A comparative photochemical study on the behavior of 3,3´-dihydroxyflavone and its complex with La(III) as generators and quenchers of reactive oxygen species

GABRIELA V. FERRARI; M. PAULINA MONTAÑA; FRIDA C. D. DIMARCO; NORA B. DEBATTISTA; NORA B. PAPPANO; WALTER MASSAD; NORMAN A. GARCÍA

JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY B-BIOLOGY

ELSEVIER SCIENCE SA

Lugar: Amsterdam; Año: 2013 vol. 124 p. 42 - 49

1011-1344

A 1:1 complex between 3,3´-dihydroxyflavone (DHF) and La(III) (DHF?La(III)) is formed in methanolic solution with the relatively high apparent stability constant value of 2.3x10^6 and a calculated standard entropy change of 88.2 J/(mol K), both at 25 C. The photophysical properties of the complex and the free flavonoid are discussed in comparison to the well known related compound 3-hydroxyflavone. The ligand photogenerates singlet molecular oxygen by energy transfer from its excited triplet state (3DHF) to dissolved ground state oxygen, with a quantum yield of 0.13. 3DHF is quenched by La(III) with a rate constant close to the diffusion-controlled value. The respective abilities of the free flavonoid and DHF?La(III) as quenchers of the riboflavin-photogenerated reactive oxygen species singlet molecular oxygen and superoxide radical anion have been investigated. Both individual compounds were photoirradiated with visible light in the presence of the flavin as the only light-absorbing compound. A detailed kinetics and mechanistic study employing polarographic monitoring of oxygen uptake and time resolved detection of singlet molecular oxygen phosphorescence indicates that DHF and the complex react with singlet molecular oxygen and superoxide radical anion by a non simple mechanism. The former deactivates singlet molecular oxygen in a predominant physical fashion, a fact that constitutes a desirable property for antioxidants. It was found that metal chelation greatly enhances the ability of DHF as an overall singlet molecular oxygen quencher.