Full Paper Electrochemical Oxidation of Rutin Mariana-Emilia Ghica, Ana Maria Oliveira Brett* Departamento de Química, Faculdade de Cie ˆncias e Tecnologia, Universidade de Coimbra, 3004-535 Coimbra, Portugal *e-mail: brett@ci.uc.pt Received: March 9, 2004 Final version: June 8, 2004 Abstract An electrochemical investigation of rutin oxidation on a glassy carbon electrode was carried out using cyclic voltammetry, differential pulse voltammetry and square-wave voltammetry over a wide pH interval. The electro- chemical oxidation is a complex process, which proceeds in a cascade mechanism, related with the 4-hydroxyl groups of the rutin molecule. The catechol 3’,4’-dihydroxyl group is the first to be oxidized by a two-electron – two-proton reversible oxidation reaction, followed by an irreversible oxidation reaction due to the 5,7-dihydroxyl group. Both mechanisms are pH dependent. An adsorption process is also observed and the oxidation products block the electrode surface. Keywords: Rutin, Electrochemistry, Oxidation, Reduction, Adsorption process 1. Introduction Flavonoids are a large family of polyphenolic compounds widely present in plants. Over the past decade, evidence has accumulated that flavonoids are an important class of antioxidants. They effectively suppress lipid peroxidation (LPO) in biological tissues and subcellular fractions, such as mitochondria, microsomes, liposomes, low-density lipopro- tein (LDL), and erythrocyte membranes [1 – 4]. They also can act as antiproliferative agents to inhibit tumor growth, as weak oestrogen agents, or as antagonists to modulate endogenous hormone activity. Particularly confusing is the apparent paradoxical behavior of flavonoids in some situations, where they may behave as prooxidants or carcinogens. All flavonoids are electroactive, easily subject to either oxidation or reduction electron transfer reactions, hence they can be investigated by electrochemical methods and flavonoids in beverages have been quantified by liquid chromatography with electrochemical detection (LC-ED) [5, 6]. A detailed electrochemical study of four structurally- related flavonoids including rutin and quercetin, with identification of their oxidation products at a glassy carbon electrode has been reported [7, 8]. Fourteen flavonoid aglycones, and the flavonoid glycoside rutin were evaluated by cyclic voltammetry [9]. Rutin (quercetin-3-O-rutinose) is one of the most bio- active flavonoids, also known as vitamin P and was thought to be an activating factor for vitamin C [10]. Rutin is 3’,4’,5,7-tetrahydroxyflavone-3b-d-rutinoside and the cor- responding chemical structure is given in Figure 1. Some investigations showed that rutin has a broad range of physiological activities such as anti-inflammatory, anti- tumor and antibacterial. It is thought to improve capillary function by reducing abnormal leakage and has been administered to reduce capillary impairment and venous insufficiency of the lower limb [11]. The purpose of this study was to investigate the oxidation of rutin by electrochemical techniques, using cyclic, differ- ential pulse and square-wave voltammetry, with respect to the pH dependent electron transfer process and the adsorption of rutin on the electrode. 2. Experimental 2.1. Reagents and Solutions Rutin was purchased from Sigma-Aldrich, Madrid, Spain and was used as received without further purification. All solutions were made up using high-purity water from a Millipore Milli-Q system (resistivity greater than or equal to 18 MW cm). Fig. 1. Structure of rutin. 313 Electroanalysis 2005, 17, No. 4 # 2005 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim DOI: 10.1002/elan.200403100