Solvent effect on the proton-binding sites in urocanic acid. A tautomeric equilibrium study

A 1H NMR study of urocanic acid dissolved in water/dimethyl sulfoxide mixtures together with potentiometric determinations of its two successive acidities in the same solutions reveal that a proton is transferred from the imidazolium ring to the carboxylate group when the amount of dimethyl sulfoxide is increased. An analysis of the potentiometric data by means of the classical Hammett linear relationship allowed the four microscopic acidity constants to be determined. At 20 degrees C, there are equal fractions of the tautomeric species, i.e. urocanic acid (AH0) and its zwitterion (AH+-) in the mixture containing 34% dimethyl sulfoxide by weight (congruent to 0.1 in mole fraction), while 50% dimethyl sulfoxide by weight (congruent to 0.2 in mole fraction) is necessary to completely bias the equilibrium: AH+- in equilibrium AH0 toward the molecular form AH0.