Thermodynamics of the aggregation of the bile anions of obeticholic and chenodeoxycholic acids in aqueous solution

Abstract A complete characterization of the aggregation process of the sodium salts of chenodeoxycholic (NaCDC) and obeticholic (NaObC) acids has been carried out by using different experimental techniques. Since it is well known that the presence of hydrophobic groups strongly reduce the critical aggregation concentrations and modifies the type of aggregation structures which can be formed, it could be expected that the introduction of an ethyl group into the structure of the chenodeoxycholic acid, to form the obeticholic derivative, could give rise to important modifications in its aggregation behavior. However, it has been observed that the aggregation numbers (determined in the range of temperatures 10–40 °C from ITC measurements) are identical to each other, a fact which was supported by the determination of the hydrodynamic radius from dynamic light scattering. This similarity was also observed for the fraction of bound counterions determined from the well-known Corrin–Harkins plot, being equal to 0.33 for both compounds. Finally, the polarity of the interior of the aggregate as measured by the ratio of the fluorescence intensities of the first and third vibronic peaks, I1/I3, of monomeric pyrene solubilized within the aggregates is also identical within the experimental error. The main differences correspond to: (1) the temperatures at which the enthalpy of demicellization is zero (31.4 (NaObC) and 26.8 °C (NaCDC)); and (2) the cmc value for NaObC is roughly half the one for NaCDC as observed from surface tension, fluorescence and ITC measurements. This implies that the free energy for the demicellization process is around 2 kJ mol−1 larger for NaObC than for NaCDC. (3) Finally, the difference in the change in the heat capacity of the aggregation (being 60 J mol−1 K−1 higher for ObCA) is due to the different number of water molecules which are lost during the monomer transfer from the bulk solution to the micellar aggregate.