Antioxidant properties of natural flavonoids: quenching and generation of singlet molecular oxygen

Abstract

Summary This paper describes the interaction between photochemically generated singlet molecular oxygen [O 2 ( 1 Δ g )] and a series of naturally occurring flavonoids (FL) (2′,4′‐dihydroxychalcone ( I ), 2′,4′‐dihydroxy, 3′‐methoxy chalcone ( II ), 7‐hydroxyflavanone ( III ) and 7‐hydroxy‐8‐methoxyflavanone ( IV ) all obtained from Zuccagnia punctata Cav. The properties of these compounds as generators and quenchers of the known oxidative species O 2 ( 1 Δ g ), upon direct and dye‐sensitized photoirradiation, were investigated. Time‐resolved phosphorescence detection, and conventional spectroscopic measurements demonstrate that compounds I‐IV are fairly good O 2 ( 1 Δ g ) deactivators with overall rate constant for the process of quenching in the range of 3.7 x 10 6 ( IV ) to 2.8 × 10 8 M −1 s −1 ( I ). Because the O 2 ( 1 Δ g ) quenching possesses an exclusively physical component, O 2 ( 1 Δ g ) scavenging is not accompanied by consumption of the flavonoid. Compounds I, III and IV are poor O 2 ( 1 Δ g ) generators upon direct (near UV) photoirradiation, whereas compound II does not generate any of the oxidative species. Experiments measuring inhibition by flavonoids I‐IV of the photosensitized lipid peroxidation employed linoleic acid as a photoxidizable substrate and Rose Bengal and Methylene Blue as photosenstizers. Results demonstrate that the interesting properties of flavonoids I‐IV as O 2 ( 1 Δ g ) scavengers cannot be straightforwardly extrapolated to their potential activity as photoprotectors for lipid peroxidation.