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 Δ 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. Δ H demic is strongly temperature-dependent but shows almost no dependence on the ionic strength. For comparison with other systems, the thermodynamic parameters Δ G demic and Δ S demic associated with the demicellization process were calculated using the pseudo-phase-separation model. From the temperature dependence of Δ H demic, the change in heat capacity Δ Cp demic for the demicellization process was determined. The data obtained for Δ Cp demic 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 nm 2 per molecule. For NaDC, Δ Cp demic decreases at 343 K to ∼250 J·mol - 1 ·K - 1, whereas Δ 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.