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Anodic Behaviour of Flavonoids Orientin, Eriodictyol and Robinin at a Glassy Carbon Electrode

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Anodic Behaviour of Flavonoids Orientin, Eriodictyol and Robinin at a Glassy Carbon Electrode Full Paper Anodic Behaviour of Flavonoids Orientin, Eriodictyol and Robinin at a Glassy Carbon Electrode Eric de Souza Gil,a, b Adrian Teodor Enache,a Ana Maria de Oliveira-Brett*a a Departamento de Qumica, Faculdade de CiÞncias e Tecnologia, Universidade de Coimbra, 3004–535 Coimbra, Portugal b Faculdade de Farmcia, Universidade Federal de Gois, 74605–220, Goinia, Gois, Brasil *e-mail: [email protected] Received: April 20, 2012;& Accepted: May 31, 2012 Abstract Orientin, eriodictyol and robinin are polyphenolic compounds, and their oxidation mechanism is pH-dependent, in two steps, involving a different number of electrons and protons. Orientin and eriodictyol first oxidation occurs at a lower potential, corresponding to the reversible oxidation of the catechol group, and is followed by an irreversible oxidation on the ring-A at more positive potential. Robenin oxidation is irreversible, with the formation of electro- active products, and occurs at ring-A and ring-B. The electrochemical characterization of their redox behaviour brought useful data about their chemical stability, antioxidant and pro-oxidant activity, enabling a comprehensive understanding of their redox mechanism. Keywords: Orientin, Eriodictyol, Robinin, Oxidation, Glassy carbon electrode. DOI: 10.1002/elan.201200211 1 Introduction Orientin is a flavone, found in passion flower, bamboo leaves, aÅai pulps and wardii berries [5–7]. Chemically is Flavonoids constitute, among other compounds, an impor- the 8-C glucoside of the widespread citrus flavone, luteo- tant class of antioxidants that inhibit the oxidative degra- lin [8]. dation of organic materials including a large number of Eriodictyol, 3’,4’,5,7-tetrahydroxyflavanone, is found in biological aerobic organisms and commercial products. several types of plants, such as yerba santa (eriodictyon As well as many polyphenols, flavonoids owe their anti- californicum), pistachio (pistacia vera), cork (quercus oxidant activity to their ability to scavenge radicals, by suber), lemon balm (melissa officinalis), rose hips (rosa hydrogen or electron transfer, in a much faster process canina), tomato and lemon species [9–13]. than the radical attack to an organic substrate [1,2]. Robinin, kaempferol 3-O-robinoside-7-O-rhamnoside, The major class of flavonoids may be divided into sev- is a glycoside of the well-studied flavonol, kaempferol eral subclasses, flavonols, flavones, flavanonols, flava- [14,15], that can be isolated from various genus of herbs, nones, isoflavonoids, catechins and anthocianidins, as well such as astragalus (flaronin, milk-vetch), pueraria a varying number of polymeric compounds [1–3]. (kudzu), vinca (periwinkle) and also from false acacia Consumption of flavonoid-rich foods, in particular (robinia pseudoacacia), a garden tree [14–16]. fruits and vegetables, is associated with a lower incidence The importance of these three distinct flavonoids is in of heart disease, ischemic stroke, cancer, and other chron- their diverse biological effects which include radioprotec- ic diseases. Additional studies also found inverse associa- tive, antitumour, antimicrobial, vasodilatory and anti-in- tions between flavonoid intake and the risk of stroke and flammatory actions [5–14]. Eriodictyol has been used as lung and colorectal cancers [2–4]. Because these chronic bitter masking in cosmetic and pharmaceutical formula- diseases are associated with increased oxidative stress and tions [13], while robinin is one of the active constituents flavonoids are strong antioxidants in vitro, it has been of the commercial drug flaronin, which has been widely suggested that dietary flavonoids exert health benefits used in the treatment of acute and chronic renal failures through antioxidant mechanisms [1–5]. Besides these [17,18]. benefits, several studies have shown that in specific condi- Regarding the structureÀactivity relationship, the tions the flavonoids can play pro-oxidant function [3,4]. mechanism of action of flavonoids is related to the physi- Among flavonoids derivatives, orientin, eryodictiol and cochemical properties of their electroactive moieties, e.g., robenin, Scheme 1, are of particular interest due their phenol, catechol and resorcinol [2–4]. great occurrence in foodstuffs, cosmetics and pharmaceut- Due to their high sensitivity, voltammetric methods ical products. have been successfully used for the investigation of oxida- tion mechanism of biological active substituted phenols, 1576 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Electroanalysis 2012, 24, No. 7, 1576 – 1583 Anodic Behaviour of Flavonoids Orientin, Eriodictyol and Robinin at a GCE Scheme 1. Chemical structures of orientin, eriodictyol and robinin. providing valuable insights into their redox behaviour 2 Experimental and their detection in various samples. The electrochemi- cal characterization of the mostly widespread flavonoids 2.1 Materials and Reagents has already been investigated at different carbon elec- trode materials: glassy carbon, carbon paste, etc. [19–23] Orientin, eriodictyol and robinin were purchased from and their oxidation mechanisms were correlated with the Extrasynthse (Genay, France) and used without further electroactive groups: phenol, resorcinol and catechol. purification. A stock solution was prepared in ethanol-de- The oxidation of phenol and resorcinol is an irreversi- ionised water (30:70, v/v) and stored at +48C. Solutions ble pH dependent process, occurring in one step, leading of different concentrations were prepared by dilution of to electroactive products [24]. The catechol oxidation is the appropriate quantity in supporting electrolyte. All also pH dependent, and occurs in a two electron two supporting electrolyte solutions, were prepared using ana- proton reversible mechanism [24]. Although the electro- lytical grade reagents and purified water from a Millipore chemical oxidation mechanisms of flavonoids is basically Milli-Q system (conductivity 0.1 mScmÀ1) [25]. addressed considering the position and number of hy- droxyl groups, the different types of chemical substitu- 2.2 Apparatus ents, OCH3, sugar, etc., can also have influence on their redox behaviour and oxidation products formation [19– Voltammetric experiments were carried out using a mAu- 24]. tolab running with GPES 4.9 software, Eco-Chemie, The present study is concerned with the investigation Utrecht, The Netherlands. Measurements were carried of the electron transfer reactions of the three flavonoids, out using a three-electrode system in a 0.5 mL one-com- orientin, eriodictyol and robinin, using cyclic, differential partment electrochemical cell (Cypress System Inc., pulse and square wave voltammetry at a glassy carbon USA). Glassy carbon electrode (GCE, d=1.0 mm) was electrode, and so far no electrochemical study of these the working electrode, Pt wire the counter electrode and compounds has been carried out. Therefore, owing to the Ag/AgCl (3 mol LÀ1 KCl) reference electrode. The their different structure, their redox mechanisms and the GCE was polished using diamond particles of 1 mm influence of non electroactive groups, like the sugar (Kemet, UK) before each electrochemical experiment. moiety and the C3=C2 double bond, will influence the After polishing, it was rinsed thoroughly with Milli-Q redox behaviour of these flavonoids, providing important water. Following this mechanical treatment, the GCE was data for the development of electroanalytical methods placed in buffer supporting electrolyte and voltammo- and also for the understanding of structure activity rela- grams were recorded until a steady state baseline voltam- tionships and antioxidant properties. mograms were obtained. This procedure ensured very re- Electroanalysis 2012, 24, No. 7, 1576 – 1583 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.electroanalysis.wiley-vch.de 1577 Full Paper E. de Souza Gil et al. producible experimental results. The experimental condi- tions for differential pulse (DP) voltammetry were: pulse amplitude 50 mV, pulse width 70 ms and scan rate 5mVsÀ1. For square wave (SW) voltammetry were: pulse 50 mV, frequency 50 Hz and potential increment 2 mV, corresponding to an effective scan rate of 100 mVsÀ1. The pH measurements were carried out with a Crison micropH 2001 pH-meter with an Ingold combined glass electrode. All experiments were done at room tempera- ture (25Æ18C) and microvolumes were measured using EP-10 and EP-100 Plus Motorized Microliter Pippettes (Rainin Instrument Co. Inc., Woburn, USA). 2.3 Acquisition and Presentation of Voltammetric Data All the voltammograms presented were background-sub- tracted and baseline-corrected using the moving average application with a step window of 5 mV included in GPES version 4.9 software. This mathematical treatment improves the visualisation and identification of peaks over the baseline without introducing any artefact, al- though the peak intensity is, in some cases, reduced (<10%) relative to that of the untreated curve. Never- theless, this mathematical treatment of the original vol- tammograms was used in the presentation of all experi- mental voltammograms for a better and clearer identifica- tion of the peaks. The values for peak current presented in all plots were determined from the original untreated voltammograms after subtraction of the baseline. 3 Results and Discussion 3.1 Cyclic Voltammetry Orientin and eriodictyol have a catechol moiety at ring-B, and two hydroxyl groups at ring-A similar to resorcinol and were studied by cyclic voltammetry. The cyclic voltammogram
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