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

Abstract

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 2.3×106 and a calculated standard entropy change of 88.2Jmol−1K−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(1Δg) 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(1Δg)) and superoxide radical anion (O2-) 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(1Δg) phosphorescence indicates that DHF and the complex react with O2(1Δg) and O2- by a non simple mechanism. The former deactivates O2(1Δg) 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(1Δg) quencher.