Singlet Oxygen-mediated Hydroxyl Radical Production in the Presence of Phenols: Whether DMPO-·OH Formation Really Indicates Production of ·OH?¶

The reaction of singlet oxygen (1O2) generated by ultraviolet-A (UVA)–visible light (λ > 330 nm) irradiation of air-saturated solutions of hematoporphyrin with phenolic compounds in the presence of a spin trap, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), gave an electron spin resonance (ESR) spectrum characteristic of the DMPO–hydroxyl radical spin adduct (DMPO–·OH). In contrast, the ESR signal of 5,5-dimethyl-2-pyrrolidone-N-oxyl, an oxidative product of DMPO, was observed in the absence of phenolic compounds. The ESR signal of DMPO–·OH decreased in the presence of either a ·OH scavenger or a quencher of 1O2 and under anaerobic conditions, whereas it increased depending on the concentration of DMPO. These results indicate both 1O2- and DMPO-mediated formation of free ·OH during the reaction. When DMPO was replaced with 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline-N-oxide (DEPMPO), no DEPMPO adduct of oxygen radical species was obtained. This suggests that 1O2, as an oxidizing agent, reacts little with DEPMPO, in which a strong electron-withdrawing phosphoryl group increases the oxidation potential of DEPMPO compared with DMPO. A linear correlation between the amounts of DMPO–·OH generated and the oxidation potentials of phenolic compounds was observed, suggesting that the electron-donating properties of phenolic compounds contribute to the appearance of ·OH. These observations indicate that 1O2 reacts first with DMPO, and the resulting DMPO–1O2 intermediate is immediately decomposed/reduced to give ·OH. Phenolic compounds would participate in this reaction as electron donors but would not contribute to the direct conversion of 1O2 to ·OH. Furthermore, DEPMPO did not cause the spin-trapping agent–mediated generation of ·OH like DMPO did.