Direct determination of thiol pKa by isothermal titration microcalorimetry.

Knowledge of the acid dissociation constant (pKa) of a molecule is a critical step toward understanding its structure and reactivity. Current methods for pKa measurement, including electrochemical, spectroscopic, and spectrophotometric titrations, have proven to be useful but also have significant limitations. To overcome these limitations, we report the use of isothermal titration calorimetry (ITC) as a new method for pKa determination. We demonstrate by the measurement of the pKa values for free cysteine, glutathione, and a cysteine residue in a protein that this method is rapid and accurate.