Caftaric acid oxidation in the presence of cell signaling regulator glutathione: Electrochemical and chromatographic analyses

New antioxidant materials in biomedical applications are an exciting research topic. However, the lack of understanding of the products/intermediates formed and the interaction between nucleophiles and these antioxidants will prevent commercial or industrial applications. To overcome this obstacle, chromatographic techniques, mass spectroscopy, and electrochemical analyses were combined to study the mechanism and products of the expected reaction between antioxidants and nucleophiles. Grape juice contains phenolic compounds such as 2-S-glutathionylcaftaric acid, which is formed as a result of oxidative action. Through UPLC and voltammetric analysis of caftaric acid (CFT) and its quinone with glutathione (GSH), the oxidation and subsequent reactions were monitored. Electrochemical techniques such as cyclic voltammetry (CV) and chronoamperometry (CA) have been used to investigate the electrochemical oxidation characteristics of the investigated CFT in a Britton-Robinson (B-R) buffer electrolyte. As a result of the analysis of ultra-high performance liquid chromatography-mass spectrometry (UHPLC-MS) results, the oxidation products were identified to suggest the optimal mechanism for the oxidation of CFTs both in the presence and absence of GSH. A reversible oxidation process is observed at pH 1.8 for CFT without glutathione (GSH) at E1/2 = 0.52 V against Ag/AgCl 3.0 M KCl) using cyclic voltammetry. During this electrochemical process, two electrons in the CFT molecule were oxidized along with two protons, forming o-quinone. In the presence of GSH, the formed quinone either reacts with glutathione to form mono- and bi-glutathione CFT conjugates or is reduced by GSH to its original state. As a result of the UPLC-MS analysis, the oxidation products of CFT were identified, and the reaction pathways were proposed.