Investigation of reaction between quercetin and Au(III) in acidic media: mechanism and identification of reaction products

The aim of the present paper was to investigate the reaction of quercetin, the flavonol very often used as a dietary supplement, with [AuCl4](-) ions. The reaction was studied spectrophotometrically using the equimolar solutions in 1 : 1 water-methanol at pH similar to 2. The spectrophotometric data indicated the formation of the products with an absorption maximum at 295 nm in all cases, characteristic of the oxidized forms of quercetin. HPLC coupled with DAD and LC-MS analysis of the reaction products suggested that the oxidation of quercetin resulted in the generation of similar metabolites including quinone and various oxidized quercetin-solvent adducts. In addition, cyclic voltammetric measurements confirmed that under applied experimental conditions, the reduction of Au(III) to Au(0) took place. The reduction species in the reaction mixture were Au(III) ions, while Au(I) disproportionates back to Au(III) and Au(0). The newly generated Au(III) ions further oxidized 3'-4'-dihydroxy groups of quercetin adducts obtained after first 2e(-) oxidation, giving the final reaction products. Based on the identification of reaction products, the reaction mechanism for the oxidation of quercetin in the presence of Au(III) which involves two 2e(-) transfer processes was proposed.