Electrochemical And Spectroelectrochemical Study Of Antioxidation Mechanism Of Kaempferol

Flavonoids are abundant in vegetables,fruits and other plants.Owing to the strong antioxidant properties,flavanoids can scavenge superfluous superoxide free radicals in human body,preventing DNA and cells from oxidative damage.Most of them show pharmacological effects in anti-tumour,anti-inflammation and anti-senility,etc.As a natural flavonoid antioxidant,kaempferol has got great attention in the nearly years.In this thesis the electro-oxidation mechanism and interaction with DNA of kaempferol were studied on graphite-wax paste electrode (GWE)in Britton-Robinson buffer solutions involving 1.0 mol·L^-1KC1 as supporting electrolyte,using cyclic voltammetry(CV),differential pulse voltammetry(DPV)and in situ UV-Vis spectroelectrochemical technique with a long-optical-path thin-layer electrolytic cell.The voltammetric results on GWE showed a well-shaped oxidation peak of kaemfrol,due to the strong adsorption of kaemfrol on GWE.From the DPV measurement,a linear relation between the peak current and concentration was obtained in the range of 1×10^-4-1×10_⁶mol·L_¹,with a detection limit of 3×10^-7mol·L^-1(S/N=3).Polyphenolic compounds usually showed different antioxidant capacities depending on the assay methods used.To determine the possible chemical cause for this assay-dependence,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 polyphenols 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.Cyclic voltabsorptomograms were recorded at three characteristic absorption wavelengths during CV scan for kaempferol,and then were differentiated with respect to time to obtain the waveform comparable with the corresponding voltammetric peaks.The result showed that both the pre-adsorbed kaempferol and those in free solution contributed to the total current,and a stable quinone methide with a united conjugated structure was generated via an EC_irrmechanism. Kaempferol was first oxidized to the corresponding phenoxyl radical via le^-and 1H⁺transfer,followed by an irreversible homogeneous transition to the final product.It is proved that multi-wavelength cyclic voltabsorptometry can provide significant information on the complex electrochemical processes coupled with adsorption and homogeneous chemical transitions.The electrochemical behavior of interaction of kaempferol with DNA was studied by cyclicvoltammetry and UV-Vis spectrum.When DNA was added in the kaempferol solution,both reduction and oxidation peak currents of kaempferol decreased with no change in the peak potentials.We concluded that the reason was an electrochemical inactive supramolecular complex DNA-kaempferol.