Proton transfer in histidine hydrochloride induced by a dipolar aprotic solvent. 
A 1H NMR study of histidine hydrochloride dissolved in water/dimethyl sulfoxide mixtures gives evidence that dimethyl sulfoxide is able to induce a proton transfer from the imidazolinium nitrogens (mainly N3) to the carboxylate group when its mole fraction becomes greater than 42%.  The new tautomeric equilibrium AH2++- in equilibrium AH2+ is quantitatively studied as a function of the solvent composition.  In pure dimethyl sulfoxide, this equilibrium is strongly biased toward the monocation AH2+, which represents 91% of the total histidine hydrochloride.