Table 3

Characteristics of flavonoid structure for most effective radical-scavenging activity

•  The catechol (O-dihydroxy) group in the B ring confers great scavenging ability, with exceptions such as those described by Ratty and Das (1983), who thought it did not contribute towards lipid peroxidation in rat brain mitochondria.
•  A pyrogallol (trihydroxy) group in ring B of a catechol, as in myricetin, produces even higher activity. The C2-C3 double bond of the C ring appears to increase scavenger activity because it confers stability to the phenoxy radicals produced.
•  The 4-oxo (keto double bond at position 4 of the C ring), especially in association with the C2-C3 double bond, increases scavenger activity by delocalizing electrons from the B ring.
•  The 3-OH group on the C ring generates an extremely active scavenger; in fact, the combination of C2-C3 double bond and 4-oxo group appears to be the best combination on top of the catechol group.
•  The 5-OH and 7-OH groups may also add scavenging potential in certain cases.