Direct evidence of α ester linkages between lignin and glucuronoxylan that reveal the robust heteropolymeric complex in plant cell walls

Abstract

All vascular plants have lignin that interacts with polysaccharides in cell walls; however, the chemical structure of covalent lignin-carbohydrate (L-C) linkages remains unresolved because of the small number of L-C linkage relative to the polymer main chain. Sample isolation that predominantly maintains intact L-C linkages followed by advanced multi-dimensional NMR showed that the critical L-C linkage between lignin and xylan involves the α-position of the β-aryl ether unit of lignin and the sixth position of the glucuronic acid residue of 4-O-methyl glucuronoxylan. The L-C ester linkages occur primarily at the α-position of lignin, supporting a theoretical synthetic pathway through a quinone methide intermediate. In contrast, the conventional γ-L-C ester was undetectable, indicating that this linkage is not the primary type. Instead, the γ-acetyl-substituted lignin coexisted. The results on primary L-C linkages contribute to plant physiology and biology and advance research into biomass conversion.

Grass xylan structural variation suggests functional specialization and distinctive interaction with cellulose and lignin

“…Therefore, GAXc could covalently cross‐link to lignin or other xylan chains through ferulic acid (Figure 8 , box C). Hardwood glucuronoxylan can be esterified to lignin via its GlcA substituents (Nishimura et al., 2022 ). It is therefore plausible that some of the GlcA residues on GAXc are esterified to lignin (Figure 8 , box D).…”

Altering the substitution and crosslinking of glucuronoarabinoxylans affects cell wall porosity and assembly inBrachypodium distachyon

“…The degree, type and distribution of substitutions on xylan are major factors affecting cell wall recalcitrance by forming cross-links with other xylan molecules and lignin (Grabber et al, 2000;Nishimura et al, 2018;Feijao et al, 2022;Nishimura et al, 2022), by influencing interactions with cellulose (Grantham et al, 2017) and by impairing the action of xylan hydrolytic enzymes (Biely et al, 2016). Xylan comprises a linear backbone of β-1,4-xylose (Xyl) residues commonly substituted with α-1,2-linked (4-O-methyl)-glucuronic acid (GlcA), α-1,3-linked arabinofuranose (Araf) and acetylation at the O-2 and/or O-3 positions (Ebringerová & Heinze, 2000).…”

“…A covalent linkage between GlcA of glucuronoxylan and lignin has been reported in eudicots (Imamura et al, 1994;Nishimura et al, 2022). Covalent links between the GlcA residues of grass GAX and lignin are also likely to exist, but their presence and importance have not been investigated.…”