Radical Coupling Reactions in Lignin Synthesis: A Density Functional Theory Study
Radical Coupling Reactions in Lignin Synthesis: A Density Functional Theory Study
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DOI：
10.1021/jp2122449
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发表时间：
2012-04-26
影响因子：
3.3
通讯作者：
Smith, Jeremy C.
中科院分区：
文献类型：
--
作者：
Sangha, Amandeep K.;Parks, Jerry M.;Smith, Jeremy C.
木质素是植物细胞壁中的复杂的非均质聚合物，其为植物茎提供机械强度并赋予对降解微生物、酶和化学品的抗性。木质素合成通过单木质醇的氧化自由基-自由基偶联开始，其中最常见的是对香豆醇、松柏醇和芥子醇。在这里，我们使用密度泛函理论来表征自由基-自由基偶联反应中所涉及的单分子二聚。我们计算这些monolamel自由基的初始自我和交叉偶联反应，形成二聚体中间体通过六个主要的连接在天然木质素的反应mixtures。计算出8-O-4、8-8和8-5偶联是最有利的，而5-O-4、5-5和8-1键不太有利。总的来说，对香豆酰自身和交叉偶联反应计算是最有利的。对于交叉偶联反应，其中每个自由基可以通过参与二聚体形成的两个位点中的任一个进行偶联，发现两个自由基中更具反应性的在其具有最高自旋密度的位点处进行偶联。
Lignin is a complex, heterogeneous polymer in plant cell walls that provides mechanical strength to the plant stem and confers resistance to degrading microbes, enzymes, and chemicals. Lignin synthesis initiates through oxidative radical-radical coupling of monolignols, the most common of which are p-coumaryl, coniferyl, and sinapyl alcohols. Here, we use density functional theory to characterize radical-radical coupling reactions involved in monolignol dimerization. We compute reaction enthalpies for the initial self- and cross-coupling reactions of these monolignol radicals to form dimeric intermediates via six major linkages observed in natural lignin. The 8-O-4, 8-8, and 8-5 coupling are computed to be the most favorable, whereas the 5-O-4, 5-5, and 8-1 linkages are less favorable. Overall, p-coumaryl self- and cross-coupling reactions are calculated to be the most favorable. For cross-coupling reactions, in which each radical can couple via either of the two sites involved in dimer formation, the more reactive of the two radicals is found to undergo coupling at its site with the highest spin density.