrodynamic radii remain almost the same in a range of 100-600 mM NaCl, indicating that the aggregate states of CHAPS are not sensitive to the change of the surrounding conditions. In addition, the dependence of NOE intensities on temperatures further demonstrates the existence of the unique staggered micellar structure of CHAPS at a concentration above the apparent second cmc.(3) The aggregate behavior of the complex micelle composed of CHAPS and nonionic detergent Triton X-100 has been investigated. The experiment results reveal that Triton X-100 has more hydrophobic characteristic, the incorporation of Triton X-100 to CHAPS may lead to a more compact and hydrophobic mixed micelle that is restricted in a relative small bilayer structure. Triton X-100 micelles usually become bigger with the increase of its concentration, however, by adding CHAPS, the mixed micelles may be kept in a relatively smaller volume, which is the same as the staggered micelle of CHAPS.The data presented in this dissertation will enrich the surfactant theory and be useful to understand the micellar stability and solubilization capability of CHAPS and CHAPS/Triton X-100 mixed detergent systems, and may provide valuable information in designing mixed micelles with desirable performance for particular applications, such as the solubilization and stabilization of membrane protein.