Abstract
Cross-linking between lignin and polysaccharide in plant cell-wall determines physical, chemical, and biological features of lignocellulosic biomass. Since Erdmann’s first report in 1866, numerous studies have suggested the presence of a bond between hemicelluloses and lignin; however, no clear evidence for this interaction has been reported. We describe the first direct proof of covalent bonding between plant cell-wall polysaccharides and lignin. Nuclear magnetic resonance spectroscopy was used to observe the long-range correlations through an α-ether bond between lignin and the primary hydroxyl group of a mannose residue in glucomannan. Complete signal assignment of the cognate structural units was also achieved. Thus, we identified lignin–carbohydrate bonds by complete connectivity analysis from the phenylpropane unit to the carbohydrate moiety.
Highlights
Lignin is a key adaptation to terrestrial life that reinforces plant cell walls; this compound forms the hydrophobic xylem vessels that transport water, and it supports plant bodies by co-localising with polysaccharides, cellulose, and hemicellulose
To obtain original LCC from wood cell walls, we have focused on the milled wood lignin (MWL) residue
MWL mostly consists of lignin, and its residue is in large part composed of polysaccharides, cellulose, and hemicellulose, but it contains LCC
Summary
Lignin is a key adaptation to terrestrial life that reinforces plant cell walls; this compound forms the hydrophobic xylem vessels that transport water, and it supports plant bodies by co-localising with polysaccharides, cellulose, and hemicellulose. Numerous investigations have been conducted to elucidate the structure and nature of LCCs in plant cell walls; no direct proof of the presence of chemical bonds between lignin and polysaccharides has been reported. To improve our understanding of plant cell wall structure and to develop efficient separation techniques to be implemented in lignocellulosic biorefineries, additional information on the chemical linkages between lignin and carbohydrates is necessary. 2D 1H–13C heteronuclear single-quantum correlation (HSQC) NMR spectroscopy has been applied to the analysis of LC bonds, and the presence of α ether and phenyl glycoside bonds has been reported[20,21,22,23,24]. Γ-ester LC linkages, instead of α esters, have been reported[22,23] These observations and assignments were based on 1H–13C HSQC correlations. In order to prepare LCC fragments possessing a number of LC bonds high enough to suit NMR sensitivity, we concentrated the LC bonds in water-soluble neutral LCCs from Pinus densiflora (Japanese red pine) wood by carrying out the enzymatic digestion of the sugar moiety and using a polyvinyl gel characterised by an affinity for lignin and size-exclusion activity[25,26]
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