Free-radical scavenging and mechanism study of flavonoids extracted from the radix of Scutellaria baicalensis Georgi

Free-radical scavenging activities of baicalein, baicalin, wogonin and wogonoside, the four major flavonoids in a traditional Chinese herb medicine, the radix of Scutellaria baicalensis Georgi, were examined by electron paramagnetic resonance (EPR). The results showed that baicalein and baicalin scavenged hydroxyl radical, superoxide anion, 2,2-diphenyl-1-picrylhydrazyl radical and alkyl radical in a dose-dependent manner, while wogonin and wogonoside showed subtle or no effect on these radicals. Baicalein was the most effective free-radical scavenger among the four tested compounds. When 10 mmol/l tested flavonoids dissolved in alkaline solution, only baicalein and baicalin form stable free radicals which can be detected by EPR technique, it was found that the signal came from their o-di-hydroxyl structure in ring A. To our knowledge, it was the first time demonstrated that flavonoids with o-di-hydroxyl structure in ring A could also form stable semiquinone free radicals. These results demonstrated that flavonoids in radix of Scutellaria baicalensis with o-di-hydroxyl group in ring A such as baicalein and baicalin are good free-radical scavengers and might contribute to some of their pharmaceutical effects.