Efficiency of Natural Phenolic Compounds Regenerating alpha-Tocopherol from alpha-Tocopheroxyl Radical

摘要

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 a-tocopherol from a-tocopheroxyl radical in comparison with ascorbyl palmitate, which is known to regenerate a-tocopherol. Ethanolic solutions of phenolic compounds were added to a homogeneous hexane medium containing a-tocopheroxyl radical generated by reaction of alpha-tocopherol in molar excess with 1,1-diphenyl-2-picry!hydrazyl radical, and the a-tocopheroxyl radical was monitored by electron spin resonance spectroscopy, p-Hydroxybenzoic acid, vanillic acid, and syringic acid (400 mu M) did not exhibit a significant effect on a-tocopheroxyl radical concentration (0.6-0.7 mu M). In contrast, 3,4-dihydroxybenzoic acid and gallic acid were able to reduce the concentration of a-tocopheroxyl radical by 16 and 64%, respectively. Epicatechin showed a reduction of a-tocopheroxyl radical similar to gallic acid, and EGCG and ascorbyl paimitate were the most effective, reducing alpha-tocopheroxyl radical completely at a much lower phenolic concentration (66.7 alpha M). The moles of a-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 a-tocopheroxyl radical was found to be ascorbyl palmitate (2142) > EGCG (151) > gallic acid (1) approx= epicatechin (1).