Impact of the phenolic O-H vs. C-ring C-H bond cleavage on the antioxidant potency of dihydrokaempferol

Kinetics and thermodynamics of the phenolic O-H and the C-ring C-H bond breaking in dihydrokaempferol, a natural flavonoid compound, was investigated using a density functional based approach. The rate constants calculated using the variational transition state theory with multidimensional tunneling corrections indicate 4 '-OH phenolic hydrogen as operative in the inactivation of a hydroperoxyl radical (OOH). The proton coupled electron transfer (PCET) mechanism appears as preferred in O-H bond breaking by OOH, while the hydrogen atom transfer (HAT) mechanism is involved in the C-H bond cleavage. Comparison of the antioxidant potency of dihydrokaempferol vs. kaempferol proves the superiority of the latter and the negligible role of the C-ring hydrogens of dihydrokaempferol in radical scavenging. The obtained results confirm the traditional view that O-H groups of polyphenols are responsible for radical inactivation contrary to recent proposals that the aliphatic C-ring hydrogens also contribute to this activity.