A deeper insight on the radical scavenger activity of two simple coumarins toward OOH radical

The antioxidant properties of 7-hyd roxy-5-methoxy-4-methyl-2H-chromen-2-one (system a ) and 5-hydroxy-7-methoxy-4-methyl-2H-chromen-2-one (systemb ) have been explored and rationalized by densityfunctional theory. Quantum mechanics based test for overall free radical scavenging activity (QMORSA)protocol has been employed to account for their radical-scavenging capacity against hydroperoxylradical in aqueous and lipid solutions. Three different reaction mechanisms have been considered. Theyare the single electron transfer (SET), the hydrogen atom transfer (HAT) and the radical adduct formation(RAF). From both, thermodynamic and kinetic points of view, our calculatio ns suggest that HAT is themost favored reaction mechanism for the free radical scavenging activity of the studied systems.Kinetic rate constants indicate that system a is a more efficient antiox idant than system b in nonpolarmedia, while the opposite trend is found in aqueous solution. These results indicate that boththe polarity of the environmen t, and the position of the methoxy and hydroxy group s in the coumarinstructure influence the activity of this kind of compounds.