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Electrochemical properties of some chlorogenic acids

Chlorogenic acids (CGAs) are one of the groups of polyphenolic compounds widely distributed in different plant materials. Chemically, CGAs are family of different esters formed between (-)- quinic acid and certain trans-cinnamic acids, most commonly caffeic, ferulic and p-coumaric acids. In this work, the electrochemical properties of six important chlorogenic acids (CGAs) isomers: three caffeoylquinic acids, CQAs (5-CQA, 4-CQA, 3-CQA) and three dicaffeoylquinic acids (3, 4-diCQA, 3, 5-diCQA, 4, 5-diCQA) were studied by square-wave voltammetry (SWV) and compared with the electrochemical behaviour of caffeic acid (CFA), precursor of investigated CGAs. The study reveals that electrochemical properties of investigated CGAs strongly depend on their chemical structures and electronic properties, particularly on electron-donating effect of −OH and −CH=CH− groups, and a strong electron- withdrawing effect of ester (−COOR) group presented in their structures. SWV measurements show that electrochemical oxidation/reduction of CGAs at a glassy carbon electrode (GCE) is reversible, pH-dependent, two electron-two proton process. The oxidation/reduction processes occurred on the catechol moiety (ortho-dihydroxyl groups) in the structure of CGAs molecules. The oxidation product(s) relatively strongly adsorb on the GCE surface, especially at higher concentration of CGAs. The electrochemical behaviour of 5-CQA was studied more in detail. It was observed that the anodic peak current of 5-CQA show maximum in solution of pH 7, due to the maximum concentration of 5- CQA- anion at this pH value. The electrode reaction proceeded in the solution of 5-CQA is diffusion controlled process. The anodic peak current of 5-CQA show linear relationship with the concentration in the concentration range 5- 50 µmol L-1, with low limit of detection (LOD) of 7.7x10-7 mol L-1. Therefore, SWV can be used as a very sensitive and selective method for determination of CQAs content (and also CGAs content) in different samples.

chlorogenic acids; dicaffeoylquinic acids; caffeic acid; square-wave voltammetry

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