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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.

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Profiles of betacyanins present in light-stressed stems of different cactus species and nonstressed but genetically aberrated red pigmented cacti of Gymnocalycium mihanovichii cv. ;Hibotan' Britton & Rose, which are known as grafted cacti, were compared. The identities of all the pigments in the cacti were characterized for the first time. The identification of acylated and nonacylated betacyanins was performed by means of mass spectrometry and UV-vis diode array detection coupled to high-performance liquid chromatography. The most indicative pigments of the stressed cacti, 5''-O-E-feruloyl-2'-O-beta-apiosyl-betanin and 5''-O-E-sinapoyl-2'-O-beta-apiosyl-betanin, as well as their diasteromers, were the prevailing betacyanins in Hylocereus polyrhizus , Epiphyllum phyllanthus , and Rhipsalis rhombea . Stressed Rhipsalis regnellii stems contained the sinapoylated betacyanins accompanied only by traces of the feruloylated derivatives. In addition, high contents of 2'-O-apiosyl-betanin were frequently observed in the samples with the highest concentration found in stressed Schlumbergera x buckleyi (T. Moore) Tjaden. These pigments were also detected, but at low levels, in the Hibotan pink, red, and violet scions, which were not light-stressed. In the Hibotan scions, the most abundant were the polar betacyanins: betanidin 5-O-beta-sophoroside and betanin. In most of the stressed samples, betanin was present at relatively low levels.