Lignocellulosic biomass, as an important natural renewable material, is a major resource for producing biofuels, construction materials, and chemical feedstocks. Plant cell walls serve as the majority of this kind of inexhaustible biomass materials, regulating cellular integrity and providing structural support for plant cells. Rigid scaffolds of cellulose microfibrils are enclosed either in a soft biopolymer matrix (hemicellulose and pectin) in primary cell walls, or in a hemicellulose and aromatic polymer (lignin) mixtures in secondary cell walls. Understanding these biopolymer networks will facilitate the development of more digestible crops and cost-effective conversion technology for biofuel production. Compared to traditional characterization methods, the use of multidimensional solid-state NMR (ssNMR) spectroscopy has become a powerful technique to investigate whole plant cells in their native environment, without pre-chemical treatments. Firstly, we employed three wood samples, that is eucalyptus and poplar (hardwood), and spruce (softwood), to achieve in vivo investigation of biopolymer contacts of lignocellulosic components in plant secondary cell walls using ssNMR. Our results show that the extent of carbohydrate-aromatic association increases sequentially across grasses, hardwoods, and softwood. Lignin principally packs with the xylan in a non-flat conformation via electrostatic interactions and partially binds the junction of flat-ribbon xylan and cellulose surface as a secondary site. All molecules are homogeneously mixed in softwoods; The dynamics and hydration level of polysaccharides in wood cell walls are also discussed. This work will guide the rational design of more digestible plants and more efficient biomass-conversion pathways. Furthermore, progress was also made with 13C natural abundance plant cell walls with the assistance of dynamic nuclear polarization (DNP) approaches and engineered biomaterials. Even with 13C-unlabeled samples, 2D 13C-13C correlation spectroscopies of carbohydrates in rice and in-vitro synthesized cellulose are now rendered available. DNP is paving the road to avoid the isotopic labeling of these biomass materials, which is expensive, time-consuming, and significantly limits the choice of samples.

Elucidating carbohydrate-aromatic interactions in plant cell walls by solid-state NMR and DNP

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