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

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 x 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 ofthe 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.