Adsorption and viscoelastic properties of cationic xylan on cellulose film using QCM-D

The adsorption and viscoelastic properties of cationic xylan layers adsorbed from an aqueous electrolyte solution (NaCl 0, 1, 10, 100 mM) on a cellulose model surface were studied using quartz crystal microbalance with dissipation (QCM-D). Three cationic xylans with different charge densities were used (molecular weight, 9,600 g/mol with degrees of substitution, DS = 0.150, 0.191, and 0.259). The influences of the electrolyte concentration and charge density of cationic xylan on its adsorption onto a cellulose surface were investigated. Low charged cationic xylan was substantially more efficient in surface adsorption on cellulose compared to high charged cationic xylan at a low concentration of electrolytes. Adsorption of low charged cationic xylan decreased with increases in electrolyte concentration. However, adsorption of high cationic xylan increased with electrolyte concentration. The conformation and viscoelastic properties of the layers were interpreted by modeling the data under the assumption that the layers can be explained by the a Voigt model. Low charged cationic xylan adsorbed relatively weakly onto the cellulose surface, and formed a thicker, softer layer than high charged cationic xylan. On the other hand, high charged cationic xylan formed a thinner adsorption layer onto the cellulose surface.     

Purification of cellulosic pulp by hot water extraction

Hot water extraction (HWE) of pulp in a flow-through reactor was evaluated as a method to purify paper-grade pulps. About 50–80 % of the xylan and up to 50 % of the lignin in unbleached birch Kraft pulp was extracted by the HWE without losses in cellulose yield. The residual xylan content in the extracted pulps was predominantly too high for dissolving-grade applications, but some of the pulps with a xylan content of 5–7 % might still be suitable as rayon-grade pulps. Increasing extraction temperature lowered the xylan content at which cellulose yield started to decrease. Furthermore, at any given xylan content, increasing extraction temperature resulted in cellulosic pulp with higher degree of polymerization. The extracted xylan was recovered almost quantitatively as xylo-oligosaccharides. The results suggest that HWEs at elevated temperatures may be applied to purify cellulosic pulps, preferably containing a low xylan content, and to recover the extracted sugars.     

On the accessibility and structure of xylan in birch kraft pulp

The structure of -(14)-xylan, both in isolated form and as a component of bleached birch kraft pulp, was studied employing CP/MAS ¹³C NMR spectroscopy. Bleached birch kraft pulp was treated with xylanases or alkali in order to distinguish between accessible and inaccessible xylan. In xylan which was alkali-extracted from bleached birch kraft pulp, the relative contents of xylose and 4-O-methylglucuronic acid were 99.4 and 0.6 weight %, respectively, and the degree of polymerization was 70. The supermolecular structure of xylan is very sensitive to the surrounding environment. All extracted xylan chains were accessible to water and methanol and the solvent molecules easily exchanged. In bleached birch kraft pulp, cellulose fibrils interact with xylan chains, causing these to adopt a conformation similar to one of the configurations observed for dry xylan. In birch pulp, about 1/3 of the xylan was found to be accessible to digestion by xylanases or extraction with 5% w/w potassium hydroxide (aq). A signal at 81.7ppm in the C-4 region of the CP/MAS ¹³C NMR spectrum of bleached birch kraft pulp originated from xylan at the accessible fibril surfaces. A portion of a broad signal at 83.5ppm reflected inaccessible xylan, which is probably present as co-aggregates with cellulose fibril aggregates.     

Cellulose crystallinity and ordering of hemicelluloses in pine and birch pulps as revealed by solid-state NMR spectroscopic methods

Solid-state ¹³C NMR spectroscopy was used to determine the degree of cellulose crystallinity (CrI) in kraft, flow-through kraft and polysulphide–anthraquinone (PS–AQ) pulps of pine and birch containing various amounts of hemicelluloses. The applicability of acid hydrolysis and the purely spectroscopic proton spin-relaxation based spectral edition (PSRE) method to remove the interfering hemicellulose signals prior to the determination of CrI were also compared. For softwood pulps, the spectroscopic removal of hemicelluloses by PSRE was found to be more efficient than the removal of hemicelluloses by acid hydrolysis. In addition to that, the PSRE method also provides information on the associations between cellulose and hemicelluloses. On the basis of the incomplete removal of xylan from the cellulose subspectra by PSRE, the deposition of xylan on cellulose fibrils and therefore an ordered ultrastructure of xylan in birch pulps was suggested. The ordered structure of xylan in birch pulps was also supported by the observed change of xylan conformation after regeneration. Similarly, glucomannan in pine pulps may have an ordered structure. According to the ¹³C CPMAS measurements conducted after acid hydrolysis, the degree of cellulose crystallinity was found to be slightly lower in birch pulps than in the pine pulps. Any significant differences in cellulose crystallinity were not found between the pulps obtained by the various pulping methods. Only in pine PS–AQ pulp, the degree of cellulose crystallinity may be slightly lower than in the kraft pulps containing less hemicelluloses.     

Cationic wood cellulose films with high strength and bacterial anti-adhesive properties

In this work, periodate oxidized birch wood pulp and microfibrillated cellulose (MFC) were cationized using Girard’s reagent T or aminoguanidine. Cationic celluloses were used to obtain films via solvent-casting method, and the effects of the cationization route and the cellulose fiber source on the properties of the films were studied. Thermal and optical properties of the films were measured using differential scanning calorimetry and UV–Vis spectrometry, and the morphology of the films was examined using an optical microscope and a field emission scanning electron microscope. Bacterial anti-adhesive properties of the films were also studied using a modified leaf print method and against Staphylococcus aureus and Escherichia coli. Both cationizing agents exhibited similar reactivity with periodate oxidized celluloses, however, MFC had significantly higher reactivity compared to birch pulp. The films with high tensile strength (39.1–45.3 MPa) and modulus (3.5–7.3 GPa) were obtained from cationized birch pulp, aminoguanidine modification producing a film with slightly better mechanical properties. Modulus of the films was significantly increased (up to 14.0 GPa) when MFC was used as a cellulose fiber source. Compared to the unmodified MFC films, the cationic MFC films were less porous and significantly more transparent; however, they had slightly lower tensile strength values. It was found that aminoguanidine modified celluloses had no culturable bacteria on its surface and also exhibited resistance to microbial degradation, whereas there were culturable bacteria on the surface of Girard’s reagent modified films and they were partially degraded by the bacteria.     

The effect of Fenton chemistry on the properties of microfibrillated cellulose

A fully bleached birch kraft pulp was treated with acidic hydrogen peroxide in the presence of ferrous ions (Fenton’s reagent) and thereafter treated mechanically in a colloid mill to produce a product containing microfibrillated cellulose (MFC). The produced MFC products were chemically and morphologically characterized and compared with MFC products produced without pretreatment as well as with enzymatic hydrolysis. Fenton treatment resulted in an increase in total charge and number of carbonyl groups while the intrinsic viscosity decreased. The Fenton treated pulps were easier to process mechanically i.e. they reached a higher specific surface area at a given mechanical treatment time and the MFC produced had a stable water-fibre suspension for at least 8 weeks compared to enzymatic pretreated pulps and pulps not subjected to any pretreatment.     

Theoretical investigation of azo dyes adsorbed on cellulose fibers: 2. Spectroscopic study

Electronic absorption spectra and the frontier orbitals of 1-arylazo-2-naphtol dyes are computed and analyzed in four models, namely in the gas phase (model I), in a solvent (model I + CPCM), adsorbed on the cellulose surface (model II), and model II in the presence of solvent (model II + CPCM) via time-dependent density functional theory (TD-DFT) and conductor-like polarizable continuum model (CPCM) at the B3LYP/6-31G** level of theory. A bathochromic shift is observed for the λ_max peak due to both short-range and long-range interactions of the non-ionic dyes with cellulose, while the ionic dyes exhibit hypsochromic shift in their λ_max peak. The results predict that the studied dyes should be nearly yellow after being adsorbed on cellulose with excellent color strength. Furthermore, the ionic dyes are suitable for the dyeing of cellulose fibers. The nuclear magnetic resonance (NMR) chemical shieldings calculated for the azo dyes in the gas phase and adsorbed states and for their tautomeric equilibrium mixtures show that the NMR technique can be used successfully to follow the dyeing process.     

The molecular basis of the adsorption of xylans on cellulose surface

In order to model the adsorption of xylan on cellulose, we have simulated, at the atomic level, the gas phase adsorption of small xylan fragments having 5 skeletal β (1 → 4) xylosyl residues (X₅), using molecular dynamics simulations. A first regime was considered, corresponding to a low surface coverage, with the adsorption of isolated X₅ in various initial orientations. In this regime, the simulation indicated that X₅ moved toward extended conformations, some of them being helical, with the possibility of either 2₁ or left-handed 3₁ helices. During the simulation, the X₅ fragments became preferentially oriented, parallel or anti parallel with respect to the cellulose chain axis. Substitution of the X₅ backbone by either GlcA and/or Araf side chains had no major influence on either the conformation or the efficiency of the interaction. However, the presence of side chains favored orientations of the X₅ backbone inclined with respect to the cellulose chain axis. In a second regime corresponding to monolayer coverage, the geometrical features of the adsorption of the xylan fragments on cellulose was roughly the same as that in the individual coverage situation. In this case, the monolayer became equilibrated at 0.14 g of xylan fragments for each g of cellulose, a figure that compared favourably with the values obtained in experimental adsorption of xylan on bacterial cellulose.     

偏氟乙烯/三氟氯乙烯交替共聚物在TATB表面吸附的分子动力学模拟

采用COMPASS力场和NVT正则系综的动力学模拟方法, 搭建了聚合度分别为10, 50和100的偏氟乙烯(VDF)/三氟氯乙烯(CTFE)交替共聚物, 对交替共聚物在1,3,5-三氨基-2,4,6-三硝基苯(TATB)的(0,0,1)晶面上的吸附和结构进行了分子动力学(MD)模拟. 结果表明, 在300～320 K温区, 聚合度为100的VDF/CTFE交替共聚物链对TATB晶体有理想的表面活性和吸附能力, 以train型构象平铺于TATB表面. 通过对聚合度为10的交替共聚物的多链体系在TATB表面吸附的MD模拟, 表明了VDF/CTFE交替共聚物具有非凝聚吸附的高表面活性特征. 对搭建的乙酸乙酯溶剂化的聚合度为50的VDF/CTFE交替共聚物在TATB晶体表面吸附的模拟, 实验证明了溶剂小分子能够降低共聚物链的吸附能力, 且链以tail型构象吸附于TATB表面.     

Cationic Xylan Derivatives with High Degree of Functionalization

Xylan from birch wood was characterized regarding both the supramolecular structure (X-ray, CP/MAS ¹³C-NMR) and the sugar composition. The reaction of the birch wood xylan with 2,3-epoxypropyltrimethylammonium chloride in 1,2-dimethoxyethane as slurry medium yields water-soluble, cationic 2-hydroxypropyltrimethylammonium xylan derivatives with high degree of substitution (DS). The DS values up to 1.6 can be controlled by adjusting the molar ratio in a one step synthesis. The structure of the cationic xylan derivatives was confirmed by means of DEPT(135) NMR spectroscopy. Film forming properties of cationic xylan derivatives were investigated with SEM measurements.     

Dry,hydrophobic microfibrillated cellulose powder obtained in a simple procedure using alkyl ketene dimer

In order to produce dry and hydrophobic microfibrillated cellulose (MFC) in a simple procedure, its modification with alkyl ketene dimer (AKD) was performed. For this purpose, MFC was solvent-exchanged to ethyl acetate and mixed with AKD dissolved in the same solvent. Curing at 130 °C for 20 h under the catalysis of 1-methylimidazole yielded a dry powder. Scanning electron microscopy of the powder indicated loss in nanofibrillar structure due to aggregation, but discrete microfibrillar structures were still present. Water contact angle measurements of films produced from modified and unmodified MFC showed high hydrophobicity after AKD treatment, which persisted even after extraction with THF for 8 h. The hydrophobized MFC was characterized by Fourier transform infrared spectroscopy, nuclear magnetic resonance and X-ray analysis. In summary, strong indications for the presence of AKD on the surface of MFC before and after extraction with solvent were found, but only a very small amount of covalent β-ketoester linkages between the modification agent and cellulose was revealed.     

Investigation of CO2 Orientational Dynamics through Simulated NMR Line Shapes**

The dynamics of carbon dioxide in third generation (i. e., flexible) Metal-Organic Frameworks (MOFs) can be experimentally observed by ¹³C NMR spectroscopy. The obtained line shapes directly correlate with the motion of the adsorbed CO₂, which in turn are readily available from classical molecular dynamics (MD) simulations. In this article, we present our publicly available implementation of an algorithm to calculate NMR line shapes from MD trajectories in a matter of minutes on any current personal computer. We apply the methodology to study an effect observed experimentally when adsorbing CO₂ in different samples of the pillared layer MOF Ni₂(ndc)₂(dabco) (ndc=2,6-naphthalene-dicarboxylate, dabco=1,4-diazabicyclo-[2.2.2]-octane), also known as DUT-8(Ni). In ¹³C NMR experiments of adsorbed CO₂ in this MOF, small (rigid) crystals result in narrower NMR line shapes than larger (flexible) crystals. The reasons for the higher mobility of CO₂ inside the smaller crystals is unknown. Our ligand field molecular mechanics simulations provide atomistic insight into the effects visible in NMR experiments with limited computational effort.     

Influence of cation on the cellulose dissolution investigated by MD simulation and experiments

Cellulose is the most abundant natural polymer on the earth, and effective solvents are essential for its wide application. Among various solvents such as alkali/urea or ionic liquids, cations all play a very important role on the cellulose dissolution. In this work, the influence of cation on the cellulose dissolution in alkali/urea via a cooling process was investigated with a combination of MD simulation and experiments, including differential scanning calorimetry (DSC) and NMR diffusometry (PFG-SE NMR). The results of DSC proved that the dissolution of cellulose in both solvents was a process within a temperature range, starting at above 0 °C and completing at low temperature (?5 °C for LiOH/urea and ?20 °C for NaOH/urea), indicating the necessity of low temperature for the cellulose dissolution. Molecular dynamic (MD) simulation suggested that the electrostatic force between OH^? and cellulose dominated the inter-molecular interactions. In our findings, Li⁺ could penetrate closer to cellulose, and displayed stronger electrostatic interaction with the biomacromolecule than Na⁺, thus possessed a greater “stabilizing” effect on the OH^?/cellulose interaction. PFG-SE NMR demonstrated a more significant binding fraction of Li⁺ than Na⁺ to cellulose, which was consistent with MD. These results indicated that the direct interactions existed between the cations and cellulose, and Li⁺ exhibited stronger interaction with cellulose, leading to stronger dissolving power.     

What induces pocket openings on protein surface patches involved in protein–protein interactions?

We previously showed for the proteins BCL-X_L, IL-2, and MDM2 that transient pockets at their protein–protein binding interfaces can be identified by applying the PASS algorithm to molecular dynamics (MD) snapshots. We now investigated which aspects of the natural conformational dynamics of proteins induce the formation of such pockets. The pocket detection protocol was applied to three different conformational ensembles for the same proteins that were extracted from MD simulations of the inhibitor bound crystal conformation in water and the free crystal/NMR structure in water and in methanol. Additional MD simulations studied the impact of backbone mobility. The more efficient CONCOORD or normal mode analysis (NMA) techniques gave significantly smaller pockets than MD simulations, whereas tCONCOORD generated pockets comparable to those observed in MD simulations for two of the three systems. Our findings emphasize the influence of solvent polarity and backbone rearrangements on the formation of pockets on protein surfaces and should be helpful in future generation of transient pockets as putative ligand binding sites at protein–protein interfaces. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

To understand the superior hydrolytic activity after polymorphic conversion from cellulose I to II from the adsorption behaviors of enzymes

The role of the cellulose ultrastructure on the relationship between cellulase binding and activity is not clear yet. In this article, a quartz crystal microbalance with dissipation (QCM-D) was employed to monitor the interactions between a given cellulase and the cellulose substrates with varied polymorphs of pure cellulose I and II and the intermediate state (I/II). Initially, cellulose nanocrystals (CNCs) with polymorphs of cellulose I, I/II and II were prepared and spin-coated on QCM sensors. The cellulose substrates’ crystallinity degree was examined by XRD, and morphology was detected by AFM. Then, a commercial cellulase from Trichoderma reesei was used to test the adsorption and hydrolysis of cellulose substrates with polymorphs of I, I/II and II, respectively. The results revealed that in the enzyme adsorption and desorption process at a temperature of 15 °C, CNC-II had the lowest adsorption capacity with a total adsorption mass of 179 ng cm^?2 but the highest reversible binding ratio of 33.7%; for comparison, the values were 235 ng cm^?2 versus 25.6% and 207 ng cm^?2 versus 26.9% for CNC-I and -I/II, respectively. And the conformation of adlayers on CNC-I, -I/II and -II derived from the QCM data became softer and softer in turn. On the other hand, CNC-II exhibited the best enzymatic hydrolytic ability among three substrates when enzymatic hydrolysis experiments were conducted at 45 °C. The results indicated that polymorphic conversion from I to II changes the affinity between the enzyme and cellulose surface; CNC-II has the lowest affinity to the enzyme, but the softer conformation of the adsorbed enzyme layer, and the more reversible adsorption may facilitate its hydrolytic activity. This article gives a perspective from the adsorption dynamics and conformation of the adsorbed enzyme layer, helping to understand the superior hydrolytic activity of cellulose with polymorph II. Thus, there is a potential of polymorphic conversion in the reduction of enzyme dosage and cost in the enzymatic hydrolysis process.     

Molecular dynamics simulations of the rehydration of folded and unfolded cytochrome C ions in the vapor phase.

Molecular dynamics (MD) simulations have been performed to study the rehydration of compact and unfolded cytochrome c ions in the vapor phase. Experimental studies have shown that the compact conformations adsorb many more water molecules than unfolded ones when exposed to water vapor. MD simulations performed with up to 150 water molecules reproduce the key experimental observations, including a partial refolding caused by hydration. According to the calculations it is more energetically favorable to hydrate the compact conformation in the initial stages of hydration, because it is easier for a water molecule to interact simultaneously with several polar groups (due to their proximity). The protonated side chains are not favored hydration sites in the simulations because they have "self-solvation" shells which must be disrupted for the water to penetrate. For both conformations, the adsorbed water molecules are mainly located in surface crevices.     

Effect of xylan in hardwood pulp on the reaction rate of TEMPO-mediated oxidation and the rheology of the final nanofibrillated cellulose gel

alkali-washed nanofibrillated cellulose (NFC) samples, obtained from hardwood kraft pulp, with different amounts of retained xylan were prepared to study the influence of xylan on the water-retention properties of NFC suspensions. In this study, NFC was produced using an oxoammonium-catalyzed oxidation reaction that converts the cellulosic substrate to a more highly oxidized material via the action of the nitroxide radical species 2,2,6,6-tetramethylpiperidine-1-oxyl. Reduction of the xylan content in NFC was achieved by cold alkali extraction of kraft pulp. The pulps were then oxidized to a set charge under constant chemical conditions, and the reaction time was determined. The xylan content of the feed pulp was found to have a large negative influence on the oxidation rate of the pulp, as the oxidation time shortened when xylan was removed, from 220 min (for 25.2 % xylan content) to 28 min (for 7.3 % xylan content). Following fibrillation by homogenization, the swelling of the NFC was determined by a two-point solute exclusion method. The distribution of hemicellulose over the fibril surface was observed by atomic force microscopy. Xylan was found to be distributed unevenly over the surface, and its presence increased the water immobilized within flocs of NFC, i.e., so-called network swelling. The swelling of the NFC had a large impact on its rheology and dewatering. Comparison of the morphological and swelling properties of the suspensions with their rheological and dynamic dewatering behavior showed that reducing the xylan content in NFC results in a weaker gel structure of the nanocellulose suspension. The results indicate that most of the water is held by the swollen structure by means of xylan particles trapped within the hemicellulose layer covering the fibril surface. Samples with high xylan content had high shear modulus and viscosity and were difficult to dewater.     

Glycosidic linkage conformation of methyl-alpha-mannopyranoside

We study the preferred conformation of the glycosidic linkage of methyl-alpha-mannopyranoside in the gas phase and in aqueous solution. Results obtained utilizing Car-Parrinello molecular dynamics (CPMD) simulations are compared to those obtained from classical molecular dynamics (MD) simulations. We describe classical simulations performed with various water potential functions to study the impact of the chosen water potential on the predicted conformational preference of the glycosidic linkage of the carbohydrate in aqueous solution. In agreement with our recent studies, we find that results obtained with CPMD simulations differ from those obtained from classical simulations. In particular, this study shows that the trans (t) orientation of the glycosidic linkage of methyl-alpha-mannopyranoside is preferred over its gauche anticlockwise (g-) orientation in aqueous solution. CPMD simulations indicate that this preference is due to intermolecular hydrogen bonding with surrounding water molecules, whereas no such information could be demonstrated by classical MD simulations. This study emphasizes the importance of ab initio MD simulations for studying the structural properties of carbohydrates in aqueous solution.     

Possibilities for Optimization of Industrial Alkaline Steeping of Wood-Based Cellulose Fibers

Steeping of cellulosic materials in aqueous solution of NaOH is a common pre-treatment in several industrial processes for production of cellulose-based products, including viscose fibers. This study investigated whether the span of commonly applied process settings has the potential for process optimization regarding purity, yield, and degree of transformation to alkali cellulose. A hardwood kraft dissolving pulp was extracted with 17–20 wt% aq. NaOH at 40−50 °C. The regenerated residue of the pulp was characterized regarding its chemical composition, molecular structure, and cellulose conformation. Yield was shown to be favored primarily by low temperature and secondly by high alkali concentration. Purity of xylan developed inversely. Both purity of xylan and yield varied over the applied span of settings to an extent which makes case-adapted process optimization meaningful. Decreasing the steeping temperature by 2 °C increased xylan content in the residue with 0.13%-units over the whole span of applied alkali concentrations, while yield increased by 0.15%-units when extracting with 17 wt% aq. NaOH, and by 0.20%-units when extracting with 20 wt%. Moreover, the yield-favoring conditions resulted in a narrower molecular weight distribution. The degree of transformation via alkali cellulose to cellulose II, as determined with Raman spectroscopy, was found to be high at all extraction settings applied.     

Impact of the nature and shape of cellulosic nanoparticles on the isothermal crystallization kinetics of poly(ε-caprolactone)

Polycaprolactone (PCL)/cellulose nanocomposites were prepared by mixing PCL with surface modified sisal nanowhiskers (CNW) and microfibrillated cellulose (MFC) extracted from sisal fibers. The influence of cellulosic nanoparticles on the crystallization behavior of PCL was investigated by differential scanning calorimetry. Isothermal crystallization data were modeled with Avrami’s kinetics, Lauritzen–Hoffman secondary nucleation theory and equilibrium melting points were determined with the Hoffman–Weeks method. The cellulose nanoparticles, acting as nucleating agents, drastically accelerate the crystallization of PCL while depressing its equilibrium melting by 9–10 °C. The crystallization of MFC-nanocomposites is slightly faster than that of CNW-nanocomposites, in agreement with the slightly lower bulk activation energy for crystallization and nucleation parameter in the former. The results are discussed based on the differences of specific surface area and surface chemistry of nanoparticles, as well as the confinement phenomenon.