Dehydrogenative polymerization of coniferyl alcohol in artificial polysaccharides matrices: effects of xylan on the polymerization

Biomimetic Dehydrogenation of Non-Conventional Lignin Monomers on Cellulose Ferulate Interfaces

Cellulose ferulate, synthesized by Mitsunobu reaction, is shaped into thin films and also used as an aqueous dispersion to perform artificial lignin polymerization on anchor groups. This biomimetic approach is carried out in a Quartz crystal microbalance with a dissipation monitoring (QCM-D) device to enable online monitoring of the dehydrogenation, applying H2O2 and adsorbed horseradish peroxidase (HRP). The systematic use of phenylpropanoids with different oxidation states, i.e., ferulic acid, coniferyl aldehyde, coniferyl alcohol, and eugenol allowed to conclude structure-property relationships. Both the deposited material, as well as the surface roughness increased with the hydrophobicity of the monomers. Beyond surface characterizations, py-GC-MS, HSQC NMR spectroscopy and Size exclusion chromatography (SEC) measurements revealed the linkage types β-β, β-5, 5-5, and β-O-4, as well as the oligomeric character of the dehydrogenation products. All samples possessed an antibacterial activity against B. subtilis and can be used in the field of antimicrobial biomaterials.

Multifaceted analyses reveal carbohydrate metabolism mainly affecting the quality of postharvest bamboo shoots

Bamboo shoot is one of nutritious vegetables in China. However, the edible quality of fresh bamboo shoots deteriorates easily after harvest. Here, morphological, physiological, transcriptomic and microRNA sequencing analyses were conducted to investigate the postharvest characteristics of moso bamboo (Phyllostachys edulis) shoots. Rapid decreases of soluble sugars, structural polysaccharides and hydrolyzed tannins, and increases of lignin and condensed tannins were observed in the postharvest bamboo shoots. Differentially expressed genes (DEGs) and miRNAs with opposite trends were mainly enriched in structural polysaccharide metabolism, starch and sucrose metabolism and glycolysis pathways, which were consistent with the changes of carbohydrates. A co-expression network of carbohydrate metabolism was constructed, which was verified by qPCR and yeast one-hybrid (Y1H) assay. Furthermore, the function of one hub glycosyltransferase gene was validated in Arabidopsis, which confirmed that it was involved in xylan biosynthesis. These results are of great significance for revealing the carbohydrate metabolism mechanisms of postharvest bamboo shoots and provide a potential candidate gene for molecular breeding related to xylan in the future.

Origins of covalent linkages within the lignin-carbohydrate network of biomass

Covalent linkages between lignin and the surrounding carbohydrate network, often referred to as lignin-carbohydrate complexes (LCCs), have been proposed to affect the organization of the biomass microstructure and directly correlate with the recalcitrant nature of biomass. However, the existence and frequency of these LCC linkages remain controversial and largely unknown, primarily due to the harsh experimental techniques available to characterize them. During the predominant lignin polymerization pathway a reactive intermediate is formed. Though this intermediate can covalently bind to the surrounding cellulose/hemicellulose matrix, it has been traditionally assumed to react exclusively with water, leading to purely physical interactions between lignin and cellulose/hemicellulose in the cell wall. This work, for the first time, provides direct evidence of the molecular mechanism of the formation of benzyl ether and benzyl ester LCC linkages via the speculated lignin polymerization pathway. The formation of these LCC linkages showed thermodynamic favorability, while remaining kinetically facile, compared to the previously assumed mechanism of the lignin intermediate reacting with water. The present work suggests that the surrounding carbohydrate matrix could play a role in the organization of lignin deposition and these covalent linkages could be a key factor in biomass recalcitrance.

Monolignol export by diffusion down a polymerization-induced concentration gradient

Lignin, the second most abundant biopolymer, is a promising renewable energy source and chemical feedstock. A key element of lignin biosynthesis is unknown: how do lignin precursors (monolignols) get from inside the cell out to the cell wall where they are polymerized? Modeling indicates that monolignols can passively diffuse through lipid bilayers, but this has not been tested experimentally. We demonstrate significant monolignol diffusion occurs when laccases, which consume monolignols, are present on one side of the membrane. We hypothesize that lignin polymerization could deplete monomers in the wall, creating a concentration gradient driving monolignol diffusion. We developed a two-photon microscopy approach to visualize lignifying Arabidopsis thaliana root cells. Laccase mutants with reduced ability to form lignin polymer in the wall accumulated monolignols inside cells. In contrast, active transport inhibitors did not decrease lignin in the wall and scant intracellular phenolics were observed. Synthetic liposomes were engineered to encapsulate laccases, and monolignols crossed these pure lipid bilayers to form polymer within. A sink-driven diffusion mechanism explains why it has been difficult to identify genes encoding monolignol transporters and why the export of varied phenylpropanoids occurs without specificity. It also highlights an important role for cell wall oxidative enzymes in monolignol export.

Growing of Artificial Lignin on Cellulose Ferulate Thin Films

Thin films of cellulose ferulate were designed to study the formation of dehydrogenation polymers (DHPs) on anchor groups of the surface. Trimethylsilyl (TMS) cellulose ferulate with degree of substitution values of 0.35 (ferulate) and 2.53 (TMS) was synthesized by sophisticated polysaccharide chemistry applying the Mitsunobu reaction. The biopolymer derivative was spin-coated into thin films to yield ferulate moieties on a smooth cellulose surface. Dehydrogenative polymerization of coniferyl alcohol was performed in a Quartz crystal microbalance with a dissipation monitoring device in the presence of H₂O₂ and adsorbed horseradish peroxidase. The amount of DHP formed on the surface was found to be independent of the base layer thickness from 14 to 75 nm. Pyrolysis-GC-MS measurements of the DHP revealed β-O-4 and β-5 linkages. Mimicking lignification of plant cell walls on highly defined model films enables reproducible investigations of structure-property relationships.

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.

Bamboo-derived carboxymethyl cellulose for liquid film as renewable and biodegradable agriculture mulching

Mulching has been extensively sought after in modern agriculture. However, massive utilization of plastics for mulching has induced severe environmental concerns. Developing biodegradable mulch thus represents an emerging need for future agriculture. By using bamboo-derived carboxymethyl cellulose (CMC), this study proposed a crosslinking strategy to prepare liquid film as quality mulch. CMC was synthesized by delignifying bamboo and etherifying resultant cellulose, which was then blended with polyvinyl alcohol (PVA) and crosslinked by glutaraldehyde to prepare a liquid film. By simply spraying on soil, mulch can quickly form on soil surface. Especially, bamboo-timber derived mulch had strong mechanical property (18.2 MPa), good transmittance (74.2%) and moisture absorption (141%), and excellent soil moisture retention. More importantly, about 64% of used mulches were biodegraded within 60-d after burring in soil, which will not need post-handling. These results highlighted that bamboo-derived mulch can be an alternative of current plastic mulch to tackle associated environmental pollution.

In-situ lignin drives lytic polysaccharide monooxygenases to enhance enzymatic saccharification

Recently low-molecular lignin was reported to activate lytic polysaccharide monooxygenases (LPMOs) to oxidize cellulose. However, whether lignin formed in cell wall can play the role as electron donor for LPMOs is still largely unknown due to the complex ultrastructure of lignocellulosic biomass. In this study, we presented a new strategy to elucidate in-situ lignin function in LPMOs reaction. A lignocellulosic mimicking model was used as substrate, which was equipped with a polysaccharide template of self-assembled bacterial cellulose film and synthesized lignin. Remarkably, it has been demonstrated that lignin polymer deposited on cellulose can reduce LPMOs in-situ for cellulose oxidation and then boost cellulose hydrolysis, and the cellulose conversion ratio of the mimicked lignocellulosic film was increased by 26.0%. More importantly, lignin in-situ might exceed the well-known reductant of ascorbic acid to drive LPMOs for cellulase enzymatic hydrolysis with equivalent cellulose oxidation efficiency and extremely lower H₂O₂ generation, avoiding the inactivation of enzymes. The maximum H₂O₂ yield from lignin-driven LPMO reaction was 75.8% lower than that from ascorbic acid-driven reaction. Therefore, by using the lignocellulosic mimicking model, we have elucidated the function of in-situ lignin in boosting enzymatic hydrolysis. Such understanding could significantly promote current utilization of LPMOs in lignocellulosic biorefinery.

Artificially lignified cell wall catalyzed by peroxidase selectively localized on a network of microfibrils from cultured cells

An artificial lignified cell wall was synthesized in three steps: (1) isolation of microfibrillar network; (2) localization of peroxidase through immunoreaction; and (3) polymerization of DHP to lignify the cell wall. Artificial woody cell wall synthesis was performed following the three steps along with the actual formation in nature using cellulose microfibrils extracted from callus derived from Cryptomeria japonica. First, we constructed a polysaccharide network on a transmission electron microscopy (TEM) grid. The preparation method was optimized by chemical treatment, followed by mechanical fibrillation to create a microfibrillated network. Morphology was examined by TEM, and chemical characterization was by Fourier transform infrared (FTIR) spectroscopy. Second, we optimized the process to place peroxidase on the microfibrils via an immunoreaction technique. Using a xyloglucan antibody, we could ensure that gold particles attached to the secondary antibodies were widely and uniformly localized along with the microfibril network. Third, we applied the peroxidase attached to secondary antibodies and started to polymerize the lignin on the grid by simultaneously adding coniferyl alcohol and hydrogen peroxide. After 30 min of artificial lignification, TEM observation showed that lignin-like substances were deposited on the polysaccharide network. In addition, FTIR spectra revealed that the bands specific for lignin had increased, demonstrating the successful artificial formation of woody cell walls. This approach may be useful for studying woody cell wall formation and for producing made-to-order biomaterials.

Unassisted solar lignin valorisation using a compartmented photo-electro-biochemical cell

Lignin is a major component of lignocellulosic biomass. Although it is highly recalcitrant to break down, it is a very abundant natural source of valuable aromatic carbons. Thus, the effective valorisation of lignin is crucial for realising a sustainable biorefinery chain. Here, we report a compartmented photo-electro-biochemical system for unassisted, selective, and stable lignin valorisation, in which a TiO₂ photocatalyst, an atomically dispersed Co-based electrocatalyst, and a biocatalyst (lignin peroxidase isozyme H8, horseradish peroxidase) are integrated, such that each system is separated using Nafion and cellulose membranes. This cell design enables lignin valorisation upon irradiation with sunlight without the need for any additional bias or sacrificial agent and allows the protection of the biocatalyst from enzyme-damaging elements, such as reactive radicals, gas bubbles, and light. The photo-electro-biochemical system is able to catalyse lignin depolymerisation with a 98.7% selectivity and polymerisation with a 73.3% yield using coniferyl alcohol, a lignin monomer.

Nativity of lignin carbohydrate bonds substantiated by biomimetic synthesis

The question of whether lignin is covalently linked to carbohydrates in native wood, forming what is referred to as lignin-carbohydrate complexes (LCCs), still lacks unequivocal proof. This is mainly due to the need to isolate lignin from woody materials prior to analysis, under conditions leading to partial chemical modification of the native wood polymers. Thus, the correlation between the structure of the isolated LCCs and LCCs in situ remains open. As a way to circumvent the problematic isolation, biomimicking lignin polymerization in vivo and in vitro is an interesting option. Herein, we report the detection of lignin-carbohydrate bonds in the extracellular lignin formed by tissue-cultured Norway spruce cells, and in modified biomimetic lignin synthesis (dehydrogenation polymers). Semi-quantitative 2D heteronuclear singular quantum coherence (HSQC)-, 31P -, and 13C-NMR spectroscopy were applied as analytical tools. Combining results from these systems, four types of lignin-carbohydrate bonds were detected; benzyl ether, benzyl ester, γ-ester, and phenyl glycoside linkages, providing direct evidence of lignin-carbohydrate bond formation in biomimicked lignin polymerization. Based on our findings, we propose a sequence for lignin-carbohydrate bond formation in plant cell walls.

Microstructure defines the electroconductive and mechanical performance of plant-derived renewable carbon fiber

A plant-derived lignin polymer has been sought-after as a low-cost carbon fiber (CF) precursor, but the underlying mechanisms defining CF performances are still elusive. This study revealed that both the electroconductive and mechanical performances of lignin-based CF were synergistically improved by enhancing the microstructures through modifying the lignin chemistry, which paved a pathway to holistically improve the lignin CF quality.

Non-Solvent Fractionation of Lignin Enhances Carbon Fiber Performance

Even though lignin carbon fiber has been sought after for several decades, the poor mechanical performance remains to be a major barrier for commercial applications. The low mechanical performance is attributed to the heterogeneity of lignin polymer. Recent advances in fractionation technologies showed the great potential to reduce lignin heterogeneity, but current fractionation methods often depend on costly chemicals and materials such as enzymes, organic solvents, membranes, and dialysis tubes. Here, a new non-solvent strategy was developed to fractionate lignin by autohydrolysis. By using only water, lignin was efficiently fractionated into water-soluble and -insoluble fractions. The latter fraction had increased molecular weight and uniformity and resulted in more β-O-4 interunitary linkages as analyzed by size-exclusion chromatography and 2D heteronuclear single quantum coherence NMR spectroscopy, respectively. In particular, the water-insoluble fraction significantly enhanced the mechanical performances of the resultant carbon fibers. Mechanistic study by differential scanning calorimetry (DSC) revealed that the miscibility of lignin with guest polyacrylonitrile molecules was improved with the reduced lignin heterogeneity. Crystallite analyses by XRD and Raman spectroscopy revealed that the crystallite size and content of the pre-graphitic turbostratic carbon structure were increased. The fundamental understanding revealed how lignin fractionation could modify lignin chemical features to enhance the mechanical performance of resultant carbon fibers. The autohydrolysis fractionation thus represents a green, economic, and efficient methodology to process lignin waste and boost lignin carbon fiber quality, which could open new horizons for lignin valorization.

Xylan in the Middle: Understanding Xylan Biosynthesis and Its Metabolic Dependencies Toward Improving Wood Fiber for Industrial Processing

Lignocellulosic biomass, encompassing cellulose, lignin and hemicellulose in plant secondary cell walls (SCWs), is the most abundant source of renewable materials on earth. Currently, fast-growing woody dicots such as Eucalyptus and Populus trees are major lignocellulosic (wood fiber) feedstocks for bioproducts such as pulp, paper, cellulose, textiles, bioplastics and other biomaterials. Processing wood for these products entails separating the biomass into its three main components as efficiently as possible without compromising yield. Glucuronoxylan (xylan), the main hemicellulose present in the SCWs of hardwood trees carries chemical modifications that are associated with SCW composition and ultrastructure, and affect the recalcitrance of woody biomass to industrial processing. In this review we highlight the importance of xylan properties for industrial wood fiber processing and how gaining a greater understanding of xylan biosynthesis, specifically xylan modification, could yield novel biotechnology approaches to reduce recalcitrance or introduce novel processing traits. Altering xylan modification patterns has recently become a focus of plant SCW studies due to early findings that altered modification patterns can yield beneficial biomass processing traits. Additionally, it has been noted that plants with altered xylan composition display metabolic differences linked to changes in precursor usage. We explore the possibility of using systems biology and systems genetics approaches to gain insight into the coordination of SCW formation with other interdependent biological processes. Acetyl-CoA, s-adenosylmethionine and nucleotide sugars are precursors needed for xylan modification, however, the pathways which produce metabolic pools during different stages of fiber cell wall formation still have to be identified and their co-regulation during SCW formation elucidated. The crucial dependence on precursor metabolism provides an opportunity to alter xylan modification patterns through metabolic engineering of one or more of these interdependent pathways. The complexity of xylan biosynthesis and modification is currently a stumbling point, but it may provide new avenues for woody biomass engineering that are not possible for other biopolymers.

Xylo-oligosaccharides enriched yeast protein feed production from reed sawdust

The aim of this study was to convert the cellulose and hemicellulose, in reed sawdust from the pulp mills, into yeast protein and xylo-oligosaccharide, then functionalize xylo-oligosaccharide as yeast feed. Both synchronous saccharification and fermentation and separate hydrolysis and fermentation of cellulase and Candida utilis were investigated to produce protein feed. By optimizing the fermentation conditions, 6.1 g/L of protein with 76.1% (7.1 g/L) xylo-oligosaccharide as the sugar was obtained. The final glucan and xylan utilization efficiencies in reed sawdust were 85.45% and 91.03%, respectively. Xylo-oligosaccharide enriched yeast protein feed from reed sawdust was thus realized by pretreatment, enzymatic hydrolysis and synchronous saccharification and fermentation.

Lignin polymerization: how do plants manage the chemistry so well?

The final step of lignin biosynthesis is the polymerization of monolignols in apoplastic cell wall domains. In this process, monolignols secreted by lignifying cells, or occasionally neighboring non-lignifying and/or other lignifying cells, are activated by cell-wall-localized oxidation systems, such as laccase/O₂ and/or peroxidase/H₂O₂, for combinatorial radical coupling to make the final lignin polymers. Plants can precisely control when, where, and which types of lignin polymers are assembled at tissue and cellular levels, but do not control the polymers' exact chemical structures per se. Recent studies have begun to identify specific laccase and peroxidase proteins responsible for lignin polymerization in specific cell types and during different developmental stages. The coordination of polymerization machinery localization and monolignol supply is likely critical for the spatio-temporal patterning of lignin polymerization. Further advancement in this research area will continue to increase our capacity to manipulate lignin content/structure in biomass to meet our own biotechnological purposes.

Bioinspired lignocellulosic films to understand the mechanical properties of lignified plant cell walls at nanoscale

The physicochemical properties of plant fibres are determined by the fibre morphology and structural features of the cell wall, which is composed of three main layers that differ in chemical composition and architecture. This composition and hierarchical structure are responsible for many of the mechanical properties that are desirable for industrial applications. As interactions between the lignocellulosic polymers at the molecular level are the main factor governing the final cohesion and mechanical properties of plant fibres, atomic force microscopy (AFM) is well suited for the observation and measurement of their physical properties at nanoscale levels. Given the complexity of plant cell walls, we have developed a strategy based on lignocellulosic assemblies with increasing complexity to understand the influence of the different polymers on the nanomechanical properties. Measurements of the indentation moduli performed on one type of lignified cell wall compared with those performed on the corresponding lignocellulosic films clearly show the importance of the lignin in the mechanical properties of cell walls. Through this strategy, we envision a wide application of bioinspired systems in future studies of the physical properties of fibres.

Peroxidases Bound to the Growing Lignin Polymer Produce Natural Like Extracellular Lignin in a Cell Culture of Norway Spruce

Lignin, an important component of plant cell walls, is a polymer of monolignols derived from the phenylpropanoid pathway. Monolignols are oxidized in the cell wall by oxidative enzymes (peroxidases and/or laccases) to radicals, which then couple with the growing lignin polymer. We have investigated the characteristics of the polymerization reaction by producing lignin polymers in vitro using different oxidative enzymes and analyzing the structures formed with NMR. The ability of the enzymes to oxidize high-molecular-weight compounds was tested using cytochrome c as a substrate. The results support an idea that lignin structure is largely determined by the concentration ratios of the monolignol (coniferyl alcohol) and polymer radicals involved in the coupling reaction. High rate of the lignin polymer oxidation compared to monolignol oxidation leads to a natural-like structure. The high relative rate can be achieved by an open active site of the oxidative enzyme, close proximity of the enzyme with the polymeric substrate or simply by high enzymatic activity that consumes monolignols rapidly. Monolignols, which are oxidized efficiently, can be seen as competitive inhibitors of polymer oxidation. Our results indicate that, at least in a Norway spruce (Picea abies L. Karst.) cell culture, a group of apoplastic, polymer-oxidizing peroxidases bind to the lignin polymer and are responsible for production of natural-like lignin in cell suspension cultures in vivo, and also in vitro. The peroxidases bound to the extracellular lignin had the highest ability to bind to various cell wall polymers in vitro. Extracellular lignin contains pectin-type sugars, making them possible attachment points for these cationic peroxidases.