Guaicolic spices curcumin and capsaicin electrochemical oxidation

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Journal of Electroanalytical Chemistry 682 (2012) 83–89 Contents lists available at SciVerse ScienceDirect Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem Guaicolic spices curcumin and capsaicin electrochemical oxidation behaviour at a glassy carbon electrode Marcia A.N. Manaia a, Victor C. Diculescu a, Eric de Souza Gil a,b, Ana Maria Oliveira-Brett a,⇑ a Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, Coimbra, Portugal b Faculdade de Farmácia, Universidade Federal de Goiás, Goiânia, Goiás, Brazil article info Article history: Received 9 April 2012 Received in revised form 1 June 2012 Accepted 12 June 2012 Available online 16 July 2012 Keywords: Curcumin Capsaicin Redox mechanism Voltammetry Radical scavenging activity Cancer 1.Introduction Curcumin, 1.7-bis (4-hydroxy-3-methoxyphenyl)-1.6-heptadie- no-3,5-dione, Scheme 1A, and capsaicin, 8-methyl-N-vanillyl-6- nonenamide, Scheme 1C, are the major phytochemicals found in some of the most consumed dietary spice ingredients turmeric (saffron) and chili peppers. Besides the flavour and taste that makes these spices very popular especially in Indian and Mexican cuisine, the curcuminoids from saffron and curry are also popular as colour- ing agents and food dyes, whereas capsaicinoids from chili peppers are particularly appreciated for the pungent action [1–3]. Due to high medicinal potential with virtually no side effects, these compounds have attracted considerable interest in recent years [1–4]. Their therapeutic potential is being explored in inflam- matory [5,6], cardiovascular [7,8], neurodegenerative [9,10], neoplasic diseases [11,12], as well as other disorders [13,14]. The presence of the 4-hydroxy-3-methoxy phenyl residue allows them to specifically interact with sensory neurons, and from this point of view, capsaicin has been used to treat peripheral painful condi- tions and neuropathies [10,15,16]. It was shown that curcumin regulates the classical and the alternative pathway of nervous system, being used in the treatment of Alzheimer, multiple sclerosis and dementia [9,10,17]. ⇑ Corresponding author. Address: Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, 3004-535 Coimbra, Portugal. Tel./fax: +351 239 835295. E-mail address: brett@ci.uc.pt (A.M. Oliveira-Brett). 1572-6657/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jelechem.2012.06.023 abstract The electrochemical behaviour of the spice biomarkers, curcumin and capsaicin, two polyphenolic com- pound with a large spectrum of medical application, has been studied by cyclic, differential pulse and square wave voltammetry at a glassy carbon electrode. The oxidation of curcumin is an irreversible pro- cess that in acidic and mild alkaline supporting electrolytes proceeds in two steps. The first irreversible oxidation step leads to the formation of a catechol moiety, and the second reversible step occurs for a higher potential. The oxidation mechanism of ferulic acid, capsaicin and dihydrocapsaicin, curcumin chemical analogues was also investigated. The oxidation of ferulic acid is similar to curcumin whereas the oxidation of capsaicin and dihydrocapsaicin led to the formation of only one oxidation product. A redox mechanism for curcumin oxidation has been proposed. Ó 2012 Elsevier B.V. All rights reserved. In fact, capsaicin and especially curcumin have received considerable attention owing to their antitumor properties, exerting their effects either at the stage of tumorigenesis or in selectively inducing apoptosis in tumor cells [12,18–20]. The complex mechanism of action of curcumin involves various biological targets such as signal transducers and activators, DNA and several kinase enzymes [18–20]. Also, modulation of intracel- lular redox state is an indirect mechanism since several critical transcription factors control cell cycle, differentiation, stress response and other physiological processes [19–23]. Curcumin has also been shown to potentiate the effect of chemotherapeutic agents [21] and of c-radiation [22] in cell culture, and capsaicin is able to trigger apoptosis in human cancer cells, and a direct inhibitory effect of tumour growth has been observed [19,23]. The presence of 4-hydroxy-3-methoxy phenyl residue also con- fers to curcumin and capsaicin, strong antioxidant activity which leads to radical scavenging ability, also useful to prevent cancer and other mentioned diseases [1–6,24,25]. However, as redox agent, they can also act as pro-oxidants thus exhibiting dual effects on carcinogenic and mutagenic processes [25,26]. Such behaviour is determined by the same structural patterns and perhaps is the explanation for all the controversy surrounding epidemiologic studies about the therapeutic use of phytoantioxidants. The electrochemical characterisation under different conditions is a promising tool to understand the redox behaviour of polyphe- nolic compounds in physiological medium and several studies are reported on the electrochemical properties of curcumin [27,28 and

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