Investigation of the applicability of cis-urocanic acid as a model for the catalytic Asp–His dyad in the active site of serine proteases based on 1H NMR hydrogen bonding studies and spectroscopic pKa measurements

Abstract

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. pK_a determinations of imidazole derivatives in DMSO have also been performed and a value of 3.2 has been observed for cis-UCA.