UVA and UVB radiation-induced oxidation products of quercetin

The flavonol quercetin is believed to provide protection against ultraviolet (UV) radiation-induced damage in plants. As part of our investigations into the potential for quercetin to protect skin against UV radiation-induced damage we have investigated the products of quercetin exposed to UV radiation in vitro. UVA (740 μW cm−2 at 365 nm) or UVB (1300 μW cm−2 at 310 nm) irradiation of quercetin in methanol results in a small conversion (less than 20%) to C-ring breakdown products over 11 h. When the triplet sensitizer benzophenone is added, greater than 90% conversion by UVA or UVB occurs within 1 h. The major photoproducts from either UVA or UVB radiation are 2,4,6-trihydroxybenzaldehyde (1), 2-(3′,4′-dihydroxybenzoyloxy)-4,6-dihydroxybenzoic acid (2) and 3,4-dihydroxyphenylethanol (3). Product 2 has previously been observed as a product of oxidative metabolism of quercetin, however products 1 and 3 appear to be the result of a unique UV-dependent pathway. In conclusion we have determined that quercetin undergoes slow decomposition to a mixture of C-ring-opened products, two of which to our knowledge have not been previously observed for quercetin decomposition, and that the presence of a triplet sensitizer greatly increases UV radiation-mediated quercetin decomposition. The presence of endogenous photosensitizers in the skin could potentially affect the UV stability of quercetin, suggesting that further study of quercetin for both its photoprotective properties and photostabilty in skin are warranted.