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Sixteen flavonoids including flavonols, flavones, flavanonol and catechins, and five aromatic compounds were examined for their ability to scavenge superoxide radical (O2-*) generated enzymatically in a xanthin-xanthinoxidase system and non-enzymatically in a phenazine methosulfate-NADH system. Pyrogallol, gallic acid and its ester, were much more efficient in scavenging O2-* than catechol. The superiority of pyrogallol over catechol in the flavonoidal nucleus is apparent from the much higher O2-* scavenging activity of myricetin and epigallocatechin, which contain 3',4',5'-trihydroxyl substitution in the B-ring, compared to quercetin and epicatechin, which contain 3',4'-dihydroxyl substitution, respectively. The strong O2-* scavenging ability of pyrogallol appears to function even in the A-ring, as in baicalein, and also in the form of a pyrogalloyl ester at the C-3 position in the C-ring, as in epicatechin gallate and epigallocatechin gallate. It can be concluded that the pyrogallol moiety is an active component of flavonoids for displaying high O2-* scavenging activity. Flavonoids and aromatics were also examined to correlate their O2-* scavenging activity with their oxidizability, which was measured on the basis of electrochemical redox potential and the reducing ability of the Cu2+ ion. Aromatics such as pyrogallol, gallic acid and its ester, and flavonoids such as baicalein, epicatechin gallate and epigallocatechin gallate, in which the O2-* scavenging activity is enhanced by the presence of a pyrogallol moiety which does not belong to the B-ring, reduced the correlation between the higher O2-* scavenging activity and the lower redox potential. The O2-* scavenging activity was well correlated with the Cu2+ reducing ability of flavonoids and aromatics.

The Contribution of the Pyrogallol Moiety to the Superoxide Radical Scavenging Activity of Flavonoids.