Preparation of synthetic lignins with superior NMR characteristics via isotopically labeled monolignols
作者:John Ralph、Yingsheng Zhang、Richard M. Ede
DOI:10.1039/a803281e
日期:——
Synthetic lignins are particularly valuable for studying aspects of lignification, plant cell wall cross-linking, and lignin structure. If they are not too highly polymeric, they are soluble in normal lignin solvents and amenable to solution-state NMR studies. However, in the application of inverse-detected correlation experiments, particularly the popular HMQC and HMBC experiments, the spectra have annoying T1-noise ridges. These artifacts make it difficult to locate correlation peaks that are near the methoxy signal in the proton dimension. One solution is to use gradient-enhanced NMR but that requires additional hardware that is not yet ubiquitous. An alternative is to produce monolignols in which the atoms of the methoxy group are NMR-invisible. We have accomplished this by preparing coniferyl and sinapyl alcohols using 13C-depleted deuterated methyl iodide (12C2H3I). The methods, which incorporate steps simpler than have been used previously for labeled monolignols, are sufficiently low cost and straightforward that these monomers can be utilized for any synthetic lignins destined for NMR studies. The NMR spectra of lignins derived from these ‘methoxy-less’ monomers are markedly superior to their normal-monomer counterparts. Several popular NMR experiments are illustrated for synthetic lignins derived from normal vs. isotopically labeled coniferyl alcohol, along with some useful experiments that have not been seen in lignin-related publications to date.
合成木质素对于研究木质化、植物细胞壁交联和木质素结构特别有价值。如果它们的聚合度不太高,则它们可溶于普通木质素溶剂并适合溶液态核磁共振研究。然而,在逆检测相关实验的应用中,特别是流行的 HMQC 和 HMBC 实验中,光谱具有令人讨厌的 T1 噪声脊。这些伪影使得很难定位质子维度中甲氧基信号附近的相关峰。一种解决方案是使用梯度增强核磁共振,但这需要尚未普及的额外硬件。另一种方法是生产木质素单体,其中甲氧基原子是 NMR 不可见的。我们通过使用 13C 贫化氘代甲基碘 (12C2H3I) 制备松柏醇和芥子醇来实现这一目标。这些方法所采用的步骤比以前用于标记木质素单体的步骤更简单,成本足够低且简单,这些单体可用于任何用于 NMR 研究的合成木质素。由这些“无甲氧基”单体衍生的木质素的核磁共振谱明显优于其正常单体对应物。几个流行的核磁共振实验说明了从正常与同位素标记的松柏醇衍生的合成木质素,以及迄今为止在木质素相关出版物中尚未见过的一些有用的实验。