Reducing Activity of Polyphenols with Stable Radicals of the TTM Series. Electron Transfer versus H-Abstraction Reactions in Flavan-3-ols
摘要:
[GRAPHICS]A new method to test the antioxidant activity of polyphenols by electron transfer reactions to a stable organic free radical, tris(2,4,6-trichloro-3,5-dinitrophenyl)methyl radical (HNTTM), is reported. Therefore, the activity of the natural flavanols, (-)-epicatechin, and two synthetic derivatives, 4beta-(S-cysteinyl)epicatechin and 4beta-(2-aminoethylthio)epicatechin, can be differentiated by their capacity to transfer hydrogen atoms to 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) and to transfer electrons to HNTTM.
The reducing activity of simple polyphenols (PhOH), catechol and pyrogallol, is tested in different solvents in front of tris(2,4,6-trichloro-3,5-dinitrophenyl)methyl (HNTTM) radical, a stable organic free radical of the TTM series. HNTTM radical is very active in electron-transfer reactions to give a very stable anion. The standard potential for the reduction of HNTTM radical by cyclic voltammetry in different solvents is E degrees = 0.60 +/- 5 V vs SCE. In hydroxylic solvents, the electron transfer is a very rapid process and the electron-donating species is the ionized PhO-, whereas in nonpolar solvents, it is suggested that the electron transfer is facilitated by the formation of an inter-mediate complex between HNTTM and PhOH.