Effect of cyclodextrin dimers with bipyridyl and biphenyl linking groups on carboxyl ester hydrolysis catalyzed by zinc complex

Abstract

Two new β-cyclodextrin dimers, biphenyl-4,4′-bis(6-monodeoxy-6-ammoniomethyl-β-cyclodextrin) (Host 1, H1) and 2,2′-bipyridyl-5,5′-bis(6-monodeoxy-6-ammoniomethyl-β-cyclodextrin) (Host 2, H2), were synthesized and further assembled with a zinc complex containing a hydrophobic group to construct two host–guest systems, ZnL( H1) and ZnL( H2) (L = 4-(4′- tert-butylbenzyl)diethylenetriamine), acting as supramolecular metallohydrolase models. In such system, the chemical equilibrium constants were determined by pH potentiometric titration at 298 ± 0.1 K. Two deprotonation constants of the Zn(II)-coordinated water, 8.68 ± 0.03 (p K a1) and 10.50 ± 0.04 (p K a2), were obtained respectively for the ZnL( H1) system whereas only one p K a1 value was obtained for ZnL( H2) (9.38 ± 0.02). The kinetics of p-nitrophenyl acetate (pNA) hydrolysis catalyzed by ZnL( H1) and ZnL( H2) were examined respectively from pH 7.56 to 10.56 at 298 ± 0.1 K. The pH profile of rate constant of pNA hydrolysis catalyzed by ZnL( H1) exhibits an exponential increase with second-order rate constants of 0.98 and 8.41 M −1 s −1 for mono- and di-hydroxyl active species, indicating a potent catalytic activity relative to the reported mononuclear and polynuclear Zn(II) species. However, the pH profile of rate constant of hydrolysis catalyzed by ZnL( H2) only shows a saturated kinetic behavior with the second-order rate constant of 1.26 M −1 s −1, suggesting a kinetic process controlled only by an acid–base equilibrium. All kinetic results are in good agreement with thermodynamic data for hydroxyl activated species in ZnL( H1) and ZnL( H2) systems.