Fini A, Roda A
J Lipid Res. 1987 Jul;28(7):755-9.
The dissociation constants for the carboxyl group of a series of glycine (N-acyl)-conjugated and unconjugated bile acids were determined by potentiometric titration using dimethylsulfoxide-water and methanol-water mixtures of varying proportions. The pKa values in water were calculated by extrapolating the experimental values determined in different mole fractions of the organic solvent mixtures. The following values were obtained: 3.9 +/- 0.1 for glycine-conjugated bile acids and 5.0 +/- 0.1 for unconjugated bile acids, as general pKa values for the two classes of bile acids, respectively. The amidation of bile acids with glycine lowers the pKa value because of the proximity of the amide bond to the terminal carboxyl group. Bile acid dissociation constants are independent of the substituents in the steroid nucleus, since inductive effects of the hydroxyl groups on the steroid nucleus are too distant from the acidic group at the end of the side chain to influence its ionization.
使用不同比例的二甲基亚砜 - 水和甲醇 - 水混合物，通过电位滴定法测定了一系列甘氨酸（N - 酰基）共轭和未共轭胆汁酸羧基的解离常数。通过外推在不同摩尔分数的有机溶剂混合物中测定的实验值，计算出在水中的pKa值。得到以下值：甘氨酸共轭胆汁酸为3.9±0.1，未共轭胆汁酸为5.0±0.1，分别作为这两类胆汁酸的一般pKa值。胆汁酸与甘氨酸的酰胺化作用由于酰胺键靠近末端羧基而降低了pKa值。胆汁酸解离常数与甾体核中的取代基无关，因为甾体核上羟基的诱导效应距离侧链末端的酸性基团太远，无法影响其电离。
