Micelle formation of sodium glyco- and taurocholates and sodium glyco- and taurodeoxycholates and solubilization of cholesterol into their micelles

Abstract

Micelle formation of sodium glyco- and taurocholates and sodium glyco- and taurodeoxycholates was studied at 308.2 K for the critical micelle concentration at various NaCl concentrations by pyrene fluorescence and scattered light intensity, and the degree of counterion binding to micelle was calculated using the Corrin-Harkins plots. The change of I1/I 3 values of the fluorescence spectrum with the conjugate bile salt concentration suggested two steps for the bile salt aggregation, which was supported by the scattered light intensity. The first step is a commencement of smaller aggregates, the first CMC, and the second one is a beginning of stable aggregates, the second CMC. Between the first and the second CMCs, the aggregates grow in size with the conjugate concentration. The aggregation number above the second CMC was determined at 308.2 K and 0.15 mol dm -3 NaCl concentration by the static light scattering: 8.7 and 6.0 for sodium glyco- and taurocholate, respectively, and 15.7 and 15.9 for sodium glyco- and taurodeoxycholate, respectively. The solubilization of cholesterol into the bile salt micelles in the presence of coexisting cholesterol phase or the maximum additive concentration (MAC) of cholesterol was determined against the bile salt concentration, and then, the standard Gibbs energy change for the solubilization was evaluated, where the micelles were regarded as a chemical species. The solubilization was stabilized in the order of sodium taurocholate