Electrochemical Properties of Chlorogenic Acids and Determination of Their Content in Coffee Using Differential Pulse Voltammetry

摘要

The electrochemical properties of nine chlorogenic acids (CGAs) isomers: three caffeoylquinic acids,CQAs (5-CQA, 4-CQA, 3-CQA), three dicaffeoylquinic acids, diCQAs (3,4-diCQA, 3,5-diCQA, 4,5- CQA) and three feruloylquinic acids, FQAs (5-FQA, 4-FQA, 3-FQA) were studied by differentialpulse voltammetry (DPV) method. The study has shown that electrochemical properties of CQAs anddiCQAs are strongly dependent on their chemical structure and electronic properties, particularly onpresence of electron-donating H and H=CHgroups, and a strong electron-withdrawing effect ofester (OOR) group presented in their structures. The electrochemical properties of FQAs wereadditionally influenced by the presence of methoxy group (CH3) attached on the aromatic ring.DPV measurements show that electrochemical oxidation of CQAs and diCQAs at a GCE is reversible,pH dependent, two- electron-two- proton process occured on catechol moiety in the structure of thesemolecules. Electrochemical oxidation of FQAs proceeded by an EC mechanism. In the first,electrochemical oxidation step, a phenoxy radical was formed by irreversible one-electron-one-protonprocess. In the second, chemical step, phenoxy radical undergoes hydrolysis generated a new moreelectroactive compound with methoxy-catechol moiety in the structure. This compound was reversibleoxidised to methoxy-ortho-quionone structure by two-electron-two-proton process. Theelectrochemical properties of 5-CQA (main CGAs in coffee) were investigated more in detail. It wasobserved that anodic oxidation peak current of 5-CQA show its maximum in PBS solution of pH 7 and-1 -6 -1 linear relationship within the concentration range of 5-50 ol L , with LOD of 1.210 mol L .DPVs of coffee show that electrochemical properties of coffee extracts were very similar to that ofinvestigated CGAs. Therefore, DPV was used for characterization of CGAs in coffees. It was shownthat DPV is a very sensitive and selective method for determination of total CGAs content in coffee.