The formation of complexes of metal ions with the flavonoids quercetin (L1), rutin (L2), galangin (L3) and catechin (L4) has been investigated by UV-visible spectroscopy. The antioxidant activities of the compounds were evaluated by using the 1,1-diphenyl-2-picrylhydrazyl (DPPH) radicalscavenging method. In this work, we have shown that the complexed flavonoids are much more effective free radical scavengers than the free flavonoids. We suggest that the higher antioxidant activity of the complexes is due to the acquisition of additional superoxide dismutating centers. Radical scavenging activities of the compounds were also investigated from an electrochemical point of view. There is a relationship between the logarithm of the antioxidant activity (represented by EC50) and the oxidation potential. The synergic effect of the complexes and ascorbic acid were studied by [13C]-NMR analyses. The results show that ascorbic acid can protect flavonoids from oxidative degradation, and reveal antioxidant synergies between ascorbic acid and the compounds.

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