Oxygenolysis of Flavonoid Compounds: DFT Description of the Mechanism for Quercetin SØbastien Fiorucci, Jerôme Golebiowski, Daniel Cabrol-Bass, and Serge Antonczak* [a] Introduction Flavonoids are naturally occurring phenol derivatives present in substantial amounts in plants, fruits and vegetables. [1] Since food products derived from plants are an integral part of the human diet, the potential bioactivity of such compounds has been investigated ever since their discovery. Their capacity to absorb oxygen radicals, their antioxidant potential [2] and radi- cal-scavenging properties contribute to the chemopreventive effect. [3] Among this family, quercetin has been reported to be the most predominant flavonol and can be found in fairly large amounts in fruits, vegetables, olive oil, red wine and tea. [4] Sim- ilar to other polyphenolics, quercetin has a pronounced ability to inhibit radical processes in cells. However, more recently it was clearly demonstrated that quercetin could behave both as an antioxidant [5] and as pro-oxidant, [6] depending on its con- centration and the radical source. This pro-oxidant activity has been especially reported when a transition metal is available. [7] Quercetin has three structural groups which determine its radi- cal-scavenging and/or antioxidative potential: the catechol moiety of the C ring, the 2,3-double bond in conjugation with a 4-oxo functionality in the A ring and 3- and 5-hydroxyl groups on the B ring. Deprotonation of catechol and 3-hydroxy- chromone [8] enhances the activity of the corresponding anions towards dioxygen, while this substrate is known to be inert in its protonated form. In several aerobic metabolic pathways, O 2 is incorporated into organic compounds by mono- or dioxygenase-catalyzed reactions. Oxygenases can also be metal-containing proteins, and a fair number of them uses copper at their active site. [9] Di- oxygenases play a key role in the complex degradation path- way of aromatic and heteroaromatic compounds. One of these is the copper-containing quercetin 2,3-dioxygenase (2,3-QD), which catalyzes the oxygenolysis of quercetin to the corre- sponding depside as a result of the oxidative cleavage of the heterocyclic ring (Scheme 1). 2,3-QD belongs to the cupin su- perfamilly [10] and is the only non-iron dioxygenase for which a crystal structure is known [11] and whose reactivity is unambigu- ously known to rely on a mononuclear copper center. [11–13] Generally, the reactivity of the copper centers in proteins is in- fluenced by their surrounding environment, which fine tunes the redox potential of the metal and confers substrate specific- ity. [11] Two recent reviews, presenting an overview of the reac- tivity of dioxygen–copper systems [14] and descriptions of elec- tronic structures of metal sites in proteins, [15] give a clear idea of the role played by the metal center in biological systems and models. The oxygenolysis reaction catalyzed by 2,3-QD is a spin-for- bidden process, since the triplet ground state of O 2 should react with the singlet ground state of quercetin to produce the singlet ground state of the phenolic carboxylic ester. For this reaction to occur, some form of activation is necessary. The most widely accepted hypothesis is that, via a tautomeri- [a] S. Fiorucci, Dr. J. Golebiowski, Prof. D. Cabrol-Bass, Dr. S. Antonczak Laboratoire Arômes, Synthses, Interactions FacultØ des Sciences, UniversitØ de Nice-Sophia Antipolis 06108 Nice Cedex 2 (France) Fax: (+ 33)492-076-125 E-mail: serge.antonczak@unice.fr Flavonoids are naturally occurring phenol derivatives present in substantial amounts in a large variety of plants, fruits and vege- tables daily eaten by humans. Most of these compounds exhibit several interesting biological activities, such as antiradical and antioxidant actions. Indeed, by complexation with specific en- zymes, flavonoids are notably liable to metabolize molecular di- oxygen. On the basis of experimental results describing oxygenol- ysis of the flavonoid quercetin, activated by the enzyme quercetin 2,3-dioxygenase (2,3-QD), our attention has focused on the role of metal center in the activation of the substrate quercetin. Thus, in the present study, by means of DFT calculations at the B3LYP/ 6-31(+ )G* level on model molecular systems, we describe differ- ent mechanisms for dioxygen metabolization by quercetin. Sta- tionary points are described, and energetic and structural analy- ses along the reaction paths are reported. Our calculations show that the copper cation must act as an oxidant towards the sub- strate and that the reaction proceeds through a 1,3-cycloaddi- tion. Scheme 1. Oxygenolysis of quercetin by quercetin 2,3-dioxygenase. 1726 # 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/cphc.200400186 ChemPhysChem 2004, 5, 1726 – 1733