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

## TL;DR

Quercetin and gallic acid generate singlet oxygen under mild conditions, leading to oxidative fragmentation of flavonols in onions and leeks.

## Contribution

This is the first demonstration of singlet oxygen generation by quercetin and gallic acid under mild, dark conditions.

## Key findings

- Quercetin and gallic acid generate singlet oxygen at 37°C and pH 7 in the absence of light.
- Quercetin fragmentation yields protocatechuic acid and 2,4,6-trihydroxybenzoic acid.
- Fragmentation products were detected in minced onion and leek samples, confirming relevance in food matrices.

## Abstract

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 % p-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.