Scavenging Effects of Dihydric and Polyhydric Phenols on Superoxide Anion Radicals, Studied by Electron Spin Resonance Spectrometry

We investigated the scavenging effects of dihydric, trihydric phenols and related polyphenols on superoxide anion radicals (O^-_2) generated form the hypoxanthine-xanthine oxidase system by ESR (electron spin resonance) spin trapping technique using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a trapping reagent.The O^-_2 scavenging activities of non-substituted dihydric phenols were in the order of pyrocatechol≫hydroquinone≫resorcinol.Trihydric phenols, pyrogallol and gallic acid, which have phenolic OH groups at ortho positions, had stronger activities than pyrocatechol; phloroglucinol, on the other hand, which has phenolic OH groups at meta positions, had much weaker effect. Polyhydric phenols like quercetin and hematoxylin, which have catechol groups, had significant O^-_2 scavenging activities. In order to clarify the physico-chemical property of these phenolic compounds which determines their O^-_2 scavenging acivities, we investigated the effects of six 4-substituents of pyrocatechol on O^-_2 scavenging effects. The effects of these substituents were well correlated with their redox potentials, indicating that O^-_2 scavenging activities of catechol derivatives are controlled by their electron donor activities.We also observed the scavenging effects of these phenolic compounds on O^-_2 from a defferent source : potassium superoxide (KO_2) solubilized in dimethyl sulfoxide by 18-crown-6. ESR analysis in this system revealed that one-electron oxidized products of these compounds were concomitantly formed accompanied by the reduction of O^-_2. The results revealed that phenolic compounds directly react with O^-_2 to form stable radicals, which account for the O^-_2 scavenging activities of the compounds.