Aqueous solubility and acidity constants of cholic,

Cholic acid, deoxycholic acid, chenodeoxycholic acid, and ursodeoxycholic acid were purified by a foam fractionation method. Using thermogravimetric analysis, the attached water molecule was found to be completely removed from solids of the latter three at 100°C, while cholic acid still had one water molecule of crystallization per two cholic acid molecules at that temperature. The acidity constants of the acids were accurately determined from aqueous solubility measurements at different pH values and at 15, 25, 35, and 45°C. The enthalpy change of dissolution from temperature dependence of solubility as undissociated acid monomer was much less than those of common ionic surfactants. This results from a smaller entropy increase on dissolution due to the hardly flexible hydrophobic structure of these bile acids.-Mod, Y., M. Kitagawa, and H. Itoh. Aqueous solubility and acidity constants of cholic, deoxycholic, chenodee xycholic, and ursodeoxycholic acids. J. Lipid Res. 1992. 53: 49-53. Supplementary key words bile acids enthalpy change of dissolution A number of papers have been published on physicochemical properties of bile acids since their physiological functions were made clear (1-15). Their vital functions are similar to ordinary surface-active agents in the sense that bile acids and their conjugates can solubilize and emulsify lipids for their transportation and absorption in vivo. They are also very sensitive to pH of living tissue, because their physiological functions are dependent on the degree of their ionization. Thus their dissociation constants should be determined as correctly as possible for insight into their physiological functions. Several papers have appeared on acidity constants of bile acids (1619). However, these constants are not only inconsistent but also dependent on bile acid concentration, which requires determination of more reliable and more accurate acidity constants. The acidity constant (&) of acids has been determined by titration, electrical conductivity, and spectrophotometric methods. These methods are, however, applicable only to acids whose aqueous solubility is high enough for their concentration to be easily controlled. In a previous paper (20), an "is* extraction" method was introduced for p% determination for acids slightly soluble in water. This method is particularly valuable for studying the association equilibria in dilute solutions (21-23). A direct solubility method (24), which is a variation of the isoextraction method from a thermodynamical point of view, has an advantage over the isoextraction method in that there is no reaction of species in a coexisting excess phase of acid; only its dissolution into the aqueous phase takes place. This method is particularly useful for dibasic acids whose solubility in aqueous solution of low pH is too low to be determined precisely by normal analytical methods, for example bilirubin (25). Hence, the solubility method is very useful for studying the physicochemical solution properties of acids that cannot be investigated by other methods. This paper then aims to determine the solubility of bile acids at lower pH values and to obtain the correct acidity constants from their solubility data.