Transformation mechanism, kinetics and ecotoxicity of kaempferol and quercetin in the gaseous and aqueous phases: A theoretical combined experimental study.

The transformation and risk assessment of flavonoids triggered by free radicals deserve extensive attention. In this work, the degradation mechanisms, kinetics, and ecotoxicity of kaempferol and quercetin mediated by ∙OH, ∙OCH, ∙OOH, and O in gaseous and aqueous environments were investigated using cell experiments and quantum chemical calculations. Three radical scavenging mechanisms, including hydrogen atom transfer (HAT), radical adduct formation (RAF) and single electron transfer (SET) were discussed. The results show that RAF and HAT are the main reaction mechanisms for the neutral kaempferol/quercetin, and SET mechanism is important for the anionic kaempferol/quercetin. The overall rate coefficient of kaempferol and quercetin with ∙OH were calculated at 273-323 K, and the aqueous rate coefficients are calculated by considering the rates of neutral and monoanionic forms multiplied with the molar fractions of each form. The values are 2.81 × 10 and 8.63 × 10 M s in the aqueous environment, and 2.31 × 10 and 1.18 × 10 cm molecule s in the gaseous environment at 298 K. Fluorescence probe and flow cytometry results show that kaempferol and quercetin can be efficiently degraded by free radicals, and quercetin has a better effect, which is consistent with the theoretical results in the aqueous environment. The transformation mechanism of Q-OH-P7a with ∙OH, O and NO was studied, and the stable product is Q-P1. Toxicology results show that most of the subsequent products of quercetin do not bioaccumulate and can be biodegraded, but most products still have toxic properties or harmful properties and show positive mutagenicity. This study provides new guidance for flavonoid degradation behavior and environmental risks.