Antioxidant activity of flavonoids isolated from Hypericum ternum

In the present work we have studied the scavenging activity of several flavonoids isolated from Hypericum ternum A. St. Hil. The evaluation of the free radical scavenging capacity was based on the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) consumption elicited by their addition. Also, we have attempted to evaluate their capacity to delay pyrogallol red consumption promoted by peroxyl radicals. The compounds isolated and characterized were I3,II8-biapigenin and five quercetin derivatives (quercetin 3-methyl ether, quercetin 3,7-dimethyl ether, hyperoside, isoquercitrin and guaijaverin). All compounds were able to scavenge DPPH radicals. The order in scavenging capacity (from highest to lowest) was: guaijaverin > hyperoside approximate to isoquercitrin > quercetin-3-methyl-ether. No protection of pyrogallol red was evidenced for all flavonoids derivatives at relatively high (100 mu M) concentrations. This lack of protection contrasts with the efficient protection afforded by 10 mu M quercetin, indicating that substitution at the 3-position in quercetin strongly reduces the capacity of the molecule to scavenge peroxyl radicals.