Helmi Neuvonen and Kari Neuvonen
The intramolecular hydrogen bond of cis-urocanic acid, (Z)-3-(1H-imidazol-4-yl)prop-2-enoic acid (cis-UCA) in DMSO has been evaluated by the 1H NMR chemical shift approach to assess the suitability of cis-UCA as a model for the Asp–His dyad in the active site of serine proteases. Both the acidic OH proton and the C2 proton of the imidazole ring of cis-UCA resonate at exceptionally low field, at ca. 17.4 and 8.2 ppm, respectively, indicating a strong N–H–O type intramolecular hydrogen bond. The nucleophilic reactivity of cis-UCA toward the acyl carbon of 4-nitrophenyl chloroacetate has been compared with that of the trans-isomer and other imidazole derivatives with the aid of kinetic studies. The non-linear kinetic behaviour observed for the reaction of cis-UCA is attributed to complex formation with the reacting acyl derivative and cis-UCA. pKa determinations of imidazole derivatives in DMSO have also been performed and a value of 3.2 has been observed for cis-UCA.