Interaction of quercetin with copper ions: complexation, oxidation and reactivity towards radicals

Quercetin, one of the most common dietary flavonols, was investigated in the presence of Cu(II) ions in methanolic solution in order to obtain some explanation on the mechanism interaction and its action against free radical-mediated damage. The spectroscopic studies (UV-VIS, IR, ESI-MS) were used to assess the extent to which it undergo complex formation through chelation or modification through oxidation. The reaction of quercetin with Cu(II) resulted in the formation of 1:1 metal-ligand complex (λ(max) = 436 nm) through the carbonyl oxygen and 3-OH group in C ring. Then quercetin is oxidized to the benzoquinone type products. The addition of EDTA destroyed the complex but did not regenerate the whole original spectrum of quercetin. From the other hand, the presence of EDTA inhibits formation of copper-quercetin complex and quercetin oxidation. The antioxidant activity of the Q + Cu solutions was evaluated by using 2,2-diphenyl-1-picrylhydrazyl radical (DPPH∙) radical scavenging method and from an electrochemical point of view. The complex is much more effective as free radical scaveninger than the free flavonoid.