| 1. |
- Angles d'Ortoli, Thibault, et al.
(författare)
-
Temperature Dependence of Hydroxymethyl Group Rotamer Populations in Cellooligomers
- 2015
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 119:30, s. 9559-9570
-
Tidskriftsartikel (refereegranskat)abstract
- Empirical force fields for computer simulations of carbohydrates are often implicitly assumed to be valid also at temperatures different from room temperature for which they were optimited: Herein, the temperature dependence of the hydroxymethyl group rotamer populations in short oligogaccharides is invegtigated using Molecular dynamics simulations and NMR spectroscopy. Two oligosaccharides, methyl beta-cellobioside and beta-cellotetraose were simulated using three different carbohydrate force fields (CHARMM C35, GLYCAM06, and GROMOS 56A(carbo)) in combination with different water models (SPC, SPC/E, and TIP3P) using replica exchange molecular dynamics simulations. For comparison, hydroxymethyl group rotamer populations were investigated for methyl beta-cellobioside and cellopentaose based- on measured NMR (3)J(H5,H6) coupling constants, in the latter case by using a chemical shift selective NMR-filter. Molecular dynamics simulations in combination with NMR spectroscopy show that the temperature dependence of the hydroxymethyl rotamer population in these short cellooligomers, in the range 263-344 K, generally becomes exaggerated in simulations when compared to experimental data, but also that it is dependent on simulation conditions, and most notably properties of the water model.
|
|
| 2. |
- Bergenstrahle-Wohlert, Malin, et al.
(författare)
-
On the anomalous temperature dependence of cellulose aqueous solubility
- 2016
-
Ingår i: Cellulose. - : Springer. - 0969-0239 .- 1572-882X. ; 23:4, s. 2375-2387
-
Tidskriftsartikel (refereegranskat)abstract
- The solubility of cellulose in water-based media is promoted by low temperature, which may appear counter-intuitive. An explanation to this phenomenon has been proposed that is based on a temperature-dependent orientation of the hydroxymethyl group. In this paper, this hypothesis is investigated using molecular dynamics computer simulations and NMR spectroscopy, and is discussed in conjunction with alternative explanations based on solvent–solute and solvent–solvent hydrogen bond formation respectively. It is shown that neither simulations nor experiments lend support to the proposed mechanism based on the hydroxymethyl orientation, whereas the two alternative explanations give rise to two distinct contributions to the hydration free energy of cellooligomers.
|
|
| 3. |
|
|
| 4. |
- Bergenstråhle-Wohlert, Malin, et al.
(författare)
-
Concentration enrichment of urea at cellulose surfaces : results from molecular dynamics simulations and NMR spectroscopy
- 2012
-
Ingår i: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 19:1, s. 1-12
-
Tidskriftsartikel (refereegranskat)abstract
- A combined solid-state NMR and Molecular Dynamics simulation study of cellulose in urea aqueous solution and in pure water was conducted. It was found that the local concentration of urea is significantly enhanced at the cellulose/solution interface. There, urea molecules interact directly with the cellulose through both hydrogen bonds and favorable dispersion interactions, which seem to be the driving force behind the aggregation. The CP/MAS (13)C spectra was affected by the presence of urea at high concentrations, most notably the signal at 83.4 ppm, which has previously been assigned to C4 atoms in cellulose chains located at surfaces parallel to the (110) crystallographic plane of the cellulose I beta crystal. Also dynamic properties of the cellulose surfaces, probed by spin-lattice relaxation time (13)CT (1) measurements of C4 atoms, are affected by the addition of urea. Molecular Dynamics simulations reproduce the trends of the T (1) measurements and lends new support to the assignment of signals from individual surfaces. That urea in solution is interacting directly with cellulose may have implications on our understanding of the mechanisms behind cellulose dissolution in alkali/urea aqueous solutions.
|
|
| 5. |
- Berglund, Jennie, et al.
(författare)
-
A molecular dynamics study of the effect of glycosidic linkage type in the hemicellulose backbone on the molecular chain flexibility
- 2016
-
Ingår i: The Plant Journal. - : John Wiley & Sons Ltd.. - 0960-7412 .- 1365-313X. ; 88:1, s. 56-70
-
Tidskriftsartikel (refereegranskat)abstract
- The macromolecular conformation of the constituent polysaccharides in lignocellulosic biomass influences their supramolecular interactions, and therefore their function in plants and their performance in technical products. The flexibility of glycosidic linkages from the backbone of hemicelluloses was studied by evaluating the conformational freedom of the φ and ψ dihedral angles using molecular dynamic simulations, additionally selected molecules were correlated with experimental data by NMR spectroscopy. Three types of β-(1→4) glycosidic linkages involving the monosaccharides (Glcp, Xylp and Manp) present in the backbone of hemicelluloses were defined. Different di- and tetrasaccharides with combinations of such sugar monomers from hemicelluloses were simulated and free energy maps of the φ - ψ space and hydrogen bonding patterns were obtained. The glycosidic linkage between Glc-Glc or Glc-Man (C-type) was the stiffest with mainly one probable conformation; the linkage from Man-Man or Man-Glc (M-type) was similar but with an increased probability for an alternative conformation making it more flexible, and the linkage between two Xyl-units (X-type) was the most flexible with two almost equally populated conformations. Glycosidic linkages of the same type showed essentially the same conformational space in both disaccharides and in the central region of tetrasaccharides. Different probabilities of glycosidic linkage conformations in the backbone of hemicelluloses can be directly estimated from the free energy maps, which to a large degree affect the overall macromolecular conformations of these polymers. The information gained contributes to an increased understanding of hemicelluloses’ function both in the cell wall and in technical products.
|
|
| 6. |
|
|
| 7. |
|
|
| 8. |
- Limaye, Mukta, V, et al.
(författare)
-
Functionalization and patterning of nanocellulose films by surface-bound nanoparticles of hydrolyzable tannins and multivalent metal ions
- 2019
-
Ingår i: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 11:41, s. 19278-19284
-
Tidskriftsartikel (refereegranskat)abstract
- Inspired by the Bogolanfini dyeing technique, we report how flexible nanofibrillated cellulose (CNF) films can be functionalized and patterned by surface-bound nanoparticles of hydrolyzable tannins and multivalent metal ions with tunable colors. Molecular dynamics simulations show that gallic acid (GA) and ellagic acid (EA) rapidly adsorb and assemble on the CNF surface, and atomic force microscopy confirms that nanosized GA assemblies cover the surface of the CNF. CNF films were patterned with tannin-metal ion nanoparticles by an in-fibre reaction between the pre-impregnated tannin and the metal ions in the printing ink. Spectroscopic studies show that the Fe-III/II ions interact with GA and form surface-bound, stable GA-Fe-III/II nanoparticles. The functionalization and patterning of CNF films with metal ion-hydrolyzable tannin nanoparticles is a versatile route to functionalize films based on renewable materials and of interest for biomedical and environmental applications.
|
|
| 9. |
- Tolonen, Lasse K., et al.
(författare)
-
Solubility of Cellulose in Supercritical Water Studied by Molecular Dynamics Simulations
- 2015
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 119:13, s. 4739-4748
-
Tidskriftsartikel (refereegranskat)abstract
- The insolubility of cellulose in ambient water and most aqueous systems presents a major scientific and practical challenge. Intriguingly though, the dissolution of cellulose has been reported to occur in supercritical water. In this study, cellulose solubility in ambient and supercritical water of varying density (0.2, 0.7, and 1.0 g cm(-3)) was studied by atomistic molecular dynamics simulations using the CHARMM36 force field and TIP3P water. The Gibbs energy of dissolution was determined between a nanocrystal (4 x 4 x 20 anhydroglucose residues) and a fully dissociated state using the two-phase thermodynamics model. The analysis of Gibbs energy suggested that cellulose is soluble in supercritical water at each of the studied densities and that cellulose dissolution is typically driven by the entropy gain upon the chain dissociation while simultaneously hindered by the loss of solvent entropy. Chain dissociation caused density augmentation around the cellulose chains, which improved water-water bonding in low density supercritical water whereas the opposite occurred in ambient and high density supercritical water.
|
|
| 10. |
|
|
| 11. |
- Wang, Yan, 1987-, et al.
(författare)
-
Hydration and dimensional stability of the intercalated galleries in xyloglucan/montmorillonite nanocomposites studied by molecular dynamics simulations
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The outstanding properties of biological composite nacre materials have for a long time inspired research and development of man-made bionanocomposites. One of the most recent nacre-mimetic bionanocomposites comprising xygloglucan (XG) and montmorillonite (Mnt) clay has been investigated by related model systems through Molecular dynamics (MD) simulations. The expansion of the inter-gallery of the XG-Mnt composites when exposed to water, has been found to be a key issue for the material property. In order to shed light on the mechanism for this swelling behavior we have investigated the relation between the hydration and the dimensional stability of the inter-gallery in XG-Mnt composites, exploring also the role of the dynamic state of the polymer XG for the dimensional change. We find that at a hydration level below 50%, XG-Mnt possesses good dimensional stability, suggesting a constant performance of the material, while at a hydration level of 75%, the expansion ratio of the composite is found to be slightly smaller than the swelling of Mnt clay. At the four-layer hydrate formation with a hydration level of 100%, the swelling ratios of clay and the2composite reach the same value, suggesting a critical point of losing dimensional stability. We conclude that the strong adhesion between the polymer XG and the Mnt clay is the main driving force for the preservation of the stability at lower hydration conditions, while the dynamics of the XG polymer is related to the losing of dimensional stability for the composite at higher hydration levels. The ramification of these results in terms of moisture sensitivity of the material is briefly discussed.
|
|
| 12. |
- Wang, Yan, et al.
(författare)
-
Molecular Adhesion at Clay Nanocomposite Interfaces Depends on Counterion Hydration-Molecular Dynamics Simulation of Montmorillonite/Xyloglucan
- 2015
-
Ingår i: Biomacromolecules. - : American Chemical Society (ACS). - 1525-7797 .- 1526-4602. ; 16:1, s. 257-265
-
Tidskriftsartikel (refereegranskat)abstract
- Nacre-mimetic clay/polymer nanocomposites with clay platelet orientation parallel to the film surface show interesting gas barrier and mechanical properties. In moist conditions, interfacial adhesion is lowered and mechanical properties are reduced. Molecular dynamic simulations (MD) have been performed to investigate the effects of counterions on molecular adhesion at montmorillonite clay (Mnt)-xyloglucan (XG) interfaces. We focus on the role of monovalent cations K+, Na+, and Li+ and the divalent cation Ca2+ for mediating and stabilizing the Mnt/XG complex formation. The conformation of adsorbed XG is strongly influenced by the choice of counterion and so is the simulated work of adhesion. Free energy profiles that are used to estimate molecular adhesion show stronger interaction between XG and clay in the monovalent cation system than in divalent cation system, following a decreasing order of K-Mnt, Na-Mnt, Li-Mnt, and Ca-Mnt. The Mnt clay hydrates differently in the presence of different counterions, leading to a chemical potential of water that is highest in the case of K-Mnt, followed by Na-Mnt and Li-Mnt, and lowest in the case of Ca-Mnt. This means that water is most easily displaced from the interface in the case of K-Mnt, which contributes to the relatively high work of adhesion. In all systems, the penalty of replacing polymer with water at the interface gives a positive contribution to the work of adhesion of between 19 and 35%. Our work confirms the important role of counterions in mediating the adsorption of biopolymer XG to Mnt clays and predicts potassium or sodium as the best choice of counterions for a Mnt-based biocomposite design.
|
|
| 13. |
- Wang, Yan, 1987-, et al.
(författare)
-
Molecular mechanisms for the adhesion of chitin and chitosan to montmorillonite clay
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- Molecular dynamics simulations have been performed to investigate molecular adhesion of chitin and chitosan oligomers to montmorillonite (Mnt) clay at different degrees of acetylation (DA, 0%, 20%, 40%, 60%, 80% and 100%) and different degree of protonation (DPr, 0%, 50%, 100% mimicking pH > 6.5, pH = 6.5, pH < 4, respectively) under fully hydrated conditions. Although the Mnt surface is negatively charged and a variation in DA also implies going from a positively charged oligomer at DA = 0% to a neutral oligomer at DA = 100%, the simulations show unexpectedly variation of the total molecular adhesion as a function of DA. From our analysis we propose that this quantitatively similar adhesion arise from two different mechanisms. At low DA, the oligomer is rich in positively charged amino groups interacting strongly with the negatively charged surface by direct electrostatic interaction. On the other hand, at high DA, electrically neutral acetyl groups are strongly correlated with the Na+ counter ions, which are in all cases stuck at the surface and the counter ions seem to act as ‘glue’ between the acetyl groups and the Mnt. However, when protonation was decreased, adhesion was significantly lowered. The reason is conclued by differences in charge distributions of the respective functional groups. A further investigation on the intramolecular hydrogen bonds formed in CHT or CHS shows that the adsorbed conformation of the polymer is also highly affected by DA. This work provides fundamental insights into adhesion mechanisms and is, of potential importance for the development of polymer-clay based composite materials.
|
|
| 14. |
- Wang, Yan, et al.
(författare)
-
Molecular mechanisms for the adhesion of chitin and chitosan to montmorillonite clay
- 2015
-
Ingår i: RSC Advances. - : RSC Publishing. - 2046-2069. ; 5:67, s. 54580-54588
-
Tidskriftsartikel (refereegranskat)abstract
- Molecular dynamics simulations have been performed to investigate molecular adhesion of chitin and chitosan oligomers to montmorillonite (Mnt) clay at different degrees of acetylation (DA, 0%, 20%, 40%, 60%, 80% and 100%) and different degree of protonation (DPr, 0%, 50%, 100% mimicking pH > 6.5, pH = 6.5, pH < 4, respectively) under fully hydrated conditions. Although the Mnt surface is negatively charged and a variation in DA also implies going from a positively charged oligomer at DA = 0% to a neutral oligomer at DA = 100%, the simulations show unexpectedly small variation of the total molecular adhesion as a function of DA. From our analysis we propose that this quantitatively similar adhesion arises from two different mechanisms. At low DA, the oligomer is rich in positively charged amino groups interacting strongly with the negatively charged surface by direct electrostatic interaction. On the other hand, at high DA, electrically neutral acetyl groups are strongly correlated with the Na+ counter ions, which are in all cases stuck at the surface and the counter ions seem to act as 'glue' between the acetyl groups and the Mnt. However, when protonation was decreased, adhesion was affected and significantly lowered at neutral conditions (DPr = 0%). The reason is concluded to be differences in charge distributions of the respective functional groups. A further investigation on the intramolecular hydrogen bonds formed in CHT or CHS shows that the adsorbed conformation of the polymer is also highly affected by DA. This work provides fundamental insights into adhesion mechanisms and is of potential importance for the development of polymer-clay based composite materials.
|
|
| 15. |
- Wang, Yan, et al.
(författare)
-
Swelling and dimensional stability of xyloglucan/montmorillonite nanocomposites in moist conditions from molecular dynamics simulations
- 2017
-
Ingår i: Computational Materials Science. - : Elsevier. - 0927-0256. ; 128, s. 191-197
-
Tidskriftsartikel (refereegranskat)abstract
- Nacre-mimetic biocomposites made from the combination of montmorillonite clay and the hemicellulose xyloglucan give materials that retain much of their material properties even at high relative humidity. Here, a model composite system consisting of two clay platelets intercalated by xyloglucan oligomers was studied at different levels of hydration using molecular dynamics simulations, and compared to the pure clay. It was found that xyloglucan inhibits swelling of the clay at low water contents by promoting the formation of nano-sized voids that fill with water without affecting the material's dimensions. At higher water contents the XG itself swells, but at the same time maintaining contact with both platelets across the gallery, thereby acting as a physical cross-linker in a manner similar to the role of XG in the plant cell wall.
|
|
| 16. |
- Wohlert, Jakob, et al.
(författare)
-
A simple model for cellulose solubility in supercritical water
- 2015
-
Ingår i: Nordic Pulp & Paper Research Journal. - : Walter de Gruyter GmbH. - 0283-2631 .- 2000-0669. ; 30:1, s. 14-19
-
Tidskriftsartikel (refereegranskat)abstract
- A simple model for the hydration of a polar molecule is developed in order to provide a description of cellulose solubility in water under ambient and supercritical conditions. The change in free energy upon hydration is regarded as the sum of the energy cost of forming a cavity and a polar contribution. The model is able to predict the existence of an optimal density for dissolution of polar solutes in supercritical water. Those results are in line with earlier experiments and simulations showing that water at high temperature and pressure dissolves cellulose, and that an optimal density for dissolution exists. The present study shows that the density dependence comes from the fact that both the cavity formation energy and the polar energy are highly density dependent but with opposing behaviour. The cavity formation energy increases with density, whereas the polar energy decreases. Based on the present model, it is possible to rationalize a few basic strategies regarding cellulose dissolution in aqueous media. To increase solubility, one can either increase the polar/electrostatic contribution, or more importantly, one can decrease the cost of cavity formation, e.g. by introducing co-solvents, changing temperature and/or pressure.
|
|
| 17. |
- Wohlert, Jakob, et al.
(författare)
-
Deformation of cellulose nanocrystals : entropy, internal energy and temperature dependence
- 2012
-
Ingår i: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 19:6, s. 1821-1836
-
Tidskriftsartikel (refereegranskat)abstract
- An in-depth analysis was performed of the molecular deformation mechanisms in cellulose during axial stretching. For the first time, it was demonstrated that entropy affects the stiffness of cellulose nanocrystals significantly. This was achieved through Molecular Dynamics simulations of model nanocrystals subject to constant stress in the axial direction, for nanocrystals of varying lateral dimensions and at different temperatures. The simulations were analyzed in terms of Young's modulus E, which is a measure of the elastic response to applied stress. A weak but significant temperature dependence was shown, with partial derivative E/partial derivative T = -0.05 Gpa K-1 at room temperature, in agreement with experimental numbers. In order to analyze the respective contributions from internal energy and entropy, a decomposition of the total response of the free energy with respect to strain was made. It was shown that the decrease in E with increasing T is due to entropy, and that the magnitude of the decrease is 6-9 % at room temperature compared to the value at 0 K. This was also shown independently by a direct calculation of the vibrational entropy of the cellulose crystal. Finally, it was found that internal hydrogen bonds are contributing to the stiffness by 20 %, mainly by stabilizing the cellulose internal structure.
|
|
| 18. |
- Wohlert, Malin, et al.
(författare)
-
Cellulose and the role of hydrogen bonds : not in charge of everything
- 2022
-
Ingår i: Cellulose. - : Springer Nature. - 0969-0239 .- 1572-882X. ; 29:1, s. 1-23
-
Tidskriftsartikel (refereegranskat)abstract
- In the cellulose scientific community, hydrogen bonding is often used as the explanation for a large variety of phenomena and properties related to cellulose and cellulose based materials. Yet, hydrogen bonding is just one of several molecular interactions and furthermore is both relatively weak and sensitive to the environment. In this review we present a comprehensive examination of the scientific literature in the area, with focus on theory and molecular simulation, and conclude that the relative importance of hydrogen bonding has been, and still is, frequently exaggerated.
|
|
| 19. |
- Bannow, J., et al.
(författare)
-
Solid nanofoams based on cellulose nanofibers and indomethacin—the effect of processing parameters and drug content on material structure
- 2017
-
Ingår i: International Journal of Pharmaceutics. - : Elsevier. - 0378-5173 .- 1873-3476. ; 526:1-2, s. 291-299
-
Tidskriftsartikel (refereegranskat)abstract
- The unique colloidal properties of cellulose nanofibers (CNF), makes CNF a very interesting new excipient in pharmaceutical formulations, as CNF in combination with some poorly-soluble drugs can create nanofoams with closed cells. Previous nanofoams, created with the model drug indomethacin, demonstrated a prolonged release compared to films, owing to the tortuous diffusion path that the drug needs to take around the intact air-bubbles. However, the nanofoam was only obtained at a relatively low drug content of 21 wt% using fixed processing parameters. Herein, the effect of indomethacin content and processing parameters on the foaming properties was analysed. Results demonstrate that a certain amount of dissolved drug is needed to stabilize air-bubbles. At the same time, larger fractions of dissolved drug promote coarsening/collapse of the wet foam. The pendant drop/bubble profile tensiometry was used to verify the wet-foam stability at different pHs. The pH influenced the amount of solubilized drug and the processing-window was very narrow at high drug loadings. The results were compared to real foaming-experiments and solid state analysis of the final cellular solids. The parameters were assembled into a processing chart, highlighting the importance of the right combination of processing parameters (pH and time-point of pH adjustment) in order to successfully prepare cellular solid materials with up to 46 wt% drug loading.
|
|
| 20. |
- Beckham, Gregg T., et al.
(författare)
-
The O-Glycosylated Linker from the Trichoderma reesei Family 7 Cellulase Is a Flexible, Disordered Protein
- 2010
-
Ingår i: Biophysical Journal. - : Elsevier BV. - 0006-3495 .- 1542-0086. ; 99:11, s. 3773-3781
-
Tidskriftsartikel (refereegranskat)abstract
- Fungi and bacteria secrete glycoprotein cocktails to deconstruct cellulose Cellulose degrading enzymes (cellulases) are often modular with catalytic domains for cellulose hydrolysis and carbohydrate binding modules connected by linkers rich in serine and threonine with O-glycosylation Few studies have probed the role that the linker and O-glycans play in catalysis Since different expression and growth conditions produce different glycosylation patterns that affect enzyme activity the structure function relationships that glycosylation imparts to linkers are relevant for understanding cellulase mechanisms Here the linker of the Trichoderma reesei Family 7 cellobiohydrolase (Cel7A) is examined by simulation Our results suggest that the Cel7A linker is an intrinsically disordered protein with and without glycosylation Contrary to the predominant view the O-glycosylation does not change the stiffness of the linker as measured by the relative fluctuations in the end to end distance rather it provides a 16 A extension thus expanding the operating range of Cel7A We explain observations from previous biochemical experiments in the light of results obtained here and compare the Cel7A linker with linkers from other cellulases with sequence based tools to predict disorder This preliminary screen indicates that linkers from Family 7 enzymes from other genera and other cellulases within T reesei may not be as disordered warranting further study
|
|
| 21. |
- Bergenstråhle, Malin, 1977-, et al.
(författare)
-
Dynamics of Cellulose-Water Interfaces : NMR Spin-Lattice Relaxation Times Calculated from Atomistic Computer Simulations
- 2008
-
Ingår i: Journal of Physical Chemistry B. - Washington : ACS Publications. - 1520-6106 .- 1520-5207. ; 112:9, s. 2590-2595
-
Tidskriftsartikel (refereegranskat)abstract
- Solid-state nuclear magnetic resonance (CP/MAS 13C NMR) spectroscopy has often been used to study cellulose structure, but some features of the cellulose NMR spectrum are not yet fully understood. One such feature is a doublet around 84 ppm, a signal that has been proposed to originate from C4 atoms at cellulose fibril surfaces. The two peaks yield different T1, differing by approximately a factor of 2 at 75 MHz. In this study, we calculate T1 from C4-H4 vector dynamics obtained from molecular dynamics computer simulations of cellulose Iβ-water interfacial systems. Calculated and experimentally obtained T1 values for C4 atoms in surface chains fell within the same order of magnitude, 3-20 s. This means that the applied force field reproduces relevant surface dynamics for the cellulose-water interface sufficiently well. Furthermore, a difference in T1 of about a factor of 2 in the range of Larmor frequencies 25-150 MHz was found for C4 atoms in chains located on top of two different crystallographic planes, namely, (110) and (10). A previously proposed explanation that the C4 peak doublet could derive from surfaces parallel to different crystallographic planes is herewith strengthened by computationally obtained evidence. Another suggested basis for this difference is that the doublet originates from C4 atoms located in surface anhydro-glucose units with hydroxymethyl groups pointing either inward or outward. This was also tested within this study but was found to yield no difference in calculated T1.
|
|
| 22. |
- Bergenstråhle, Malin, 1977-, et al.
(författare)
-
Simulation studies of the insolubility of cellulose
- 2010
-
Ingår i: Carbohydrate Research. - : Elsevier BV. - 0008-6215 .- 1873-426X. ; 345:14, s. 2060-2066
-
Tidskriftsartikel (refereegranskat)abstract
- Molecular dynamics simulations have been used to calculate the potentials of mean force for separating short cellooligomers in aqueous solution as a means of estimating the contributions of hydrophobic stacking and hydrogen bonding to the insolubility of crystalline cellulose. A series of four potential of mean force (pmf) calculations for glucose, cellobiose, cellotriose, and cellotetraose in aqueous solution were performed for situations in which the molecules were initially placed with their hydrophobic faces stacked against one another, and another for the cases where the molecules were initially placed adjacent to one another in a co-planar, hydrogen-bonded arrangement, as they would be in cellulose ID. From these calculations, it was found that hydrophobic association does indeed favor a crystal-like structure over solution, as might be expected. Somewhat more surprisingly, hydrogen bonding also favored the crystal packing, possibly in part because of the high entropic cost for hydrating glucose hydroxyl groups, which significantly restricts the configurational freedom of the hydrogen-bonded waters. The crystal was also favored by the observation that there was no increase in chain configurational entropy upon dissolution, because the free chain adopts only one conformation, as previously observed, but against intuitive expectations, apparently due to the persistence of the intramolecular O3-O5 hydrogen bond.
|
|
| 23. |
- Berglund, Jennie, et al.
(författare)
-
Acetylation and Sugar Composition Influence the (In)Solubility of Plant beta-Mannans and Their Interaction with Cellulose Surfaces
- 2020
-
Ingår i: ACS Sustainable Chemistry and Engineering. - : AMER CHEMICAL SOC. - 2168-0485. ; 8:27, s. 10027-10040
-
Tidskriftsartikel (refereegranskat)abstract
- Plant beta-mannans are complex heteropolysaccharides that represent an abundant resource from lignocellulosic biomass. The influence of the molecular motifs of plant mannans on the backbone flexibility, solubility, and the interaction with cellulose was investigated by computational and experimental approaches. The regioselectivity of the acetyl substitutions at C2 and C3 distinctively influenced backbone flexibility in aqueous media, as revealed by molecular dynamic simulations. The molecular weight and degree of acetylation were tailored for two model seed mannans (galactomannan and glucomannan) and compared to spruce acetylated galactoglucomannan. The thermal stability was enhanced with increasing acetyl substitutions, independently of the type of mannan. Dynamic light scattering and atomic force microscopy revealed that the occurrence of galactosylation and a low degree of acetylation (similar to that of native acetylated galactoglucomannans) enhanced solubility/dispersibility of mannans, whereas the solubility/dispersibility decreased for higher degrees of acetylation. Mannan solubility influenced their interactions with cellulose at water-cellulose interfaces in terms of adsorbed mass and viscoelastic properties of the adsorbed mannan layers. Our results reveal that modulating the molecular motifs of plant beta-mannans influences their macromolecular conformation and physicochemical properties, with fundamental implications for their role in the plant cell wall and the design of wood-based materials.
|
|
| 24. |
|
|
| 25. |
|
|
| 26. |
|
|
| 27. |
|
|
| 28. |
- Berglund, Jennie, et al.
(författare)
-
The structure of galactoglucomannan impacts the degradation under alkaline conditions
- 2018
-
Ingår i: Cellulose. - : Springer. - 0969-0239 .- 1572-882X.
-
Tidskriftsartikel (refereegranskat)abstract
- Galactoglucomannan (GGM) from sprucewas studied with respect to the degradation behavior inalkaline solution. Three reference systems includinggalactomannan from locust bean gum, glucomannanfrom konjac and the linear water-soluble carboxymethylcellulose were studied with focus onmolecular weight, sugar composition, degradationproducts, as well as formed oligomers, to identifyrelative structural changes in GGM. Initially allmannan polysaccharides showed a fast decrease inthe molecular weight, which became stable in the laterstage. The degradation of the mannan polysaccharidescould be described by a function corresponding to thesum of two first order reactions; one slow that wasascribed to peeling, and one fast that was connectedwith hydrolysis. The galactose side group wasstable under conditions used in this study (150 min,90 C, 0.5 M NaOH). This could suggest that, apartfrom the covalent connection to C6 in mannose, thegalactose substitutions also interact non-covalentlywith the backbone to stabilize the structure againstdegradation. Additionally, the combination of differentbackbone sugars seems to affect the stability of thepolysaccharides. For carboxymethyl cellulose thedegradation was linear over time which furthersuggests that the structure and sugar composition playan important role for the alkaline degradation. Moleculardynamics simulations gave details about theconformational behavior of GGM oligomers in watersolution, as well as interaction between the oligomersand hydroxide ions.
|
|
| 29. |
- Berglund, Jennie
(författare)
-
Wood Hemicelluloses - Fundamental Insights on Biological and Technical Properties
- 2018
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Hemicelluloses are a group of heterogeneous polysaccharides representing around 30 % of wood where the dominating types are xylans, glucomannans and xyloglucans. Hemicelluloses complex molecular structure makes it difficult to understand the relationship between structure and properties entirely, and their biological role is not yet fully verified. Additionally, hemicelluloses are sensitive to chemical processing and are not utilized to their full potentials for production of value-added products such as materials, additives to food and pharmaceutical products, etc. Increased knowledge regarding their functions is important for the development of both processes and products. The aim with this work has therefore been to increase the fundamental understanding about how the structure and properties of wood hemicelluloses are correlated, and properties such as flexibility, interaction with cellulose, solubility, resistance to chemical-, thermal-, and enzymatic degradation have been explored.Molecular dynamics (MD) simulations were used to, in detail, study the structures found in wood hemicelluloses. The flexibility was evaluated by comparing the impact of backbone sugars on the conformational space and also the impact of side groups was considered. Based on the conformational space of backbone glycosidic linkages the flexibility order of hemicelluloses in an aqueous environment was determined to be: xylan > glucomannan > xyloglucan. Additionally, the impact of xylan structure on cellulose interaction was evaluated by MD methods.Hemicelluloses were extracted from birch and spruce, and were used to fabricate different composite hydrogels with bacterial cellulose. These materials were studied with regards to mechanical properties, and it was shown that galactoglucomannans mainly contributed to an increased modulus in compression, whereas the most significant effect from xylan was increased strain under uniaxial tensile testing. Besides, other polysaccharides of similar structure as galactoglucomannans were modified and used as pure, well defined, models. Acetyl groups are naturally occurring decorations of wood hemicelluloses and can also be chemically introduced. Here, mannans with different degrees of acetylation were prepared and the influence of structure on solubility in water and the organic solvent DMSO were evaluated. Furthermore, the structure and water solubility influenced the interaction with cellulose. Acetylation also showed to increase the thermal and biological stability of mannans.With chemical pulping processes in mind, the degradability of spruce galactoglucomannans in alkaline solution were studied with regards to the structure, and the content of more or less stable structural regions were proposed.
|
|
| 30. |
- Cederholm, Linnea, et al.
(författare)
-
“Like Recycles Like” : Selective Ring-Closing Depolymerization of Poly(L-Lactic Acid) to L-Lactide
- 2022
-
Ingår i: Angewandte Chemie International Edition. - : Wiley. - 1433-7851 .- 1521-3773. ; 61:33
-
Tidskriftsartikel (refereegranskat)abstract
- Chemical recycling of poly(L-lactic acid) to the cyclic monomer L-lactide is hampered by low selectivity and by epimerization and elimination reactions, impeding its use on a large scale. The high number of side reactions originates from the high ceiling temperature (Tc) of L-lactide, which necessitates high temperatures or multistep reactions to achieve recycling to L-lactide. To circumvent this issue, we utilized the impact of solvent interactions on the monomer–polymer equilibrium to decrease the Tc of L-lactide. Analyzing the observed Tc in different solvents in relation to their Hildebrand solubility parameter revealed a “like recycles like” relationship. The decreased Tc, obtained by selecting solvents that interact strongly with the monomer (dimethyl formamide or the green solvent γ-valerolactone), allowed chemical recycling of high-molecular-weight poly(L-lactic acid) directly to L-lactide, within 1–4 h at 140 °C, with >95 % conversion and 98–99 % selectivity. Recycled L-lactide was isolated and repolymerized with high control over molecular weight and dispersity, closing the polymer loop.
|
|
| 31. |
- Chen, Mo, et al.
(författare)
-
Molecular Dynamics Simulations of the Ionic Liquid 1-n-Butyl-3-Methylimidazolium Chloride and Its Binary Mixtures with Ethanol
- 2014
-
Ingår i: Journal of Chemical Theory and Computation. - : American Chemical Society (ACS). - 1549-9618 .- 1549-9626. ; 10:10, s. 4465-4479
-
Tidskriftsartikel (refereegranskat)abstract
- Room temperature ionic liquids (ILs) of the imidazolium family have attracted much attention during the past decade for their capability to dissolve biomass. Besides experimental work, numerous compuational studies have been concerned with the physical properties of both neat ILs and their interactions with different solutes, in particular, carbohydrates. Many classical force fields designed specifically for ILs have been found to yield viscosities that are too high for the liquid state, which has been attributed to the fact that the effective charge densities are too high due to the lack of electronic polarizability. One solution to this problem has been uniform scaling of the partial charges by a scale factor in the range 0.6-0.9, depending on model. This procedure has been shown to improve the viscosity of the models, and also to positively affect other properties, such as diffusion constants and ionic conductivity. However, less attention has been paid to how this affects the overall thermodynamics of the system, and the problems it might create when the IL models are combined with other force fields (e.g., for solutes). In the present work, we employ three widely used IL force fields to simulate 1-n-buty1-3-methyl-imidazolium chloride in both the crystal and the liquid state, as well as its binary mixture with ethanol. Two approaches are used: one in which the ionic charge is retained at its full integer value and one in which the partial charges are uniformly reduced to 85%. We investigate and calculate crystal and liquid structures, molar heat capacities, heats of fusion, self-diffusion constants, ionic conductivity, and viscosity for the neat IL, and ethanol activity as a function of ethanol concentration for the binary mixture. We show that properties of the crystal are less affected by charge scaling compared to the liquid. In the liquid state, transport properties of the neat IL are generally improved by scaling, whereas values for the heat of fusion are unaffected, and results for the heat capacity are ambiguous. Neither full nor reduced charges could reproduce experimental ethanol activities for the whole range of compositions.
|
|
| 32. |
|
|
| 33. |
- Chen, Pan, et al.
(författare)
-
Hydration-Dependent Dynamical Modes in Xyloglucan from Molecular Dynamics Simulation of C-13 NMR Relaxation Times and Their Distributions
- 2018
-
Ingår i: Biomacromolecules. - : American Chemical Society (ACS). - 1525-7797 .- 1526-4602. ; 19:7, s. 2567-2579
-
Tidskriftsartikel (refereegranskat)abstract
- Macromolecular dynamics in biological systems, which play a crucial role for biomolecular function and activity at ambient temperature, depend strongly on moisture content. Yet, a generally accepted quantitative model of hydration-dependent phenomena based on local relaxation and diffusive dynamics of both polymer and its adsorbed water is still missing. In this work, atomistic-scale spatial distributions of motional modes are calculated using molecular dynamics simulations of hydrated xyloglucan (XG). These are shown to reproduce experimental hydration-dependent C-13 NMR longitudinal relaxation times (T-1) at room temperature, and relevant features of their broad distributions, which are indicative of locally heterogeneous polymer reorientational dynamics. At low hydration, the self-diffusion behavior of water shows that water molecules are confined to particular locations in the randomly aggregated XG network while the average polymer segmental mobility remains low. Upon increasing water content, the hydration network becomes mobile and fully accessible for individual water molecules, and the motion of hydrated XG segments becomes faster. Yet, the polymer network retains a heterogeneous gel-like structure even at the highest level of hydration. We show that the observed distribution of relaxations times arises from the spatial heterogeneity of chain mobility that in turn is a result of heterogeneous distribution of water-chain and chain chain interactions. Our findings contribute to the picture of hydration-dependent dynamics in other macromolecules such as proteins, DNA, and synthetic polymers, and hold important implications for the mechanical properties of polysaccharide matrixes in plants and plant-based materials.
|
|
| 34. |
|
|
| 35. |
- Chen, Pan, et al.
(författare)
-
Quantifying Localized Macromolecular Dynamics within Hydrated Cellulose Fibril Aggregates
- 2019
-
Ingår i: Macromolecules. - : AMER CHEMICAL SOC. - 0024-9297 .- 1520-5835. ; 52:19, s. 7278-7288
-
Tidskriftsartikel (refereegranskat)abstract
- Molecular dynamics (MD) simulations of C-13 NMR longitudinal relaxation (T-1) distributions were recently established as a powerful tool for characterizing moisture adsorption in natural amorphous polymers. Here, such computational-experimental synergy is demonstrated in a system with intrinsically high structural heterogeneity, namely crystalline cellulose nanofibrils (CNFs) in highly hydrated aggregated state. In such a system, structure-function properties on the nanoscale remain largely uncovered by experimental means alone. In this work, broadly polydispersed experimental C-13 NMR T-1 distributions could be successfully reproduced in simulations and, for the first time, were decomposed into contributions from distinct molecular sources within the aggregated CNFs, namely, (i) the core and (ii) the less-accessible and accessible surface regions of the CNFs. Furthermore, within the surface groups structurally different sites such as (iii) residues with different hydroxymethyl orientations and (iv) center and origin chains could be discerned based on their distinct molecular dynamics. The MD simulations unravel a direct correlation between dynamical and structural heterogeneity at an atomistic-level resolution that cannot be accessed by NMR experiments. The proposed approach holds the potential to enable quantitative interpretation of NMR data from a range of multicomponent high-performance nanocomposites with significantly heterogeneous macromolecular structure.
|
|
| 36. |
- Chen, Pan, et al.
(författare)
-
Quantifying the Contribution of the Dispersion Interaction and Hydrogen Bonding to the Anisotropic Elastic Properties of Chitin and Chitosan
- 2022
-
Ingår i: Biomacromolecules. - : American Chemical Society (ACS). - 1525-7797 .- 1526-4602. ; 23:4, s. 1633-1642
-
Tidskriftsartikel (refereegranskat)abstract
- The elastic tensors of chitin and chitosan allomorphs were calculated using density functional theory (DFT) with and without the dispersion correction and compared with experimental values. The longitudinal Young's moduli were 114.9 or 126.9 GPa for alpha-chitin depending on the hydrogen bond pattern: 129.0 GPa for beta-chitin and 191.5 GPa for chitosan. Furthermore, the moduli were found to vary between 17.0 and 52.8 GPa in the transverse directions and between 2.2 and 15.2 GPa in shear. Switching off the dispersion correction led to a decrease in modulus by up to 63%, depending on the direction. The transverse Young's moduli of a-chitin strongly depended on the hydroxylmethyl group conformation coupled with the dispersion correction, suggesting a synergy between hydrogen bonding and dispersion interactions. The calculated longitudinal Young's moduli were, in general, higher than experimental values obtained in static conditions, and the Poisson's ratios were lower than experimental values obtained in static conditions.
|
|
| 37. |
- Chen, P., et al.
(författare)
-
Quantifying the influence of dispersion interactions on the elastic properties of crystalline cellulose
- 2021
-
Ingår i: Cellulose. - : Springer Nature. - 0969-0239 .- 1572-882X. ; 28:17, s. 10777-10786
-
Tidskriftsartikel (refereegranskat)abstract
- Dispersion and electrostatic interactions both contribute significantly to the tight assembly of macromolecular chains within crystalline polysaccharides. Using dispersion-corrected density functional theory (DFT) calculation, we estimated the elastic tensor of the four crystalline cellulose allomorphs whose crystal structures that are hitherto available, namely, cellulose Iα, Iβ, II, IIII. Comparison between calculations with and without dispersion correction allows quantification of the exact contribution of dispersion to stiffness at molecular level.
|
|
| 38. |
- Chen, Pan, et al.
(författare)
-
Surface modification effects on nanocellulose - molecular dynamics simulations using umbrella sampling and computational alchemy
- 2020
-
Ingår i: Journal of Materials Chemistry A. - : Royal Society of Chemistry (RSC). - 2050-7496 .- 2050-7488. ; 8:44, s. 23617-23627
-
Tidskriftsartikel (refereegranskat)abstract
- Topochemical modification of nanocellulose particles, in particular acetylation, is commonly used to reduce hygroscopicity and improve their dispersibility in non-polar polymers. Despite enormous experimental efforts on cellulose surface modification, there is currently no comprehensive model which considers both (a) the specific interactions between nanocellulose particles and the surrounding liquid or polymer matrix, and (b) the interactions between the particles themselves. The second mechanism is therefore frequently ignored. The present approach is based on atomistic molecular dynamics (MD) simulations, where computational alchemy is used to calculate the changes in interactions between nanocellulose and the surrounding medium (liquid or polymer) upon modification. This is combined with another method, based on potential of mean force, to calculate interactions between particles. Results show that both contributions are of equal importance for nanoparticle surface acetylation effects. The proposed method is not restricted to either cellulose or acetylation, and has the prospect to find application in a broad context of nanomaterials design.
|
|
| 39. |
- Chen, Pan, et al.
(författare)
-
Translational Entropy and Dispersion Energy Jointly Drive the Adsorption of Urea to Cellulose
- 2017
-
Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 121:10, s. 2244-2251
-
Tidskriftsartikel (refereegranskat)abstract
- The adsorption of urea on cellulose at room temperature has been studied using adsorption isotherm experiments and molecular dynamics (MD) simulations. The immersion of cotton cellulose into bulk urea solutions with concentrations between 0.01 and 0.30 g/mL led to a decrease in urea concentration in all solutions, allowing the adsorption of urea on the cellulose surface to be measured quantitatively. MD simulations suggest that urea molecules form sorption layers on both hydrophobic and hydrophilic surfaces. Although electrostatic interactions accounted for the majority of the calculated interaction energy between urea and cellulose, dispersion interactions were revealed to be the key driving force for the accumulation of urea around cellulose. The preferred orientation of urea and water molecules in the first solvation shell varied depending on the nature of the cellulose surface, but urea molecules were systematically oriented parallel to the hydrophobic plane of cellulose. The translational entropies of urea and water molecules, calculated from the velocity spectrum of the trajectory, are lower near the cellulose surface than in bulk. As urea molecules adsorb on cellulose and expel surface water into the bulk, the increase in the translational entropy of the water compensated for the decrease in the entropy of urea, resulting in a total entropy gain of the solvent system. Therefore, the cellulose urea dispersion energy and the translational entropy gain of water are the main factors that drive the adsorption of urea on cellulose.
|
|
| 40. |
- Chen, Pan, et al.
(författare)
-
Water as an Intrinsic Structural Element in Cellulose Fibril Aggregates
- 2022
-
Ingår i: The Journal of Physical Chemistry Letters. - : American Chemical Society (ACS). - 1948-7185. ; 13:24, s. 5424-5430
-
Tidskriftsartikel (refereegranskat)abstract
- While strong water association with cellulose in plant cell walls and man-made materials is well-established, its molecular scale aspects are not fully understood. The thermodynamic consequences of having water molecules located at the microfibril-microfibril interfaces in cellulose fibril aggregates are therefore analyzed by molecular dynamics simulations. We find that a thin layer of water molecules at those interfaces can be in a state of thermal equilibrium with water surrounding the fibril aggregates because such an arrangement lowers the free energy of the total system. The main reason is enthalpic: water at the microfibril- microfibril interfaces enables the cellulose surface hydroxyls to experience a more favorable electrostatic environment. This enthalpic gain overcomes the entropic penalty from strong immobilization of water molecules. Hence, those particular water molecules stabilize the cellulose fibril aggregates, akin to the role of water in some proteins. Structural and functional hypotheses related to this finding are presented.
|
|
| 41. |
- Chen, Yu, et al.
(författare)
-
The thermodynamics of enhanced dope stability of cellulose solution in NaOH solution by urea
- 2023
-
Ingår i: Carbohydrate Polymers. - : Elsevier BV. - 0144-8617 .- 1879-1344. ; 311, s. 120744-
-
Tidskriftsartikel (refereegranskat)abstract
- The addition of urea in pre-cooled alkali aqueous solution is known to improve the dope stability of cellulose solution. However, its thermodynamic mechanism at a molecular level is not fully understood yet. By using molecular dynamics simulation of an aqueous NaOH/urea/cellulose system using an empirical force field, we found that urea was concentrated in the first solvation shell of the cellulose chain stabilized mainly by dispersion interaction. When adding a glucan chain into the solution, the total solvent entropy reduction is smaller if urea is present. Each urea molecule expelled an average of 2.3 water molecules away from the cellulose surface, releasing water entropy that over-compensates the entropy loss of urea and thus maximizing the total entropy. Scaling the Lennard-Jones parameter and atomistic partial charge of urea revealed that direct urea/cellulose interaction was also driven by dispersion energy. The mixing of urea solution and cellulose solution in the presence or absence of NaOH are both exothermic even after correcting for the contribution from dilution.
|
|
| 42. |
|
|
| 43. |
|
|
| 44. |
- Djahedi, Cyrus, et al.
(författare)
-
Molecular deformation mechanisms in cellulose allomorphs and the role of hydrogen bonds
- 2015
-
Ingår i: Carbohydrate Polymers. - : Elsevier BV. - 0144-8617 .- 1879-1344. ; 130, s. 175-182
-
Tidskriftsartikel (refereegranskat)abstract
- Differences in tensile properties between cellulose crystal allomorphs cannot be rationalized by simply counting hydrogen bonds. From molecular dynamics computer simulations the cooperative nature of energy contributions to axial cellulose crystal modulus becomes apparent. Using a decomposition of inter and intrarnolecular forces as a function of tensile strain, the three allomorphs show dramatic differences in terms of how the contributions to elastic energy are distributed between covalent bonds, angles, dihedrals, electrostatic forces, dispersion and steric forces.
|
|
| 45. |
|
|
| 46. |
|
|
| 47. |
- Heinonen, Emilia
(författare)
-
Structural basis for the recalcitrance and molecular packing of hemicelluloses
- 2024
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The properties of wood cell walls are determined by the composition and the molecular structures of the cell wall polymers (cellulose, hemicelluloses, pectins and lignin) and the interactions between them. In particular, hemicelluloses are an underutilized source of biopolymers that constitute around 30 % of wood. In hardwoods, the main hemicellulose is acetylated glucuronoxylan and the variation in the patterns of acetylation and glucuronidation between tree species is not fully understood. This study aimed to increase understanding of the xylan structure in particular and more generally, the interaction between the cell wall matrix polysaccharides and cellulose, which are important for the preparation of more accurate cell wall models as well as for the development of hemicellulose-based products.The effect of matrix polysaccharides’ backbone on the assembly with cellulose fibril in water was studied using atomistic simulations. First, several setups were compared with xylo-oligosaccharides (XOs) as model hemicellulose motifs. Anti-parallel alignment together with the conformational change to a 2-fold helix as well as the formation of a distinct hydrogen bonding network were characteristics of spontaneous adsorption of XOs to cellulose. The established simulation setup was further used to expand the scope to other hemicelluloses and pectins. The parameters investigated included mobility on the cellulose surface, alignment, conformation in water and on cellulose and the interaction strength. Alignment along the fibril and conformational change upon adsorption defined the adaptability of hemicelluloses and distinguished them from the other oligosaccharides. In this sense, the mixed-linked β-glucan with a β-(1→3)-linkage was shown to be comparable to β-(1→4)-linked hemicelluloses.To investigate the structure of hemicelluloses, sequential subcritical water (SWE) and alkaline extraction methods were applied to aspen and Eucalyptus wood. Buffered SWE released acetylated glucuronoxylan (acGX) with distinct patterns of acetylation and glucuronidation and enabled the extraction of galactosylated acGX from Eucalyptus without the need for prior delignification. In particular, a small amount of consecutive glucuronidation in both Eucalyptus and aspen xylan showed a more complex substitution pattern than previously thought, with similarity to arabinoglucuronoxylan from softwoods. The structure of GX affected its biological degradability by xylanases. Regions of low degree of glucuronidation were shown to aggregate in water, which hindered their accessibility to GH30 glucuronoxylanase. This was particularly relevant to alkali-extracted GX devoid of acetylation. In Eucalyptus, galactosylation impeded the digestion by GH30. GH10 could release galactosylated XOs but the cleavage site preference was shown to be altered. The large difference in the amount of Gal-MeGlcA from incubations with GH30 and GH10 suggests that the Gal-MeGlcA motifs may be clustered in xylan. Furthermore, the terminal galactose was released by a β- but not an α-galactose confirming the presence of galactose as a β-anomer.The degree and the pattern of glucuronidation were further shown to affect the aggregation of glucuronoxylan at acidic pH 2.0. Separation of beech glucuronoxylan in two fractions based on their solubility at pH 2.0 was achieved by freezing and subsequent thawing. The precipitated GX had a higher DP and a lower degree of glucuronidation with an enrichment in the motifs with an even and lengthily-spaced glucuronidation pattern (X>6U) compared to the soluble GX. The fractionation of these GX populations in acidic conditions revealed the occurrence of interpolymer variability of glucuronidation, possibly corresponding to different activities of the two known glucuronyltransferases GUX1 and GUX2.Overall, these results provide insights into the fundamentals of the molecular structure of xylans and the interaction of matrix polysaccharides with cellulose fibrils, which can increase our understanding of their biological functions and be utilized in the development of more accurate cell wall models and hemicellulose-based products.
|
|
| 48. |
|
|
| 49. |
- Heinonen, Emilia, et al.
(författare)
-
Xylan adsorption on cellulose : Preferred alignment and local surface immobilizing effect
- 2022
-
Ingår i: Carbohydrate Polymers. - : Elsevier BV. - 0144-8617 .- 1879-1344. ; 285, s. 119221-119221
-
Tidskriftsartikel (refereegranskat)abstract
- Interaction between xylan and cellulose microfibrils is required to maintain the integrity of secondary cell walls. However, the mechanisms governing their assembly and the effects on cellulose surface polymers are not fully clear. Here, molecular dynamics simulations are used to study xylan adsorption onto hydrated cellulose fibrils. Based on multiple spontaneous adsorption simulations it is shown that an antiparallel orientation is thermodynamically preferred over a parallel one, and that adsorption is accompanied by the formation of regular but orientation-dependent hydrogen bond patterns. Furthermore, xylan adsorption restricts the local dynamics of the adjacent glucose residues in the surface layer to a level of the crystalline core, which is manifested as a three-fold increase in their 13C NMR T1 relaxation time. These results suggest that xylan forms a rigid and ordered layer around the cellulose fibril that functions as a transition phase to more flexible and disordered polysaccharide and lignin domains.
|
|
| 50. |
|
|
