A Macrocyclic Enzyme Model System. Deacylation of p-Nitrophenyl Carboxylates as Effected by a [20]Paracyclophane Bearing an Imidazole Group

Abstract

Abstract The kinetic effect of a [20]paracyclophane bearing an imidazole moiety (1) on the deacylation of various p-nitrophenyl carboxylates was investigated in 10.9% (v/v) ethanol-1.0%(v/v) dioxane-water of μ 0.10 (KCl) at 40.0±0.1 °C. The facile reaction of 1 with a hydrophobic ester resulted in the accumulation of the NIm-acyl derivative of 1 along the progress of reaction, and the rate of regeneration of 1 (turnover) was negligibly small under present experimental conditions compared to the corresponding acylation rate. The overall feature for deacylation reactions of p-nitrophenyl carboxylates bearing a hydrophobic acyl moiety in the presence of 1 is consistent with the reaction scheme which involves the reactions of the monomeric and aggregated forms of 1 with substrate esters. The second-order rate constants for reactions of the monomeric species of 1 with relatively hydrophobic esters exceed considerably the corresponding second-order rate constants for imidazole-catalyzed reactions, suggesting the significant hydrophobic interaction between 1 and each hydrophobic ester. The binding constants (K) for complex formation of a micelle (aggregation number, N) with significantly hydrophobic substrates are greater than 105 M−1. The second-order rate constants (kmK⁄N) for reactions of the imidazole group of 1 with the long-chain carboxylates in micellar phase are the largest ever achieved for reactions of synthetic 4-substituted imidazoles with p-nitrophenyl esters.