Abstract
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.
Highlights
A 'H NMR study of urocanic acid dissolved in water/ dimethyl sulfoxide mixtures together with potentiometric determinations oifts two successive aciditiesin the same solutionsreveal that a protonis transferred from the imidazolium ring to the carboxylate group when the amount of dimethyl sulfoxide is increased
Previous studies have revealed that addition of a dipolar aprotic solvent such as Me2S0 to water solutions decreases the acidity of R-COOH type acids [2,3,4] while it increases the acidity of the NH+functionality of pyridium and imidazolium cations [1,5,6,7]
This suggeststhat theproton shift of Scheme 1 is the reflection of a change in the relative acidities of the COOH and imidazolium moieties of the histidine dication AH;+ on transfer from water to Me2S0
Summary
Jean-ClaudeHalleSO, Clotilde PichonS, and Frangois Terrier From the $Laboratoirede Physicochimie des Solutions, E.N.S.C.P., 11 Rue Pierre et Marie Curie, F75231 Paris Cedex 05 and the VDepartment of Chemistry, Facult des Sciences de Rouen, F76130 Mont Saint Aignan, France. Previous studies have revealed that addition of a dipolar aprotic solvent such as Me2S0 to water solutions decreases the acidity of R-COOH type acids [2,3,4] while it increases the acidity of the NH+functionality of pyridium and imidazolium cations [1,5,6,7] This suggeststhat theproton shift of Scheme 1 is the reflection of a change in the relative acidities of the COOH and imidazolium moieties of the histidine dication AH;+ on transfer from water to Me2S0. The information that we have obtained is of interest with respect to both the biological role of the imidazole ring [8]
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