Structural Characterization of AHP-Pretreated Biomass
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Alkaline hydrogen peroxide (AHP) pretreatment is exceptionally well-suited to grasses, yielding high digestibilities at low enzyme loadings, while generating relatively few fermentation innhibitors. For AHP pretreatment, the question of how structural and chemical compositional changes within the plant cell wall correlate to pretreatment effectiveness has not been effectively resolved, while knowledge of how selective modification of lignin during pretreatment may improve digestibility may yield insights into both improving pretreatments and tailoring plant cell walls for deconstruction. This study presents a comprehensive chemical and structural characterization of the changes in plant cell walls associated with switchgrass (Panicum virgatum cv. Cave-In-Rock), corn stovers (a commercial hybrid and inbred brown midrib lines bm1 and bm3), sugar maple (Acer saccharum) and hybrid aspen (Populus tremula x tremuloides) that were AHP pretreated at varied severities. Both the remaining solids and solubilized biomass hydrolyzates were subjected to a number of characterizations including total polysaccharide composition (monosaccharides, acetate, uronic acids), lignin content as both Klason and acetyl bromide lignin, and the ratio of H/G/S monolignols as determined by thioacidolysis GC/MS. Solid-state 13CCP/MAS NMR and HSQC 2D NMR were applied to determine changes associated with intra-lignin, carbohydrate-carbohydrate, carbohydrate-lignin linkages and functional groups. Apparent molecular weight distributions of soluble polymeric lignins and hemicellulose aggregates were determined by HP-SEC. Py-GC/MS was utilized to characterize changes in the volatilized compounds before and after pretreatment implying pretreatment-induced alterations in pyrolysis-labile monolignol linkages.