Antioxidant activity of betanidin: electrochemical study in aqueous media.

The antioxidative mechanism of action of betalains is of significant interest because these pigments are recently emerging as highly bio-active natural compounds with potential benefits to human health. Betanidin, the basic betacyanin, comprises the 5,6-dihydroxyl moiety, which results in its high antioxidant activity. Oxidation of betanidin by voltammetric techniques and chro matographic identification of the oxidation products with spectrophotometric and mass spectrometric detection (LC-DAD-MS/MS) were performed. Two main oxidation peaks for betanidin are observable at pH 3-5. These peaks become merged at higher pH, suggesting a different mechanism of oxidation at higher and lower pH values. The low oxidation potential of betanidin confirms its very strong reduction properties. The presence of two prominent oxidized products, 2-decarboxy-2,3-dehydrobetanidin and 2,17-bidecarboxy-2,3-dehydrobetanidin, indicates their generation through two reaction routes with two different quinonoid intermediates: dopachrome derivative and quinone methide. Both lead to the decarboxylative dehydrogenation of betanidin. Subsequent oxidation and rearrangement of the conjugated chromophoric system results in formation of 14,15-dehydrogenated derivatives.