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 H-1-NMR HYDROGEN-BONDING STUDIES AND SPECTROSCOPIC PK(A) MEASUREMENTS

The intramolecular hydrogen bond of cis-urocanic acid, (Z)-3-(1H-imida zol-4-yl)prop-2-enoic acid (cis-UCA) in DMSO has been evaluated by the H-1 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 protease s, Both the acidic OH proton and the C2 proton of the imidazole ring o f 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 o f 4-nitrophenyl chloroacetate has been compared with that of the trans -isomer and other imidazole derivatives with the aid of kinetic studie s. The non-linear kinetic behaviour observed for the reaction of cis-U CA is attributed to complex formation with the reacting acyl derivativ e and cis-UCA, pK(a) determinations of imidazole derivatives in DMSO h ave also been performed and a value of 3.2 has been observed for cis-U CA.