EFFECT OF FLAVONOIDS ON THE PHOTOOXIDATION OF FATS - A STUDY ON THEIR ACTIVITY AS SINGLET MOLECULAR-OXYGEN [O-2((1)DELTA-G)] GENERATORS AND QUENCHERS

The protective effect of the polyhydroxyflavones Fisetin and Baicalein on the sensitized [O-2((1) Delta(g))] - 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 O-2((1) Delta(g)). The rate constants for such a process of quenching were 1.9 and 1.4 x 10(8) M(-1)s(-1) 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 O-2((1) Delta(g)) quenching. The presence of the -OH groups in the aromatic rings of the hydroxyflavones confer to these compounds the ability as O-2((1) Delta(g)) quenchers. The parent compound Flavone does not quench that oxy gen excited species. Only Flavone generates O-2((1) Delta(g)) 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.