Singlet oxygen quenching- and chain-breaking antioxidant-properties of a quercetin dimer able to prevent age-related macular degeneration

A dimer of quercetin prepared through a Mannich reaction protects pyridinium bisretinoid A2E from photo oxidation at 430 nm in aqueous medium at pH 7.4. In the presence of light and O-2, A2E is quickly attacked by 10(2) produced in situ (by excited A2E) to give nonaoxirane and other oxygenated compounds which can be involved in diseases of the macula. Peroxyl radicals might have only a marginal role on the photooxidation of A2E. The dimer is actually a potent quencher of O-1(2) with a rate constant k(Q) of 8.5 x 10(8) M-1 s(-1) in methanol at room temperature. On the other hand, its antioxidant abilities against ROO. radicals are quite limited since k(ROO). = 7.3 x 10(8) M-1 s(-1) (in buffer solution at pH 7.4), the value being essentially identical to that of quercetin. The quenching of O-1(2) by the dimer is therefore the main reason for the A2E protection and prevention of age-related macular degeneration.