Singlet Oxygen Generated by Quercetin and Gallic Acid Leads to Oxidative Fragmentation of Flavonols in Onions and Leek

For the first time, singlet oxygen was shown to be generated by quercetin and gallic acid under mild conditions (37 °C, pH 7) and exclusion of light. In contrast to kaempferol, quercetin induced its own oxidative fragmentation, yielding protocatechuic acid (3,4-dihydroxybenzoic acid) and 2,4,6-trihydroxybenzoic acid as the corresponding counterparts. If a 4-molar excess of gallic acid was coincubated, quercetin fragmentation reached 25 mol %, but now also kaempferol gave 7 mol % <i>p</i>-hydroxybenzoic acid. The hydroxybenzoic acids formed always showed the B-ring substitution pattern of the precursor flavonol. This reactive oxygen chemical quenching mechanism initiated by pericyclic cycloadditions was underlined by the use of singlet oxygen released from endoperoxides. Isolation and characterization of a quercetin-methanol solvent adduct pointed toward the parallel occurrence of physical quenching. The importance of singlet oxygen-mediated flavonol degradation in food matrices was verified by the detection of the fragmentation products in minced onion and leek samples.