Full Paper On the Electrochemical Oxidation of Resveratrol Oana Corduneanu, Patricia Janeiro, Ana Maria Oliveira Brett* Departamento de Quimica, Faculdade de Ciencias e Tecnologia, Universidade de Coimbra, 3004-535 Coimbra, Portugal *e-mail: brett@ci.uc.pt Received: December 13, 2005 Accepted: February 08, 2006 Abstract Resveratrol (3,5,4’-trihydroxystilbene) is an organic metabolite produced by plants in response to fungal infection. It is found in various plant fruits and is abundant in the skins of unripe grapes and related products. This photosensitive molecule exists in two isomeric forms, trans and cis-resveratrol. The antioxidant activity of resveratrol, both trans and cis forms, was evaluated by means of cyclic, differential pulse and square-wave voltammetry over a wide pH range, using a glassy carbon electrode. Voltammograms of resveratrol presented two oxidation peaks; the first oxidation peak corresponds to the oxidation of the phenol group and was shown to undergo an irreversible oxidation reaction. The second oxidation peak corresponds to the oxidation of the resorcinol moiety and is also irreversible. The influence of pH on the electrochemical oxidation process of resveratrol was investigated. Keywords: trans-Resveratrol, cis-Resveratrol, Electrochemistry, pH dependent antioxidant activity DOI: 10.1002/elan.200503469 1. Introduction Resveratrol (3,5,4’-trihydroxy-trans-stilbene) is a naturally occurring phytoalexin produced by a limited number of plants as a response to stress or injury, fungal attack or UV exposure [1, 2]. It can be found in vines, eucalyptus, some pines and berries, in the roots of the Asian medicinal plant Polygonum cuspidatum [3], grapes, peanuts and their derived products. Various studies reported resveratrol to have anticancer activity [4, 5], to prevent heart and blood diseases, to be a neuroprotective agent [6, 7], hormone [8], gene and enzyme modulator [9], and possibly to be a longevity activator [10]. Being a phenolic compound, it may act as an antioxidant that can protect cell membranes and cellular DNA from the damaging effects of free radical induced oxidation [11]. Resveratrol exists as trans- and cis-isomers (Scheme 1) and both isomeric forms have been detected in wines [12], with their concentration in a direct relationship to the weather conditions [1 – 3]. Physiologically, the cis- form seems to act in the same way as trans-resveratrol, although it is less effective [13]. Electrochemical measurements leading to the quantifi- cation and determination of physicochemical parameters of antioxidants are relevant for the evaluation of the antioxi- dative properties of polyphenols [14]. In order to act as an antioxidant, resveratrol itself has to undergo oxidation. In a scavenging reaction a hydrogen atom is donated to the radical that becomes a nonradical. The hydrogen atoms are supplied by dissociation of the  OH groups, a process that takes place both in scavenging reactions and electrochem- ical oxidation. Because the hydroxyl groups are electron donating, the phenoxyl radicals are preferred targets for the oxidants, but they may also form polymeric products that adsorb on the surface of the electrode [15, 16]. Solid state voltammetry consists in mechanically immo- bilizing microparticles of resveratrol onto the glassy carbon surface. Abrasive stripping voltammetry can be employed Scheme 1. Molecular structure of resveratrol trans- and cis-isomers. 757 Electroanalysis 18, 2006, No. 8, 757 – 762 # 2006 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim