Free radicals produced by the oxidation of gallic acid and catechin derivatives

Abstract

Gallic acid and catechin derivatives were oxidized in alkaline solution (pH 10.0–13.0) with K₃[Fe(CN)₆] under anaerobic conditions, and electron spin resonance (ESR) spectra of the radicals produced were measured. Gallic acid, epicatechin gallate, gallocatechin gallate, epigallocatechin, and epigallocatechin gallate showed hyperfine structures. Gallic acid was found to be oxidatively C—O coupled in alkaline solution (pH 10.5–12.0). It was found that an unpaired electron delocalized over gallocatechin gallate and epigallocatechin molecules, but was localized on the galloyl group and the A-ring of epicatechin gallate and epigallocatechin gallate. The galloyl group of gallo-catechin gallate was readily alkali-hydrolyzed but those of epicatechin gallate and epigallocatechin gallate were resistant to alkaline hydrolysis.