Abstract
The oxidation of the natural bioflavonoid rhamnetin was studied by a multi-analytical approach. It was widely used as a dye for colouring tapestries, and it is known for degrading in the presence of atmospheric oxygen and light. The oxidation mechanism was studied by cyclic voltammetry and UV–Vis spectroelectrochemical methods in aqueous solution on glassy carbon and platinum electrodes. Two-electron and two-proton oxidation proceeds in acidic solution, while two-electron and one-proton oxidation was found for a dissociated molecule of rhamnetin, and one electron oxidative process was obtained for the dianion present in alkaline solution. The complicated oxidation mechanism involves not only electron transfer, but also several coupled chemical reactions. The products were identified by HPLC-DAD and HPLC-ESI-MS/MS techniques. The distribution of oxidation products was strongly dependent on the duration of exposure to atmospheric oxygen. The main oxidation product was identified as 2-[(3,4-dihydroxyphenyl)carbonyl]-2,4-dihydroxy-6-methoxy-1-benzofuran-3(2H)-one. Because rhamnetin in solution was fully degraded by oxygen in 6min, it was necessary to prepare its solutions under argon atmosphere prior any electrochemical measurement.
Published Version
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