Study Of Ellagic Acid By Thin-layer Spectral And Electrophoretic Electrochemistry

Ellagic acid is a biologically active polyphenol found widely in fruits,vegetables, seeds and some nuts in free form or in a bound state. It is a naturalantioxidant and anticarcinogenic agent with the ability to scavenge superfluoussuperoxide free radicals. It is usually used as an ingredient in cosmetic productsfor skin whitening. Over the last few years, a number of in vivo and in vitrostudies of ellagic acid have provided evidences of important pharmacologicalproperties including anti-inflammatory, anti-coagulatory and anticarcinogenicactivities. The oxidative metabolism mechanism of ellagic acid is verycomplicated and further investigation is required for better understanding ofmetabolism mechanisms or the the potential toxicity. In this thesis, the oxidationprocesses of ellagic acid at different pH values were studied by cyclicvoltammetry, in situ UV–vis spectral analysis, cyclic voltabsorptometry andcapillary electrophoretic detection. A parallel consecutive reaction mechanismwas proposed.The results show that a total of four stable products were obtained in acidic,physiological, and alkaline pH media. It was initially oxidized to a reactivephenoxyl radical and then to a disemiquinone intermediate via two steps ofone-electron one-proton transfers. The latter intermediate may undergoisomerization and lactone-hydrolysis reactions leading to two types of finalproducts. The major product is possibly a diquinonemethide with redox stabilityin the studied potential range. In alkaline pH media, the diquinonemethidecontains two carboxyl groups generated from two-lactone-ring opening. Theminor product is considered as an o-quinone derivative with alactone-ring-opening, which can be reduced to the corresponding diphenol atlower potentials, or, further oxidized at higher potentials in physiological andalkaline pH media.The chip-type thin layer electrochemical cells, which contain an electrolytesolution usually in a microliter scale volume, can be used to realize completeconversion of electroactive species in a short time. A new application of thinlayer cell was developed by coupling it with high-performance capillaryelectrophoresis and UV–vis detection. This coupling method allows rapidexhaustive electrolysis, direct sampling and online electrophoretic separation of the electrolysis products. The ellagic acid solid carbon paste electrode was firstused by coupling with high performance capillary electrophoresis for thedetection of oxidation products of solid ellagic acid, free of the interference ofinsoluble reactants.