Electrochemical Study Of Structure-activity Relationship Of Flavonoid Antioxidants

Natural flavonoid antioxidants have got great attention in the past decade years, because they can scavenge superfluous superoxide free radicals in human body and show pharmacological effects in anti-virus, anti-tumour, anti-inflammation and anti-senility, etc. In this thesis, cyclic voltammetry (CV), differential pulse voltammetry (DPV) and in situ spectroelectrochemical techniques were used to investigate the electro-oxidation mechanism and the structure-activity relationship of some flavonoids, including the electro-oxidation of morin and interaction with DNA, comparison of the electro-oxidation and chemical oxidation by H₂O₂ for two groups of flavoniods, and comparison in electro-oxidation mechanism of luteolin and quercetin. In situ UV-Vis spectroelectrochemical measurements were carried out in a long-optical-path thin-layer electrolytic cell.The voltammetric data for morin indicated that the graphite-wax paste electrode (GWE) displayed great power to adsorb morin. A well-shaped oxidation peak of 4'-hydroxy at B-ring occurred firstly in the lower potentials and a second oxidation in relatively positive potentials. The first oxidation was a qusasi-reversible reaction involving 1-electron and 1-proton, probably generating a phenoxy radical intermediate. The latter further transformed into the final stable products. Addition of DNA in the morin solution caused the derease in the current of redox peaks, without the change in the peak potential. This effect of DNA suggested the formation of super-molecular compounds without electrochemical activity, leading to the decrease in the amount of morin in free solution.Some flavoniod antioxidants were comparatively investigated for the structure-activity relationship by recording the dynamic thin-layer UV-Vis spectroscopy during their electro-oxidation or chemical oxidation by H₂O₂. One group of flavoniods such as morin could be easily oxidized by H₂O₂ and showed the same spectral change as in electro-oxidation; both the electro-oxidation and the chemical oxidation led to the mergence of the two conjugated systems present in the parent compounds into a more stable A-B-C-ring united conjugated system. However, the others such as rutin were hardly oxidized by H₂O₂ with little spectral change different from that in electro-oxidation, especially in acidic medium. The former compounds have common structure features, including 4'-OH, 3-OH, 2-3 double bond and 4-carbonyl, all of which are indispensable for the stable A-B-C-ring united conjugated system in the oxidation products.As one of flavonols with 3-OH group in their chemical structures, quercetin (3,5,7,3',4'-pentahydroxyflavone) shows much stronger antioxidant activity than does its corresponding flavone luteolin (5,7,3',4'-tetrahydroxyflavone). Cyclic voltammetry and in situ UV-Vis spectroelectrochemical method were used to investigate the electro-oxidation processes of both flavonoids, for the purpose of discovering the reaction mechanism by which the 3-OH group promotes the antioxidant activity. The results indicates that both compounds were oxidized to the corresponding ortho-quinones via 2e–/2H⁺ reactions, and then only the quercetin o-quinone was further isomerized to the more stable united conjugated structure, in the presence of the 3-OH group. This subsequent chemical transformation gives an additional contribution to the antioxidant activity.