Solubilization of aromatics in aqueous bile salts: III. Thermodynamic model for solubilization of benzene in sodium cholate based on 1H NMR chemical shifts

Abstract

A thermodynamic model for benzene solubilization in aqueous sodium cholate was developed based on the 1H NMR chemical shift changes of angular methyl on the hydrophobic side of the cholate anion. As the first step of the aggregation process of cholate the dimer of cholate anions was selected as the solubilizing site for benzene molecules. A two-step solubilization model was not able to explain the chemical-shift behavior observed, and therefore three successive equilibria for benzene binding in the cholate dimer are needed to reproduce the experimental data. In spite of many limiting constraints used to reduce the number of variables only the relative values for the Gibbs free energies could be evaluated. They do not show any clear cooperativity, as detected for deoxycholate in another study. A similar feature of cholate and deoxycholate is that the third equilibrium constant is clearly smaller than the first and second.