Efficiency of Natural Phenolic Compounds Regenerating α-Tocopherol from α-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 α-tocopherol from α-tocopheroxyl radical in comparison with ascorbyl palmitate, which is known to regenerate α-tocopherol. Ethanolic solutions of phenolic compounds were added to a homogeneous hexane medium containing α-tocopheroxyl radical generated by reaction of α-tocopherol in molar excess with 1,1-diphenyl-2-picrylhydrazyl radical, and the α-tocopheroxyl radical was monitored by electron spin resonance spectroscopy. p-Hydroxybenzoic acid, vanillic acid, and syringic acid (400 μM) did not exhibit a significant effect on α-tocopheroxyl radical concentration (0.6−0.7 μM). In contrast, 3,4-dihydroxybenzoic acid and gallic acid were able to reduce the concentration of α-tocopheroxyl radical by 16 and 64%, respectively. Epicatechin showed a reduction of α-tocopheroxyl radical similar to gallic acid, and EGCG and ascorbyl palmitate were the most effective, reducing α-tocopheroxyl radical completely at a much lower phenolic concentration (66.7 μM). The moles of α-tocopheroxyl radical reduced per mole of ascorbyl palmitate (0.93), EGCG (0.066), gallic acid (4.3 × 10^-4), and epicatechin (4.5 × 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 α-tocopheroxyl radical was found to be ascorbyl palmitate (2142) > EGCG (151) > gallic acid (1) ≈ epicatechin (1).

Keywords: α-Tocopherol regeneration; α-tocopheroxyl radical; synergism; benzoic acid-derived phenolics; polyphenols