Abstract
The monolignols, p-coumaryl, coniferyl, and sinapyl alcohol, arise from the general phenylpropanoid biosynthetic pathway. Increasingly, however, authentic lignin monomers derived from outside this process are being identified and found to be fully incorporated into the lignin polymer. Among them, hydroxystilbene glucosides, which are produced through a hybrid process that combines the phenylpropanoid and acetate/malonate pathways, have been experimentally detected in the bark lignin of Norway spruce (Picea abies). Several interunit linkages have been identified and proposed to occur through homo-coupling of the hydroxystilbene glucosides and their cross-coupling with coniferyl alcohol. In the current work, the thermodynamics of these coupling modes and subsequent rearomatization reactions have been evaluated by the application of density functional theory (DFT) calculations. The objective of this paper is to determine favorable coupling and cross-coupling modes to help explain the experimental observations and attempt to predict other favorable pathways that might be further elucidated via in vitro polymerization aided by synthetic models and detailed structural studies.
Highlights
Lignin is canonically formed by the oxidation and polymerization of p-coumaryl, coniferyl, and sinapyl alcohols derived from the general phenylpropanoid biosynthetic pathway (Ralph et al, 2019; Vanholme et al, 2019)
The other proposed crosscoupled products of the hydroxystilbene glucosides with coniferyl alcohol in the lignin of Norway spruce bark have not been definitively identified yet, but the current results show that there should be no thermodynamic impediments to the reactions and formation of such structures
Hydroxystilbene glucosides have been identified in the lignin of Norway spruce and, based on NMR experiments, homocoupling, and cross-coupling with coniferyl alcohol are observed
Summary
Lignin is canonically formed by the oxidation and polymerization of p-coumaryl, coniferyl, and sinapyl alcohols derived from the general phenylpropanoid biosynthetic pathway (Ralph et al, 2019; Vanholme et al, 2019). As of a 2019 review, 35 phenolic monomers had been found in natural lignins (Vanholme et al, 2019) These monomers, like the cinnamyl alcohols, arise through the general phenylpropanoid pathway (del Río et al, 2020), and include monolignol acetates (del Río et al, 2007), benzoates (Kim et al, 2020), p-hydroxybenzoates (Lu et al, 2015), p-coumarates (del Río et al, 2008) and ferulates (Karlen et al, 2016), caffeyl alcohol (Chen et al, 2012), 5-hydroxyconiferyl alcohol (Ralph et al, 2001), hydroxycinnamaldehydes, and dihydroconiferyl alcohol (Ralph et al, 1997). Hydroxystilbene Glucosides DFT to these, there have been recent reports of phenolic compounds that are fully incorporated into the lignin polymer, produced through hybrid processes that combine the phenylpropanoid and acetate/malonate ketide or the amino acid pathway (del Río et al, 2020). The lignin monomers that have been identified include flavonoids and hydroxystilbenes, that are produced by the former biosynthetic pathway, and hydroxycinnamic amides that are formed by the latter pathway
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