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 photooxidation at 430 nm in aqueous medium at pH 7.4. In the presence of light and O2 , A2E is quickly attacked by 1 O2 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 1 O2 with a rate constant kQ of 8.5 × 108  M-1  s-1 in methanol at room temperature. On the other hand, its antioxidant abilities against ROO· radicals are quite limited since kROO·  = 7.3 × 105  M-1  s-1 (in buffer solution at pH 7.4), the value being essentially identical to that of quercetin. The quenching of 1 O2 by the dimer is therefore the main reason for the A2E protection and prevention of age-related macular degeneration.