Copper Dioxygenation Chemistry Relevant to Quercetin Dioxygenase

Abstract

Biological oxygenations catalyzed by oxygenases are very important processes in nature for the metabolism of various organic substances. The oxygenases are metal-containing proteins and a fair number of them utilizes copper at their active sites.¹ The dioxygenases as a subclass of these enzymes degrade cyclic organic substrates such as tryptophan, catechol, protocatechuic acid and quercetin and have the characteristics of incorporating both oxygen atoms of dioxygen into the substrate. Biomimetic chemistry is a useful tool for determination of the possible structure of the active site, binding mode of the substrate and the reaction pathways of the enzyme actions. Quercetin 2,3-dioxygenase is a copper-containing dioxygenase^2–6 with copper(II) in the active site, which catalyzes the oxygenolysis of 3-hydroxyflavones (1,2) to the corresponding depsides (3,4) as a result of the oxidative cleavage of the heterocyclic ring (equation 1). Quercetin was assumed to coordinate to Cu^II as a chelating ligand at the 3-hydroxy and 4-carbonyl groups.^7–9 In a few cases copper(II) compounds have been used in model catalytic oxygenations of 3-hydroxyflavones and related compounds.^10–12 Since no copper flavonolate or quercetin complexes were known in the literature we set the ultimate goal to prepare and characterize such complexes which would serve as structural models and give possibilities to study relevant stoichiometric and catalytic oxygenation reactions in order to disclose the possible pathway of the enzymatic reaction.