The subject of lignin structure, critical for fundamental and practical reasons, is addressed in this study that includes a review of the methods applied to elucidate macromolecular branching. The recently available approaches for determination of the absolute molecular mass of spruce milled wood lignin (MWL) along with the quantification of terminal groups clearly indicate that MWL is significantly branched and cross-linked (with ∼36% lignin units partaking in these linkages). Results from independent methods imply that about half of the branching and crosslinking linkages involve aromatic rings, predominantly 5–5′ etherified units; meanwhile, a significant number of linkages are located in the side chains. Quantitative ¹³C NMR analyses suggest that the branches involve different aliphatic ether (alkyl-O-alkyl) types at the α- and γ-positions of the side chain, with intact β-O-4 linkages. While the exact structures of these moieties require further investigation, our results point to the fact that conventional lignification theory disagrees with the presence of such key moieties in softwood MWL and the observed high degree of branching/crosslinking. Potential reasons for the noted discrepancies are discussed.

中文翻译：

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云杉磨碎的木质素：直链，支链或交联的？

木质素结构的主题，对于基本和实际原因至关重要，在本研究中得到解决，其中包括对阐明大分子分支的方法的综述。确定云杉磨木屑木质素（MWL）的绝对分子量以及端基的定量的最新可用方法清楚地表明，MWL是显着分支和交联的（这些连接中约有36％的木质素单元参与其中）。独立方法的结果表明，大约一半的支链和交联键都涉及芳香环，主要是5-5'醚化单元。同时，在侧链中存在大量的连接。定量¹³13 C NMR分析表明，支链在侧链的α和γ位上具有不同的脂肪族醚（烷基-O-烷基）类型，具有完整的β-O-4键。尽管这些部分的确切结构需要进一步研究，但我们的结果指出了这样一个事实，即常规的木质化理论与软木MWL中此类关键部分的存在以及所观察到的高度支化/交联不同。讨论了引起差异的潜在原因。