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

This paper describes the interaction between photochemically generated sing let molecular oxygen [O-2((1)Delta (g))] and a series of naturally occurrin g 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 compo unds as generators and quenchers of the known oxidative species O-2((1)Delt a (g)), upon direct and dye-sensitized photoirradiation, were investigated. Time-resolved phosphorescence detection, and conventional spectroscopic me asurements demonstrate that compounds I-IV are fairly good O-2((1)Delta (g) ) deactivators with overall rate constant for the process of quenching in t he range of 3.7 x 10(6) (IV) to 2.8 x 10(8) M(-1)s(-1) (I). Because the O-2 ((1)Delta (g)) quenching possesses an exclusively physical component, O-2(( 1)Delta (g)) scavenging is not accompanied by consumption of the flavonoid. Compounds I, III and IV are poor O-2((1)Delta (g)) generators upon direct (near UV) photoirradiation, whereas compound II does not generate any of th e oxidative species. Experiments measuring inhibition by flavonoids I-IV of the photosensitized lipid peroxidation employed linoleic acid as a photoxi dizable substrate and Rose Bengal and Methylene Blue as photosenstizers. Re sults demonstrate that the interesting properties of flavonoids I-IV as O-2 ((1)Delta (g)) scavengers cannot be straightforwardly extrapolated to their potential activity as photoprotectors for lipid peroxidation.