Free radicals generated during oxidation of green tea polyphenols: Electron paramagnetic resonance spectroscopy combined with density functional theory calculations
Author: Joyce Ferreira Severino and Bernard A. Goodman and Christopher W.M. Kay and Klaus Stolze and Daniel Tunega and Thomas G. Reichenauer and Katharina F. Pirker
Electron paramagnetic resonance spectroscopy and density functional theory calculations have been used to investigate the redox properties of the green tea polyphenols (GTPs) (−)-epigallocatechin gallate (EGCG), (−)-epigallocatechin (EGC), and (−)-epicatechin gallate (ECG). Aqueous extracts of green tea and these individual phenols were autoxidized at alkaline pH and oxidized by superoxide anion (O2−) radicals in dimethyl sulfoxide. Several new aspects of the free radical chemistry of GTPs were revealed. EGCG can be oxidized on both the B and the D ring. The B ring was the main oxidation site during autoxidation, but the D ring was the preferred site for O2− oxidation. Oxidation of the D ring was followed by structural degradation, leading to generation of a radical identical to that of oxidized gallic acid. Alkaline autoxidation of green tea extracts produced four radicals that were related to products of the oxidation of EGCG, EGC, ECG, and gallic acid, whereas the spectra from O2− oxidation could be explained solely by radicals generated from EGCG. Assignments of hyperfine coupling constants were made by DFT calculations, allowing the identities of the radicals observed to be confirmed.