Comparative analysis of quercetin oxidation by electrochemical, enzymatic, autoxidation, and free radical generation techniques: A mechanistic study

Quercetin, the most abundant flavonoid in dietary fruits and vegetables, acts as antioxidant or prooxidant depending on the environmental conditions. The antioxidant behavior is believed to involve initial oxidative steps with subsequent changes in the flavonoid skeleton, which ultimately alters the chemical and biological properties of these molecules. Although the mechanism is still unclear, it has been suggested to be strongly influenced by the surrounding media. This paper reports the oxidation of quercetin by air oxygen or autoxidation, bulk electrolysis, mushroom tyrosinase, and azodiisobutyronitrile (AIBN). The central aim of this study is to systematically examine how the similarities and differences of quercetin transformation can be affected by the nature of the oxidation systems. Using a range of molecular and structural characterization techniques (UV-vis, LC-MS, GC-MS, and NMR), the oxidation of quercetin was found to result in the generation of somewhat similar metabolites including depside, phenolic acids, and quercetin-solvent adducts, although the transformation process and quantities of each product depend on the type of oxidation method employed. The rate of quercetin autoxidation can be fitted to a monoexponential first-order decay with a k value of 6.45 × 10^-2 M^-1 s^-1. Comparison of quercetin oxidative products in the different systems provides a deeper insight into the underlying mechanism involved in the oxidation process. This work demonstrates that the presence of water and/or nucleophiles as well as different catalysts (tyrosinase, AIBN, or air oxygen in solution) may have very important implications for the formation of quinone with subsequent oxidative cleavage or polymerization. Moreover, the apparent first-order kinetics of autoxidation can indicate a rate-determining, one-electron oxidation of quercetin anions followed by two fast steps of radical disproportionation and solvent addition on the resulting quinone.