Lignin–ferulate cross-links in grasses. Part 4.1–3 Incorporation of 5–5-coupled dehydrodiferulate into synthetic lignin
作者:Stéphane Quideau、John Ralph
DOI:10.1039/a701808h
日期:——
Ferulates and dehydrodiferulates have a significant role in
cross-linking polysaccharides to lignin in grass cell walls. Among the
various ferulate dehydrodimers, the 5â5-coupled dehydrodimer
(E,E)-4,4â²-dihydroxy-3,3â²-dimethoxy-5,5
â²-bicinnamate is capable of the most extensive cross-linking into
lignin, forming major branch-points. However, current literature fails
to recognise the role of radical cross-coupling reactions with lignin
monomers/oligomers. Here we demonstrate that a synthetic model for
5â5-coupled dehydrodiferulate polysaccharide esters in grass cell
walls biomimetically incorporates into synthetic lignins via
radical coupling mechanisms to produce a range of cross-coupled
structures. The incorporation profile is remarkably similar to that for
ferulate. Importantly, significant coupling at the cinnamoyl 8-position
readily occurs. Evidence for the expected incorporation of the
5â5-coupled dehydrodiferulate into the newly discovered
dibenzodioxocine structures is readily apparent in HMQC or HSQC spectra.
Since some of the structures that dehydrodiferulates are involved in
cannot be hydrolytically cleaved, their current
âquantificationâ is a significant underestimation of the
importance of these species. What is clear is that dehydrodiferulates
can have a powerful role in effecting ligninâpolysaccharide
cross-linking.
阿魏酸酯和脱氢阿魏酸酯在将多糖与草细胞壁中的木质素交联方面发挥着重要作用。在各种阿魏酸酯脱氢二聚体中,5â5-偶联脱氢二聚体(E,E)-4,4â²-二羟基-3,3â²-二甲氧基-5,5â²-肉桂酸酯能够与木质素发生最广泛的交联,形成主要的支点。然而,目前的文献没有认识到与木质素单体/异构体发生自由基交联反应的作用。在这里,我们证明了草细胞壁中 5â5 偶联脱氢多糖酯的合成模型可通过自由基偶联机制生物模拟地与合成木质素结合,产生一系列交叉偶联结构。其结合情况与阿魏酸酯极为相似。重要的是,肉桂酰 8 位很容易发生明显的耦合。在 HMQC 或 HSQC 光谱中,5â5-偶联脱氢二蕨酸酯与新发现的二苯并二氧杂环辛结构的预期结合证据显而易见。 由于脱氢二蕨酸酯参与的某些结构无法水解,因此目前对它们的 "量化 "大大低估了这些物种的重要性。但显而易见的是,脱氢增活素能在木质素-多糖交联过程中发挥重要作用。