Electrochemical And Spectroelectrochemical Study Of Myricetin And Dihydroquercetin

As a series of natural antioxidants, flavonoids are abundant in vegetables, fruits and medicinal plants. Myricetin and dihydroquercetin belong to flavonoids, which can scavenge superfluous superoxide free radicals in human body and show pharmacological effects in anti-tumour, anti-inflammation and anti-senility, etc. Therefore, they have got great attention in recent years. In this paper, these two compounds were investigated as model compounds to test their electric redox behavior and spectral changes using cyclic voltammetry, differential pulse voltammetry and in situ UV-Vis spectroelectrochemical methods, and then explored the influence of 2,3 C-C saturated bond in electro-oxidation mechanism and anti-oxidation properties. We also used double potential chronoabsorptometry and cyclic voltabsorptometry to test electric redox behavior of the two compounds. In addition, the interactions between metal ions and flavonoids were deeply investigated by spectroelectrochemical methods.Electrochemical tests showed that CPE displayed great power to adsorb myricetin and dihydroquercetin. The two flavonoids both had a couple of reversible redox peaks A1 and C1 at lower potentials, which corresponded to the oxidations of 3',4'-OHs at B-ring involving 2-electrons and 2-protons. Curves at different scan rates indicated that the reactions of myricetin and dihydroquercetin were both controlled by adsorption and diffusion on CPE surface. Through the examinations at different pH values, we found that acidic conditions were suitable for oxidations of the two flavonoids.Cyclic voltammetry and thin layer spectroelectrochemistry test results indicated that the adsorption and oxidation of myricetin and dihydroquercetin on a CPE in a longoptical-path thin-layer electrochemical cell. The characteristic bands of the two flavonoids showed potential-dependent variations under controlled-potential oxidation. Cyclic voltammetry and in situ UV-Vis spectroelectrochemical methods were used to investigate the electro-oxidation mechanisms of myricetin and dihydroquercetin, in order to understand the relationship between flavonols'antioxidant activity and their chemical structures. The results indicated that the presence of the 5'-OH at the B-ring of myricetin may, to a certain extent, hinder the subsequent chemical transformation, while the 2,3 C-C saturated bond in dihydroquercetin can completely stop the subsequent step, leading to the decrease in antioxidant activity, less for myricetin and more for dihydroquercetin. This suggests that the subsequent chemical transformation gives very important contribution to the antioxidant activity of flavonols.In this work, the interactions between flavonoids and metal ions were studied by spectroelectrochemical methods. We also studied the effect of hydrogen peroxide, one of the reactive oxygen species, on the complexation of flavonoids and metal ions. We have shown that myricetin forms 1:2 complexes with Cu2+, in which 3-OH and 4-oxo groups and 3',4'-OH groups are coordination sites. The same result was obtained for Fe3+. Spectral changes were hardly obtained for the complexation of dihydroquercetin and metal ions, which corresponded to the 2,3 C-C saturated bond in dihydroquercetin. Hydrogen peroxide can oxidize myricetin to quinine in acidic solutions slowly, whose characteristic absorption peak appears at 294 nm. The slow myricetin oxidation by hydrogen peroxide is accelerated in the presence of Fe3+.