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Susana C r i a d o , Sonia G. B e r t o l o t t i , A . T S o l t e r m a n n , I/: A v i l a a n d N . A . G a r c i a *The protective effect of the polyhydroxyflavones Fisetin and Baicalein on the sensitized [Oz('$)]mediated photooxidation of fats was investigated through a kinetic study. These flavonoids at concentrations in the order of a few ppm efficiently inhibit the photoperoxidation of linoleic acid, which was chosen as an example of photooxidizable fat. This property was attributed to the ability of Fisetin and Baicalein to quench photochemically generated Oz('4). The rate constants for such a process of quenching were 1.9 and 1.4 x lo8 M -k ' for Fisetin and Baicalein respectively. The ratio between the overall and the chemical rate constants were in the order of 0.01, indicating that the flavones are practically not consumed in the process of Oz('4) quenching. The presence of the -OH groups in the aromatic rings of the hydroxyflavones confer to these compounds the ability as Oz ('4) quenchers. The parent compound Flavone does not quench that oxygen excited species. Only Flavone generates Oz('4) upon direct irradiation, at 337 nm. in the absence of added sensitizers, with a quantum yield of 0.16. Fisetin and Baicalein were totally inefficient for such a process. This fact constitutes a very convenient property from the point of view of the protective (anti oxidative) activity of the hydroxyflavones. Fat Sci. Technol. 97. Jahrgang Nr. 718 lYY5 Fat SCI. Technol. 97. Jahrgang Nr. 718 1995

This paper describes the interaction between photochemically generated singlet molecular oxygen [O 2 ( 1 D g )] and a series of naturally occurring flavonoids (FL) (29,49-dihydroxychalcone (I), 29,49-dihydroxy, 39-methoxy chalcone (II), 7-hydroxyflavanone (III) and 7-hydroxy-8methoxyflavanone (IV) all obtained from Zuccagnia punctata Cav. The properties of these compounds as generators and quenchers of the known oxidative species O 2 ( 1 D 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 D g ) deactivators with overall rate constant for the process of quenching in the range of 3.7 x10 6 (IV) to 2.8 310 8 M 21 s 21 (I). Because the O 2 ( 1 D g ) quenching possesses an exclusively physical component, O 2 ( 1 D g ) scavenging is not accompanied by consumption of the flavonoid. Compounds I, III and IV are poor O 2 ( 1 D g ) generators upon direct (near UV) photoirradiation, whereas compound II does not generate any of the oxidative species. Experiments measuring inhibition by flavonoids I-IVof 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 D g ) scavengers cannot be straightforwardly extrapolated to their potential activity as photoprotectors for lipid peroxidation.

The visible-light irradiation of the system Rivoflavin plus 3-hydroxyflavone or plus 7-hydroxyflavone, under aerobic conditions, produces a series of competitive processes that depend on the relative concentrations of the pigment and the flavones. The picture comprises photochemical mechanisms that potentially operate in nature. They mainly include the quenching of Rf singlet (1Rf*) and triplet (3Rf*) excited states (with bimolecular rate constants in the order of 109 M–1 s–1) and superoxide radical anion-mediated reactions. The participation of the oxidative species singlet molecular oxygen was not detected. The overall result shows chemical transformations in both Rf and 3-hydroxyflavone. No experimental evidence was found indicating any chemical reaction involving 7-hydroxyflavone. The fate of the pigment also depends on the amount of the dissolved flavonoid. At 50 mM concentrations of these compounds or higher, practically no photochemistry occurs, owing to the extensive quenching of 1Rf*. When the concentration of the flavones is in the mM range or lower, 3Rf* is photogenerated. Then, the excited triplet species can be quenched mainly by the flavones through an electron-transfer process, yielding the semireduced pigment. The latter interacts with dissolved oxygen producing O2·–, which reacts with both the pigment and 3-hydroxyflavone. In summary, 3-hydroxyflavone and 7-hydroxyflavone participate in the generation of superoxide ion in an Rf-sensitized process, and simultaneously 3-hydroxyflavone constitutes a degradable quencher of the oxidative species.Key words: flavones, riboflavin, sensitization, singlet molecular oxygen, superoxide radical anion.