Study Of Electrooxidation Processes Of Resveratrol And Pterostilbene By Spectroelectrochemistry And Capillary Electrophoresis

Resveratrol and pterostilbene are two bioactive stilbene compounds that arewidely found in the plant kingdom. Pterostilbene is a3,5-dimethylether derivativeof resveratrol. Both of them attract growing attention in the pharmaceutical andfood fields, because of their physiological effects, such as cardiovascular andcerebrovascular disease prevention, anti-cancer, anti-inflammatory, antioxidant andfree radical scavenging activities. In this thesis, the oxidation mechanisms ofresveratrol and pterostilbene were studied at a carbon paste working electrode usingelectrochemical and in situ UV-Vis spectroelectrochemical methods, for thepurpose of clarifying the structural origin of the relative antioxidative differencebetween the two stilbene compounds. In addition, the soluble oxidation productswere on line separated and detected using a home-made chip thin-layer electrolyticcell coupled with capillary electrophoresis apparatus.Voltammetric results indicate that the oxidation of both the compoundsshowed two irreversible anodic peaks under mixed adsorption-diffusion control. Forthe first oxidation peak, pterostilbene has less positive peak potential but higherpeak current than resveratrol, which suggests that the antioxidant activity wasenhanced due to the substitution of two o-hydroxyls of resveratrol by methoxyls. Inalkaline media, the oxidation products of pterostilbene block the electrode surfacemuch more seriously than those of resveratrol, which can be attributed to thestronger lipophilicity of pterostilbene than resveratrol. The final products ofpterostilbene were adsorbed on the electrode surface and then hindered the chargetransfer.The in situ UV-Vis spectra showed the decrease in characteristic absorptionbands of both the compounds during their oxidation in neutral and acidic media,without the presence of new absorption bands. This suggests that the oxidationproducts of them were deposited on the electrode surfaces, without stripping intothe solution. In alkaline media, soluble oxidation products were determined only forresveratrol, which produced a new absorption band at around250nm. Resveratrolwas transformed to soluble small molecules via the oxidative cleavage of the centerdouble bond, whereas pterostilbene was subjected to radical polymerization to yield dimers that covered the electrode surface. The chemical oxidation of them by airshowed the similar spectral changes as their respective electro-oxidation, but withmuch slower oxidation rates than the latter. Lipophilicity of pterostilbene superiorto resveratrol might be the important reason for its higher antioxidant activity, sincethe lipophilicity increases the adsorptive accumulation as well as accelerates theconversion of the oxidized radical intermediates.Capillary electrophoresis (CE) experiments achieved satisfactory results forthe on line separation of the oxidation products of resveratrol and pterostilbene.Parents and products were well separated without dead volume and delayphenomenon. CE chromatograms indicated that resveratrol and pterostilbene couldbe oxidized completely by air, while their electrochemical oxidation was inhibiteddue to the electrode passivation by the oxidation products. A new peak appeared atresveratrol CE chromatogram only under alkaline condition and at250nmwavelength. For pterostilbene there was a nearby product peak at any pH value.