Efficiency of natural phenolic compounds regenerating alpha-tocopherol from alpha-tocopheroxyl radical

Benzoic acid-derived phenolics (p-hydroxybenzoic acid, 3,4-dihydroxybenzoic acid, vanillic acid, syringic acid, and gallic acid) and the polyphenols epicatechin and epigallocatechin gallate (EGCG) were evaluated for their efficiency in regenerating alpha-tocopherol from alpha-tocopheroxyl radical in comparison with ascorbyl palmitate, which is known to regenerate alpha-tocopherol. Ethanolic solutions of phenolic compounds were added to a homogeneous hexane medium containing alpha-tocopheroxyl radical generated by reaction of alpha-tocopherol in molar excess with 1,1-diphenyl-2-picrylhydrazyl radical, and the alpha-tocopheroxyl radical was monitored by electron spin resonance spectroscopy. p-Hydroxybenzoic acid, vanillic acid, and syringic acid (400 microM) did not exhibit a significant effect on alpha-tocopheroxyl radical concentration (0.6-0.7 microM). In contrast, 3,4-dihydroxybenzoic acid and gallic acid were able to reduce the concentration of alpha-tocopheroxyl radical by 16 and 64%, respectively. Epicatechin showed a reduction of alpha-tocopheroxyl radical similar to gallic acid, and EGCG and ascorbyl palmitate were the most effective, reducing alpha-tocopheroxyl radical completely at a much lower phenolic concentration (66.7 microM). The moles of alpha-tocopheroxyl radical reduced per mole of ascorbyl palmitate (0.93), EGCG (0.066), gallic acid (4.3 x 10(-4)), and epicatechin (4.5 x 10(-4)) were determined, and the logarithm of these stoichoimetric ratios showed a negative linear relationship with the bond dissociation enthalpies of the O-H bond of the phenolics. The relative capacity to reduce alpha-tocopheroxyl radical was found to be ascorbyl palmitate (2142) > EGCG (151) > gallic acid (1) approximately epicatechin (1).