Bile Acids and Bile Salts: Ionization and Solubility Properties

Abstract

The number, position and orientation of nuclear OH substituents profoundly influence the equilibrium solubilities of undissociated bile acids in water. Estimates from several studies range from 5 × 10″⁸ for lithocholic acid to 1.6 × 10⁻³ Af for ursocholic acid at 37°C. Fully dissociated sodium bile salts are extremely soluble in water, attaining values as high as 1 to 2 M. However, ionized unconjugated bile salts are appreciably less soluble than their glycine and taurine conjugates. In contrast to aqueous solutions of typical soaps and detergents, aqueous bile salt systems in concentrations beyond the micellar phase limit exhibit no liquid-crystalline phases. The critical micellar temperatures of the common di- and trihydroxy bile salts lie below 0°C, hence most bile salts form micelles at all ambient temperatures. With many monohydroxy and certain uncommon dihydroxy bile salts, the critical micellar temperatures lie above body temperature, and a micellar phase is observed only at temperatures in excess of 50°C. The pKa' values of aqueous micellar concentrations of bile salts are influenced by the nuclear substituents, the state of conjugation and bile salt concentration. In contrast, the intrinsic thermodynamic pK'a values of all monocarboxy-lated bile salts in monomeric solution are similar, are not influenced by nuclear substituents and are equivalent to that expected for propionic acid (pKa' ∼4.8 to 5.0). These values can be deduced by extrapolation from acidimetric titrations of bile salts in aqueous methanol mixtures or from acidometric aqueous titration of the carboxylate groups of pansulfated bile salts which remain water soluble when the ionic group of the side chain is protonated. The precipitation pH values of the common protonated bile acids are influenced by bile salt concentration, the state of conjugation and the solubilizing capacity of bile salt micelles for the otherwise sparingly soluble undissociated bile acid species.