Nanocomposite films based on xylan-rich hemicelluloses and cellulose nanofibers with enhanced mechanical properties

Interest in xylan-rich hemicelluloses (XH) film is growing, and efforts have been made to prepare XH films with improved mechanical properties. This work described an effective approach to produce nanocomposite films with enhanced mechanical properties by incorporation of cellulose nanofibers (CNFs) into XH. Aqueous dispersions of XH (64-75 wt %), sorbitol (16-25 wt %), and CNF (0-20 wt %) were cast at a temperature of 23 °C and 50% relative humidity. The surface morphology of the films was revealed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The thermal properties and crystal structure of the films were evaluated by thermal analysis (TG) and X-ray diffraction (XRD). The surface of XH films with and without CNF was composed primarily of nanonodules, and CNFs were embedded in the XH matrix. Freeze-dried XH powder was amorphous, whereas the films with and without CNF showed a distinct peak at around 2θ = 18°, which suggested that XH molecules aggregated or reordered in the casting solution or during water evaporation. Furthermore, the nanocomposite films had improved thermal stability. XH film with 25 wt % plasticizer (sorbitol, based on dry XH weight) showed poor mechanical properties, whereas incorporation of CNF (5-20 wt %, based on the total dry mixture) into the film resulted in enhanced mechanical properties due to the high aspect ratio and mechanical strength of CNF and strong interactions between CNF and XH matrix. This effective method makes it possible to produce hemicellulose-based biomaterials of high quality.     

Mechanical properties of primary plant cell wall analogues

Mechanical effects of turgor pressure on cell walls were simulated by deforming cell wall analogues based on Acetobacter xylinus cellulose under equi-biaxial tension. This experimental set-up, with associated modelling, allowed quantitative information to be obtained on cellulose alone and in composites with pectin and/or xyloglucan. Cellulose was the main load-bearing component, pectin and xyloglucan leading to a decrease in modulus when incorporated. The cellulose-only system could be regarded as an essentially linear elastic material with a modulus ranging from 200 to 500 MPa. Pectin incorporation modified extensibility properties of the system by topology/architecture changes of cellulose fibril assemblies, but the cellulose/pectin composites could still be described as a linear elastic material with a modulus ranging from 120 to 250 MPa. The xyloglucan/cellulose composite could not be modelled as a linear elastic material. Introducing xyloglucan into a cellulose network or a cellulose/pectin composite led to very compliant materials characterised by time-dependent creep behaviour. Modulus values obtained for the composite materials were compared with mechanical data found for plant-derived systems. After comparing bi-axial and uni-axial behaviour of the different composites, structural models were proposed to explain the role of each polysaccharide in determining the mechanical properties of these plant primary cell wall analogues.     

Xyloglucan and xyloglucan endo-transglycosylases (XET): Tools for ex vivo cellulose surface modification

Wood fibres constitute a renewable raw material for the production of novel biomaterials. The development of efficient methods for cellulose surface modification is essential for expanding the properties of wood fibres for increased reactivity and compatibility with other materials. By combining the high affinity between xyloglucan and cellulose, the unique mechanistic property of xyloglucan endo-transglycosylases (XET, EC 2.4.1.207) to catalyze polysaccharide-oligosaccharide coupling reactions, and traditional carbohydrate synthesis, a new system for the attachment of a wide variety of functional groups to wood pulps has been generated. An overview of recent developments is presented in the context of the structure, physical properties, and historical applications of xyloglucan.     

Xyloglucan and xyloglucan endo-transglycosylases (XET): Tools for ex vivo cellulose surface modification

Wood fibres constitute a renewable raw material for the production of novel biomaterials. The development of efficient methods for cellulose surface modification is essential for expanding the properties of wood fibres for increased reactivity and compatibility with other materials. By combining the high affinity between xyloglucan and cellulose, the unique mechanistic property of xyloglucan endo-transglycosylases (XET, EC 2.4.1.207) to catalyze polysaccharide-oligosaccharide coupling reactions, and traditional carbohydrate synthesis, a new system for the attachment of a wide variety of functional groups to wood pulps has been generated. An overview of recent developments is presented in the context of the structure, physical properties, and historical applications of xyloglucan.     

Mechanical effects of plant cell wall enzymes on cellulose/xyloglucan composites

Xyloglucan-acting enzymes are believed to have effects on type I primary plant cell wall mechanical properties. In order to get a better understanding of these effects, a range of enzymes with different in vitro modes of action were tested against cell wall analogues (bio-composite materials based on Acetobacter xylinus cellulose and xyloglucan). Tomato pericarp xyloglucan endo transglycosylase (tXET) and nasturtium seed xyloglucanase (nXGase) were produced heterologously in Pichia pastoris. Their action against the cell wall analogues was compared with that of a commercial preparation of Trichoderma endo-glucanase (EndoGase). Both 'hydrolytic' enzymes (nXGase and EndoGase) were able to depolymerise not only the cross-link xyloglucan fraction but also the surface-bound fraction. Consequent major changes in cellulose fibril architecture were observed. In mechanical terms, removal of xyloglucan cross-links from composites resulted in increased stiffness (at high strain) and decreased visco-elasticity with similar extensibility. On the other hand, true transglycosylase activity (tXET) did not affect the cellulose/xyloglucan ratio. No change in composite stiffness or extensibility resulted, but a significant increase in creep behaviour was observed in the presence of active tXET. These results provide direct in vitro evidence for the involvement of cell wall xyloglucan-specific enzymes in mechanical changes underlying plant cell wall re-modelling and growth processes. Mechanical consequences of tXET action are shown to be complimentary to those of cucumber expansin.     

Antioxidant activity and antitumor activity (in vitro) of xyloglucan selenious ester and surfated xyloglucan

Two kinds of xyloglucan derivatives (xyloglucan selenious ester and sulfated xyloglucan) were prepared and evaluated on antioxidant activity and antitumor activity. Compared with xyloglucan, xyloglucan derivatives have new bioactivity against oxidative damage and tumor. Furthermore, xyloglucan selenious ester is more potent than sulfated xyloglucan at antioxidant activity and antitumor activity in vitro. The current data suggest for the first time that selenition of xyloglucan significantly increases its bioactivity and the chemical modification of polysaccharide may allow the preparation of derivatives with new properties and a variety of applications.     

Effect of hypergravity stimulus on XTH gene expression in Arabidopsis thaliana.

There are several reports indicating that hypergravity and microgravity influence the mechanical properties of cell walls in shoots, resulting in changes in the growth rate. The mechanical properties of cell walls in dicots are mainly determined by the physicochemical properties of xyloglucan, a matrix polysaccharide. An increase in the molecular mass of xyloglucan correlated with a decrease in cell wall extensibility. Hypergravity is known to increase the molecular mass of xyloglucan. The cell wall enzyme, xyloglucan endotransglucosylase/hydrolase (XTH) is involved in xyloglucan metabolism. Using Arabidopsis, it was examined whether or not the expression of XTH genes in the floral stem and rosette leaf is influenced by hypergravity. RT-PCR analysis revealed that the expression of XTH genes changes in response to hypergravity of 300 g.     

Study on thermomechanical properties of cross-linked epoxy resin

In this paper, molecular dynamics simulation was carried out to investigate the thermomechanical properties of cross-linked epoxy resin. The glass transition temperature, coefficients of thermal and moisture expansion, mechanical property parameters and so on are studied with the influence of temperature, water concentration and polymer conversion taken into account. The simulation results were in good agreement with existing experimental data.     

Novel CaO/polylactic acid nanoscaffold as dental resin nanocomposites and the investigation of physicochemical properties

In this study, for the first time, calcium oxide (CaO)/polylactic acid nanoscaffolds were synthesized by co‐precipitation assistant reverse micelles method. The physical and chemical (physicochemical) properties of the structures as dental resin composites were also studied. Nanocomposite materials as primary and basic dental compounds can be conveniently applied as dental filling materials with a high esthetic quality. In this research nanoscaffolds act as a bed for nanoparticles and improve the mechanical and chemical (mechanochemical) properties, CaO nanoparticles were loading in polylactic acid nanoscaffold as a bioactivity polymer for usage in the dental resin composites. Mechanical properties of the dental resin composite containing CaO/polylactic acid nanoscaffold were calculated: the flexural strength (137.2 MPa), modulus (12.9GPa) and compressive strength (344.2 MPa). Potential of the basic nanoparticle and the products were characterized by X‐ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), dynamic light scattering (DLS), ultraviolet‐visible spectroscopy (UV‐visible) and atomic force microscopy (AFM) showed the size of the optimized nanostructures was about 85 to 120 nm. According to TGA results of polylactic acid nanofibers with thermal stability below 300°C these high thermal stability materials can be used as dental resin composites.     

Freezing Characteristics of Cultured Catharanthus roseus (L). G. Don Cells Treated with Dimethylsulfoxide and Sorbitol in Relation to Cryopreservation

The freezing behavior of dimethylsulfoxide (DMSO) and sorbitol solutions and periwinkle (Catharanthus roseus) cells treated with DMSO and sorbitol alone and in combination was examined by nuclear magnetic resonance and differential thermal analysis. Incorporation of DMSO or sorbitol into the liquid growth medium had a significant effect in the temperature range for initiation to completion of ice crystallization. Compared to the control, less water crystallized at temperatures below −30°C in DMSO-treated cells. Similar results were obtained with sorbitol-treated cells, except sorbitol had less effect on the amount of water crystallized at temperatures below −25°C. There was a close association between the per cent unfrozen water at −40°C and per cent cell survival after freezing for 1 hour in liquid nitrogen. It appears that, in periwinkle suspension cultures, the amount of liquid water at −40°C is critical for a successful cryopreservation. The combination of DMSO and sorbitol was the most effective in preventing water from freezing. The results obtained may explain the cryoprotective properties of DMSO and sorbitol and why DMSO and sorbitol in combination are more effective as cryoprotectants than when used alone.     

Application of xyloglucan to improve the gluten membrane on breadmaking

Effects of xyloglucan (XG) on the physical properties of dough and bread quality were studied. XG was fractionated to water-soluble (WS-XG) by enzymatic hydrolysis with cellulase, and the four kinds of WS-XG: WS-XG-A (average degree of polymerization; 17), WS-XG-B (32), WS-XG-C (78) and WS-XG-D (223) were obtained. XG without enzymatic hydrolysis was termed water-insoluble xyloglucan (WI-XG). Additions of WS-XG-A (3%), WS-XG-D (1–5%) and WI-XG (3%) increased the stability of the dough and improved the loaf and softness of bread samples. Especially, the WS-XG-D (1–5%) significantly improved the various factors of the final products, such as loaf volume, storage properties and good appearances with fine distribution of small size gas cells, and its addition of low level (1%) still showed improving effects, as compared with other additives. The more viscous gluten matrix could be observed in the mixed doughs with WS-XG-D, than the control sample without XG. WS-XG-D increased the water activity of the dough, and therefore the gluten matrix of dough became strong and uniform. Since the WS-XG-D had the higher degree of polymerization, it might be polymerized during mixing or fermentation, followed by the formation of new insoluble-XG after baking. Appropriate amount of the new WI-XG formed from WS-XG-D was considered to improve the storage properties with higher water holding property than WS-XG-D alone.     

A Barley xyloglucan xyloglucosyl transferase covalently links xyloglucan, cellulosic substrates, and (1,3;1,4)-beta-D-glucans

Molecular interactions between wall polysaccharides, which include cellulose and a range of noncellulosic polysaccharides such as xyloglucans and (1,3;1,4)-beta-D-glucans, are fundamental to cell wall properties. These interactions have been assumed to be noncovalent in nature in most cases. Here we show that a highly purified barley xyloglucan xyloglucosyl transferase HvXET5 (EC 2.4.1.207), a member of the GH16 group of glycoside hydrolases, catalyzes the in vitro formation of covalent linkages between xyloglucans and cellulosic substrates and between xyloglucans and (1,3;1,4)-beta-D-glucans. The rate of covalent bond formation catalyzed by HvXET5 with hydroxyethylcellulose (HEC) is comparable with that on tamarind xyloglucan, whereas that with (1,3; 1,4)-beta-D-glucan is significant but slower. Matrix-assisted laser desorption ionization time-of-flight mass spectrometric analyses showed that oligosaccharides released from the fluorescent HEC:xyloglucan conjugate by a specific (1,4)-beta-D-glucan endohydrolase consisted of xyloglucan substrate with one, two, or three glucosyl residues attached. Ancillary peaks contained hydroxyethyl substituents (m/z 45) and confirmed that the parent material consisted of HEC covalently linked with xyloglucan. Similarly, partial hydrolysis of the (1,3;1,4)-beta-D-glucan:xyloglucan conjugate by a specific (1,3;1,4)-beta-D-glucan endohydrolase revealed the presence of a series of fluorescent oligosaccharides that consisted of the fluorescent xyloglucan acceptor substrate linked covalently with 2-6 glucosyl residues. These findings raise the possibility that xyloglucan endo-transglucosylases could link different polysaccharides in vivo and hence influence cell wall strength, flexibility, and porosity.     

A Cell-Surface GH9 Endo-Glucanase Coordinates with Surface Glycan-Binding Proteins to Mediate Xyloglucan Uptake in the Gut Symbiont Bacteroides ovatus

Dietary fiber is an important food source for members of the human gut microbiome. Members of the dominant Bacteroidetes phylum capture diverse polysaccharides via the action of multiple cell surface proteins encoded within polysaccharide utilization loci (PUL). The independent activities of PUL-encoded glycoside hydrolases (GHs) and surface glycan-binding proteins (SGBPs) for the harvest of various glycans have been studied in detail, but how these proteins work together to coordinate uptake is poorly understood. Here, we combine genetic and biochemical approaches to discern the interplay between the BoGH9 endoglucanase and the xyloglucan-binding proteins SGBP-A and SGBP-B from the Bacteroides ovatus xyloglucan utilization locus (XyGUL). The expression of BoGH9, a weakly active xyloglucanase in isolation, is required in a strain that expresses a non-binding version of SGBP-A (SGBP-A*). The crystal structure of the BoGH9 enzyme suggests the molecular basis for its robust activity on mixed-linkage β-glucan compared to xyloglucan. However, catalytically inactive site-directed mutants of BoGH9 fail to complement the deletion of the active BoGH9 in a SGBP-A* strain. We also find that SGBP-B is needed in an SGBP-A* background to support growth on xyloglucan, but that the non-binding SGBP-B* protein acts in a dominant negative manner to inhibit growth on xyloglucan. We postulate a model whereby the SGBP-A, SGBP-B, and BoGH9 work together at the cell surface, likely within a discrete complex, and that xyloglucan binding by SGBP-B and BoGH9 may facilitate the orientation of the xyloglucan for transfer across the outer membrane.     

Mechanism of the stabilization of ribonuclease A by sorbitol: preferential hydration is greater for the denatured then for the native protein.

The effect of interactions of sorbitol with ribonuclease A (RNase A) and the resulting stabilization of structure was examined in parallel thermal unfolding and preferential binding studies with the application of multicomponent thermodynamic theory. The protein was stabilized by sorbitol both at pH 2.0 and pH 5.5 as the transition temperature, Tm, was increased. The enthalpy of the thermal denaturation had a small dependence on sorbitol concentration, which was reflected in the values of the standard free energy change of denaturation, delta delta G(o) = delta G(o) (sorbitol) - delta G(o)(water). Measurements of preferential interactions at 48 degrees C at pH 5.5, where protein is native, and pH 2.0 where it is denatured, showed that sorbitol is preferentially excluded from the denatured protein up to 40%, but becomes preferentially bound to native protein above 20% sorbitol. The chemical potential change on transferring the denatured RNase A from water to sorbitol solution is larger than that for the native protein, delta mu(2D) > delta mu(2N), which is consistent with the effect of sorbitol on the free energy change of denaturation. The conformity of these results to the thermodynamic expression of the effect of a co-solvent on denaturation, delta G(o)(W) + delta mu(D)(2)delta G(o)(S) + delta mu(2D), indicates that the stabilization of the protein by sorbitol can be fully accounted for by weak thermodynamic interactions at the protein surface that involve water reversible co-solvent exchange at thermodynamically non-neutral sites. The protein structure stabilizing action of sorbitol is driven by stronger exclusion from the unfolded protein than from the native structure.     

Theoretical design and characterisation on the fluorinated nitrophenyl azidotriazoles

Fluorinated molecules containing pyridyl- or phenyl-triazole moiety are useful materials in high-energy applications. In this work, based on the structure of N-(2, 4-dinitrophenyl)-3-azido-1H-1, 2, 4-triazole (I), two fluorinated nitrophenyl azidotriazoles (II and III) were designed by modifying I with the –CF₃ or –NF₂ group and were studied using the density functional theory (DFT). Compound I is a synthesised nitrophenyl azidotriazole with high heat of formation and acceptable detonation performance and thermal stability. Compared with I, the density and detonation properties of II and III are obviously improved, especially those of II. To better characterise II, the spectra (IR and UV/vis), thermal stability and pyrolysis mechanism were investigated. The thermal decomposition mechanism of II is similar to that of I, i.e. the azido triazole fragment is the initial spot of decomposition, while the thermal stability of II is better. The better performance of II suggests that it is worth further experimental investigations.     

AtBGAL10 is the main xyloglucan β-galactosidase in Arabidopsis, and its absence results in unusual xyloglucan subunits and growth defects

In growing cells, xyloglucan is thought to connect cellulose microfibrils and regulate their separation during wall extension. In Arabidopsis (Arabidopsis thaliana), a significant proportion of xyloglucan side chains contain β-galactose linked to α-xylose at O2. In this work, we identified AtBGAL10 (At5g63810) as the gene responsible for the majority of β-galactosidase activity against xyloglucan. Xyloglucan from bgal10 insertional mutants was found to contain a large proportion of unusual subunits, such as GLG and GLLG. These subunits were not detected in a bgal10 xyl1 double mutant, deficient in both β-galactosidase and α-xylosidase. Xyloglucan from bgal10 xyl1 plants was enriched instead in XXLG/XLXG and XLLG subunits. In both cases, changes in xyloglucan composition were larger in the endoglucanase-accessible fraction. These results suggest that glycosidases acting on nonreducing ends digest large amounts of xyloglucan in wild-type plants, while plants deficient in any of these activities accumulate partly digested subunits. In both bgal10 and bgal10 xyl1, siliques and sepals were shorter, a phenotype that could be explained by an excess of nonreducing ends leading to a reinforced xyloglucan network. Additionally, AtBGAL10 expression was examined with a promoter-reporter construct. Expression was high in many cell types undergoing wall extension or remodeling, such as young stems, abscission zones, or developing vasculature, showing good correlation with α-xylosidase expression.     

Structure-function relationships affecting the insecticidal and miticidal activity of sugar esters

Synthetic sugar esters are a relatively new class of insecticidal compounds that are produced by reacting sugars with fatty acids. The objective of this research was to determine how systematic alterations in sugar or fatty acid components of sugar ester compounds influenced their insecticidal properties. Sucrose octanoate, sorbitol octanoate, sorbitol decanoate, sorbitol caproate, xylitol octanoate, xylitol decanoate and xylitol dodecanoate were synthesized and evaluated against a range of arthropod pests. Dosage-mortality studies were conducted on pear psylla (Cacopsylla pyricola Foerster) on pear, tobacco aphid (Myzus nicotianae) Blackman and tobacco hornworm (Manduca sexta [Johannson]) on tobacco, and twospotted spider mite (Tetranychus urticae Koch) on apple in laboratory bioassays. These sugar esters were compared with insecticidal soap (M-Pede, Dow AgroSciences L.L.C., San Diego, CA), to determine how toxicologically similar these materials were against the arthropod pests. Substitutions in either the sugar or fatty acid component led to significant changes in the physical properties and insecticidal activity of these compounds. The sugar esters varied in their solubility in water and in emulsion stability, yet, droplet spread upon pear leaves occurred at low concentrations of 80-160 ppm and was strongly correlated with psylla mortalities (R2 = 0.73). Sequentially altering the sugar or fatty acid components from lower to higher numbers of carbon chains, or whether the sugar was a monosaccharide or disaccharide did not follow a predictable relationship to insecticidal activity. Intuitively, changing the hydrophile from sorbitol (C6) to xylitol (C5) would require a decrease in lipophile chain length to maintain hydrophilic-lipophilic balance (HLB) relationships, yet an increase in lipophile chain length was unexpectedly needed for increasing insecticidal activity. Thus, the HLB of these materials did not correlate with pear psylla mortalities. Initial insect bioassays and dosage-mortality data found significant differences among sugar ester compounds' toxicity to the range of arthropod species. Sucrose octanoate high in monoester content had the highest activity against the range of arthropod pests at low concentrations of 1200-2400 ppm. No single chemical structure for the xylitol or sorbitol esters were optimally effective against the range of arthropods we tested and sorbitol octanoate and xylitol decanoate had the highest insecticidal activity of this group. All of the sugar ester materials produced high T. urticae mortalities on apple at very low concentrations of 400 ppm. Overall, most of the sugar esters that were examined had superior insecticidal activity compared with insecticidal soap. Sugar ester chemistry offers a unique opportunity to design an insecticide or miticide specific to certain arthropod pests which would be valuable in crop integrated pest management (IPM) programs. Sucrose esters are currently used as additives in the food industry which makes them especially attractive as safe and effective insecticides.     

Understanding the thermal properties of amorphous solids using machine-learning-based interatomic potentials

Abstract

Understanding the thermal properties of disordered systems is of fundamental importance for condensed matter physics - and for practical applications as well. While quantities such as the thermal conductivity are usually well characterised experimentally, their microscopic origin is often largely unknown - hence the pressing need for molecular simulations. However, the time and length scales involved with thermal transport phenomena are typically well beyond the reach of ab initio calculations. On the other hand, many amorphous materials are characterised by a complex structure, which prevents the construction of classical interatomic potentials. One way to get past this deadlock is to harness machine-learning (ML) algorithms to build interatomic potentials: these can be nearly as computationally efficient as classical force fields while retaining much of the accuracy of first-principles calculations. Here, we discuss neural network potentials (NNPs) and Gaussian approximation potentials (GAPs), two popular ML frameworks. We review the work that has been devoted to investigate, via NNPs, the thermal properties of phase-change materials, systems widely used in non-volatile memories. In addition, we present recent results on the vibrational properties of amorphous carbon, studied via GAPs. In light of these results, we argue that ML-based potentials are among the best options available to further our understanding of the vibrational and thermal properties of complex amorphous solids.     

Top-down grafting of xyloglucan to gold monitored by QCM-D and AFM: enzymatic activity and interactions with cellulose

This study focuses on the manufacture and characterization of model surfaces consisting of end-grafted xyloglucan (XG), a naturally occurring polysaccharide, onto a gold substrate. The now well-established XET-technology was utilized for enzymatic incorporation of a thiol moiety at one end of the xyloglucan backbone. This functionalized macromolecule was subsequently top-down grafted to gold, forming a thiol-bonded xyloglucan brushlike layer. The grafting was monitored in situ with QCM-D, and a significant difference in the adsorbed/grafted amount between unmodified xyloglucan and the thiol-functionalized polymer was observed. The grafted surface was demonstrated to be accessible to enzyme digestion using the plant endo-xyloglucanase TmNXG1. The nanotribological properties toward cellulose of the untreated crystal, brush-modified surface, and enzyme-exposed surfaces were compared with a view to understanding the role of xyloglucan in friction reduction. Friction coefficients obtained by the AFM colloidal probe technique using a cellulose functionalized probe on the xyloglucan brush showed an increase of a factor of 2 after the enzyme digestion, and this result is interpreted in terms of surface roughness. Finally, the brush is shown to exhibit binding to cellulose despite its highly oriented nature.