Voltammetric and spectral characterization of two flavonols for assay-dependent antioxidant capacity

Polyphenolic compounds usually showed different antioxidant capacities depending on the assay methods used. To determine the possible chemical cause for this assay-dependence, two flavonols, kaempferol and morin, were selected as the model molecules for the comparative investigation between electrochemical and chemical oxidations. The electro-oxidation of the flavonols was studied using cyclic voltammetry and in situ UV–Vis spectroelectrochemical technique with a long-optical-path thin-layer electrolytic cell. The spectral changes recorded in different potentials were compared with those in the chemical oxidation by H2O2 or ABTS•+ radical in the same thin-layer cell. The 4′-OH group of either sample was first oxidized at lower potentials or induced by H2O2, and in this case kaempferol was somewhat more active than morin. With an additional 2′-OH group, morin underwent the secondary oxidation in moderately higher potentials or by ABTS•+, showing antioxidant capacity about twice of that of kaempferol. This study clarified that the chemical oxidation of a polyphenolic compound by the oxidants with different reactivities, which corresponded to its electro-oxidation in different anodic peaks, had a difference in number of oxidizable OH-groups, leading to the difference in antioxidant capacity.