Studies on Inhibition Mechanism of Autoxidation by Tannins and Flavonoids: V: Radical-Scavenging Effects of Tannins and Related Polyphenols on l,l-Diphenyl-2-picrylhydrazyl Radical

Radical scavenging effects of tannins and related polyphenols on l,l-diphenyl-2-picrylhydrazyl (DPPH) radical were evaluated by colorimetry. All of the polyphenols examined showed effects much stronger than that of a-tocopherol. The hydrolyzable tannins having galloyl groups in the molecule exhibited stronger effects than those having modified galloyl groups, such as hexahydroxydiphenoyl (HHDP), dehydrohexahydroxydiphenoyl (DHHDP) and chebuloyl groups. (-)-Epigallocatechin gallate, (-)-epigallocatechin, (-)-epicatechin gallate and methyl gallate also showed fairly significant effects, even though they are small molecules. The predominant reaction products upon the treatment of various alkyl gallates with DPPH radical on a preparative scale were dialkyl hexahydroxydiphenates, which should be formed by mutual coupling of C-centered galloyl radicals. Evidence for the formation of the alkyl gallate radicals was also obtained by the electron spin resonance spectroscopy.