Metalloenzyme Models. Divalent Metal Ion Catalyzed Hydrolysis of p-Nitrophenyl Picolinate in the Presence of Imidazoles and Pyridines Having Hydroxyl Groups in Their Side Chains

Abstract

Abstract Rate constants for hydrolysis of p-nitrophenyl picolinate at 25 °C in the pH range 6.5–8.5 were measured in the absence and presence of divalent metal ions (Ni(II), Zn(II), Co(II), Ca, Mg) and substituted imidazoles or pyridines as ligands having alcoholic hydroxyl groups in their side chains. In the presence of either metal ion or ligand, the rate is slow and the pseudo-first-order rate constant (kobsd) increases linearly in a first-order manner with respect to the concentration of metal ion or ligand until it gives the second-order rate constant, kM or kL, respectively. In the presence of both a metal ion (Ni(II) or Zn(II)) and a ligand, rate increase is remarkable for some ligands and the increase in kobsd values constructs saturation curves with respect to increase in either metal ion or ligand concentration. The saturation curves were analyzed based upon rate equations formulated by assuming the formation of 1 : 1 complex of metal ion and ligands as the catalyst, leading to evaluation of the association constant K for complexes and the second-order rate constant kc for the reaction of complex with substrate. Values of kobsd, kc, and K are dependent greatly upon the structure of ligands and pH. The ligands complexed with Zn(II) ion appear to be simple but highly active models of hydrolytic metalloenzymes.