Eslami Angelique C, Pasanphan Wanvimol, Wagner Brett A, Buettner Garry R
Free Radical and Radiation Biology, Med Labs B180, The University of Iowa, Iowa City, IA 52242-1181, USA.
Chem Cent J. 2010 Aug 5;4:15. doi: 10.1186/1752-153X-4-15.
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.
没食子酸（3,4,5 - 三羟基苯甲酸）存在于多种植物中；它广泛应用于鞣革、油墨染料以及纸张制造。没食子酸酯部分是许多功能性植物化学物质的关键成分。在这项工作中，电子顺磁共振光谱（EPR）被用于检测没食子酸空气氧化产生的自由基。
我们发现没食子酸根据pH值产生两种不同的自由基。在pH值为7 - 10的范围内，没食子酸酯自由基的光谱是三重态的双重峰（aH = 1.00 G，aH = 0.23 G，aH = 0.28 G）。这与三个氢提供超精细分裂一致。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D2O作为溶剂，我们证明在较低pH值下的第三个氢（即aH = 0.28 G）是一个缓慢交换的质子，参与与没食子酸酯环上邻位的两个氧形成氢键。该质子的pKa已确定为10。
这种简单而新颖的方法有助于理解由没食子酸产生的半醌自由基的质子转移平衡，没食子酸是一种普遍存在的化合物，为其氧化及后续反应提供了新的见解。
