Journal of Lipid Research, Vol 33, 49-53, Copyright © 1992 by Lipid Research, Inc.
Aqueous solubility and acidity constants of cholic, deoxycholic, chenodeoxycholic, and ursodeoxycholic acids
Y Moroi, M Kitagawa and H Itoh
Department of Chemistry, Faculty of Science, Kyushu University, Fukuoka, Japan.
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 degrees 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 degrees 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.