Biomimetic oxidation of quercetin: Isolation of a naturally occurring quercetin heterodimer and evaluation of its in vitro antioxidant properties

Quercetin was oxidized with potassium ferricyanide under alkaline conditions, a reaction that has been previously shown to generate the same set of products deriving from oxidation by mushroom polyphenol oxidase (PPO) and horseradish peroxidase (POD). The major oxidation product was isolated and characterized by means of 1H NMR and LC–MS, and it was found to be a quercetin heterodimer [1,3,11a-trihydroxy-9-(3,5,7-trihydroxy-4H-1-benzopyran-4-on-2-yl)-5a-(3,4-dihydroxyphenyl)-5,6,11-hexahydro-5,6,11-trioxanapthacene-12-one], occurring in onion skins. Its in vitro antioxidant properties were evaluated in comparison with the parent molecule and other natural polyphenols, employing three representative tests and the alterations in the activity observed were discussed on the basis of structure–activity relationships. Biomimetic chemical oxidations of natural polyphenols may help understand oxidation pathways in plant food commodities and evaluate the consequences on their nutritional value.