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 (O2-) generated from 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 O2- 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 O2- scavenging activities. In order to clarify the physico-chemical property of these phenolic compounds which determines their O2- scavenging activities, we investigated the effects of six 4-substituents of pyrocatechol on O2- scavenging effects. The effects of these substituents were well correlated with their redox potentials, indicating that O2- scavenging activities of catechol derivatives are controlled by their electron donor activities. We also observed the scavenging effects of these phenolic compounds on O2- from a different source: potassium superoxide (KO2) 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 O2-. The results revealed that phenolic compounds directly react with O2- to form stable radicals, which account for the O2- scavenging activities of the compounds.