| 1. |
- Angles d'Ortoli, Thibault, et al.
(författare)
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Temperature Dependence of Hydroxymethyl Group Rotamer Populations in Cellooligomers
- 2015
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 119:30, s. 9559-9570
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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.
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| 2. |
- Bergenstråhle-Wohlert, Malin, et al.
(författare)
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Concentration enrichment of urea at cellulose surfaces : results from molecular dynamics simulations and NMR spectroscopy
- 2012
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Ingår i: Cellulose. - : Springer Science and Business Media LLC. - 0969-0239 .- 1572-882X. ; 19:1, s. 1-12
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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.
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| 3. |
- Berglund, Jennie, et al.
(författare)
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A molecular dynamics study of the effect of glycosidic linkage type in the hemicellulose backbone on the molecular chain flexibility
- 2016
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Ingår i: The Plant Journal. - : John Wiley & Sons Ltd.. - 0960-7412 .- 1365-313X. ; 88:1, s. 56-70
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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.
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| 4. |
- Tolonen, Lasse K., et al.
(författare)
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Solubility of Cellulose in Supercritical Water Studied by Molecular Dynamics Simulations
- 2015
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 119:13, s. 4739-4748
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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.
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| 5. |
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| 6. |
- Wang, Yan, 1987-, et al.
(författare)
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Hydration and dimensional stability of the intercalated galleries in xyloglucan/montmorillonite nanocomposites studied by molecular dynamics simulations
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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.
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| 7. |
- Wang, Yan, et al.
(författare)
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Molecular Adhesion at Clay Nanocomposite Interfaces Depends on Counterion Hydration-Molecular Dynamics Simulation of Montmorillonite/Xyloglucan
- 2015
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Ingår i: Biomacromolecules. - : American Chemical Society (ACS). - 1525-7797 .- 1526-4602. ; 16:1, s. 257-265
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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.
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| 8. |
- Wang, Yan, 1987-, et al.
(författare)
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Molecular mechanisms for the adhesion of chitin and chitosan to montmorillonite clay
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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.
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| 9. |
- Wang, Yan, et al.
(författare)
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Molecular mechanisms for the adhesion of chitin and chitosan to montmorillonite clay
- 2015
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Ingår i: RSC Advances. - : RSC Publishing. - 2046-2069. ; 5:67, s. 54580-54588
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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.
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| 10. |
- Wang, Yan, et al.
(författare)
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Swelling and dimensional stability of xyloglucan/montmorillonite nanocomposites in moist conditions from molecular dynamics simulations
- 2017
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Ingår i: Computational Materials Science. - : Elsevier. - 0927-0256. ; 128, s. 191-197
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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.
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