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

• Published in: New journal of chemistry 2021-05, Vol.45 (18), p.7977-7986
• Main Authors: Ami, Ana, Milenkovi, Dejan, Markovi, Zoran, Cagardová, Denisa, Rodríguez-Guerra Pedregal, Jaime, Dimitri Markovi, Jasmina M
• Format: Article
• ISSN: 1144-0546; 1369-9261
• DOI: 10.1039/d1nj00690h

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 (&z.rad;OOH). The proton coupled electron transfer (PCET) mechanism appears as preferred in O-H bond breaking by &z.rad;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. In order to correctly estimate hydrogen atom abstraction from polyphenols, kinetic analysis including suitable tunneling effects should be mandatory.