Free radicals produced by the oxidation of gallic acid: An electron paramagnetic resonance study

被引:119
作者
Eslami, Angelique C. [1 ]
Pasanphan, Wanvimol [2 ]
Wagner, Brett A. [1 ]
Buettner, Garry R. [1 ]
机构
[1] Univ Iowa, Med Labs B180, Iowa City, IA 52242 USA
[2] Kasetsart Univ, Fac Sci, Dept Appl Radiat & Isotopes, Bangkok 10900, Thailand
来源
CHEMISTRY CENTRAL JOURNAL | 2010年 / 4卷
关键词
GALLATE;
D O I
10.1186/1752-153X-4-15
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
Background: Gallic acid (3,4,5-trihydroxybenzoic acid) is found in a wide variety of plants; it is extensively used in tanning, ink dyes, as well as in the manufacturing of paper. The gallate moiety is a key component of many functional phytochemicals. In this work electron paramagnetic spectroscopy (EPR) was used to detect the free radicals generated by the air-oxidation of gallic acid. Results: We found that gallic acid produces two different radicals as a function of pH. In the pH range between 7-10, the spectrum of the gallate free radical is a doublet of triplets (a(H) = 1.00 G, a(H) = 0.23 G, a(H) = 0.28 G). This is consistent with three hydrogens providing hyperfine splitting. However, in a more alkaline environment, pH > 10, the hyperfine splitting pattern transforms into a 1: 2: 1 pattern (a(H) (2) = 1.07 G). Using D2O as a solvent, we demonstrate that the third hydrogen (i.e. a(H) = 0.28 G) at lower pH is a slowly exchanging hydron, participating in hydrogen bonding with two oxygens in ortho position on the gallate ring. The pK(a) of this proton has been determined to be 10. Conclusions: This simple and novel approach permitted the understanding of the prototropic equilibrium of the semiquinone radicals generated by gallic acid, a ubiquitous compound, allowing new insights into its oxidation and subsequent reactions.
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页数:4
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