Electrochimica Acta 90 (2013) 27–34 Contents lists available at SciVerse ScienceDirect Electrochimica Acta jou rn al hom epa ge: www.elsevier.com/locate/electacta Surface electrochemical oxidation and polymerization mechanism of epicatechin Shubo Han ∗ , Kaodi Umera, Xiaoyan Han, Justin W. Graham Department of Chemistry and Physics, Fayetteville State University, NC 28301, United States a r t i c l e i n f o Article history: Received 28 August 2012 Received in revised form 5 November 2012 Accepted 7 November 2012 Available online 17 December 2012 Keywords: Epicatechin Cyclic voltammetry Polymerization Flavonoid Computation a b s t r a c t Electrochemical process of epicatechin, one of the flavonoids antioxidants, was studied here by cyclic voltammetry and semiempirical molecular orbital computation (MOPAC). Electrochemical oxidation of epicatechin showed a multistep mechanism with two anodic peaks being recognized at about +0.14 V and +0.52 V (vs. Ag/AgCl). The first peak is strong concentration dependent, showing an adsorptive feature between 1 × 10 -8 M and 2 × 10 -7 M, a diffusion controlled feature between 2 × 10 -7 M and 1 × 10 -5 M, and a surface polymerization feature between 1 × 10 -5 M and 1 × 10 -3 M. Computation showed that the first electron was released at 4 ′ -hydroxyl group in B-ring. No charge delocalization occurs between A- and B-rings. Higher pH medium favors oxidation. The oxidation rate is faster in strong acidic or basic medium and slower in a weak acidic medium. This research may help to explain the complexity of antioxidant activity of flavonoids and as a complement method to characterize the role of flavonoids antioxidants in treating oxidative stress diseases. © 2013 Published by Elsevier Ltd. 1. Introduction Flavonoids, a group of ubiquitous polyphenols found in plants, have been associated with reduced risk of a variety of diseases, due to their capability as potent antioxidants, as chelators of redox- active metals and as inhibitors of lipid peroxidation [1]. Flavonoids might be particularly effective in the prevention of neurodegen- erative diseases [2,3]. However, controversial results have been reported, with some positive findings, many null findings, and some suggestions of harm in certain high-risk populations, due to the intrinsic diversity of the multiple-step antioxidant reactions among the different flavonoids at varied conditions [4,5]. In addition, the variance of the methods used in oxidative process research often lead to disagreeable conclusions, too. Three assays are widely used for standardization of the antiox- idant capacities measurement in foods and dietary supplements: the oxygen radical absorbance capacity assay, the Folin–Ciocalteu method, and the Trolox equivalent antioxidant capacity assay. However, it is still a challenging task to express the details of an antioxidant process (e.g., the multiple step mechanisms and multiple roles of flavonoids, the impact of pH, ionic strength and other surface or solution conditions, etc.), and in the mean time to consider the specialty of a bimolecular target. A more detailed and convenient method is needed to observe antioxidant process ∗ Corresponding author. Tel.: +1 910 672 1303. E-mail address: shan@uncfsu.edu (S. Han). in vivo or in vitro. Electrochemical analysis, particularly cyclic voltammetry (CV), has been validated for quantitative analysis of the antioxidant capacity of blood plasma, tissue homogenates, and plant extracts [6–8]. Chevion et al. proposed that the area under the anodic current wave, rather than I a , is a better parameter reflecting the antioxidant capacity [7]. Janeiro et al. investigated the electro- chemical oxidation of the flavonoids (+)-catechin over a wide range of conditions, using CV, differential and square wave voltammetry [9]. Zielinska et al. evaluated the antioxidant activity of quercitin, its glucosides, and onion by using CV and spectrophotometric methods based on free radical-scavenging activities and reducing power, suggesting that CV assay is an efficient tool for describing the reducing activity of quercetin and its glucosides based on their redox properties. Meanwhile, care should be taken during ana- lytical work to avoid the adsorption of oxidative products on the carbon electrode surface [10]. Noticeably the surface adsorption is very common in protein interactions that should play roles in neurodegenerative disease development or prevention. Therefore, further investigations on the surface phenomena of the flavonoids, along with other coexisting interactions, were discussed together with electrochemical processes in this work. Epicatechin, a flavonoid compound rich in cocoa, teas, wines and fruits, which is well recognized to be effective in lowering the risk of four com- mon killer diseases, stroke, heart disease, cancer and diabetes, is selected as a model flavonoid in this study. Mechanisms of epicate- chin antioxidant activity in solution and at surface were studied by CV and the electroactive sites are predicted by quantum chemical computation. 0013-4686/$ – see front matter © 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.electacta.2012.11.019