Phenolic syringyl end groups in 13C-enriched hardwoods detected and quantified by solid-state NMR

IF 1.8 3区 化学 Q4 CHEMISTRY, PHYSICAL
Zhaoxi Zheng, Klaus Schmidt-Rohr
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引用次数: 0

Abstract

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 13C-enriched young hardwoods, from beech and oak, the syringyl units were quantitatively investigated by advanced solid-state 13C NMR. Small signals of OH-terminated syringyl units were resolved in spectrally edited two-dimensional 13C–13C 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 13C-beechwood. In a two-dimensional 13C–13C 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 13C chemical shifts of regular and end-group syringyl units were obtained. Through spectrally edited 2D NMR after 1H 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.

Abstract Image

通过固态核磁共振检测和量化富含 13C 的硬木中的酚类丁炔基末端基团。
虽然丁香基单元是硬木木质素中最丰富的单木质素,但其酚醛(即羟基)末端基团的浓度尚未测量过。我们采用先进的固态 13C NMR 对两种 13C 富集均匀的硬木(榉木和橡木)中的丁炔基单元进行了定量研究。在这两种硬木经光谱编辑的二维 13C-13C NMR 光谱中,分辨出了羟基末端丁香基单元的小信号。通过水解 13C -山毛榉木材中丰富的羟基末端丁香基单元,验证了根据文献数据预测的它们的不同峰位。在对角线-棱线抑制的二维 13C-13C 交换谱图中,在丁香基环的特征 C-H 峰位置观察到了酚丁香基单元的分辨良好的峰值,与愈创木基峰无明显重叠。此外,还获得了常规和端基丁香基单元的准确 13C 化学位移。通过 1H 反转恢复后的光谱编辑 2D NMR,仔细分析了与精氨酸复配的缩合单宁的酚类,结果表明与酚类丁香基单元的信号重叠极少。醚(链)和羟基(末端基团)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,因此可以通过峰积分或解卷积进行量化,结果表明酚醛丁香基末端基团在榉木中占丁香基单元的 2 ± 1%,在橡木中占 5 ± 2%。硬木木质素结构的现实分子模型需要考虑到所观察到的低端基团浓度。
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来源期刊
CiteScore
5.30
自引率
9.40%
发文量
42
审稿时长
72 days
期刊介绍: The journal Solid State Nuclear Magnetic Resonance publishes original manuscripts of high scientific quality dealing with all experimental and theoretical aspects of solid state NMR. This includes advances in instrumentation, development of new experimental techniques and methodology, new theoretical insights, new data processing and simulation methods, and original applications of established or novel methods to scientific problems.
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