An investigation on structural aspects influencing product formation in enzymic and chemical oxidation of quercetin and related flavonols

Quercetin oxidation, with mushroom polyphenol oxidase (PPO), horseradish peroxidase (POD) and potassium ferricyanide, was studied by means of reversed-phase high-performance liquid chromatography (RP-HPLC). In order to establish structural requirements for product formation, two other flavonols, morin and rutin (quercetin 3-O-rutinoside), which possess key structural differences, were also treated. Oxidations were performed, either in aqueous acetonitrile (MeCN) or aqueous N,N-dimethylformamide (DMF), due to poor solubility of flavonols in aqueous media. The chromatographic profiles obtained from quercetin solutions, treated with PPO, POD, or potassium ferricyanide, were very similar, yielding the same set of products. In contrast, morin and rutin oxidations proceeded via different pathways. From the examinations carried out, it became clear that some products which possess significantly lower polarity compared with quercetin have, as a strict requirement, both the o-catechol function and the 3-hydroxyl group.