Effect of Nuclear Hydroxy Substituents on Aqueous Solubility and Acidic Strength of Bile Acids

Abstract

The aqueous solubility and thermodynamic dissociation constants of a representative series of bile acids with varying numbers and configuration of nuclear hydroxyl substituents were determined. The pKa values were calculated by extrapolating pKa’ values determined in solutions of aqueous methanol of different mole fractions at 25^o C. All bile acids had the same acidic strength in water, indicating that the pattern of hydroxyl nuclear substituents does not affect the acidic strength probably because of the distance between the hydroxyl and ionizing groups. In contrast, aqueous solubility was dependent on the number, position and orientation of the hydroxyl groups. The solubility values ranged from 5 x 10⁻⁸ M (0.05 μM) for 3αr-hydroxy-5β-cholanoic acid (lithocholic acid) to 1.67 x 10⁻³ M (1,670 μM) for 3α,7β,12α-trihydroxy-5β-cholanoic acid (urso-cholic acid). The hydroxy groups notably affect solubility by forming hydrogen bonds with water but also by reducing, according to their orientation, hydrophobicity of the steroid nucleus.