Abstract Biphenyl linkage is the second most abundant linkage in softwood lignin; the β-O-4′ linkage being the most abundant linkage. That about 20–28% of monolignols are linked by biphenyl linkage has been demonstrated by UV spectroscopy, permanganate oxidation, 13C NMR, and thioacidolysis followed by 31P NMR. Most, if not all, of the biphenyl structures in softwood lignin are etherified, indicating that biphenyl structures are formed while monolignols are being transported to the lignifying zone during lignin biosynthesis. Along with 4-O-5′ (∼3/100 C9 units), β-6′/α-6′ (∼3/100 C9) and noncyclic α-O-4′ structures (4/100 C9), biphenyl structures are the major branching points of lignin structure. Thus, at least one out of every three C9 units in softwood lignin are branching units. This fact indicates that softwood lignin is most likely a network polymer. Many alkyl-aryl ether bonds are cleaved during ball milling for the isolation of Milled Wood Lignin (MWL), but the number of branching points remains the same. Therefore, softwood milled wood lignin, with a number average DP of 20, exists most likely as at least a highly branched polymer and unlikely as a linear polymer. Harwood lignin has less biphenyl structures (∼9%), but has more 4-O-5′ (∼7%), noncyclic α-O-4′ (6%) and similar β-6′/α-6′ (∼2%) linkages as compared with softwood lignin. Thus, only one out of every four C9 units is a branching point, indicating that hardwood lignin is most likely a highly branched polymer. Hardwood MWL is most likely a branched polymer, but one cannot rule out the chance of finding a few linear oligomers in MWL. One potential branching point is not considered in the above discussion. The lignin-carbohydrate complexes are also branching points in lignin. The number of LCC linkages in native lignin cannot be determined precisely.

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联苯结构及其对木质素大分子结构的影响：批判性评论

摘要 联苯键是针叶木木质素中第二丰富的键；β-O-4' 键是最丰富的键。紫外光谱、高锰酸盐氧化、13C NMR 和硫代酸解，然后是 31P NMR 已经证明，大约 20-28% 的木质素单体通过联苯键连接。大多数（如果不是全部）针叶木木质素中的联苯结构都被醚化，这表明在木质素生物合成过程中单木质素被运输到木质化区时形成了联苯结构。除了 4-O-5'（~3/100 C9 单元）、β-6'/α-6'（~3/100 C9）和非环状 α-O-4' 结构（4/100 C9），联苯结构是木质素结构的主要分支点。因此，软木木质素中每三个 C9 单元中至少有一个是支化单元。这一事实表明软木木质素很可能是一种网络聚合物。许多烷基-芳基醚键在球磨过程中被裂解以分离磨碎木木质素 (MWL)，但支化点的数量保持不变。因此，数均 DP 为 20 的针叶木碾磨木木质素最有可能至少以高度支化的聚合物存在，而不太可能以线性聚合物的形式存在。Harwood 木质素具有较少的联苯结构（~9%），但具有更多的 4-O-5'（~7%）、非环状 α-O-4'（6%）和类似的 β-6'/α-6'（ ∼2%) 与软木木质素相比的联系。因此，每四个 C9 单元中只有一个是支化点，表明硬木木质素很可能是高度支化的聚合物。硬木 MWL 很可能是一种支化聚合物，但不能排除在 MWL 中发现一些线性低聚物的可能性。上述讨论中没有考虑一个潜在的分支点。木质素-碳水化合物复合物也是木质素的分支点。无法精确确定天然木质素中 LCC 连接的数量。