The influence of competing factors on the endothermic nature of bile salt binding by cationic adsorbent

Abstract

The binding of sodium chenodeoxycholate, a hydrophobic bile salt, by a polyacrylamide resin with N,N,N-trimethylammonium dodecyl chloride (QPDA12) pendant groups was studied in the presence of elevated concentrations of competing anions. The equilibrium bile salt concentration was determined from HPLC data for a range of initial bile salt concentrations. Binding constants extracted from the fit of the isotherms to the Langmuir equation were obtained for data for temperatures from 24 to 60°C. A reduction in chenodeoxycholate binding affinity was observed in comparison with that reported previously at lower overall anion concentrations. A van't Hoff analysis of the data revealed that a decrease in the favourable entropy of binding was responsible for this reduced affinity. Decomposition of the binding constants into specific contributions for the competing buffer anions and the chenodeoxycholate anion returned positive values of ΔH° and ΔS° for the binding of both the competing ions and bile salt. These findings reveal that the weakening of the bile salt binding that occurs with added salt is not due to Debye screening of an attraction between ions of opposite charge. A binding mechanism consistent with the thermodynamics observed was proposed in which the dominant role of hydration changes that occur on binding provides the principal driving force for the process.Key words: bile salts, endothermic binding, polymer sorbents, solvent randomization.