Imidazole Catalyses in Aqueous Systems. III. Formation of the Catalyst-Substrate Complex in the Hydrolysis of a Phenyl Ester Catalyzed by a Naphthylimidazole Derivative

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Catalytic hydrolysis of p-acetoxybenzoic acid by 1-[4(5)-imidazolyl]-4-aminomethylnaphthalene was carried out at 30°C in aqueous systems using a pH-stat. The catalytic rate leveled off at high substrate concentrations and increased remarkably at high pH. The data indicated that both the neutral catalytic species (AmIm) and the monoprotonated catalytic species (Am+Im) acted as enzyme-like catalysts, the catalytic rate conforming to the Michaelis-Menten kinetics, although their catalytic efficiencies were quite different: Km (dissociation constant of the catalyst-substrate complex)=0.011M and k3 (pseudo-intramolecular rate constant of the product-formation)=0.19 min−1 for AmIm, and Km=0.041M and k3=0.036 min−1 for Am+Im. The binding function was attributed to hydrophobic interaction, on the ground that Km was smaller for AmIm than for Am+Im and that an increase in the ionic strength of the medium caused rate enhancement. The hydrolysate showed an inhibitory action. The wide variation of the catalytic efficiency between AmIm and Am+Im may be related to the structural difference of the Michaelis complexes. 1-Aminomethylnaphthalene similarly catalyzed the hydrolysis and showed a substrate binding phenomenon, though the saturation phenomenon was much less pronounced as compared with the imidazole catalyst.
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