Department of Analytical Chemistry, Institute C-1, Faculty of Chemical Engineering and Technology , Cracow University of Technology , ul. Warszawska 24 , Cracow 31-155 , Poland.
NanoBioMedical Centre , Adam Mickiewicz University in Poznań , Umultowska 85 , 61-614 Poznań , Poland.
J Agric Food Chem. 2019 Jul 3;67(26):7455-7465. doi: 10.1021/acs.jafc.9b01168. Epub 2019 Jun 24.
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 Cu 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 Cu-catalyzed oxidation of betanin and 2-decarboxy-betanin results in generation of neo-derivatives of betanin. In contrast, Cu-catalyzed oxidation of 17-decarboxy-betanin and 2,17-bidecarboxy-betanin resulted mostly in formation of betanin xan-derivatives. A relevant mechanism of Cu-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.
天然色素如甜菜红素在食品和与健康相关的产品中的应用常常受到这些色素在产品或生理基质中的相对氧化稳定性的限制。氧化机制的确定可能为未来更好地稳定分子以改善结果的开发和交付提供信息。为了最好地确定甜菜红素（一种天然食用色素）的氧化机制，我们努力对由于甜菜红素、脱羧甜菜红素和新甜菜红素氧化而产生的先前暂定鉴定的关键脱氢产物进行结构阐明（LCMS-IT-TOF 和 NMR）由 2,2'-偶氮-双（3-乙基苯并噻唑啉-6-磺酸）（ABTS）阳离子自由基。所得氧化产物，即新衍生物，是最稳定的，并且在制备分离和纯化过程中幸存下来。结构分析随后证实，这些化合物以及新甜菜红素，也是在接近中性 pH 的水溶液中 Cu 阳离子催化下，甜菜红素和 2-脱羧甜菜红素氧化的替代途径的关键产物。因此，以下五个新或 xanneo-衍生物（分别为 14、15-或 2、3、14、15-脱氢衍生物）的结构得到确认：新甜菜红素、2-脱羧新甜菜红素、2-脱羧 xanneobetanin、2,17-双脱羧 xanneobetanin 和 2,15,17-三脱羧 xanneobetanin。这项研究证实，Cu 催化的甜菜红素和 2-脱羧甜菜红素的氧化导致甜菜红素的新衍生物的产生。相比之下，Cu 催化的 17-脱羧甜菜红素和 2,17-双脱羧甜菜红素的氧化主要导致甜菜红素 xan-衍生物的形成。本文提出了一种相关的 Cu 催化氧化机制，该机制表明甜菜红素的氧化可能发生在二氢吡啶环区域，并且可以省略二氢吲哚系统中甲化物醌形成的阶段。
