A comparative photochemical study on the behavior of 3,30 -dihydroxyflavone and its complex with La(III) as generators and quenchers of reactive oxygen species

Abstract

A 1:1 complex between 3,30-dihydroxyflavone (DHF) and La(III) (DHF?La(III)) is formed in methanolic solution with the relatively high apparent stability constant value of 2.3e6 and a calculated standard entropy change of 88.2 J mol-1 K-1, 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 O2(1Dg) 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 (O2(1Dg)) and superoxide radical anion (O 2 ) 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 O2(1Dg) phosphorescence indicates that DHF and the complex react with O2(1Dg) and O 2 by a non simple mechanism. The former deactivates O2(1Dg) 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 O2(1Dg) quencher.