硫醇酸解法中木质素降解机理和提高木质素结构分析的准确度研究

As an important analytical method for characterization of lignin, thioacidolysis has been successfully applied to a wide array of lignocellulosic materials and isolated lignins in various studies related to the biosynthesis of monolignols, biological processing of plant biomass, and the pulping industry. Dimeric products released from analytical thioacidolysis by selectively cleaving β-aryl ethers reflect the distribution of recalcitrant lignin units, such as β–5-, β–β-, 5–5-, β–1-, 4–O–5-linkages. In view of lignins’ complexity and heterogeneity, the reaction pathways of condensed lignin structures in thioacidolysis followed by Raney nickel desulfurization remains unclear. In addition, there are still some dimeric products that haven’t been identified or quantitated due to the lack of authenticated compounds. In this study, a series of dimeric or tetrameric model compounds representing condensed structures in lignins including β–5-, β–β-, β–1-, 5–O–4-, and 5–5-linked dimers or tetramers will be designed and synthesized. By using each of these model com¬pounds as substrates, thioacidolysis will be performed and the degradation products after Raney nickel desulfurization will be analyzed by GC-MS and NMR. Based on results from these model study, where all those lignin-derived thioacidolysis dimers of a lignin-containing sample come from will be confirmed, and the mechanism of thioacidolysis reactions on lignins’ condensed structures will be elucidated. Subsequently, a serial of authenticated dimers will be synthesized and used as standards for the reevaluation and quantification of lignin-derived dimeric products released from thioacidolysis. With the aid of authenticated dimeric compounds, the accuracy of lignin structural analysis will be greatly improved. Most accurate lignin structural information will be beneficial for better evaluation of biological processing for plant breeding, genetic modification etc., and thereby the utilization of lignocellulosic biomass.

硫醇酸解法是一种重要的木质素结构分析方法，广泛应用于木质素单体的生物合成、植物生理机制以及制浆等研究。经硫醇酸解法选择性断裂木质素中的β芳基醚键释放出的二聚体结构，反映了木质素中“缩合”单元（β–5-、β–β-、5–5-、β–1-和4–O–5-等）结构信息。由于木质素结构复杂，硫醇酸解法对木质素的降解机制尚不完善；同时，缺少标准物影响了硫醇酸解法对二聚体产物定性与定量的准确性。本研究将合成一系列具有代表性的“缩合”二聚体或四聚体模型物作为反应底物进行硫醇酸解法降解研究，采用GC-MS和NMR分析手段对产物进行解析；对比木质素降解产物，确认二聚体产物的来源，阐明硫醇酸解法中木质素降解的作用机制；指导合成二聚体标准物，对木质素二聚体产物进行重评估和定量，提高木质素结构分析的准确度。木质素不同结构单元的有效鉴定和准确定量，可对林业育种、基因改造等手段进行有效评估，指导植物纤维原料的开发利用。

作为一种重要的木质素结构分析方法，硫醇酸解法广泛应用于木质素生物合成、植物的生理机制以及制浆等相关研究。由于木质素结构复杂、不均一，在硫醇酸解法降解木质素过程中产生的二聚体产物种类繁多、难分离，且缺少相应的标准物，致使木质素二聚体结构单元的定性和定量一直是相关研究中长期存在的难题。针对上述问题，本项目设计、合成了一系列具有结构代表性的G型或S/G型（β–5、β–β、5–5、β–1或4–O–5等）木质素模型物与标准物，并应用木质素模型物进行了硫醇酸解降解产物鉴定与结构单元定量的研究，阐明了硫醇酸解法中木质素特征二聚体碎片产物的主要结构来源与形成机制，建立了快捷、高灵敏度的硫醇酸解法/多反应监测模式（MRM）的木质素单体、二聚体基本结构单元的定量体系，提升了对木质素结构单元定性与定量解析的准确度与灵敏性。S/G型木质素模型物合成过程中，首次在芥子醇（SA）与阿魏酸乙酯（FA）体外自由基仿生系统中鉴定出了SA-(β-O-4)-FA结构，纠正了以往报导对SA自身偶合产物SA-(β-O-4)-SA得率的高估；发现了一种半开环结构的丁香脂醇产物以及由SA-(β-5)-FA结构脱氢形成的新型化合物，增加了对于禾本科植物细胞壁木质化过程及木质素二聚体键型连接信息的认知；发现4-O-5和5-5模型物经硫醇酸解法处理后会发生侧链断裂的现象，确认了部分或全部4-O-5和5-5结构的侧链“残缺”产物来自于正常侧链结构；闭环的β-5二聚体硫醇酸解产物形成于雷尼镍脱硫反应过程中，而非硫醇酸解处理过程；应用快捷、灵敏的硫醇酸解法/ MRM定量体系，揭示了拟南芥与毛白杨全生长周期的木质素结构变化规律等；发表学术论文12篇（SCI论文11篇、中文核心论文1篇）、英文专著1部等，研究成果为木质素新结构的解译、木质纤维原料评价及高值转化利用提供了重要的依据。