ESI-TOF MS analysis of complexes formed between quercetin and five metal ions in hot water and a study into their DNA cleavage activity

Quercetin is a flavonoid compound widely found in vegetables, fruits, and medicinal plants. It has carbonyl and phenolic hydroxyl groups in its structure that can easily form complexes with metal ions. In this study, we reacted quercetin in water at 95 °C with five metal trace elements commonly found in plants, namely calcium, magnesium, zinc, iron(III), and copper. Each supernatant was then examined by electrospray ionization-time-of-flight mass spectrometry (ESI-TOF MS). The results showed that quercetin can form complexes with Ca²⁺, Mg²⁺, Zn²⁺, and Cu²⁺ in ratios of 2:1, 3:1, and 3:2. Interestingly, after reaction with Fe³⁺ in water at 95 °C, not only can quercetin‑iron(III) complexes be formed in ratios of 2:1 and 3:1, but small amounts of quercetin‑iron(II) complexes can also be formed in the same ratios. Furthermore, DNA cleavage experiments showed that when acting alone, quercetin and Cu²⁺ have weak or no cleavage effects on DNA, but the complex formed after reaction in hot water cleaves DNA in a time- and concentration-dependent manner. These results indicate that complexes may form between quercetin and a variety of metal trace elements in a water decoction of plants, and that these metal complexes may be the material basis for the health-promoting and therapeutic effects of edible or medicinal plants.