We have developed an environmentally benign large-scale (50 kg wood meal per batch) lignin production plant, operating based on acid-catalyzed polyethylene glycol (PEG) solvolysis of softwood biomass. The motivation for the proposed process was to promote technological innovation in biomass utilization systems in Japanese rural areas based on widely abundant Japanese cedar (sugi) biomass. In this study, the process was evaluated by investigating the effects of the source sugi wood meal size and the solvent PEG molecular mass on the yield, chemical structure, molecular mass, and thermal properties of the resultant PEG-modified lignin derivatives, glycol lignins (GLs). Reducing the source wood meal size and PEG solvent molecular mass not only promoted lignin PEGylation but also the subsequent acid-induced chemical rearrangements of the GLs as demonstrated by chemical analyses, 2D NMR, and size exclusion chromatography (SEC). Reducing the source wood meal size and/or increasing the solvent PEG molecular mass enhanced the thermal properties of GLs as determined by thermomechanical analysis (TMA) and thermogravimetric analysis (TGA). We considered that the proposed process can efficiently produce lignin derivatives with substantial control over the chemical structure and thermal properties to meet commercial and industrial needs for lignin-based advanced material production.
Termites represent one of the most efficient lignocellulose decomposers on earth. The mechanism by which termites overcome the recalcitrant lignin barrier to gain access to embedded polysaccharides for assimilation and energy remains largely unknown. In the present study, softwood, hardwood, and grass lignocellulose diets were fed to Coptotermes formosanus workers, and structural differences between the original lignocellulose diets and the resulting feces were examined by solution-state multidimensional nuclear magnetic resonance (NMR) techniques as well as by complementary wet-chemical methods. Overall, our data support the view that lignin polymers are partially decomposed during their passage through the termite gut digestive system, although polysaccharide decomposition clearly dominates the overall lignocellulose deconstruction process and the majority of lignin polymers remain intact in the digestive residues. High-resolution NMR structural data suggested preferential removal of syringyl aromatic units in hardwood lignins, but non-acylated guaiacyl units as well as tricin end-units in grass lignins. In addition, our data suggest that termites and/or their gut symbionts may favor degradation of C–C-bonded β–5 and resinol-type β–β lignin inter-monomeric units over degradation of ether-bonded β–O–4 units, which is in contrast to what has been observed in typical lignin biodegradation undertaken by wood-decaying fungi.
The residual wood meal left after milled wood lignin (MWL) isolation [milled wood residue (MWR)] of 5-year-old Eucalyptus globulus was fractionated to afford a xylan-lignin fraction (X-L) in 2.9 % yield (based on MWR) by the method reported previously. X-L was further fractionated with the lignin solvent 1,4-dioxane/water (9:1, v/v) to give a soluble fraction (XL-F1; 24.0 % ) and an insoluble fraction (XL-F1-residue; 74.6 % ; both yields based on X-L). XL-F1-residue was further extracted with the good xylan solvent dimethyl sulfoxide and the soluble fraction was termed XL-F2 (43.0 % ; based on the XL-F1-residue). XL-F1 was mainly composed of lignin with a small amount of xylan and it is similar to purified MWL, whereas XL-F2 was mainly composed of xylan with some amount of lignin and it is similar to a fraction that was prepared by the extraction of crude MWL with acetic acid [lignin-carbohydrate complex (LCC)-AcOH]. The twodimensional heteronuclear single quantum coherence nuclear magnetic resonance spectra of XL-F1 and XL-F2 were interpreted that the former has α -ether-type lignincarbohydrate (LC) linkages and the latter might have LC linkages of the phenyl glycoside type, which are different from those in LCC-AcOH.
The existence and formation of covalent lignin-carbohydrate (LC) linkages in plant cell walls has long been a matter of debate in terms of their roles in cell wall development and biomass use. Of the various putative LC linkages proposed to date, evidence of the native existence and formation mechanism of phenyl glycoside (PG)-type LC linkages in planta is particularly scarce. The present study aimed to explore previously overlooked mechanisms for the formation of PG-type LC linkages through the incorporation of monolignol glucosides, which are possible lignin precursors, into lignin polymers during lignification. Peroxidase-catalyzed lignin polymerization of coniferyl alcohol in the presence of coniferin and syringin in vitro resulted in the generation of PG-type LC linkages in synthetic lignin polymers, possibly via nucleophilic addition onto quinone methide (QM) intermediates formed during polymerization. Biomimetic lignin polymerization of coniferin via the b-glucosidase/peroxidase system also resulted in the generation of PG-type as well as alkyl glycoside-type LC linkages. This occurred via non-enzymatic QM-involving reactions and also via enzymatic transglycosylations involving b-glucosidase, which was demonstrated by in-depth structural analysis of the synthetic lignins by two-dimensional NMR. We collected heteronuclear single-quantum coherence (HSQC) NMR for native cell wall fractions prepared from pine (Pinus taeda), eucalyptus (Eucalyptus camaldulensis), acacia (Acacia mangium), poplar (Populus 3 eurarnericana) and bamboo (Phyllostachys edulis) wood samples, which exhibited correlations, albeit at low levels, that were well matched with those of the PG-type LC linkages in synthetic lignins incorporating monolignol glucosides. Overall, our results provide a molecular basis for feasible mechanisms for the generation of PG-type LC linkages from monolignol glucosides and further substantiates their existence in planta.
Synthesis and fundamental HSQC NMR data of monolignol β-glycosides, dihydromonolignol β-glycosides and p-hydroxybenzaldehyde derivative β-glycosides for the analysis of phenyl glycoside type lignin-carbohydrate complexes (LCCs) Abstract: Twelve monolignol (coniferyl alcohol, sinapyl alcohol and p-coumaryl alcohol) β-glycosides (β-glucosides, β-galactosides, β-xylosides and β-mannosides) were synthesised to obtain fundamental NMR data for the analysis of phenyl glycoside type lignincarbohydrate complexes (LCCs). That is, the 1,2-trans glycosides (the β-glucosides, β-galactosides and β-xylosides) and the 1,2-cis glycosides (the β-mannosides) were synthesized by means of Koenig-Knorr glycosylation and β-selective Mitsunobu glycosylation strategies, respectively. In addition, dihydromonolignol and p-hydroxybenzaldehyde derivative β-glycosides were also prepared from the corresponding monolignol glycosides and their intermediates, respectively. The correlation observed for the C 1β -H 1β bonds of the sugar moieties in the HSQC spectra of the all β-glycosides varied and were in the range of δ C /δ H 96-104/4.7-5.4 ppm. Especially, it was found that the correlations derived from the C 1β -H 1β bonds of the guaiacyl and p-hydroxyphenyl β-mannosides were close to those derived from the C 1α -H 1α bonds of the 4-O-methyl-α-Dglucuronic acid moieties described in the literature.
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