Thermodynamic Characterization of Bile Salt Aggregation as a Function of Temperature and Ionic Strength Using Isothermal Titration Calorimetry

Abstract

The critical micellar concentration (cmc) and the demicellization enthalpy ΔHdemic 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. ΔHdemic is strongly temperature-dependent but shows almost no dependence on the ionic strength. For comparison with other systems, the thermodynamic parameters ΔGdemic and ΔSdemic associated with the demicellization process were calculated using the pseudo-phase-separation model. From the temperature dependence of ΔHdemic, the change in heat capacity ΔCpdemic for the demicellization process was determined. The data obtained for ΔCpdemic are 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 nm2 per molecule. For NaDC, ΔCpdemic decreases at 343 K to ∼250 J·mol-1·K-1, whereas ΔCpdemic 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.