Free radicals produced by the oxidation of gallic acid: An electron paramagnetic resonance study.
Free radicals produced by the oxidation of gallic acid: An electron paramagnetic resonance study.
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DOI：
10.1186/1752-153x-4-15
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发表时间：
2010-08-05
影响因子：
--
通讯作者：
Buettner GR
中科院分区：
文献类型：
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作者：
Eslami AC;Pasanphan W;Wagner BA;Buettner GR
没食子酸（3，4，5-三羟基苯甲酸）存在于多种植物中;它广泛用于制革，油墨染料以及造纸。没食子酸酯部分是许多功能性植物化学物质的关键组分。本文采用电子顺磁共振（EPR）技术对没食子酸空气氧化过程中产生的自由基进行了研究。我们发现没食子酸产生两种不同的自由基作为pH的函数。在pH 7-10之间的范围内，没食子酸自由基的光谱是一个双峰的三重峰（aH = 1.00 G，aH = 0.23 G，aH = 0.28 G）。这与提供超精细分裂的三个氢一致。然而，在更碱性的环境中，pH >10，超精细分裂模式转变为1：2：1模式（aH（2）= 1.07 G）。使用D2 O作为溶剂，我们证明，第三个氢（即aH = 0.28 G）在较低的pH值是一个缓慢交换的氢，参与氢键与两个氧原子在邻位上的没食子酸环。该质子的pKa已被确定为10。这种简单而新颖的方法允许了解由没食子酸（一种普遍存在的化合物）产生的半醌自由基的亲质子平衡，从而对其氧化和随后的反应有新的见解。
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. 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 (aH = 1.00 G, aH = 0.23 G, aH = 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 (aH (2) = 1.07 G). Using D2O as a solvent, we demonstrate that the third hydrogen (i.e. aH = 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 pKa of this proton has been determined to be 10. 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.