Thermodynamic characterization of bile salt aggregation as a function of temperature and ionic strength using isothermal titration calorimetry

The critical micellar concentration (cmc) and the demicellization enthalpy Delta H-demic of the primary aggregates of sodium cholate (NaC) and sodium deoxycholate (NaDC) in water and 0.1 M NaCl at pH 7.5 were determined by isothermal titration calorimetry (ITC). The cmc of NaC and NaDC in water and 0.1 M NaCl at pH 7.5 shows a minimum between 295 and 300 K. With increasing ionic strength, the cmc of the bile salts decreases. Delta H-demic is strongly temperature-dependent but shows almost no dependence on the ionic strength. For comparison with other systems, the thermodynamic parameters Delta G(demic) and Delta S-demic associated with the demicellization process were calculated using the pseudo-phase-separation model. From the temperature dependence of Delta H-demic, the change in heat capacity Delta Cp-demic for the demicellization process was determined. The data obtained for Delta Cp-demi care positive and at 298 K have values of 250 J.mol(-1).K-1 for NaC and 350 J.mol(-1).K-1 for NaDC. These values correspond to changes in the exposed hydrophobic surface area of 1.1-1.5 nm(2) per molecule. For NaDC, Delta Cp-demic decreases at 343 K to similar to 250 J.mol(-1).K-1, whereas Delta Cp-demic for NaC remains essentially unchanged. The calorimetric titration curves were simulated using a mass action model including counterion condensation for the aggregation process. The simulation of the titration curves yielded values for the aggregation number n. In the concentration region of the cmc, n is approximately 4-6 for NaC in water or 0.1 M NaCl and independent of temperature. For NaDC in water values of n of 7 and 12 were obtained at low temperature (284 K) in water and 0.1 M NaCl, respectively. For NaDC in water and 0.1 M NaCl, the aggregation number n decreases to 5 and 7, respectively, at 328 K.