Electrochimica Acta 50 (2005) 4350–4355 Electrochemistry of chlorogenic acid: experimental and theoretical studies Mansoor Namazian ∗ , Hamid R. Zare Department of Chemistry, Yazd University, P.O. Box 89195-741, Yazd, Iran Received 2 December 2004; received in revised form 23 January 2005; accepted 30 January 2005 Available online 24 February 2005 Abstract Cyclic voltammetry, chronoamperometry and rotating disk electrode voltammetry as well as quantum chemical methods, are used for electrochemical study of chlorogenic acid, as an important biological molecule. The standard formal potential, diffusion coefficient, and heterogeneous electron transfer rate constant of chlorogenic acid in aqueous solution are investigated. Acidic dissociation constant of chloro- genic acid is also obtained. Quantum mechanical calculations on oxidation of chlorogenic acid in aqueous solution, using density functional theory are presented. The change of Gibbs free energy and entropy of oxidation of chlorogenic acid are calculated using thermochemistry calculations. The calculations in aqueous solution are carried out with the use of polarizable continuum solvation method. Theoretical standard electrode potential of chlorogenic acid is achieved to be 0.580 V versus standard calomel electrode (SCE) which is in agreement with the experimental value of 0.617V obtained experimentally in this work. The difference is consistent with the values we previously reported for other quinone derivatives. © 2005 Elsevier Ltd. All rights reserved. Keywords: Chlorogenic acid; Standard formal potential; Cyclic voltammetry; Electrochemical oxidation; DFT calculations 1. Introduction Chlorogenic acid (CGA) is the major phenolic compound found in many fruits such as apple, pear, peach, plum, cherry and apricot [1]. This compound is well-known substrate of polyphenol oxides (PPOs). Phenolic compounds give bitter- ness and astringency to the juices and contribute to the flavor of processed apple product like cider, although they are in lower proportion [2]. Moreover, browning of apples is due to enzymatic oxidation of phenolic compounds to quinone, which subsequently polymerize to brown product. Among the hydroxycinnamic acids presents in apple juice, chloro- genic acid is the most abundant. This acid is considered as the preferential natural substrate for polyphenol oxidase and therefore it may have an effect on the oxidation process and color development during cider making [3]. Therefore, redox ∗ Corresponding author. Tel.: +98 351 8214778; fax: +98 351 7250110. E-mail address: namazian@yazduni.ac.ir (M. Namazian). potential of CGA is an important factor which affects on its biological behavior. We previously reported the use of CGA for the electro- catalytic oxidation-reduction of nicotinamide adenine dinu- cleotide (NADH) [4] and hydrazine [5] and found the role of CGA very important in the redox reactions of the studied molecules [4,5]. In the present work, we report the electro- chemical study of CGA in aqueous solution. We have inves- tigated the standard formal potential, dissociation constant and transfer coefficient of CGA as well as charge transfer rate constant between glassy carbon electrode and CGA. We have recently showed that standard electrode potentials can be calculated using quantum mechanical calculations [6–8], in which we theoretically studied the standard electrode poten- tials of quinone derivatives in aqueous solution [7,8] and non- aqueous solution of DMSO [6]. The employed theories were able to predict the electrode potential of studied molecules in non-aqueous solvent of DMSO and aqueous solution. In this study, we also present the result of quantum mechanical cal- culations for the oxidation of CGA in aqueous solution using 0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2005.01.043