Department of Chemistry, University of Kentucky, Lexington, KY, 40506, USA.
Anal Bioanal Chem. 2022 Aug;414(19):5755-5771. doi: 10.1007/s00216-022-04111-6. Epub 2022 Jun 3.
Structural elucidation of lignin degradation products is a requirement for successfully developing lignin valorization technology. Most of mass spectrometry-based techniques have utilized negative ion mode mass spectrometry for structural elucidation of lignin-derived compounds. Unfortunately, simple deprotonation can lead to in-source fragmentation and may not be suitable for condensed lignin structures without acidic moieties. Herein, we present a lithium cationization methodology for mass spectrometry sequencing of advanced lignin oligomers having β-β' and β-O-4' bonding motifs. To do so, two advanced lignin oligomers were first synthesized through a step-by-step synthetic route, and then subjected to two different ESI mass spectrometry techniques in positive ion mode using lithium cations for ionization. An orbitrap mass spectrometer was used to obtain exact mass information, and higher-energy collisional dissociation (HCD) was used to sequence the lignin model oligomers. Based on the sequence-specific fragment ions, sequence rules were proposed. Multi-stage (MS) collision-induced dissociation (CID) using an ion trap mass spectrometer provided data to investigate the origin of each fragment ion and to further confirm proposed fragmentation pathways. In addition to β-O-4' bond cleavage, the presented lithium cationization approach led to cleavage of β-β' bonds on the model oligomers in both ion trap and orbitrap mass spectrometry experiments. Additionally, MS experiments were used to investigate possible lithium cationization sites on the model oligomers. Lithium cationization in positive ion mode mass spectrometry proved to be a robust tool for characterization and sequencing of advanced lignin oligomers with different bonding motifs.
木质素降解产物的结构解析是成功开发木质素增值技术的要求。大多数基于质谱的技术都利用负离子模式质谱来解析木质素衍生化合物的结构。不幸的是，简单的去质子化可能导致源内碎裂，并且对于没有酸性部分的缩合木质素结构可能不合适。在此，我们提出了一种用于具有β-β'和β-O-4'键合模式的先进木质素低聚物的质谱测序的锂离子化方法。为此，首先通过逐步合成路线合成了两种先进的木质素低聚物，然后在正离子模式下使用锂离子进行两种不同的 ESI 质谱技术进行离子化。使用轨道阱质谱仪获得精确质量信息，并使用更高能量的碰撞解离（HCD）对木质素模型低聚物进行测序。基于序列特异性片段离子，提出了序列规则。使用离子阱质谱仪进行多级（MS）碰撞诱导解离（CID）提供了数据来研究每个片段离子的起源，并进一步证实了所提出的碎裂途径。除了β-O-4'键的断裂外，所提出的锂离子化方法还导致模型低聚物在离子阱和轨道阱质谱实验中β-β'键的断裂。此外，MS 实验用于研究模型低聚物上可能的锂离子化位点。在正离子模式质谱中，锂离子化被证明是一种用于具有不同键合模式的先进木质素低聚物的表征和测序的强大工具。
