Abstract
Plant cell walls constitute the majority of lignocellulosic biomass and serve as a renewable resource of biomaterials and biofuel. Extensive interactions between polysaccharides and the aromatic polymer lignin make lignocellulose recalcitrant to enzymatic hydrolysis, but this polymer network remains poorly understood. Here we interrogate the nanoscale assembly of lignocellulosic components in plant stems using solid-state nuclear magnetic resonance and dynamic nuclear polarization approaches. We show that the extent of glycan-aromatic association increases sequentially across grasses, hardwoods, and softwoods. Lignin principally packs with the xylan in a non-flat conformation via non-covalent interactions and partially binds the junction of flat-ribbon xylan and cellulose surface as a secondary site. All molecules are homogeneously mixed in softwoods; this unique feature enables water retention even around the hydrophobic aromatics. These findings unveil the principles of polymer interactions underlying the heterogeneous architecture of lignocellulose, which may guide the rational design of more digestible plants and more efficient biomass-conversion pathways.
Highlights
Plant cell walls constitute the majority of lignocellulosic biomass and serve as a renewable resource of biomaterials and biofuel
With solar energy and carbon dioxide transformed into carbohydrate-rich cell walls, terrestrial plants constitute 80% of the biomass distributed in the biosphere[1]
The secondary cell wall is a lignocellulosic composite deposited once the cellular expansion has ceased, which has evolved into a major source of biopolymers and biofuels[2,3]
Summary
Polymorphic structure of carbohydrates in woody stems. We obtained 13C-labeled plant stems by providing poplar, eucalyptus (gum tree), and spruce with 13CO2. Atomic-level information of polysaccharide structure was obtained using two-dimensional (2D) 13C–13C correlation experiments (Supplementary Fig. 1), which spotted 69 allomorphs of monosaccharide residues in three samples (Supplementary Data 1 and Supplementary Table 1). These sugar units, with a wide range of linkages and conformations, were mainly found in cellulose and four hemicelluloses, including glucuronoxylan, arabinoxylan, galactoglucomannan, and a very low amount of xyloglucan (Fig. 1a). The multiplicity should be triggered by the varied oxidation states (for example, S′ is a Cα-oxidized form of S unit), the assorted linkages, and presumably, the wide-ranging conformation and packing in native solids.
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