Breakdown of cell wall nanostructure in dilute acid pretreated biomass

Ectopic Production of 3,4-Dihydroxybenzoate in Planta Affects Cellulose Structure and Organization

Lignocellulosic biomass is a highly sustainable and largely carbon dioxide neutral feedstock for the production of biofuels and advanced biomaterials. Although thermochemical pretreatment is typically used to increase the efficiency of cell wall deconstruction, genetic engineering of the major plant cell wall polymers, especially lignin, has shown promise as an alternative approach to reduce biomass recalcitrance. Poplar trees with reduced lignin content and altered composition were previously developed by overexpressing bacterial 3-dehydroshikimate dehydratase (QsuB) enzyme to divert carbon flux from the shikimate pathway. In this work, three transgenic poplar lines with increasing QsuB expression levels and different lignin contents were studied using small-angle neutron scattering (SANS) and wide-angle X-ray scattering (WAXS). SANS showed that although the cellulose microfibril cross-sectional dimension remained unchanged, the ordered organization of the microfibrils progressively decreased with increased QsuB expression. This was correlated with decreasing total lignin content in the QsuB lines. WAXS showed that the crystallite dimensions of cellulose microfibrils transverse to the growth direction were not affected by the QsuB expression, but the crystallite dimensions parallel to the growth direction were decreased by ∼20%. Cellulose crystallinity was also decreased with increased QsuB expression, which could be related to high levels of 3,4-dihydroxybenzoate, the product of QsuB expression, disrupting microfibril crystallization. In addition, the cellulose microfibril orientation angle showed a bimodal distribution at higher QsuB expression levels. Overall, this study provides new structural insights into the impact of ectopic synthesis of small-molecule metabolites on cellulose organization and structure that can be used for future efforts aimed at reducing biomass recalcitrance.

A state-of-the-art review on plant-derived cellulose-based green hydrogels and their multifunctional role in advanced biomedical applications

The most prevalent carbohydrate on Earth is cellulose, a polysaccharide composed of glucose units that may be found in diverse sources, such as cell walls of wood and plants and some bacterial and algal species. The inherent availability of this versatile material provides a natural pathway for exploring and identifying novel uses. This study comprehensively analyzes cellulose and its derivatives, exploring their structural and biochemical features and assessing their wide-ranging applications in tissue fabrication, surgical dressings, and pharmaceutical delivery systems. The use of diverse cellulose particles as fundamental components gives rise to materials with distinct microstructures and characteristics, fulfilling the requirements of various biological applications. Although cellulose boasts substantial potential across various sectors, its exploration has predominantly unfolded within industrial realms, leaving the biomedical domain somewhat overlooked in its initial stages. This investigation, therefore, endeavors to shed light on the contemporary strides made in synthesizing cellulose and its derivatives. These innovative techniques give rise to distinctive attributes, presenting a treasure trove of advantages for their compelling integration into the intricate tapestry of biomedical applications.

Critical Techniques for Overcoming the Diffusion Limitations in Heterogeneously Catalytic Depolymerization of Lignin

Heterogeneously catalyzed depolymerization of lignin to value-added chemicals is increasingly attractive but highly challengeable. Particularly, the diffusion limitation of lignin macromolecule to the solid catalyst surface is a big barrier, which significantly decreases the yield of monomer while increasing char formation. Therefore, for the potential industrial utilization of lignin, new knowledge focused on the size of lignin particles is of great importance to offer guidance for promoting lignin depolymerization and suppressing condensation in the heterogeneously catalytic systems. In this Review, the size of lignin particles and macromolecules are summarized. Previous approaches for improving the mass diffusion including enhancing the solubility of lignin and exploitation of hierarchical and "solubilized" materials are also discussed. Based on these, a constructive perspective is proposed. Thus, this work provides a new insight on the rational design of heterogeneous catalytic techniques for efficient utilization of the aromatic polymer of lignin.

How lignin sticks to cellulose-insights from atomic force microscopy enhanced by machine-learning analysis and molecular dynamics simulations

Elucidating cellulose-lignin interactions at the molecular and nanometric scales is an important research topic with impacts on several pathways of biomass valorization. Here, the interaction forces between a cellulosic substrate and lignin are investigated. Atomic force microscopy with lignin-coated tips is employed to probe the site-specific adhesion to a cellulose film in liquid water. Over seven thousand force-curves are analyzed by a machine-learning approach to cluster the experimental data into types of cellulose-tip interactions. The molecular mechanisms for distinct types of cellulose-lignin interactions are revealed by molecular dynamics simulations of lignin globules interacting with different cellulose Iβ crystal facets. This unique combination of experimental force-curves, data-driven analysis, and molecular simulations opens a new approach of investigation and updates the understanding of cellulose-lignin interactions at the nanoscale.

Morphology of lignin structures on fiber surfaces after organosolv pretreatment

The redeposition of lignin to the fiber surface after organosolv pretreatment was studied using two different reactor types. Results from the conventional autoclave reactor suggest that redeposition occurs during the cooling down stage. Redeposited particles appeared to be spherical in shape. The size and population density of the particles depends on the concentration of organosolv lignin in the cooking liquor, which is consistent with the hypothesis that reprecipitation of lignin occurs when the system is cooled down. The use of a displacement reactor showed that displacing the spent cooking liquor with fresh cooking liquor helps in reducing the redeposition and the inclusion of a washing stage with fresh cooking liquor reduced the reprecipitation of lignin, particularly on the outer fiber surfaces. Redeposition of lignin was still observed on regions that were less accessible to washing liquid, such as fiber lumens, suggesting that complete prevention of redeposition was not achieved.

Nanostructural Changes Correlated to Decay Resistance of Chemically Modified Wood Fibers

Reactive chemical modifications have been shown to impart decay resistance to wood. These modifications change hydroxyl availability, water uptake, surface energy, and the nanostructure of wood. Because fungal action occurs on the micro and nano scale, further investigation into the nanostructure may lead to better strategies to prevent fungal decay. The aim of this article is to introduce our findings using small angle neutron scattering (SANS) to probe the effects of chemical modifications on the nanostructure of wood fibers. Southern pine wood fiber samples were chemically modified to various weight percentage gains (WPG) using propylene oxide (PO), butylene oxide (BO), or acetic anhydride (AA). After modification, the samples were water leached for two weeks to remove any unreacted reagents, homopolymers or by-products and then the equilibrium moisture content (EMC) was determined. Laboratory soil-block-decay evaluations against the brown rot fungus Gloeophyllum trabeum were performed to determine weight loss and decay resistance of the modifications. To assist in understanding the mechanism behind fungal decay resistance, SANS was used to study samples that were fully immersed in deuterium oxide (D2O). These measurements revealed that modifying the fibers led to differences in the swollen wood nanostructure compared to unmodified wood fibers. Moreover, the modifications led to differences in the nanoscale features observed in samples that were exposed to brown rot fungal attack compared to unmodified wood fibers and solid wood blocks modified with alkylene oxides.

Lessons from Biomass Valorization for Improving Plastic-Recycling Enzymes

Synthetic polymers such as plastics exhibit numerous advantageous properties that have made them essential components of our daily lives, with plastic production doubling every 15 years. The relatively low cost of petroleum-based polymers encourages their single use and overconsumption. Synthetic plastics are recalcitrant to biodegradation, and mismanagement of plastic waste leads to their accumulation in the ecosystem, resulting in a disastrous environmental footprint. Enzymes capable of depolymerizing plastics have been reported recently that may provide a starting point for eco-friendly plastic recycling routes. However, some questions remain about the mechanisms by which enzymes can digest insoluble solid substrates. We review the characterization and engineering of plastic-eating enzymes and provide some comparisons with the field of lignocellulosic biomass valorization. Expected final online publication date for the Annual Review of Chemical and Biomolecular Engineering, Volume 13 is October 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Ultrastructural change in lignocellulosic biomass during hydrothermal pretreatment

In recent years, visualization and characterization of lignocellulose at different scales elucidate the modifications of its ultrastructural and chemical features during hydrothermal pretreatment which include degradation and dissolving of hemicelluloses, swelling and partial hydrolysis of cellulose, melting and redepositing a part of lignin in the surface. As a result, cell walls are swollen, deformed and de-laminated from the adjacent layer, lead to a range of revealed droplets that appear on and within cell walls. Moreover, the certain extent morphological changes significantly promote the downstream processing steps, especially for enzymatic hydrolysis and anaerobic fermentation to bioethanol by increasing the contact area with enzymes. However, the formation of pseudo-lignin hinders the accessibility of cellulase to cellulose, which decreases the efficiency of enzymatic hydrolysis. This review is intended to bridge the gap between the microstructure studies and value-added applications of lignocellulose while inspiring more research prospects to enhance the hydrothermal pretreatment process.

Acid-based lignocellulosic biomass biorefinery for bioenergy production: Advantages, application constraints, and perspectives

The production of chemicals and fuels from renewable biomass with the primary aim of reducing carbon footprints has recently become one of the central points of interest. The use of lignocellulosic biomass for energy production is believed to meet the main criteria of maximizing the available global energy source and minimizing pollutant emissions. However, before usage in bioenergy production, lignocellulosic biomass needs to undergo several processes, among which biomass pretreatment plays an important role in the yield, productivity, and quality of the products. Acid-based pretreatment, one of the existing methods applied for lignocellulosic biomass pretreatment, has several advantages, such as short operating time and high efficiency. A thorough analysis of the characteristics of acid-based biomass pretreatment is presented in this review. The environmental concerns and future challenges involved in using acid pretreatment methods are discussed in detail to achieve clean and sustainable bioenergy production. The application of acid to biomass pretreatment is considered an effective process for biorefineries that aim to optimize the production of desired products while minimizing the by-products.

Multifunctional lignin-based nanocomposites and nanohybrids

Significant progress in lignins valorization and development of high-performance sustainable materials have been achieved in recent years. Reports related to lignin utilization indicate excellent prospects considering green chemistry, chemical engineering, energy, materials and polymer science, physical chemistry, biochemistry, among others. To fully realize such potential, one of the most promising routes involves lignin uses in nanocomposites and nanohybrid assemblies, where synergistic interactions are highly beneficial. This review first discusses the interfacial assembly of lignins with polysaccharides, proteins and other biopolymers, for instance, in the synthesis of nanocomposites. To give a wide perspective, we consider the subject of hybridization with metal and metal oxide nanoparticles, as well as uses as precursor of carbon materials and the assembly with other biobased nanoparticles, for instance to form nanohybrids. We provide cues to understand the fundamental aspects related to lignins, their self-assembly and supramolecular organization, all of which are critical in nanocomposites and nanohybrids. We highlight the possibilities of lignin in the fields of flame retardancy, food packaging, plant protection, electroactive materials, energy storage and health sciences. The most recent outcomes are evaluated given the importance of lignin extraction, within established and emerging biorefineries. We consider the benefit of lignin compared to synthetic counterparts. Bridging the gap between fundamental and application-driven research, this account offers critical insights as far as the potential of lignin as one of the frontrunners in the uptake of bioeconomy concepts and its application in value-added products.

Multiscale analysis of lignocellulose recalcitrance towards OrganoCat pretreatment and fractionation

BACKGROUND

Biomass recalcitrance towards pretreatment and further processing can be related to the compositional and structural features of the biomass. However, the exact role and relative importance to those structural attributes has still to be further evaluated. Herein, ten different types of biomass currently considered to be important raw materials for biorefineries were chosen to be processed by the recently developed, acid-catalyzed OrganoCat pretreatment to produce cellulose-enriched pulp, sugars, and lignin with different amounts and qualities. Using wet chemistry analysis and NMR spectroscopy, the generic factors of lignocellulose recalcitrance towards OrganoCat were determined.

RESULTS

The different materials were processed applying different conditions (e.g., type of acid catalyst and temperature), and fractions with different qualities were obtained. Raw materials and products were characterized in terms of their compositional and structural features. For the first time, generic correlation coefficients were calculated between the measured chemical and structural features and the different OrganoCat product yields and qualities. Especially lignin-related factors displayed a detrimental role for enzymatic pulp hydrolysis, as well as sugar and lignin yield exhibiting inverse correlation coefficients. Hemicellulose appeared to have less impact, not being as detrimental as lignin factors, but xylan-O-acetylation was inversely correlated with product yield and qualities.

CONCLUSION

These results illustrate the role of generic features of lignocellulosic recalcitrance towards acidic pretreatments and fractionation, exemplified in the OrganoCat strategy. Discriminating between types of lignocellulosic biomass and highlighting important compositional variables, the improved understanding of how these parameters affect OrganoCat products will ameliorate bioeconomic concepts from agricultural production to chemical products. Herein, a methodological approach is proposed.

Progress and Opportunities in the Characterization of Cellulose - An Important Regulator of Cell Wall Growth and Mechanics

The plant cell wall is a dynamic network of several biopolymers and structural proteins including cellulose, pectin, hemicellulose and lignin. Cellulose is one of the main load bearing components of this complex, heterogeneous structure, and in this way, is an important regulator of cell wall growth and mechanics. Glucan chains of cellulose aggregate via hydrogen bonds and van der Waals forces to form long thread-like crystalline structures called cellulose microfibrils. The shape, size, and crystallinity of these microfibrils are important structural parameters that influence mechanical properties of the cell wall and these parameters are likely important determinants of cell wall digestibility for biofuel conversion. Cellulose-cellulose and cellulose-matrix interactions also contribute to the regulation of the mechanics and growth of the cell wall. As a consequence, much emphasis has been placed on extracting valuable structural details about cell wall components from several techniques, either individually or in combination, including diffraction/scattering, microscopy, and spectroscopy. In this review, we describe efforts to characterize the organization of cellulose in plant cell walls. X-ray scattering reveals the size and orientation of microfibrils; diffraction reveals unit lattice parameters and crystallinity. The presence of different cell wall components, their physical and chemical states, and their alignment and orientation have been identified by Infrared, Raman, Nuclear Magnetic Resonance, and Sum Frequency Generation spectroscopy. Direct visualization of cell wall components, their network-like structure, and interactions between different components has also been made possible through a host of microscopic imaging techniques including scanning electron microscopy, transmission electron microscopy, and atomic force microscopy. This review highlights advantages and limitations of different analytical techniques for characterizing cellulose structure and its interaction with other wall polymers. We also delineate emerging opportunities for future developments of structural characterization tools and multi-modal analyses of cellulose and plant cell walls. Ultimately, elucidation of the structure of plant cell walls across multiple length scales will be imperative for establishing structure-property relationships to link cell wall structure to control of growth and mechanics.

Hemicellulose-Cellulose Composites Reveal Differences in Cellulose Organization after Dilute Acid Pretreatment

Model hemicellulose-cellulose composites that mimic plant cell wall polymer interactions were prepared by synthesizing deuterated bacterial cellulose in the presence of glucomannan or xyloglucan. Dilute acid pretreatment (DAP) of these materials was studied using small-angle neutron scattering, X-ray diffraction, and sum frequency generation spectroscopy. The macrofibril dimensions of the pretreated cellulose alone were smaller but with similar entanglement of macrofibrillar network as native cellulose. In addition, the crystallite size dimension along the (010) plane increased. Glucomannan-cellulose underwent similar changes to cellulose, except that the macrofibrillar network was more entangled after DAP. Conversely, in xyloglucan-cellulose the macrofibril dimensions and macrofibrillar network were relatively unchanged after pretreatment, but the cellulose I_β content was increased. Our results point to a tight interaction of xyloglucan with microfibrils while glucomannan only interacts with macrofibril surfaces. This study provides insight into roles of different hemicellulose-cellulose interactions and may help in improving pretreatment processes or engineering plants with decreased recalcitrance.

Preliminary study of Cell Wall Structure and its Mechanical Properties of C3H and HCT RNAi Transgenic Poplar Sapling

This research focused on the cell wall structure and its mechanical properties of down-regulated Coumaroyl shikimate 3-hydroxylase (C3H) transgenic poplar and down-regulated hydroxycinnamoyl CoA: shikimate hydroxycinnamoyl transferase (HCT) transgenic poplar (Populus alba × P. glandulosa cv ‘84 k’). The wood samples with respect to microstructure, the longitudinal elastic modulus (MOE) and hardness of wood fiber secondary cell wall were investigated. The results show that the lignin contents in the two transgenic poplar woods were lower than non-modified wood. The C3H transgenic poplar and HCT transgenic poplar have more than 18.5% and 16.1% cellulose crystalline regions than non-modified poplar respectively. The diameter of the fiber cell and the vessel element of transgenic poplars are smaller. Double radial vessel cell wall thicknesses of both transgenic poplars were smaller than non-modified poplar. Cell wall ratios for the transgenic poplar were higher than non-modified poplar and cell wall density was significantly lower in both C3H and HCT transgenic poplar. The cell wall MOEs of C3H and HCT transgenic poplar was 5.8% and 7.0% higher than non-modified poplar. HCT can be more effective than C3H to modify the trees by considerably increasing mechanical properties of the cell wall.

Effects of combined pretreatment of dilute acid pre-extraction and chemical-assisted mechanical refining on enzymatic hydrolysis of lignocellulosic biomass

An efficient enzymatic hydrolysis of lignocellulosic biomass into fermentable sugars depends greatly on the pretreatment of raw materials. In this study, a combination of dilute acid pre-extraction and chemical-assisted mechanical refining was used to pretreat wood lignocellulosic biomass for subsequent enzymatic hydrolysis. This work analyzed the surface lignin concentration, specific surface area, crystallinity, fines content, fiber length, and kink index of the resultant pulp substrates and their effects on the enzymatic hydrolysis. The results showed that the combined pretreatment significantly enhanced the enzymatic hydrolysis efficiency, and the maximum glucose conversion yield and glucose concentration were 93.32% and 21.41 g L⁻¹, respectively. It is found that the surface lignin concentration, specific surface area, and fines content significantly affected the enzymatic hydrolysis.

The combined pretreatment of dilute acid pre-extraction and mechanical refining significantly improved the enzymatic hydrolysis performance of lignocellulosic biomass.

Tension wood structure and morphology conducive for better enzymatic digestion

BACKGROUND

Tension wood is a type of reaction wood in response to bending or leaning stem as a corrective growth process. Tension wood is formed by both natural and man-made processes. Most attractively, tension wood contains higher glucan content and undergoes higher enzymatic conversion to fermentable sugars. Here, we have employed structural techniques, small-angle neutron scattering (SANS) and wide-angle X-ray diffraction (WAXD) to elucidate structural and morphological aspects of tension wood conducive to higher sugar yields.

RESULTS

Small-angle neutron scattering data exhibited a tri-modal distribution of the fibril cross-sectional dimension. The smallest size, 22 Å observed in all samples concurred with the WAXD results of the control and opposite side samples. This smallest and the most abundant occurring size was interpreted as the cellulose elementary microfibril diameter. The intermediate size of 45 Å, which is most pronounced in the tension side sample and consistent with WAXD results for tension side sample, indicates association of neighboring elementary microfibrils to form larger crystallite bundles. The largest size 61 Å observed by SANS was however not observed by WAXD and therefore associated to mesopores.

CONCLUSIONS

Structure and morphology of tension wood is different from control wood. Cellulose crystallinity increases, lignin content is lower and the appearance of mesopores with 61 Å diameter is observed. Despite the presence of higher crystalline cellulose content in tension side, the lower lignin content and may be combined with the abundance of mesopores, substantially improves enzyme accessibility leading to higher yields in cellulose digestion.

Oxygen-radical pretreatment promotes cellulose degradation by cellulolytic enzymes

BACKGROUND

The efficiency of cellulolytic enzymes is important in industrial biorefinery processes, including biofuel production. Chemical methods, such as alkali pretreatment, have been extensively studied and demonstrated as effective for breaking recalcitrant lignocellulose structures. However, these methods have a detrimental effect on the environment. In addition, utilization of these chemicals requires alkali- or acid-resistant equipment and a neutralization step.

RESULTS

Here, a radical generator based on non-thermal atmospheric pressure plasma technology was developed and tested to determine whether oxygen-radical pretreatment enhances cellulolytic activity. Our results showed that the viscosity of carboxymethyl cellulose (CMC) solutions was reduced in a time-dependent manner by oxygen-radical pretreatment using the radical generator. Compared with non-pretreated CMC, oxygen-radical pretreatment of CMC significantly increased the production of reducing sugars in culture supernatant containing various cellulases from Phanerochaete chrysosporium. The production of reducing sugar from oxygen-radical-pretreated CMC by commercially available cellobiohydrolases I and II was 1.7- and 1.6-fold higher, respectively, than those from non-pretreated and oxygen-gas-pretreated CMC. Moreover, the amount of reducing sugar from oxygen-radical-pretreated wheat straw was 1.8-fold larger than those from non-pretreated and oxygen-gas-pretreated wheat straw.

CONCLUSIONS

Oxygen-radical pretreatment of CMC and wheat straw enhanced the degradation of cellulose by reducing- and non-reducing-end cellulases in the supernatant of a culture of the white-rot fungus P. chrysosporium. These findings indicated that oxygen-radical pretreatment of plant biomass offers great promise for improvements in lignocellulose-deconstruction processes.

Modification of the nanostructure of lignocellulose cell walls via a non-enzymatic lignocellulose deconstruction system in brown rot wood-decay fungi

Wood decayed by brown rot fungi and wood treated with the chelator-mediated Fenton (CMF) reaction, either alone or together with a cellulose enzyme cocktail, was analyzed by small angle neutron scattering (SANS), sum frequency generation (SFG) spectroscopy, Fourier transform infrared (FTIR) analysis, X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). Results showed that the CMF mechanism mimicked brown rot fungal attack for both holocellulose and lignin components of the wood. Crystalline cellulose and lignin were both depolymerized by the CMF reaction. Porosity of the softwood cell wall did not increase during CMF treatment, enzymes secreted by the fungi did not penetrate the decayed wood. The enzymes in the cellulose cocktail also did not appear to alter the effects of the CMF-treated wood relative to enhancing cell wall deconstruction. This suggests a rethinking of current brown rot decay models and supports a model where monomeric sugars and oligosaccharides diffuse from the softwood cell walls during non-enzymatic action. In this regard, the CMF mechanism should not be thought of as a "pretreatment" used to permit enzymatic penetration into softwood cell walls, but instead it enhances polysaccharide components diffusing to fungal enzymes located in wood cell lumen environments during decay. SANS and other data are consistent with a model for repolymerization and aggregation of at least some portion of the lignin within the cell wall, and this is supported by AFM and TEM data. The data suggest that new approaches for conversion of wood substrates to platform chemicals in biorefineries could be achieved using the CMF mechanism with >75% solubilization of lignocellulose, but that a more selective suite of enzymes and other downstream treatments may be required to work when using CMF deconstruction technology. Strategies to enhance polysaccharide release from lignocellulose substrates for enhanced enzymatic action and fermentation of the released fraction would also aid in the efficient recovery of the more uniform modified lignin fraction that the CMF reaction generates to enhance biorefinery profitability.

Precise pretreatment of lignocellulose: relating substrate modification with subsequent hydrolysis and fermentation to products and by-products

BACKGROUND

Pretreatment is a crucial step for valorization of lignocellulosic biomass into valuable products such as H₂, ethanol, acids, and methane. As pretreatment can change several decisive factors concurrently, it is difficult to predict its effectiveness. Furthermore, the effectiveness of pretreatments is usually assessed by enzymatic digestibility or merely according to the yield of the target fermentation products. The present study proposed the concept of "precise pretreatment," distinguished the major decisive factors of lignocellulosic materials by precise pretreatment, and evaluated the complete profile of all fermentation products and by-products. In brief, hemicellulose and lignin were selectively removed from dewaxed rice straw, and the cellulose was further modified to alter the crystalline allomorphs. The subsequent fermentation performance of the selectively pretreated lignocellulose was assessed using the cellulolytic, ethanologenic, and hydrogenetic Clostridium thermocellum through a holistic characterization of the liquid, solid, and gaseous products and residues.

RESULTS

The transformation of crystalline cellulose forms from I to II and from I_α to I_β improved the production of H₂ and ethanol by 65 and 29%, respectively. At the same time, the hydrolysis efficiency was merely improved by 10%, revealing that the crystalline forms not only influenced the accessibility of cellulose but also affected the metabolic preferences and flux of the system. The fermentation efficiency was independent of the specific surface area and degree of polymerization. Furthermore, the pretreatments resulted in 43-45% of the carbon in the liquid hydrolysates unexplainable by forming ethanol and acetate products. A tandem pretreatment with peracetic acid and alkali improved ethanol production by 45.5%, but also increased the production of non-ethanolic low-value by-products by 136%, resulting in a huge burden on wastewater treatment requirements.

CONCLUSION

Cellulose allomorphs significantly affected fermentation metabolic pathway, except for hydrolysis efficiency. Furthermore, with the increasing effectiveness of the pretreatment for ethanol production, more non-ethanolic low-value by-products or contaminants were produced, intensifying environmental burden. Therefore, the effectiveness of the pretreatment should not only be determined on the basis of energy auditing and inhibitors generated, but should also be assessed in terms of the environmental benefits of the whole integrated system from a holistic view.

Fibrillar assembly of bacterial cellulose in the presence of wood-based hemicelluloses

Composite materials mimicking the plant cell wall structure were made by culturing cellulose-producing bacteria together with secondary-wall hemicelluloses from wood. The effects of spruce galactoglucomannan (GGM) and beech xylan on the nanoscale morphology of bacterial cellulose were studied in the original, hydrated state with small-angle X-ray scattering (SAXS). The SAXS intensities were fitted with a model covering multiple levels of the hierarchical structure. Additional information on the structure of dried samples was obtained using scanning and transmission electron microscopy and infra-red spectroscopy. Both hemicelluloses induced a partial conversion of the cellulose crystal structure from I_α to I_β and a reduction of the cross-sectional dimensions of the cellulose microfibrils, thereby affecting also their packing into bundles. The differences were more pronounced in samples with xylan instead of GGM, and they became more significant with higher hemicellulose concentrations.

Current Understanding of the Correlation of Lignin Structure with Biomass Recalcitrance

Lignin, a complex aromatic polymer in terrestrial plants, contributes significantly to biomass recalcitrance to microbial and/or enzymatic deconstruction. To reduce biomass recalcitrance, substantial endeavors have been exerted on pretreatment and lignin engineering in the past few decades. Lignin removal and/or alteration of lignin structure have been shown to result in reduced biomass recalcitrance with improved cell wall digestibility. While high lignin content is usually a barrier to a cost-efficient application of bioresources to biofuels, the direct correlation of lignin structure and its concomitant properties with biomass remains unclear due to the complexity of cell wall and lignin structure. Advancement in application of biorefinery to production of biofuels, chemicals, and bio-derived materials necessitates a fundamental understanding of the relationship of lignin structure and biomass recalcitrance. In this mini-review, we focus on recent investigations on the influence of lignin chemical properties on bioprocessability-pretreatment and enzymatic hydrolysis of biomass. Specifically, lignin-enzyme interactions and the effects of lignin compositional units, hydroxycinnamates, and lignin functional groups on biomass recalcitrance have been highlighted, which will be useful not only in addressing biomass recalcitrance but also in deploying renewable lignocelluloses efficiently.

Diffraction pattern simulation of cellulose fibrils using distributed and quantized pair distances

Intensity simulation of X-ray scattering from large twisted cellulose molecular fibrils is important in understanding the impact of chemical or physical treatments on structural properties such as twisting or coiling. This paper describes a highly efficient method for the simulation of X-ray diffraction patterns from complex fibrils using atom-type-specific pair-distance quantization. Pair distances are sorted into arrays which are labelled by atom type. Histograms of pair distances in each array are computed and binned and the resulting population distributions are used to represent the whole pair-distance data set. These quantized pair-distance arrays are used with a modified and vectorized Debye formula to simulate diffraction patterns. This approach utilizes fewer pair distances in each iteration, and atomic scattering factors are moved outside the iteration since the arrays are labelled by atom type. This algorithm significantly reduces the computation time while maintaining the accuracy of diffraction pattern simulation, making possible the simulation of diffraction patterns from large twisted fibrils in a relatively short period of time, as is required for model testing and refinement.

Ferricyanide-based analysis of aqueous lignin suspension revealed sequestration of water-soluble lignin moieties

A simple and reproducible ferricyanide-based technique for routine qualitative and semi-quantitative comparative analysis of aqueous lignin extracts.

Production of bacterial cellulose with controlled deuterium-hydrogen substitution for neutron scattering studies

Isotopic enrichment of biomacromolecules is a widely used technique that enables the investigation of the structural and dynamic properties to provide information not accessible with natural abundance isotopic composition. This study reports an approach for deuterium incorporation into bacterial cellulose. A media formulation for growth of Acetobacter xylinus subsp. sucrofermentans and Gluconacetobacter hansenii was formulated that supports cellulose production in deuterium (D) oxide. The level of D incorporation can be varied by altering the ratio of deuterated and protiated glycerol used during cell growth in the D2O-based growth medium. Spectroscopic analysis and mass spectrometry show that the level of deuterium incorporation is high (>90%) for the perdeuterated form of bacterial cellulose. The small-angle neutron scattering profiles of the cellulose with different amounts of D incorporation are all similar indicating that there are no structural changes in the cellulose due to substitution of deuterium for hydrogen. In addition, by varying the amount of deuterated glycerol in the media it was possible to vary the scattering length density of the deuterated cellulose. The ability to control deuterium content of cellulose extends the range of experiments using techniques such as neutron scattering to reveal information about the structure and dynamics of cellulose, and its interactions with other biomacromolecules as well as synthetic polymers used for development of composite materials.

Development of approaches for deuterium incorporation in plants

Soon after the discovery of deuterium, efforts to utilize this stable isotope of hydrogen for labeling of plants began and have proven successful for natural abundance to 20% enrichment. However, isotopic labeling with deuterium ((2)H) in higher plants at the level of 40% and higher is complicated by both physiological responses, particularly water exchange through transpiration, and inhibitory effects of D2O on germination, rooting, and growth. The highest incorporation of 40-50% had been reported for photoheterotrophic cultivation of the duckweed Lemna. Higher substitution is desirable for certain applications using neutron scattering and nuclear magnetic resonance (NMR) techniques. (1)H(2)H NMR and mass spectroscopy are standard methods frequently used for determination of location and amount of deuterium substitution. The changes in infrared (IR) absorption observed for H to D substitution in hydroxyl and alkyl groups provide rapid initial evaluation of incorporation. Short-term experiments with cold-tolerant annual grasses can be carried out in enclosed growth containers to evaluate incorporation. Growth in individual chambers under continuous air perfusion with dried sterile-filtered air enables long-term cultivation of multiple plants at different D2O concentrations. Vegetative propagation from cuttings extends capabilities to species with low germination rates. Cultivation in 50% D2O of annual ryegrass and switchgrass following establishment of roots by growth in H2O produces samples with normal morphology and 30-40% deuterium incorporation in the biomass. Winter grain rye (Secale cereale) was found to efficiently incorporate deuterium by photosynthetic fixation from 50% D2O but did not incorporate deuterated phenylalanine-d8 from the growth medium.

Production of deuterated switchgrass by hydroponic cultivation

The bioenergy crop switchgrass was grown hydroponically from tiller cuttings in 50 % D 2 O to obtain biomass with 34 % deuterium substitution and physicochemical properties similar to those of H 2 O-grown switchgrass controls. Deuterium enrichment of biological materials can potentially enable expanded experimental use of small angle neutron scattering (SANS) to investigate molecular structural transitions of complex systems such as plant cell walls. Two key advances have been made that facilitate cultivation of switchgrass, an important forage and biofuel crop, for controlled isotopic enrichment: (1) perfusion system with individual chambers and (2) hydroponic growth from tiller cuttings. Plants were grown and maintained for several months with periodic harvest. Photosynthetic activity was monitored by measurement of CO2 in outflow from the growth chambers. Plant morphology and composition appeared normal compared to matched controls grown with H2O. Using this improved method, gram quantities of switchgrass leaves and stems were produced by continuous hydroponic cultivation using growth medium consisting of basal mineral salts in 50 % D2O. Deuterium incorporation was confirmed by detection of the O-D and C-D stretching peaks with FTIR and quantified by (1)H- and (2)H-NMR. This capability to produce deuterated lignocellulosic biomass under controlled conditions will enhance investigation of cell wall structure and its deconstruction by neutron scattering and NMR techniques.

Application of X-ray and neutron small angle scattering techniques to study the hierarchical structure of plant cell walls: a review

Plant cell walls present an extremely complex structure of hierarchically assembled cellulose microfibrils embedded in a multi-component matrix. The biosynthesis process determines the mechanism of cellulose crystallisation and assembly, as well as the interaction of cellulose with other cell wall components. Thus, a knowledge of cellulose microfibril and bundle architecture, and the structural role of matrix components, is crucial for understanding cell wall functional and technological roles. Small angle scattering techniques, combined with complementary methods, provide an efficient approach to characterise plant cell walls, covering a broad and relevant size range while minimising experimental artefacts derived from sample treatment. Given the system complexity, approaches such as component extraction and the use of plant cell wall analogues are typically employed to enable the interpretation of experimental results. This review summarises the current research status on the characterisation of the hierarchical structure of plant cell walls using small angle scattering techniques.

Supramolecular Interactions in Secondary Plant Cell Walls: Effect of Lignin Chemical Composition Revealed with the Molecular Theory of Solvation

Plant biomass recalcitrance, a major obstacle to achieving sustainable production of second generation biofuels, arises mainly from the amorphous cell-wall matrix containing lignin and hemicellulose assembled into a complex supramolecular network that coats the cellulose fibrils. We employed the statistical-mechanical, 3D reference interaction site model with the Kovalenko-Hirata closure approximation (or 3D-RISM-KH molecular theory of solvation) to reveal the supramolecular interactions in this network and provide molecular-level insight into the effective lignin-lignin and lignin-hemicellulose thermodynamic interactions. We found that such interactions are hydrophobic and entropy-driven, and arise from the expelling of water from the mutual interaction surfaces. The molecular origin of these interactions is carbohydrate-π and π-π stacking forces, whose strengths are dependent on the lignin chemical composition. Methoxy substituents in the phenyl groups of lignin promote substantial entropic stabilization of the ligno-hemicellulosic matrix. Our results provide a detailed molecular view of the fundamental interactions within the secondary plant cell walls that lead to recalcitrance.

In situ micro-spectroscopic investigation of lignin in poplar cell walls pretreated by maleic acid

BACKGROUND

In higher plant cells, lignin provides necessary physical support for plant growth and resistance to attack by microorganisms. For the same reason, lignin is considered to be a major impediment to the process of deconstructing biomass to simple sugars by hydrolytic enzymes. The in situ variation of lignin in plant cell walls is important for better understanding of the roles lignin play in biomass recalcitrance.

RESULTS

A micro-spectroscopic approach combining stimulated Raman scattering microscopy and fluorescence lifetime imaging microscopy was employed to probe the physiochemical structure of lignin in poplar tracheid cell walls. Two forms of lignins were identified: loosely packed lignin, which had a long (4 ns) fluorescence lifetime and existed primarily in the secondary wall layers; and dense lignin, which had a short (0.5-1 ns) fluorescence lifetime and was present in all wall layers, including the cell corners, compound middle lamellae, and secondary wall. At low maleic acid concentration (0.025 and 0.05 M) pretreatment conditions, some of the dense lignin was modified to become more loosely packed. High acid concentration removed both dense and loosely packed lignins. These modified lignins reformed to make lignin-carbohydrate complex droplets containing either dense or loosely packed lignin (mostly from secondary walls) and were commonly observed on the cell wall surface.

CONCLUSIONS

We have identified dense and loosely packed lignins in plant cell walls. During maleic acid pretreatment, both dense lignin droplets and loosely packed lignin droplets were formed. Maleic acid pretreatment more effectively removes loosely packed lignin in secondary walls which increases enzyme accessibility for digestion.

Breakdown of hierarchical architecture in cellulose during dilute acid pretreatments

Cellulose is an attractive candidate as a feedstock for sustainable bioenergy because of its global abundance. Pretreatment of biomass has significant influence on the chemical availability of cellulose locked in recalcitrant microfibrils. Optimizing pretreatment depends on an understanding of its impact on the microscale and nanoscale molecular architecture. X-ray scattering experiments have been performed on native and pre-treated maize stover and models of cellulose architecture have been derived from these data. Ultra small-angle, very small-angle and small-angle X-ray scattering (USAXS, VSAXS and SAXS) probe three different levels of architectural scale. USAXS and SAXS have been used to study cellulose at two distinct length scales, modeling the fibrils as ~30 Å diameter rods packed into ~0.14 μm diameter bundles. VSAXS is sensitive to structural features at length scales between these two extremes. Detailed analysis of diffraction patterns from untreated and pretreated maize using cylindrical Guinier plots and the derivatives of these plots reveals the presence of substructures within the ~0.14 μm diameter bundles that correspond to grouping of cellulose approximately 30 nm in diameter. These sub-structures are resilient to dilute acid pretreatments but are sensitive to pretreatment when iron sulfate is added. These results provide evidence of the hierarchical arrangement of cellulose at three length scales and the evolution of these arrangements during pre-treatments.

Augmented digestion of lignocellulose by steam explosion, acid and alkaline pretreatment methods: a review

Lignocellulosic materials can be explored as one of the sustainable substrates for bioethanol production through microbial intervention as they are abundant, cheap and renewable. But at the same time, their recalcitrant structure makes the conversion process more cumbersome owing to their chemical composition which adversely affects the efficiency of bioethanol production. Therefore, the technical approaches to overcome recalcitrance of biomass feedstock has been developed to remove the barriers with the help of pretreatment methods which make cellulose more accessible to the hydrolytic enzymes, secreted by the microorganisms, for its conversion to glucose. Pretreatment of lignocellulosic biomass in cost effective manner is a major challenge to bioethanol technology research and development. Hence, in this review, we have discussed various aspects of three commonly used pretreatment methods, viz., steam explosion, acid and alkaline, applied on various lignocellulosic biomasses to augment their digestibility alongwith the challenges associated with their processing.

The Bio-SANS instrument at the High Flux Isotope Reactor of Oak Ridge National Laboratory

Small-angle neutron scattering (SANS) is a powerful tool for characterizing complex disordered materials, including biological materials. The Bio-SANS instrument of the High Flux Isotope Reactor of Oak Ridge National Laboratory (ORNL) is a high-flux low-background SANS instrument that is, uniquely among SANS instruments, dedicated to serving the needs of the structural biology and biomaterials communities as an open-access user facility. Here, the technical specifications and performance of the Bio-SANS are presented. Sample environments developed to address the needs of the user program of the instrument are also presented. Further, the isotopic labeling and sample preparation capabilities available in the Bio-Deuteration Laboratory for users of the Bio-SANS and other neutron scattering instruments at ORNL are described. Finally, a brief survey of research performed using the Bio-SANS is presented, which demonstrates the breadth of the research that the instrument's user community engages in.

Multiscale deconstruction of molecular architecture in corn stover

Lignocellulosic composite in corn stover is a candidate biofuel feedstock of substantial abundance and sustainability. Its utilization is hampered by resistance of constituent cellulose fibrils to deconstruction. Here we use multi-scale studies of pretreated corn stover to elucidate the molecular mechanism of deconstruction and investigate the basis of recalcitrance. Dilute acid pretreatment has modest impact on fibrillar bundles at 0.1 micron length scales while leading to significant disorientation of individual fibrils. It disintegrates many fibrils into monomeric cellulose chains or small side-by-side aggregates. Residual crystalline fibrils lose amorphous surface material, change twist and where still cross-linked, coil around one another. Yields from enzymatic digestion are largely due to hydrolysis of individual cellulose chains and fragments generated during pretreatments. Fibrils that remain intact after pretreatment display substantial resistance to enzymatic digestion. Optimization of yield will require strategies that maximize generation of fragments and minimize preservation of intact cellulosic fibrils.