Solid-state Nuclear Magnetic Resonance Studies of Secondary Plant Cell Wall Polymer Arrangement: Mechanically Induced Recalcitrance Markers in Sorghum bicolor
Unlocking one third of plant biomass as a renewable feedstock for fuels and materials depends on the effective deconstruction of the secondary plant cell wall. Over 90% of the native secondary plant cell wall is composed of cellulose and hemicellulose polysaccharides and the lignin heteroaromatic polymers. Deconstruction refers to the processes which digest polymers into desired subunits. Present deconstruction processes are centered on lignin-first extraction to overcome the recalcitrance of biomass: the accumulation of indigestible plant polymers during deconstruction. Recent availability of 13C-enriched plant biomass has enabled the use of solid-state Nuclear Magnetic Resonance (NMR) experiments to refine the model of native cell wall structure in plant tissue in biofuel relevant crops. Solid-state NMR has the advantageous ability to non-invasively probe the structure of the secondary plant cell wall throughout deconstruction pathways and potentially refine methods development for the conversion of plant biomass to sustainable products. The mechanical preprocessing of plant material to solubilize the plant cell wall could introduce recalcitrance at the beginning of the deconstruction pathway. In this research solid-state NMR of Sorghum bicolor stems mechanically preprocessed on the lab scale (by vibratory ballmilling at 30 Hz for 2 and 15 minutes) supported recalcitrant reorganization in lignin and hemicellulose. Recalcitrance related to lignin becoming more rigid, potentially trapping other polymers was supported by a reduction in highly dynamic signals of arabinosyl hemicellulose (correlated in lignin-hemicellulose cross linkages), lignin linkages, and lignin probed in the refocused Incredible Nuclear Enhancement by Polarization Transfer (rINEPT) experiment. Cross polarized experiments targeting rigid polymers supported potential recalcitrance from structural hemicellulose as signals: which persisted after milling at a greater signal intensity than cellulose. Recalcitrance from cellulose polymers converting from crystalline to amorphous cellulose as fibril structures are broken down was not supported here, the cellulose carbon 4 peaks was a valuable marker for identifying morphology changes for the cellulose fibril along with scanning electron microscopy.