While syringyl units are the most abundant monolignols in hardwood lignin, their phenolic (i.e. hydroxyl) end group concentration has not been measured. In two uniformly C-enriched young hardwoods, from beech and oak, the syringyl units were quantitatively investigated by advanced solid-state C NMR. Small signals of OH-terminated syringyl units were resolved in spectrally edited two-dimensional C–C NMR spectra of the two hardwoods. Their distinct peak positions predicted based on literature data were validated via the abundant OH-terminated syringyl units in hydrolyzed C-beechwood. In a two-dimensional C–C exchange spectrum with diagonal-ridge suppression, a well-resolved peak of phenolic syringyl units was observed at the characteristic C–H peak position of syringyl rings, without significant overlap from guaiacyl peaks. Accurate C chemical shifts of regular and end-group syringyl units were obtained. Through spectrally edited 2D NMR after H inversion recovery, phenols of condensed tannin complexed with arginine were carefully analyzed and shown to overlap minimally with signals from phenolic syringyl units. The local structure and resulting spin dynamics of ether (chain) and hydroxyl (end-group) syringyl units are nearly the same, enabling quantification by peak integration or deconvolution, which shows that phenolic syringyl end groups account for 2 ± 1 % of syringyl units in beechwood and 5 ± 2 % in oakwood. The observed low end-group concentration needs to be taken into account in realistic molecular models of hardwood lignin structure.

中文翻译：

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通过固态 NMR 检测和定量富含 13C 的硬木中的酚紫丁香基端基


虽然紫丁香基单元是硬木木质素中最丰富的木质素单体，但尚未测量它们的酚（即羟基）端基浓度。通过先进的固态 13C NMR 对两种均匀富含 C 的年轻硬木（山毛榉和橡木）中的紫丁香基单元进行了定量研究。 OH 封端紫丁香基单元的小信号在两种硬木的光谱编辑二维 13C NMR 光谱中得到解析。根据文献数据预测的不同峰位置通过水解 C-山毛榉木中丰富的 OH 封端紫丁香基单元进行了验证。在具有对角线脊抑制的二维 C-C 交换光谱中，在紫丁香基环的特征 C-H 峰位置处观察到酚类紫丁香基单元的良好分辨峰，与愈创木基峰没有明显重叠。获得了规则和端基紫丁香基单元的准确 C 化学位移。通过 H 反转恢复后的光谱编辑 2D NMR，对与精氨酸复合的缩合单宁的酚进行了仔细分析，并显示与酚紫丁香基单元的信号重叠最小。醚（链）和羟基（端基）紫丁香基单元的局部结构和由此产生的自旋动力学几乎相同，可以通过峰积分或解卷积进行定量，这表明酚类紫丁香基端基占紫丁香基单元的 2 ± 1%山毛榉木中含量为 5 ± 2 %，橡木中含量为 5 ± 2 %。在硬木木质素结构的实际分子模型中需要考虑观察到的低端基浓度。