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Electroanalytical oxidation of p-coumaric acid

Polyphenols are a diverse group of secondary plant metabolites naturally present in plants. The two main subclasses of polyphenols are the flavonoids and the non-flavonoids (i.e. phenolic acids, stilbens and others). Hydroxycinnamic (HCA) and hydroxybenzoic acids (HBA) belong to the phenolic acids group. The HCAs are among the widely distributed phenylpropanoids in the plant kingdom, with fruits, vegetables, beverages, and cereal being abundant dietary sources. The interest in HCAs is also related to their pharmacological, anti-mutagenic and anti-carcinogenic activity. These biological effects are related to radical scavenging (antioxidant) properties of such compounds. In wines, hydroxycinamic acids can affect the colour, astringency, biterness, oxidation level and clarity of the beverage. The p-coumaric acid can be electrochemically oxidized due to the hydroxyl group attached to the aromatic ring. The mechanism of the electrochemical oxidation of p-coumaric acid on a glassy carbon electrode was investigated using cyclic, differential pulse and square wave voltammetry at different pHs. The oxidation of p-coumaric acid is irreversible over the whole pH range. After successive scans the p-coumaric acid oxidation product deposits on the electrode surface forming a polymeric film that undergoes reversible oxidation at a lower potential than p-coumaric acid. This polymeric film increases in thickness with the number of scans, covering the electrode surface and impeding the diffusion of p-coumaric acid and its oxidation on the electrode surface. The oxidation of p-coumaric acid is pH dependent up until values close to the pKa. For pHs higher than pKa, the p-coumaric acid oxidation process is pH independent. The electroanalytical determination of p-coumaric acid in pH 8.7 0.2 M ammonium buffer was performed and a detection limit, LOD = 82.6 nM, and limit of quantification, LOQ = 250 nM, were obtained.

p-coumaric acid; oxidation; radical scavenging activity; hydroxycinnamic acids; free radicals.

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