Nucleophilic reaction by carbonic anhydrase model zinc compound: Characterization of intermediates for CO2 hydration and phosphoester hydrolysis

Abstract

The partially hydrophilic and hydrophobic tripodal ligands, tris(hydroxy-2-benzimidazolylmethyl)amine L1h and tris(2-benzimidazolyl)amine L1 were used for the preparation of biomimetic complex of carbonic anhydrase. The CO2 hydration using [L1hZn(OH)]ClO4·1.5H 2O provided the zinc-bound and free HCO3-s, which were formed by nucleophilic attack of Zn-OH toward CO2 in dimethyl sulfoxide (DMSO). The phenolic OH in L1h can recognize water molecules through hydrogen bonds to facilitate the collection of the water molecules around a biomimetic zinc compound; the molecular structure of [L1hZn(OH)] + was revealed. The packing diagram has demonstrated the all the water molecules are hydrogen bonded to each phenolic OH. The nucleophilic attack of zinc-bound OH- to substrate is used to catalyze the CO 2 hydration and phosphoester hydrolysis. The carbonic anhydrase model compound [L1Zn(OH2)]2+ was applied for the hydrolysis of phosphoesters, parathion and bis(p-nitrophenyl)phosphate (BNPP-). The low reactivity of [L1Zn(OH)]+ for parathion hydrolysis is attributed to the stability of the intermediate [L1Zn(OP(S)(OEt)2)] +. Since the structures of the intermediates [L1Zn(OH 2)](BNPP)2 (1) and [L1Zn(OP(S)(OEt)2)]ClO 4 (2) formed on the way of hydrolysis are too stable to realize the catalytic cycle and are not active for hydrolysis, carbonic anhydrase model compound [L1Zn(OH2)]2+ was not suitable for phosphoester hydrolysis; the zinc model compound surrounded by three benzimidazolyl groups is used to have the steric hindrance for bulky substrate, such as parathion and BNPP-. © 2004 Elsevier Inc. All rights reserved.