Relative acidity scale of bile acids through ESI-MS measurements.

The anion proton affinity of the most important human bile acids and those of the corresponding keto bile acids have been examined in order to establish a true (intrinsic) relative acidity scale. The measurements have been carried out in the gas-phase using the Cooks' kinetic method. The remarkably high acidity of cholic acid with respect to the other bile acids was confirmed. Rationalization of the differences found for the various acids and comparisons with the available solution-phase data are discussed with the help of theoretical calculations.