Quantification of wheat straw lignin structure by comprehensive NMR analysis

Lignin hydrogenolysis: Tuning the reaction by lignin chemistry

Replacing fossil resource with biomass is one of the promising approaches to reduce our carbon footprint. Lignin is one of the three major components of lignocellulosic biomass, accounting for 10-35 wt% of dried weight of the biomass. Hydrogenolytic depolymerization of lignin is attracting increasing attention because of its capacity of utilizing lignin in its uncondensed form and compatibility with the biomass fractionation processes. Lignin is a natural aromatic polymer composed of a variety of monolignols associated with a series of lignin linkage motifs. Hydrogenolysis cleaves various ether bonds in lignin and releases phenolic monomers which can be further upgraded into valuable products, i.e., drugs, terephthalic acid, phenol. This review provides an overview of the state-of-the-art advances of the reagent (lignin), products (hydrol lignin), mass balance, and mechanism of the lignin hydrogenolysis reaction. The chemical structure of lignin is reviewed associated with the free radical coupling of monolignols and the chemical reactions of lignin upon isolation processes. The reactions of lignin linkages upon hydrogenolysis are discussed. The components of hydrol lignin and the selectivity production of phenolic monomers are reviewed. Future challenges on hydrogenolysis of lignin are proposed. This article provides an overview of lignin hydrogenolysis reaction which shows light on the generation of optimized lignin ready for hydrogenolytic depolymerization.

Beyond low lignin: Identifying the primary barrier to plant biomass conversion by fermentative bacteria

Renewable alternatives for nonelectrifiable fossil-derived chemicals are needed and plant matter, the most abundant biomass on Earth, provide an ideal feedstock. However, the heterogeneous polymeric composition of lignocellulose makes conversion difficult. Lignin presents a formidable barrier to fermentation of nonpretreated biomass. Extensive chemical and enzymatic treatments can liberate fermentable carbohydrates from plant biomass, but microbial routes offer many advantages, including concomitant conversion to industrial chemicals. Here, testing of lignin content of nonpretreated biomass using the cellulolytic thermophilic bacterium, Anaerocellum bescii, revealed that the primary microbial degradation barrier relates to methoxy substitutions in lignin. This contrasts with optimal lignin composition for chemical pretreatment that favors high S/G ratio and low H lignin. Genetically modified poplar trees with diverse lignin compositions confirm these findings. In addition, poplar trees with low methoxy content achieve industrially relevant levels of microbial solubilization without any pretreatments and with no impact on tree fitness in greenhouse.

Equilibrium Moisture Mediated Esterification Reaction to Achieve Over 100% Lignocellulosic Nanofibrils Yield

Lignocellulosic nanofibrils (LCNFs) isolation is recognized as an efficient strategy for maximizing biomass utilization. Nevertheless, achieving a 100% yield presents a formidable challenge. Here, an esterification strategy mediated by the equilibrium moisture in biomass is proposed for LCNFs preparation without the use of catalysts, resulting in a yield exceeding 100%. Different from anhydrous chemical thermomechanical pulp (CTMP0%), the presence of moisture (moisture content of 7 wt%, denoted as CTMP7%) introduces a notably distinct process for the pretreatment of CTMP, comprising the initial disintegration and the post-esterification steps. The maleic acid, generated through maleic anhydride (MA) hydrolysis, degrades the recalcitrant lignin-carbohydrate complex (LCC) structures, resulting in esterified CTMP7% (E-CTMP7%). The highly grafted esters compensate for the mass loss resulting from the partial removal of hydrolyzed lignin and hemicellulose, ensuring a high yield. Following microfluidization, favorable LCNF7% with a high yield (114.4 ± 3.0%) and a high charge content (1.74 ± 0.09 mmol g-1) can be easily produced, surpassing most previous records for LCNFs. Additionally, LCNF7% presented highly processability for filaments, films, and 3D honeycomb structures preparation. These findings provide valuable insights and guidance for achieving a high yield in the isolation of LCNFs from biomass through the mediation of equilibrium moisture.

Carboxyalkylated Lignin as a Sustainable Dispersant for Coal Water Slurry

Coal water slurry (CWS) has been considered a cleaner and sustainable alternative to coal. However, the challenging suspension of coal particles in CWS has created a major obstacle to its use in industry. This study presents a novel approach to enhance the stability and rheological properties of coal water slurry (CWS) through the utilization of carboxyalkylated lignin (CL) as a dispersant. The generated CL samples had high water solubility of around 9 g/L and a charge density of around 2 mmol/g. All CLs were able to stabilize the coal suspension, and their performance decreased due to the increase in the alkyl chain length of carboxyalkylated lignin. Carboxymethylated lignin (CL-1) improved the stability of the coal suspensions with the lowest instability index of less than 0.6. The addition of CLs reduced the contact angle of the coal surface from 45.3° to 34.6°, and the increase in the alkyl chain length hampered its effect on contact angle changes. The zeta potential measurements confirmed that the adsorption of CL enhanced the electrostatic repulsion between coal particles in suspensions, and the zeta potential decreased with the increased alkyl chain length of CLs due to increased steric hindrance. The rheology results indicated that CLs demonstrated shear thinning behavior. This innovative method showcases the affinity of carboxyalkylated lignin to improve the performance of CWS, offering an environmentally friendly alternative for producing a cleaner product, i.e., sustainable coal water slurry, with improved suspension stability.

Visualization of lignification in flax stem cell walls with novel click-compatible monolignol analogs

Introduction

As an essential part of plant cell walls, lignin provides mechanical support for plant growth, enhances water transport, and helps to defend against pathogens. As the most abundant natural aromatic-based renewable resource on earth, its biosynthesis has always been a research focus, and it is still currently under study.

Methods

In this study, the p-coumaryl alcohol analog (HALK) and the coniferyl alcohol analog (GALK) containing an alkyne group at the ortho position were synthesized and applied to lignification in vivo and in vitro. The incorporation of these novel lignin monomers was observed via fluorescence imaging.

Results and Discussion

It was found that the two monolignol analogs could be incorporated in dehydrogenated polymers (DHPs) in vitro and in flax cell walls in vivo. The results showed that as the cultivation time and precursor concentration varied, the deposition of H and G-type lignin exhibited differences in deposition mode. At the subcellular scale, the deposited lignin first appears in the cell corner and the middle lamella, and then gradually appears on the cell walls. Furthermore, lignin was also found in bast fiber. It was demonstrated that these new molecules could provide high-resolution localization of lignin during polymerization.

From 13C-lignin to 13C-mycelium: Agaricus bisporus uses polymeric lignin as a carbon source

Plant biomass conversion by saprotrophic fungi plays a pivotal role in terrestrial carbon (C) cycling. The general consensus is that fungi metabolize carbohydrates, while lignin is only degraded and mineralized to CO2. Recent research, however, demonstrated fungal conversion of 13C-monoaromatic compounds into proteinogenic amino acids. To unambiguously prove that polymeric lignin is not merely degraded, but also metabolized, carefully isolated 13C-labeled lignin served as substrate for Agaricus bisporus, the world's most consumed mushroom. The fungus formed a dense mycelial network, secreted lignin-active enzymes, depolymerized, and removed lignin. With a lignin carbon use efficiency of 0.14 (g/g) and fungal biomass enrichment in 13C, we demonstrate that A. bisporus assimilated and further metabolized lignin when offered as C-source. Amino acids were high in 13C-enrichment, while fungal-derived carbohydrates, fatty acids, and ergosterol showed traces of 13C. These results hint at lignin conversion via aromatic ring-cleaved intermediates to central metabolites, underlining lignin's metabolic value for fungi.

Solubilization and structural changes of lignin in naked oat stems during subcritical water autohydrolysis

In this study, the solubilization and structural changes of lignin in naked oat stems were investigated under subcritical water autohydrolysis systems (170-210 °C, 0.68-1.85 MPa). In this system, Hemicellulose was preferentially hydrolyzed in the liquid water at elevated temperatures, leading to the production of acetic acid and glucuronic acid, which acidified the reaction system. Under acidic and high-temperature conditions, lignin primarily underwent degradation and condensation reactions. At autohydrolysis temperatures below 190 °C and autohydrolysis pressures below 1.22 MPa, lignin degradation was predominant, realizing a maximum lignin removal of 47.8 % and breakage of numerous β-O-4 bonds from lignin. At autohydrolysis temperatures above 190 °C and autohydrolysis pressures above 1.22 MPa, lignin condensation dominated, with an increase in the amount of organic acids generated upon hemicellulose degradation, leading to condensation reactions with the degraded low-molecular-weight lignin. The degree of lignin condensation was positively correlated with the temperature of the reaction system. This study provides essential insights into the dynamic changes in the structure of lignin in both the hydrolysis residue and hydrolysis solution during subcritical water autohydrolysis.

Revealing the structural changes of lignin in Chinese quince (Chaenomeles sinensis) fruit as it matures

Chinese quince fruits (Chaenomeles sinensis) contain substantial amounts of lignin; however, the exact structure of lignin remains to be investigated. In this study, milled wood lignins (Milled wood lignin (MWL)-1, MWL-2, MWL-3, MWL-4, MWL-5, and MWL-6) were extracted from fruits harvested once a month from May to October 2019 to investigate their structural evolution during fruit growth. The samples were characterized via High-performance anion exchange chromatography (HPAEC), Fourier transform-infrared spectroscopy (FT-IR), gel permeation chromatography (GPC), thermogravimetric (TGA), pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) and NMR (2D-heteronuclear single quantum coherence (HSQC) and 31P). The MWL samples in all fruit growth stages were GS-type lignin and lignin core undergoing minimal alterations during fruit development. The predominant linkage in the lignin structure was β-O-4', followed by β-β' and β-5'. Galactose and glucose were the main monosaccharides associated with MWL. In MWL-6, the lignin exhibited the highest homogeneity and thermal stability. As the fruit matured, a gradual increase in the β-O-4' proportion and the ratio of S/G was observed. The results provide comprehensive characterization of the cell wall lignin of quince fruit as it matures. This study could inspire innovative applications of quince fruit lignin and provide the optimal harvest time for lignin utilization.

Simultaneous suppression of lignin, tricin and wall-bound phenolic biosynthesis via the expression of monolignol 4-O-methyltransferases in rice

Grass lignocelluloses feature complex compositions and structures. In addition to the presence of conventional lignin units from monolignols, acylated monolignols and flavonoid tricin also incorporate into lignin polymer; moreover, hydroxycinnamates, particularly ferulate, cross-link arabinoxylan chains with each other and/or with lignin polymers. These structural complexities make grass lignocellulosics difficult to optimize for effective agro-industrial applications. In the present study, we assess the applications of two engineered monolignol 4-O-methyltransferases (MOMTs) in modifying rice lignocellulosic properties. Two MOMTs confer regiospecific para-methylation of monolignols but with different catalytic preferences. The expression of MOMTs in rice resulted in differential but drastic suppression of lignin deposition, showing more than 50% decrease in guaiacyl lignin and up to an 90% reduction in syringyl lignin in transgenic lines. Moreover, the levels of arabinoxylan-bound ferulate were reduced by up to 50%, and the levels of tricin in lignin fraction were also substantially reduced. Concomitantly, up to 11 μmol/g of the methanol-extractable 4-O-methylated ferulic acid and 5-7 μmol/g 4-O-methylated sinapic acid were accumulated in MOMT transgenic lines. Both MOMTs in vitro displayed discernible substrate promiscuity towards a range of phenolics in addition to the dominant substrate monolignols, which partially explains their broad effects on grass phenolic biosynthesis. The cell wall structural and compositional changes resulted in up to 30% increase in saccharification yield of the de-starched rice straw biomass after diluted acid-pretreatment. These results demonstrate an effective strategy to tailor complex grass cell walls to generate improved cellulosic feedstocks for the fermentable sugar-based production of biofuel and bio-chemicals.

Chemometric Combination of Ultrahigh Resolving Mass Spectrometry and Nuclear Magnetic Resonance Spectroscopy for a Structural Characterization of Lignin Compounds

In recent years, the potential of lignins as a resource for material-based applications has been highlighted in many scientific and nonscientific publications. But still, to date, a lack of detailed structural knowledge about this ultracomplex biopolymer undermines its great potential. The chemical complexity of lignin demands a combination of different, powerful analytical methods, in order to obtain these necessary information. In this paper, we demonstrate a multispectroscopic approach using liquid-state and solid-state Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and nuclear magnetic resonance (NMR) spectroscopy to characterize a fractionated LignoBoost lignin. Individual FT-ICR-MS, tandem MS, and NMR results helped to determine relevant information about the different lignin fractions, such as molecular weight distributions, oligomer sizes, linkage types, and presence of specific functional groups. In addition, a hetero spectroscopic correlation approach was applied to chemometrically combine MS, MS/MS, and NMR data sets. From these correlation analyses, it became obvious that a combination of tandem MS and NMR data sets gives the opportunity to comprehensively study and describe the general structure of complex biopolymer samples. Compound-specific structural information are obtainable, if this correlation approach is extended to 1D-MS and NMR data, as specific functional groups or linkages are verifiable for a defined molecular formula. This enables structural characterization of individual lignin compounds without the necessity for tandem MS experiments. Hence, these correlation results significantly improve the depth of information of each individual analysis and will hopefully help to structurally elucidate entire lignin structures in the near future.

Structural variations of lignin and lignin-carbohydrate complexes from the fruit shells of Camellia oleifera during ripening

Camellia oleifera fruit shell (CFS), a waste lignocellulosic biomass resulting from Camellia oleifera oil production industry, is abundantly available in Southern China. Herein, to understand the structural variations of CFS lignins and lignin-carbohydrate complexes (LCC) during ripening, the native lignin and LCC fractions from CFS (harvested every seven days from October 1 to 30, 2022) were isolated and characterized systematically. The molecular weights of both MWL and DEL fractions steadily increased during ripening. CFS lignins contained abundance of β-O-4' linkages (maximum of 58.6 per 100Ar in DEL-2), and had low S/G ratios (S/G < 0.6). Moreover, the amounts of β-O-4' linkages in MWL, DEL, and LCC-AcOH fractions increased first and then decreased during ripening. The main lignin-carbohydrate linkages in the LCC-AcOH fractions were benzyl-ether (7.0-9.4 per 100Ar) and phenyl-glycoside (4.5-5.2 per 100Ar) bonds. Based on the quantitative results, the potential structural diagrams of lignins from different ripening stages of CFS were proposed. Additionally, the LCC-AcOH fractions exhibited pronounced antioxidant capacity and were promising as natural antioxidants. The properties and functions of lignin in plant cell walls, as well as its further appreciation, are crucial for the design and selection of feasible pretreatment strategies for the lignocellulosic materials.

Oxidation-driven lignin removal by Agaricus bisporus from wheat straw-based compost at industrial scale

Fungi are main lignin degraders and the edible white button mushroom, Agaricus bisporus, inhabits lignocellulose-rich environments. Previous research hinted at delignification when A. bisporus colonized pre-composted wheat straw-based substrate in an industrial setting, assumed to aid subsequent release of monosaccharides from (hemi-)cellulose to form fruiting bodies. Yet, structural changes and specific quantification of lignin throughout A. bisporus mycelial growth remain largely unresolved. To elucidate A. bisporus routes of delignification, at six timepoints throughout mycelial growth (15 days), substrate was collected, fractionated, and analyzed by quantitative pyrolysis-GC-MS, 2D-HSQC NMR, and SEC. Lignin decrease was highest between day 6 and day 10 and reached in total 42 % (w/w). The substantial delignification was accompanied by extensive structural changes of residual lignin, including increased syringyl to guaiacyl (S/G) ratios, accumulated oxidized moieties, and depleted intact interunit linkages. Hydroxypropiovanillone and hydroxypropiosyringone (HPV/S) subunits accumulated, which are indicative for β-|O-4' ether cleavage and imply a laccase-driven ligninolysis. We provide compelling evidence that A. bisporus is capable of extensive lignin removal, have obtained insights into mechanisms at play and susceptibilities of various substructures, thus we were contributing to understanding fungal lignin conversion.

Structural characterization of lignin-carbohydrate complexes from Chinese quince fruits extracted after enzymatic hydrolysis pretreatment

Chinese quince fruit (CQF) contains abundant pectin; however, the pectin cannot be efficiently separated by conventional approaches because of strong lignin-carbohydrate complexes (LCC). In this study, to elucidate the structural characteristics of the original LCC formed by lignin and pectin in CQF, single and multiple enzymatic hydrolysis pretreatments were innovatively performed, and the resulting LCC preparations were comprehensively characterized using a series of techniques. The enzymatic hydrolysis pretreatments significantly increase the LCC yield, releasing LCC fractions with low molecular weights (Mw = 4660-8288 Da). LCC-4, isolated by pretreatment with cellulase plus xylanase, had the highest galacturonic acid content (15.5 %), followed by LCC-2 (isolated by xylanase pretreatment) of 14.0 %. In CQF, lignin develops lignin-carbohydrate (LC) bonds with pectin to form LCC, with phenyl-glycoside bond being the dominant linkage. Although the pectinase pretreatment reduced the pectin content, signals of the LC linkages in the 2D-HSQC spectra were enhanced. LCC-4 could be considered as the most representative of the original LCC in CQF due to its high pectin content and multiple LCC signals in the 2D-HSQC spectrum. The structural understanding of the original LCC in CQF will lay a foundation for designing appropriate methods for extracting pectin from CQF.

Optimization of Approaches to Analysis of Lignin by Thermal Decomposition

The ratio of monomeric units is one of the main characteristics of lignin, which affects the possibilities and strategies for further processing. Pyrolytic and thermal desorption decomposition of lignins followed by mass detection of macromolecule fragments are the most common methods for determining the amount of lignin structural units. Two methods of thermal decomposition of lignin were studied: thermal desorption-GC/MS (TD-GC/MS) and pyrolysis-GC/MS (Py-GC/MS). It was noted that, when using different thermal decomposition modes, the composition of the products changes, which affects the accuracy of determining the amount of lignin structural fragments. This article investigated the influence of the sample weight, the thermal decomposition temperature, and the duration of the process in various modes on the quantitation of the lignin structural units. The optimal process conditions were established. It was shown that the DS-Py-GC/MS with cryofocusing, a sample weight of 0.2-0.4 mg, and heating from 50 to 400 °C at a rate of 120 °C/min are preferable. The HSQC NMR was used as a comparison method to obtain the content of the S/G/H units. The results showed the applicability of the proposed approaches to hardwood lignins close to native.

Evaluation of oxy-organosolv pretreatment on lignin extraction from wheat straw

To develop a characteristic "Lignin-first" strategy, the oxy-organosolv delignification processes under mild conditions were comprehensively investigated. Results showed that lignin yield could achieve about 50 % under the optimum process conditions of ethanol concentration 80 %, temperature 90 °C, liquid to wheat straw ratio 25:1 for powdery-scale substrates, which was 65.0 % higher than that for rod-scale substrates under the same conditions. The lignin structural and carbohydrate component results demonstrated the employment of oxygen induced great quantities of lignin dissolving out on the premise of little carbohydrate component (<1 %) and lignin structural (mainly β-O-4 units) changes. Moreover, based on the molecular weight and polydiversity comparison results, the aqueous oxygen could transfer homogeneously in mild organosolv system and result in lignin degradation uniformly. Besides, the employment of oxygen assisted in not only extending the massive lignin removal stage to 30 min and 50 min for P-OEEL and R-OEEL respectively, but also boost the delignification rate with comparison to P-EL and R-EL. Lastly, the excellent anti-oxidant properties of lignin from oxy-organosolv process were demonstrated by scavenging DPPH and ABTS radicals. The economic calculations showed that the cost for lignin production were about 1.58USD/g lignin from powdery-scale wheat straw, providing a competitive route for high-value utilize waste biomass.

Chemical structure change of lignin extracted from bamboo biomass by maleic acid

Maleic acid (MA) is an effective solvent for lignin extraction from raw biomass under mild conditions through acidolysis and hydrophobic interaction. To improve the valorization of lignin, the lignin extracted with MA was subjected to successive organic solvent fractionation to make the narrow M_w and M_n distribution and the homogeneity structure of lignin fragments. After successive organic solvent fractionation, the dispersity index (Đ) of lignin reduced from 2.86 to 1.25, and the purity and homogeneity were greatly improved. The phenolic hydroxyl content of lignin extracted by ethyl acetate was the highest (2.45 mmol/g), and the aliphatic-OH concentration was the lowest (0.65 mmol/g). While the βO4 bonds were partially cleaved, more hydrophobic substrates with CC linkages were recovered by ethyl acetate. Lignin fragments with a variety of main structures and functional groups were obtained effectively with successive fractionation. This method supplied broad prospects for the preparation of lignin and its applications in epoxy resin, rubber, thermoplastic additives and polymer raw materials.

Valorization of hydrolysis lignin from a spruce-based biorefinery by applying γ-valerolactone treatment

Hydrolysis lignin, i.e., the hydrolysis residue of cellulosic ethanol plants, was extracted with the green solvent γ-valerolactone (GVL). Treatments at 170-210 °C were performed with either non-acidified GVL/water mixtures (NA-GVL) or with mixtures containing sulfuric acid (SA-GVL). SA-GVL treatment at 210 °C resulted in the highest lignin solubilization (64% (w/w) of initial content), and 76% of the solubilized mass was regenerated by water-induced precipitation. Regenerated lignins were characterized through compositional analysis with sulfuric acid, as well as using pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS), high-performance size-exclusion chromatography (HPSEC), solid-state cross-polarization/magic angle spinning ¹³C nuclear magnetic resonance (CP/MAS ¹³C NMR) spectroscopy, ¹H-¹³C heteronuclear single-quantum coherence NMR (HSQC NMR), and Fourier-transform infrared (FTIR) spectroscopy. The characterization revealed that the main difference between regenerated lignins was their molecular weight. Molecular weight averages increased with treatment temperature, and they were higher and had broader distribution for SA-GVL lignins than for NA-GVL lignins.

Structural properties and antioxidation activities of lignins isolated from sequential two-step formosolv fractionation

In order to investigate the solubility behavior of lignin in formic acid (FA) solution Phragmites australis biomass was subjected to a sequential two-step formosolv fractionation using 88% FA followed by 70% FA to obtain four specific lignin fractions, designated as IFL-88%, IFSL-70%, IFIL-70% and IFL-EtAc. The structures of the four isolated lignin fractions were successfully characterized by gel permeation chromatography (GPC), Fourier transform infrared (FTIR) spectroscopy, two-dimensional heteronuclear single quantum coherence nuclear magnetic resonance spectroscopy (2D-HSQC NMR), thermogravimetric analysis (TGA), and gas chromatography-mass spectroscopy (GC/MS). Furthermore, the total phenolic content of the four isolated lignin samples was assessed by Folin-Ciocalteu analysis. The data from structural properties revealed that depolymerization of the isolated lignin fractions occurred via β-O-4 cleavage, accompanied by competitive condensation reaction. Interestingly, 70% aqueous FA could separate specific lignin fractions with different antioxidant capacities of ABTS˙+ and DPPH radical scavenging activity. Due to the high total phenolic hydroxyl content (25%) and low molecular weight (Mw = 2760 Da) and polydispersity index (PDI = 1.5), IFL-EtAc lignin showed excellent antioxidant activity at the same concentration of 2.0 mg mL-1 in comparison with three other isolated lignin fractions, and it was even equal to that of commercial antioxidant butylated hydroxytoluene (BHT). These findings are helpful to separate specific lignins with higher value as potential antioxidants by sequential two-step formosolv fractionation in lignin chemistry.

Modifying Surface Charges of a Thermophilic Laccase Toward Improving Activity and Stability in Ionic Liquid

The multicopper oxidase enzyme laccase holds great potential to be used for biological lignin valorization alongside a biocompatible ionic liquid (IL). However, the IL concentrations required for biomass pretreatment severely inhibit laccase activity. Due to their ability to function in extreme conditions, many thermophilic enzymes have found use in industrial applications. The thermophilic fungal laccase from Myceliophthora thermophila was found to retain high levels of activity in the IL [C₂C₁Im][EtSO₄], making it a desirable biocatalyst to be used for lignin valorization. In contrast to [C₂C₁Im][EtSO₄], the biocompatibility of [C₂C₁Im][OAC] with the laccase was markedly lower. Severe inhibition of laccase activity was observed in 15% [C₂C₁Im][OAc]. In this study, the enzyme surface charges were modified via acetylation, succinylation, cationization, or neutralization. However, these modifications did not show significant improvement in laccase activity or stability in [C₂C₁Im][OAc]. Docking simulations show that the IL docks close to the T1 catalytic copper, likely interfering with substrate binding. Although additional docking locations for [OAc]^- are observed after making enzyme modifications, it does not appear that these locations play a role in the inhibition of enzyme activity. The results of this study could guide future enzyme engineering efforts by showing that the inhibition mechanism of [C₂C₁Im][OAc] toward M. thermophila laccase is likely not dependent upon the IL interacting with the enzyme surface.

Biomass Pre-Extraction as a Versatile Strategy to Improve Biorefinery Feedstock Flexibility, Sugar Yields, and Lignin Purity

Feedstock flexibility is highly advantageous for the viability of (solvent-based) biorefineries but comes with the considerable challenge of having to cope with the varying nature and typically high abundance of nonlignocellulose compounds in the most readily available residual biomass streams. Here, we demonstrate that mild aqueous acetone organosolv fractionation of various complex lignocellulosic raw materials (roadside grass, wheat straw, birch branches, almond shells, and a mixed stream thereof) is indeed negatively affected by these compounds and present a versatile strategy to mitigate this bottleneck in biorefining. A biomass pre-extraction approach has been developed to remove the detrimental extractives with (aqueous) acetone prior to fractionation. Pre-extraction removed organic extractives as well as minerals, primarily reducing acid dose requirements for fractionation and loss of hemicellulose sugars by degradation and improved the purity of the isolated lignin. We show how pre-extraction affects the effectiveness of the biorefinery process, including detailed mass balances for pretreatment, downstream processing, and product characteristics, and how it affects solvent and energy use with a first conceptual process design. The integrated biorefining approach allows for the improved compatibility of biorefineries with sustainable feedstock supply chains, enhanced biomass valorization (i.e., isolation of bioactive compounds from the extract), and more effective biomass processing with limited variation in product quality.

A combination of deep eutectic solvent and ethanol pretreatment for synergistic delignification and enhanced enzymatic hydrolysis for biorefinary process

A novel pretreatment system containing deep eutectic solvents and ethanol (DES-E) for synergistic carbohydrate conversion and delignification was reported in this study. The DES-E pretreatment resulted in an enhanced glucose yield compared to individual DES and ethanol pretreatment for the three tested biomass, including Broussonetia papyrifera, corn stover and pine. To further explore the delignification mechanism, the solubilized lignin and residual lignin from Broussonetia papyrifera was recovered and extracted, then thoroughly characterized. The highest total OH content was found in the DES-E solubilized lignin, which could be used as antioxidant. The presence of ethanol in pretreatment liquor could protect the β-O-4 substructure from breakage and reduce lignin condensation, which favors the subsequent enzymatic hydrolysis. Comparable glucose yield and delignification performance was achieved by recycled DES. DES-E pretreatment offers a promising method for lignin isolation and cellulose digestibility improvement simultaneously.

Physico-chemical interactions within lignocellulosic biomass and their importance in developing solvent based deconstruction methods

Fundamental understanding of physico-chemical interactions among the biopolymers in lignocellulosic biomass is crucial to develop atom-efficient deconstruction methods.

Hydrothermal oxidative valorisation of lignin into functional chemicals: A review

Lignin is a waste by-product of bio-refineries and paper-pulp industries. It has an attractive potential to produce numerous valuable chemicals due to its highly aromatic character. At present, large amount of lignin is burnt as a source of energy due to lack of suitable efficient lignin valorisation processes. The challenge exists in handling its complex heterogeneous structure and bond breaking at selective locations. The production of high value chemicals/petrochemical feedstocks will improve the economic viability of a bio-refinery. Oxidative depolymerization is a promising way to produce functional compounds from lignin. The aim of the current review is to present the novel methodologies currently used in the area of lignin oxidative depolymerization including effect of temperature, residence time, solvent, oxidizing agents, homogeneous and heterogeneous catalysis etc. It aims to present an insight into the structure of lignin and its breakdown mechanism.

A comprehensive study of the promoting effect of manganese on white rot fungal treatment for enzymatic hydrolysis of woody and grass lignocellulose

BACKGROUND

The efficiency of biological systems as an option for pretreating lignocellulosic biomass has to be improved to make the process practical. Fungal treatment with manganese (Mn) addition for improving lignocellulosic biomass fractionation and enzyme accessibility were investigated in this study. The broad-spectrum effect was tested on two different types of feedstocks with three fungal species. Since the physicochemical and structural properties of biomass were the main changes caused by fungal degradation, detailed characterization of biomass structural features was conducted to understand the mechanism of Mn-enhanced biomass saccharification.

RESULTS

The glucose yields of fungal-treated poplar and wheat straw increased by 2.97- and 5.71-fold, respectively, after Mn addition. Particularly, over 90% of glucose yield was achieved in Mn-assisted Pleurotus ostreatus-treated wheat straw. A comparison study using pyrolysis gas chromatography mass spectrometry (Py-GC/MS) and two-dimensional 1H-13C heteronuclear single quantum coherence (2D HSQC) nuclear magnetic resonance (NMR) spectroscopy was conducted to elucidate the role of Mn addition on fungal disruption of the cross-linked structure of whole plant cell wall. The increased Cα-oxidized products was consistent with the enhanced cleavage of the major β-O-4 ether linkages in poplar and wheat straw lignin or in the wheat straw lignin-carbohydrate complexes (LCCs), which led to the reduced condensation degree in lignin and decreased lignin content in Mn-assisted fungal-treated biomass. The correlation analysis and principal component analysis (PCA) further demonstrated that Mn addition to fungal treatment enhanced bond cleavage in lignin, especially the β-O-4 ether linkage cleavage played the dominant role in removing the biomass recalcitrance and contributing to the glucose yield enhancement. Meanwhile, enhanced deconstruction of LCCs was important in reducing wheat straw recalcitrance. The findings provided not only mechanistic insights into the Mn-enhanced biomass digestibility by fungus, but also a strategy for improving biological pretreatment efficiency of lignocellulose.

CONCLUSION

The mechanism of enhanced saccharification of biomass by Mn-assisted fungal treatment mainly through Cα-oxidative cleavage of β-O-4 ether linkages further led to the decreased condensation degree in lignin, as a result, biomass recalcitrance was significantly reduced by Mn addition.

Valorization of Rice Straw via Hydrotropic Lignin Extraction and Its Characterization

Rice straw hydrotropic lignin was extracted from p-Toluene sulfonic acid (p-TsOH) fractionation with a different combined delignification factor (CDF). Hydrotropic lignin characterization was systematically investigated, and alkaline lignin was also studied for the contrast. Results showed that the hydrotropic rice straw lignin particle was in nanometer scopes. Compared with alkaline lignin, the hydrotropic lignin had greater molecular weight. NMR analysis showed that β-aryl ether linkage was well preserved at low severities, and the unsaturation in the side chain of hydrotropic lignin was high. H units and G units were preferentially degraded and subsequently condensed at high severity. High severity also resulted in the cleavage of part β-aryl ether linkage. ³¹P-NMR showed the decrease in aliphatic hydroxyl groups and the increasing carboxyl group content at high severity. The maximum weight loss temperature of the hydrotropic lignin was in the range of 330–350 °C, higher than the alkaline lignin, and the glass conversion temperature (T_g) of the hydrotropic lignin was in the range of 107–125 °C, lower than that of the alkaline lignin. The hydrotropic lignin has high β-aryl ether linkage content, high activity, nanoscale particle size, and low T_g, which is beneficial for its further valorization.

Lignin-derived 3D porous graphene on carbon cloth for flexible supercapacitors

In this work, we reported a new method to fabricate flexible carbon-based supercapacitor electrodes derived from a commercialized and low-cost lignin. The fabrication process skips traditional stabilization/carbonization/activation for lignin-based carbon production. Also, the process reported here was green and facile, with minimum solvent use and no pretreatment required. Characterization of the lignin showed that it has common properties among all types of lignin. The lignin was impregnated on carbon cloth and then subjected to direct laser writing to form the desired electrodes (LLC). The results showed that lignin was successfully bonded to carbon cloth. The LLC has a good porous carbon structure with a high I _G/I _D ratio of 1.39, and a small interlayer spacing d ₀₀₂ of 0.3436 nm, which are superior to most of the reported lignin-based carbons. Although not optimized, the fabricated LLC showed good supercapacitance behavior with an areal capacitance of 157.3 mF cm^-2 at 0.1 mA cm^-2. In addition, the superior flexibility of LLC makes it a promising electrode that can be used more widely in portable devices. Conceptually, this method can be generalized to all types of lignin and can define intriguing new research interests towards lignin applications.

Plant cell wall chemistry: implications for ruminant utilisation

Ruminants have adapted to cope with bulky, fibrous forage diets by accommodating a large, diverse microbial population in the reticulo-rumen. Ruminants are dependent on forages as their main sources of energy and other nutrients. Forages are comprised of a complex matrix of cellulose, hemicellulose, protein, minerals and phenolic compounds (including lignin and tannins) with various linkages; many of which are poorly defined. The composition and characteristics of polysaccharides vary greatly among forages and plant cell walls. Plant cell walls are linked and packed together in tight configurations to resist degradation, and hence their nutritional value to animals varies considerably, depending on composition, structure and degradability. An understanding of the inter-relationship between the chemical composition and the degradation of plant cell walls by rumen microorganisms is of major economic importance to ruminant production. Increasing the efficiency of fibre degradation in the rumen has been the subject of extensive research for many decades. This review summarises current knowledge of forage chemistry in order to develop strategies to increase efficiency of forage utilisation by ruminants.

Evaluating the bio-application of biomacromolecule of lignin-carbohydrate complexes (LCC) from wheat straw in bone metabolism via ROS scavenging

Lignin-carbohydrate complexes (LCC) arebiomacromolecules that can be obtained from different biomass. Even some works have shown the LCC can efficiently scavenge the intracellular and endogenous reactive oxygen species (ROS), while little work has been carried out to investigate the potential application of LCC for ROS-related treatment in biological filed, especially for the treatment of periprosthetic osteolysis in vivo. In this work, Lignin-rich (LCC-A) and carbohydrate-rich (LCC-B) fractions in wheat straw are isolated and used as the ROS scavenger to promote osteoblast differentiation and inhibit osteoclast differentiation. The chemical composition and structures are characterized by high performance anion exchange chromatography (HPAEC) and nuclear magnetic resonance (NMR) technologies (quantitative ¹³C NMR and 2D-HSQC NMR), respectively. The results showed LCC-A possesses higher in vitro ROS-scavenging ability than LCC-B (89.8% vs 57.8%) and to inhibit osteoclast differentiation, whereas LCC-B more significantly activates cellular antioxidant activities via the KEAP1-NRF2-ARE pathway (218.5% vs 438.0% in the level of HO-1), thus promoting osteoblast differentiation in an inflammatory environment. Moreover, the therapeutic administration of LCC-A and LCC-B for Ti-particle-induced osteolytic murine calvariae showed both of them positively regulate and restore the bone metabolism, while preventing calvaria impairment. Hence, LCC from wheat straw exhibits efficient bone protective effects, suggesting it may be used as the promising ROS scavenger for clinical treatment of periprosthetic osteolysis.

Tracing Sweetgum Lignin's Molecular Properties through Biorefinery Processing

Changes to the molecular properties of lignin over the course of biorefinery processing were investigated by using sweetgum as a feedstock. Hydrothermal pretreatment has been used because it is an economically attractive, green process. Three representative biorefinery lignin preparations were obtained, with about 70 % yield based on raw lignin. The three fractions included soluble lignin adsorbed on resin (XADL), solvent-extracted lignin (HTCELp), and an additional ball-milled residual lignin (HTRELp). By comparing the raw and biorefinery lignin preparations, it can be concluded that lignin undergoes both degradation and condensation throughout the various stages of the hydrothermal-based biorefinery process. The two fractions made soluble by biorefinery processing, XADL and HTCELp, were found to be low-molecular-weight degradation products enriched with free phenolic hydroxyl groups. In addition, about 15 % of noncondensed phenolic units were involved in condensation reactions. Quantitative NMR spectroscopy analysis revealed that at least about 28 % of β-O-4' substructures were cleaved. Hibbert's ketones were identified in XADL and HTRELp, which provided evidence of lignin undergoing acidolysis. The contents of β-5' and β-β' did not change significantly upon biorefinery processing. Finally, episyringaresinol was detected in XADL and HTCELp. It is hoped that these findings will help to further demonstrate the specific effects of biorefinery processing on lignin in hardwood and facilitate its utilization to improve biorefinery economics.

Characterization of Lignin Structures in Phyllostachys edulis (Moso Bamboo) at Different Ages

Bamboo is a gramineous plant widely distributed in China and has great prospects. Normally, local people cut bamboo culm at first year for paper milling or at six years for construction. Understanding lignin changes in bamboo with aging is necessary for better exploring the application of bamboo at different ages and can also promote the application of bamboo more effectively. Based on the previous study, the chemical structure and the lignin content of bamboo at different ages were further explored by FT-IR, GPC, NMR and other chemical methods in this paper. Results showed that the lignin structures of bamboo at different ages were similar with three monomers of S, G and H, but the molecular weight increased with age. Quantitative structure estimation further confirmed that S-type lignin content and S/G ratio of bamboo lignin constantly increased with age.

Structural Motifs of Wheat Straw Lignin Differ in Susceptibility to Degradation by the White-Rot Fungus Ceriporiopsis subvermispora

The white-rot fungus Ceriporiopsis subvermispora delignifies plant biomass extensively and selectively and, therefore, has great biotechnological potential. We previously demonstrated that after 7 weeks of fungal growth on wheat straw 70% w/w of lignin was removed and established the underlying degradation mechanisms via selectively extracted diagnostic substructures. In this work, we fractionated the residual (more intact) lignin and comprehensively characterized the obtained isolates to determine the susceptibility of wheat straw lignin's structural motifs to fungal degradation. Using 13C IS pyrolysis gas chromatography-mass spectrometry (py-GC-MS), heteronuclear single quantum coherence (HSQC) and 31P NMR spectroscopy, and size-exclusion chromatography (SEC) analyses, it was shown that β-O-4' ethers and the more condensed phenylcoumarans and resinols were equally susceptible to fungal breakdown. Interestingly, for β-O-4' ether substructures, marked cleavage preferences could be observed: β-O-4'-syringyl substructures were degraded more frequently than their β-O-4'-guaiacyl and β-O-4'-tricin analogues. Furthermore, diastereochemistry (threo > erythro) and γ-acylation (γ-OH > γ-acyl) influenced cleavage susceptibility. These results indicate that electron density of the 4'-O-coupled ring and local steric hindrance are important determinants of oxidative β-O-4' ether degradation. Our findings provide novel insight into the delignification mechanisms of C. subvermispora and contribute to improving the valorization of lignocellulosic biomass.

From waste to food: Optimising the breakdown of oil palm waste to provide substrate for insects farmed as animal feed

Waste biomass from the palm oil industry is currently burned as a means of disposal and solutions are required to reduce the environmental impact. Whilst some waste biomass can be recycled to provide green energy such as biogas, this investigation aimed to optimise experimental conditions for recycling palm waste into substrate for insects, farmed as a sustainable high-protein animal feed. NMR spectroscopy and LC-HRMS were used to analyse the composition of palm empty fruit bunches (EFB) under experimental conditions optimised to produce nutritious substrate rather than biogas. Statistical pattern recognition techniques were used to investigate differences in composition for various combinations of pre-processing and anaerobic digestion (AD) methods. Pre-processing methods included steaming, pressure cooking, composting, microwaving, and breaking down the EFB using ionic liquids. AD conditions which were modified in combination with pre-processing methods were ratios of EFB:digestate and pH. Results show that the selection of pre-processing method affects the breakdown of the palm waste and subsequently the substrate composition and biogas production. Although large-scale insect feeding trials will be required to determine nutritional content, we found that conditions can be optimised to recycle palm waste for the production of substrate for insect rearing. Pre-processing EFB using ionic liquid before AD at pH6 with a 2:1 digestate:EFB ratio were found to be the best combination of experimental conditions.

Modular Engineering of Biomass Degradation Pathways

Production of fuels and chemicals from renewable lignocellulosic feedstocks is a promising alternative to petroleum-derived compounds. Due to the complexity of lignocellulosic feedstocks, microbial conversion of all potential substrates will require substantial metabolic engineering. Non-model microbes offer desirable physiological traits, but also increase the difficulty of heterologous pathway engineering and optimization. The development of modular design principles that allow metabolic pathways to be used in a variety of novel microbes with minimal strain-specific optimization will enable the rapid construction of microbes for commercial production of biofuels and bioproducts. In this review, we discuss variability of lignocellulosic feedstocks, pathways for catabolism of lignocellulose-derived compounds, challenges to heterologous engineering of catabolic pathways, and opportunities to apply modular pathway design. Implementation of these approaches will simplify the process of modifying non-model microbes to convert diverse lignocellulosic feedstocks.

PbrMYB169 positively regulates lignification of stone cells in pear fruit

Stone cells negatively affect fruit quality because of their firm and lignified cell walls, so are targets for reduction in pear breeding programmes. However, there is only limited knowledge of the molecular mechanisms underlying the formation of stone cells. Here, we show that PbrMYB169, an R2R3 MYB transcription factor, of Pyrus bretschneideri positively regulates lignification of stone cells in pear fruit. PbrMYB169 was shown to be co-expressed with lignin biosynthesis genes during pear fruit development, and this co-expression pattern was coincident with stone cell formation in the fruit of Pyrus bretschneideri 'Dangshansuli'. The PbrMYB169 expression level was also positively correlated with stone cell content in 36 pear cultivars tested. PbrMYB169 protein significantly activated the promoter of lignin genes C3H1, CCR1, CCOMT2, CAD, 4CL1, 4CL2, HCT2, and LAC18 via binding to AC elements [ACC(T/A)ACC] in these promoters. Furthermore, overexpression of PbrMYB169 in transgenic Arabidopsis plants enhanced the expression of lignin genes, and increased lignin deposition and cell wall thickness of vessel elements, but did not change the ratio of syringyl and guaiacyl lignin monomers. In conclusion, PbrMYB169 appears to be a transcriptional activator of lignin biosynthesis and regulates secondary wall formation in fruit stone cells. This study advances the understanding of the regulation of lignin biosynthesis and provides valuable molecular genetic information for reducing stone cell content in pear fruit.

Rapid Nondestructive Fractionation of Biomass (≤15 min) by using Flow-Through Recyclable Formic Acid toward Whole Valorization of Carbohydrate and Lignin

Whole valorization of carbohydrate and lignin from biomass was achieved by rapid flow-through fractionation (RFF) within 15 min. Wheat straw was effectively deconstructed into its principle components without degradation by using easily recyclable aqueous formic acid (72 wt %) at 130 °C. The obtained cellulose-rich solid showed a nearly complete glucan recovery and 73.8 % glucose conversion after enzymatic hydrolysis. Xylan also reached full recovery with negligible furfural formation with a sum of 80 % of oligo/mono xylose in spent liquor and 20 % of xylan remaining in the solid. Up to 75.4 % lignin was dissolved in the spent liquor and further fractionated into water-insoluble (WIL) and water-soluble lignin (WSL) by dilution with water. WIL showed a non-condensed and well-preserved structure with 84.5 % β-O-4 remaining, which is believed to be beneficial for catalytic conversion into low-molecular-weight chemicals and fuels. The concentration of employed formic acid was below the formic acid/water azeotrope, and therefore the reaction medium could be restored through simple distillation. Together with the joint valorization of lignin and carbohydrates, the presented RFF is a promising process for sustainable biorefinery.

PbrmiR397a regulates lignification during stone cell development in pear fruit

Lignified stone cells substantially reduce fruit quality. Therefore, it is desirable to inhibit stone cell development using genetic technologies. However, the molecular mechanisms regulating lignification are poorly understood in fruit stone cells. In this study, we have shown that microRNA (miR) miR397a regulates fruit cell lignification by inhibiting laccase (LAC) genes that encode key lignin biosynthesis enzymes. Transient overexpression of PbrmiR397a, which is the miR397a of Chinese pear (Pyrus bretschneideri), and simultaneous silencing of three LAC genes reduced the lignin content and stone cell number in pear fruit. A single nucleotide polymorphism (SNP) identified in the promoter of the PbrmiR397a gene was found to associate with low levels of fruit lignin, after analysis of the genome sequences of sixty pear varieties. This SNP created a TCA element that responded to salicylic acid to induce gene expression as confirmed using a cell-based assay system. Furthermore, stable overexpression of PbrmiR397a in transgenic tobacco plants reduced the expression of target LAC genes and decreased the content of lignin but did not change the ratio of syringyl- and guaiacyl-lignin monomers. Consistent with reduction in lignin content, the transgenic plants showed fewer numbers of vessel elements and thinner secondary walls in the remaining elements compared to wild-type control plants. This study has advanced our understanding of the regulation of lignin biosynthesis and provided useful molecular genetic information for improving pear fruit quality.

Reactivity improvement by phenolation of wheat straw lignin isolated from a biorefinery process

This work describes an effective phenolation process to improve wheat straw biorefinery lignin reactivity.

Structural characterization of lignin and its carbohydrate complexes isolated from bamboo (Dendrocalamus sinicus)

Isolation of earth abundant biopolymer, Lignin, from Dendrocalamus sinicus and their structural properties were investigated to achieve its large-scale practical applications in value-added products. Two lignin fractions (MWL, DSL) were isolated with successive treatments of dioxane and dimethylsulfoxide (DMSO) from dewaxed and ball milled bamboo (D. sinicus) sample. The two-step treatments yielded 52.1% lignin based on the total lignin content in the dewaxed bamboo sample. Spectroscopy analyses indicated that the isolated bamboo lignin was a typical grass lignin, consisting of p-hydroxyphenyl, guaiacyl, and syringyl units. The major interunit linkages presented in the obtained bamboo lignin were β-O-4' aryl ether linkages, together with lower amounts of β-β', β-5', and β-1' linkages. The tricin was detected to be linked to lignin polymer through the β-O-4' linkage in the bamboo. In addition, phenyl glycoside and benzyl ether lignin-carbohydrate complexes (LCC) linkages were clearly detected in bamboo (D. sinicus), whereas the γ-ester LCC linkages were ambiguous due to the overlapping NMR signals with other substructures. The detailed structural properties of the obtained lignin fraction together with the light-weight will benefit efficient utilization of natural polymers as a possibly large-scale bio-based precursor for making polymeric materials, biochemicals, functional carbon and biofuels, and multifunctional polymer nanocomposites.

Polychlorinated dibenzo-p-dioxin and dibenzofuran precursors and formation mechanisms during non-woodpulp chlorine bleaching process

There is great concern about decreasing the amounts of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) that are formed and emitted during the chlorine bleaching of pulp. The formation of PCDD/Fs during non-woodpulp chlorine bleaching was investigated in the study described here. Wheat straw was separated into three components, cellulose, hemicellulose, and lignin. Chlorination experiments were performed, and lignin and hemicellulose contributed more than cellulose to PCDD/F formation when the pulp was bleached using chlorine. The chemical components of lignin were identified by gas chromatography mass spectrometry, and nine possible PCDD/F precursors were quantified by gas chromatography tandem mass spectrometry. Spiked chlorination experiments were performed to investigate the effects of these compounds on PCDD/F formation. 4-Ethyl-2-methoxyphenol had the strongest effect on PCDD/F formation, followed by p-chlorophenol, and guaiacol. All the test compounds promoted polychlorinated dibenzofuran formation but had limited effects on polychlorinated dibenzo-p-dioxin formation. The results allowed mechanisms for the formation of PCDD/Fs from phenol, chlorophenol, catechol, and guaiacol to be proposed.

Mechanistic insight in the selective delignification of wheat straw by three white-rot fungal species through quantitative 13C-IS py-GC-MS and whole cell wall HSQC NMR

BACKGROUND

The white-rot fungi Ceriporiopsis subvermispora (Cs), Pleurotus eryngii (Pe), and Lentinula edodes (Le) have been shown to be high-potential species for selective delignification of plant biomass. This delignification improves polysaccharide degradability, which currently limits the efficient lignocellulose conversion into biochemicals, biofuels, and animal feed. Since selectivity and time efficiency of fungal delignification still need optimization, detailed understanding of the underlying mechanisms at molecular level is required. The recently developed methodologies for lignin quantification and characterization now allow for the in-depth mapping of fungal modification and degradation of lignin and, thereby, enable resolving underlying mechanisms.

RESULTS

Wheat straw treated by two strains of Cs (Cs1 and Cs12), Pe (Pe3 and Pe6) and Le (Le8 and Le10) was characterized using semi-quantitative py-GC-MS during fungal growth (1, 3, and 7 weeks). The remaining lignin after 7 weeks was quantified and characterized using 13C lignin internal standard based py-GC-MS and whole cell wall HSQC NMR. Strains of the same species showed similar patterns of lignin removal and degradation. Cs and Le outperformed Pe in terms of extent and selectivity of delignification (Cs ≥ Le >> Pe). The highest lignin removal [66% (w/w); Cs1] was obtained after 7 weeks, without extensive carbohydrate degradation (factor 3 increased carbohydrate-to-lignin ratio). Furthermore, though after treatment with Cs and Le comparable amounts of lignin remained, the structure of the residual lignin vastly differed. For example, Cα-oxidized substructures accumulated in Cs treated lignin up to 24% of the total aromatic lignin, a factor two higher than in Le-treated lignin. Contrarily, ferulic acid substructures were preferentially targeted by Le (and Pe). Interestingly, Pe-spent lignin was specifically depleted of tricin (40% reduction). The overall subunit composition (H:G:S) was not affected by fungal treatment.

CONCLUSIONS

Cs and Le are both able to effectively and selectively delignify wheat straw, though the underlying mechanisms are fundamentally different. We are the first to identify that Cs degrades the major β-O-4 ether linkage in grass lignin mainly via Cβ-O-aryl cleavage, while Cα-Cβ cleavage of inter-unit linkages predominated for Le. Our research provides a new insight on how fungi degrade lignin, which contributes to further optimizing the biological upgrading of lignocellulose.

Synthesis and Characterization of Carboxyethylated Lignosulfonate

Lignosulfonate is a byproduct of the sulfite pulping process and has limited use in industry. The main objective of this study was to investigate the carboxyethylation of lignosulfonate to increase its charge density to broaden its applications. The carboxyethylation of lignosulfonate was optimized under the conditions of 30 wt % NaOH, 2.0 mol mol^-1 2-chloropropinic acid/lignosulfonate, 90 °C, 0.5 h, and 0.03 mol 2-chloropropinic acid, which produced carboxyethylated lignosulfonate with a charge density and molecular weight of -3.51 meq g^-1 and 46 493 g mol^-1 , respectively. The mechanism of the carboxyethylation reaction using 2-chloropropinic acid by an S_N 1 pathway in an alkaline solution was discussed. Methylation was also used to mask the phenolic hydroxide groups of lignosulfonate to investigate if carboxyethylation occurred on aliphatic hydroxide groups of lignosulfonate. The produced carboxyethylated lignosulfonate was characterized by using FTIR spectroscopy, NMR spectroscopy, gel permeation chromatography, and elemental and functional group analyses. Basic ¹ H-¹ H 2 D COSY NMR spectroscopy was used to record the coupled spins of the carboxyethyl group on carboxyethylated lignosulfonate. The information from 1 D ¹ H NMR and 2 D NMR COSY spectroscopy provided evidence for the existence of a 1-carboxyethyl group on the carboxyethylated lignosulfonate structure.

Elucidating Tricin-Lignin Structures: Assigning Correlations in HSQC Spectra of Monocot Lignins

Tricin [5,7-dihydroxy-2-(4-hydroxy-3,5-dimethoxyphenyl)-4H-chromen-4-one] is a flavone that has been found to be incorporated in grass lignin polymers via 4'⁻O⁻β coupling. Herein, we investigated the tricin-lignin structure using nuclear magnetic resonance (NMR) methods by comparing the 1H⁻13C heteronuclear correlation (HSQC) NMR spectra of the isolated lignin with a series of dimeric and trimeric tricin-4'⁻O⁻β-ether model compounds. Results showed that the tricin moiety significantly affects the chemical shift of the Cβ/Hβ of 4'⁻O⁻β unit, producing peaks at around δC/δH 82.5⁻83.5/4.15⁻4.45, that differ from the Cβ/Hβ correlations from normal 4⁻O⁻β units formed solely by monolignols, and that have to date been unassigned.