C. V. Hooidonk, D. Korte, M. A. C. R. Meereboer
Sep 2, 2010
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Recueil des Travaux Chimiques des Pays-Bas
Abstract
A three-step synthesis was utilized to introduce the nucleophilic oxime group, 2(hydroxyimino)ethyl, into cyclohexaamylose (CHA) at the 2-O or 3-O position. The rates of deacylation of p-nitrophenyl acetate (PNPA) in aqueous solutions were determined without and in the presence of varying concentrations of O-[2-(hydroxyimino)ethyl]-CHA (CHA-Ox). From the observed Michaelis-Menten-like kinetics, the pH dependence and competitive inhibition experiments with cyclohexanol it was concluded that the deacylation is consistent with a reaction scheme involving a 1: 1 inclusion complex followed by a rate-determining nucleophilic displacement of the p-nitrophenoxide ion by the ionized oxime group. The CHA-Ox cavity provides an effective binding site for PNPA resulting in a favourable orientation of the acyl residue with respect to the oxime group. The rate accelerations can be ascribed to an apparent increase in the local concentration of the reactive species. CHA-Ox displays substrate specificity illustrated by using m-nitrophenyl acetate and p-nitrophenyl dimethylphosphinate as substrates. The CHA-Ox/ester system represents a model for the acylation of hydrolytic enzymes.