Singlet Oxygen Photo-oxidation of Flavonoids in Etanol Solution

F.E. MORÁN VIEYRA; C.D. BORSARELLI; C. ZAMPINI; R.M. ORDOÑEZ; M.I. ISLA

Vaquerias, Cordoba

Simposio; 4to ENCUENTRO NACIONAL DE INVESTIGADORES EN TEMAS RELACIONADOS CON SUSTANCIAS PEROXIDICAS; 2006

Universidad Nacional de Cordoba

The kinetic of the O2(1Dg)-photosensitized oxidation of several naturally occurring flavonoids (glycosilated or not) in neutral and alkaline ethanol solutions was studied using Rose Bengal as photosensitizer. The rate constants of O2(1Dg) by the flavonoids for the chemical quenching (kr) were determined using either UV-Vis absorption spectroscopy or HPLC techniques, and for the total (physical + chemical) quenching (kt) were determined using time-resolved phosphorescence detection of O2(1Dg) at 1270 nm. Considering the flavonoid classification, the O2(1Dg) reactivity order was flavone > chalcone > flavanone, and in all cases the reactivity was at least one-order of magnitude larger in alkaline than in neutral ethanol. Furthermore, for the same type of flavonoid the O2(1Dg) reactivity was increased with the number of –OH groups and the presence of glycoside substituent. A correlation between the values of kr and kt and the reactivity against radical species, such as DPPHŸ and ABTSŸ+, of flavone, chalcone and flavanone present in Zuccagnia punctata Cav. was found, indicating parallel antioxidant properties of the flavonoids against either radical or neutral oxidant species. The effect on O2(1Dg)-reactivity and the photo-oxidation product pattern of the carbanion mediated flavanone-chalcone isomerization equilibrium in alkaline media was also analysed. These results are expected to have significance in biosynthesis, antioxidant properties and stability of naturally occurring flavonoids. Thanks are given to ANPCyT, CONICET, SECyT-UNSE, and SECyT- UNT for financial support.