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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 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 inactivation of hydroperoxyl radical (•OOH). Proton coupled electron transfer (PCET) mechanism appears as preferred in O−H bond breaking by the •OOH, while hydrogen atom transfer (HAT) mechanism is involved in the C−H bond cleavage. Comparison of antioxidant potency of dihydrokaempferol vs kaempferol proves superiority of the latter and negligible role of the C-ring hydrogens of dihydrokaempferol in radical scavenging. 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.

flavonoids ; computational chemistry ; radical scavenging ; kinetics ; tunneling

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