Alternative Mechanisms of Betacyanin Oxidation by Complexation and Radical Generation

Abstract

The use of natural pigments such as betalains in food and health-related products is often limited by said pigments’ relative oxidative stabilities in the products or physiological matrices. Determination of the mechanism of oxidation may inform future development and delivery of better stabilized molecules for improved outcomes. In order to best determine the oxidation mechanism of betanin, a natural food colorant, our efforts were directed toward structural elucidation (LCMS-IT-TOF and NMR) of previously tentatively identified key dehydrogenation products that had been generated as a result of betanin, decarboxylated betanin, and neobetanin oxidation by 2,2′-azino-bis­(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) cation radicals. The resultant oxidation products, the neo-derivatives, were the most stable and survived the preparative isolation and purification process. Structural analyses subsequently confirmed that these compounds, as well as neobetanin, were also the key products of alternative pathways of betanin and 2-decarboxy-betanin oxidation when catalyzed by Cu2+ cations in aqueous solutions at pH close to neutral. Therefore, the structures of the following five neo- or xanneo-derivatives (14,15- or 2,3,14,15-dehydrogenated derivatives, respectively) were confirmed: neobetanin, 2-decarboxy-neobetanin, 2-decarboxy-xanneobetanin, 2,17-bidecarboxy-xanneobetanin, and 2,15,17-tridecarboxy-xanneobetanin. This research confirmed that Cu2+-catalyzed oxidation of betanin and 2-decarboxy-betanin results in generation of neo-derivatives of betanin. In contrast, Cu2+-catalyzed oxidation of 17-decarboxy-betanin and 2,17-bidecarboxy-betanin resulted mostly in formation of betanin xan-derivatives. A relevant mechanism of Cu2+-catalyzed oxidation of the pigments is proposed herein that suggests that the oxidation of betanin can possibly occur in the region of the dihydropyridinic ring and can omit the stage of methide quinone formation in the dihydroindolic system.

Dehydrogenation of Betacyanins in Heated Betalain-Rich Extracts of Red Beet (Beta vulgaris L.)

“…In this context, it is necessary to mention that due to previous oxidation structural studies on betacyanins, with the use of enzymatic [40] and chemical agents [44], several dehydrogenation pathways have been better recognized. Furthermore, the key oxidation products were isolated and identified by NMR confirming the 2,17-decarboxylation routes [46]. The initial mechanism of betanin oxidation as well as principal directions of decarboxylation are presented in Figure 1.…”

“…Another betanin oxidation mechanism is also possible by catalysis with heavy metal cations, especially Cu 2+ , resulting in generation of neobetanin (Figure 1) [46]. As demonstrated previously [46], 2-decarboxylation is accompanied mainly by 17-decarboxylation, however, the impact of initial 15-decarboxylation of betanin should be also taken into account, as should the formation of high quantities of 2,15-bidecarboxy-betanin during the heating. This pathway seems to be equally important for considering the whole betanin reaction scheme.…”

“…In addition, after initial decarboxylation of betanin at carbon C-2, the reaction follows a similar oxidation pathway [42,44]. Another betanin oxidation mechanism is also possible by catalysis with heavy metal cations, especially Cu 2+ , resulting in generation of neobetanin (Figure 1) [46]. As demonstrated previously [46], 2-decarboxylation is accompanied mainly by 17-decarboxylation, however, the impact of initial 15-decarboxylation of betanin should be also taken into account, as should the formation of high quantities of 2,15-bidecarboxy-betanin during the heating.…”

“…Based on these assumptions, it can be concluded that the mechanism of betacyanin degradation may be different, and its variants were initially presented in our publications [41,[43][44][45]. In the course of many years of work on decarboxylated compounds, the above-mentioned and many other structures have been confirmed by LC-MS and/or NMR methods [6,46,47].…”

Winemaking Characteristics of Red‐Fleshed Dragon Fruit from Three Locations in Guizhou Province, China

“…Furthermore, while satisfactory stability of betacyanin in a low alcohol (1.5%vol) dragon fruit beverage during eight weeks storage at 4℃ has been demonstrated in a laboratory study (Choo e t al ., 2018), that result is yet to be confirmed in large scale production. Consequently, while the mechanisms behind the instability of betacyanin remain unclear (Kumorkiewicz et al ., 2019), further study is needed to understand this phenomenon; especially in regard to dragon fruit wine‐related factors.…”