Carboxy ester hydrolysis catalysed by a dinuclear, hexaazamacrocyclic zinc(II) complex. A model for zinc(II) aminopeptidases

Abstract

In order to investigate a mimic of aminopeptidase enzymes a novel macrocyclic dizinc complex was prepared. A protonated form of the dinucleating hexaazamacrocycle L, [H₄L][NO₃]₄·2H₂O, was characterized by X-ray crystallography. The macrocycle acts as a bis(tridentate) ligand in [Zn₂L(NO₃)₄]·2H₂O 1. Complex formation of L with Zn²⁺ in aqueous solution was investigated by potentiometric equilibrium measurements. Analysis of the titration data indicated that at pH > 7 the dinuclear, hydroxo-bridged species [Zn₂L(µ-OH)]³⁺ and [Zn₂L(µ-OH)(OH)]²⁺ dominate. Complex 1 is a catalyst for the hydrolysis of the activated carboxy ester p-nitrophenyl acetate, with no loss of activity for at least 2.7 catalytic cycles. The second-order rate constant for cleavage of p-nitrophenyl acetate by 1 at 20 °C and pH 8.65 is (6.2 ± 0.6)× 10^–3 dm³ mol^–1 s^–1. The pH dependence of the hydrolysis rate suggests that [Zn₂L(µ-OH)(OH)]²⁺ is the catalytically active species whereas [Zn₂L(µ-OH)]³⁺ displays little or no reactivity. The relevance of the investigations to dizinc active sites in aminopeptidases is discussed.