Hydrolysis of 3-nitrophenyl acetate by β-cyclodextrin in substituted imidazole buffers

Abstract

In order to examine the effects of substituted imidazoles upon cyclodextrin esterolysis, the title hydrolysis has been carried out at 30 °C using imidazole, 2-methyl-, 2-isopropyl-, 2,4,5-trimethyl-, and 2-(1,1-dimethyl-2-hydroxyethyl)-imidazole. Observed pseudo-first-order rate constants (k_obs) are analysed in terms of a Lineweaver–Burk-type equation to give the first-order rate constant (k_cat) for a cyclodextrinester complex (CD-S) and an apparent dissociation constant (K_app). The latter gives the dissociation constants (K_d and K_i) for CD-S and a cyclodextrin–imidazole complex. Using the equation k_cat=k_cat–OH[OH]+k_cat–Im[Im]_f, the second-order rate constants for CD-S due to hydroxide ion (k_cat–OH) and due to an imidazole base (k_cat–Im) are determined. A plot of k_cat–Imversus pK_a of the imidazoles gives β 0.62, and solvent D₂O effects upon k_cat–Im for 2-isopropyl- and 2,4,5-trimethyl-imidazole are estimated to be ca. 2. From these results, it is suggested that the imidazoles used here act as a general base catalyst for the cyclodextrin esterolysis.