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  • 1. Bannow, J.
    et al.
    Benjamins, J. -W
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Löbmann, K.
    Svagan, Anna J.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Solid nanofoams based on cellulose nanofibers and indomethacin—the effect of processing parameters and drug content on material structure2017In: International Journal of Pharmaceutics, ISSN 0378-5173, E-ISSN 1873-3476, Vol. 526, no 1-2, 291-299 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 2.
    Bergenstrahle-Wohlert, Malin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    d'Ortoli, Thibault Angles
    Sjoberg, Nils A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Widmalm, Goran
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    On the anomalous temperature dependence of cellulose aqueous solubility2016In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 23, no 4, 2375-2387 p.Article in journal (Refereed)
  • 3.
    Bergenstrahle-Wohlert, Malin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Vibrational spectrum of the cellulose-water interface investigated by atomistic simulations2014In: Abstract of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 247, 190-CELL- p.Article in journal (Other academic)
  • 4.
    Bergenstråhle, Malin
    et al.
    Cornell University.
    Wohlert, Jakob
    Cornell University.
    Brady, John
    Cornell University.
    Himmel, Michael
    National Renewable Energy Laboratory.
    Simulation studies of the insolubility of cellulose2010In: Carbohydrate Research, ISSN 0008-6215, E-ISSN 1873-426X, Vol. 345, no 14, 2060-2066 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 5.
    Bergenstråhle, Malin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Larsson, Per Tomas
    STFI-PACKFORSK AB.
    Mazeau, Karim
    CERMAV-CNRS.
    Berglund, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Dynamics of Cellulose-Water Interfaces: NMR Spin-Lattice Relaxation Times Calculated from Atomistic Computer Simulations2008In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 112, no 9, 2590-2595 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 6.
    Bergenstråhle-Wohlert, Malin
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Berglund, Lars A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Brady, John W.
    Larsson, Per Tomas
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Westlund, Per-Olof
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Concentration enrichment of urea at cellulose surfaces: results from molecular dynamics simulations and NMR spectroscopy2012In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 19, no 1, 1-12 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 7. Berglund, Jennie
    et al.
    Angles d’Ortoli, Thibault
    Vilaplana, Francisco
    KTH, School of Biotechnology (BIO), Glycoscience.
    Widmalm, Göran
    Bergenstråhle-Wohlert, Malin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Lawoko, Martin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Henriksson, Gunnar
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Lindström, Mikael
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    A molecular dynamics study of the effect of glycosidiclinkage type in the hemicellulose backbone on the molecularchain flexibility2016In: The Plant Journal, ISSN 0960-7412, E-ISSN 1365-313XArticle in journal (Refereed)
    Abstract [en]

    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 nuclear magnetic resonance 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 the function of hemicelluloses both in the cell wall and in technical products.

  • 8. Chen, Mo
    et al.
    Pendrill, Robert
    Widmalm, Göran
    Brady, John W.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Molecular Dynamics Simulations of the Ionic Liquid 1-n-Butyl-3-Methylimidazolium Chloride and Its Binary Mixtures with Ethanol2014In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 10, no 10, 4465-4479 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 9.
    Chen, Pan
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Nishiyama, Yoshiharu
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Lu, Ang
    Mazeau, Karim
    Ismail, Ahmed E.
    Translational Entropy and Dispersion Energy Jointly Drive the Adsorption of Urea to Cellulose2017In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 121, no 10, 2244-2251 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 10.
    Djahed, Cyrus
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Berglund, Lars A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Molecular scale deformation mechanisms in cellulose crystals (I and II) by molecular dynamics - synergy between covalent and hydrogen bondsManuscript (preprint) (Other academic)
  • 11. Djahedi, Cyrus
    et al.
    Bergenstrahle-Wohlert, Malin
    Berglund, Lars A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Role of hydrogen bonding in cellulose deformation: the leverage effect analyzed by molecular modeling2016In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 23, no 4, 2315-2323 p.Article in journal (Refereed)
  • 12.
    Djahedi, Cyrus
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Berglund, Lars A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Molecular deformation mechanisms in cellulose allomorphs and the role of hydrogen bondsManuscript (preprint) (Other academic)
  • 13.
    Djahedi, Cyrus
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Berglund, Lars
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Molecular deformation mechanisms in cellulose allomorphs and the role of hydrogen bonds2015In: Carbohydrate Polymers, ISSN 0144-8617, E-ISSN 1879-1344, Vol. 130, 175-182 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 14. d'Ortoli, Thibault Angles
    et al.
    Sjöberg, Nils A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Vasiljeva, Polina
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Lindman, Jonas
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Widmalm, Göran
    Bergenstråhle-Wohlert, Malin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Temperature Dependence of Hydroxymethyl Group Rotamer Populations in Cellooligomers2015In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 119, no 30, 9559-9570 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 15. Kapla, Jon
    et al.
    Engstrom, Olof
    Stevensson, Baltzar
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Widmalm, Goran
    Maliniak, Arnold
    Molecular dynamics simulations and NMR spectroscopy studies of trehalose-lipid bilayer systems2015In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 17, no 34, 22438-22447 p.Article in journal (Refereed)
    Abstract [en]

    The disaccharide trehalose (TRH) strongly affects the physical properties of lipid bilayers. We investigate interactions between lipid membranes formed by 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and TRH using NMR spectroscopy and molecular dynamics (MD) computer simulations. We compare dipolar couplings derived from DMPC/TRH trajectories with those determined (i) experimentally in TRH using conventional high-resolution NMR in a weakly ordered solvent (bicelles), and (ii) by solid-state NMR in multilamellar vesicles (MLV) formed by DMPC. Analysis of the experimental and MD-derived couplings in DMPC indicated that the force field used in the simulations reasonably well describes the experimental results with the exception for the glycerol fragment that exhibits significant deviations. The signs of dipolar couplings, not available from the experiments on highly ordered systems, were determined from the trajectory analysis. The crucial step in the analysis of residual dipolar couplings (RDCs) in TRH determined in a bicelle-environment was access to the conformational distributions derived from the MD trajectory. Furthermore, the conformational behavior of TRH, investigated by J-couplings, in the ordered and isotropic phases is essentially identical, indicating that the general assumptions in the analyses of RDCs are well founded.

  • 16. Kapla, Jon
    et al.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Stevensson, Baltzar
    Engström, Olof
    Widmalm, Göran
    Maliniak, Arnold
    Molecular Dynamics Simulations of Membrane-Sugar Interactions2013In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 117, no 22, 6667-6673 p.Article in journal (Refereed)
    Abstract [en]

    It is well documented that disaccharides in general and trehalose (TRH) in particular strongly affect physical properties and functionality of lipid bilayers. We investigate interactions between lipid membranes formed by 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and TRH by means of molecular dynamics (MD) computer simulations. Ten different TRH concentrations were studied in the range W-TRH = 0-0.20 (w/w). The potential of mean force (PMF) for DMPC bilayer TRH interactions was determined using two different force fields, and was subsequently used in a simple analytical model for description of sugar binding at the membrane interface. The MD results were in good agreement with the predictions of the model. The net affinities of TRH for the DMPC bilayer derived from the model and MD simulations were compared with experimental results. The area per lipid increases and the membrane becomes thinner with increased TRH concentration, which is interpreted as an intercalation effect of the TRH molecules into the polar part of the lipids, resulting in conformational changes in the chains. These results are consistent with recent experimental observations. The compressibility modulus related to the fluctuations of the membrane increases dramatically with increased TRH concentration, which indicates higher order and rigidity of the bilayer. This is also reflected in a decrease (by a factor of 15) of the lateral diffusion of the lipids. We interpret these observations as a formation of a glassy state at the interface of the membrane, which has been suggested in the literature as a hypothesis for the membrane sugar interactions.

  • 17. Löbmann, Korbinian
    et al.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Müllertz, Anette
    Wågberg, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Svagan, Anna J.
    Cellulose Nanopaper and Nanofoam for Patient-Tailored Drug Delivery2017In: Advanced Materials Interfaces, ISSN 2196-7350, Vol. 4, no 9, 1600655Article in journal (Refereed)
    Abstract [en]

    The development of drug delivery systems with tailored drug release can be very challenging especially in the case of problematic drugs. To address this problem, pharmaceutical scientists frequently use different formulation approaches and excipients, often involving a complex and multistep preparation. In this study, new cellulose nanofiber (CNF) based drug formulations are developed that allow controlled drug release in a facile and fast way, i.e., by simply casting drug/CNF dispersions. Altering the processing conditions and utilizing the unique inherent chemicophysical properties of cationic CNF at interfaces, it is possible to produce either drug-loaded CNF nanopapers (containing 21 or 51 wt% drug) or nanofoams (containing 21 wt% drug). The different formulations exhibit tailored release kinetics of the poorly watersoluble model drug indomethacin from immediate (nanopapers, 10-20 min) to slow release (nanofoams, approximate to 24 h). The fast release, from the nanopapers, is a result of the interplay of the molecular and supramolecular structure of indomethacin in addition to observed enhanced intrinsic dissolution of drug in the presence of CNF. The slower drug release is achieved by changing the hierarchical structure, i.e., creating a CNF based foam (porosity 99.2 wt%), and the prolonged release is mainly due to an extended drug diffusion path.

  • 18.
    Saito, Tsuguyuki
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Kuramae, R.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Berglund, Lars A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Isogai, A.
    An ultrastrong nanofibrillar biomaterial: The strength of single cellulose nanofibrils revealed via sonication-induced fragmentation2013In: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 14, no 1, 248-253 p.Article in journal (Refereed)
    Abstract [en]

    We report the mechanical strength of native cellulose nanofibrils. Native cellulose nanofibrils, purified from wood and sea tunicate, were fully dispersed in water via a topochemical modification of cellulose nanofibrils using 2,2,6,6-tetramethylpiperidinyl-1-oxyl (TEMPO) as a catalyst. The strength of individual nanofibrils was estimated based on a model for the sonication-induced fragmentation of filamentous nanostructures. The resulting strength parameters were then analyzed based on fracture statistics. The mean strength of the wood cellulose nanofibrils ranged from 1.6 to 3 GPa, depending on the method used to measure the nanofibril width. The highly crystalline, thick tunicate cellulose nanofibrils exhibited higher mean strength of 3-6 GPa. The strength values estimated for the cellulose nanofibrils in the present study are comparable with those of commercially available multiwalled carbon nanotubes.

  • 19.
    Saito, Tsuguyuki
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Kuramae, Ryota
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Berglund, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Isogai, Akira
    Mechanical strength of single cellulose nanofibrils estimated from sonication-induced fragmentation2013In: Abstract of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 245Article in journal (Other academic)
  • 20.
    Sjöstedt, Anna
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Larsson, Per Tomas
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center. Innventia AB, Sweden.
    Wågberg, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Fibre Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Structural changes during swelling of highly charged cellulose fibres2015In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 22, no 5, 2943-2953 p.Article in journal (Refereed)
    Abstract [en]

    Structural changes of fibrils and fibril aggregates in the fibre wall were studied after oxidation of the cellulose by 2,2,6,6-tetramethyl-1-piperidinyloxy to high charge densities (highest charge density: 1300 mu eq/g). The increase in pore volume was measured by mini-WRV at two different pH levels, and the supramolecular structure in the fibre wall in terms of aggregate size, specific surface area and average pore size was measured by solid state NMR, DVS desorption and BET N-2 gas adsorption. A structural change in the arrangement of the fibrils inside the fibril aggregates was observed although the oxidation did not lead to a complete liberation of individual fibrils, i.e. they still exist as an aggregated structure after oxidation. Theoretical estimates suggest that the electrostatic repulsion energy connected with the increase in surface charge of the fibrils can be sufficient to gradually separate the fibrils enough to expose all fibril surfaces to oxidation chemicals.

  • 21. Tolonen, Lasse
    et al.
    Bergenstråhle-Wohlert, Malin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Sixta, Herbert
    Swelling and dissolution of crystalline cellulose in subcritical and supercritical water2014In: Abstract of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 247, 192-CELL- p.Article in journal (Other academic)
  • 22. Tolonen, Lasse K.
    et al.
    Bergenstråhle-Wohlert, Malin
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Sixta, Herbert
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Solubility of Cellulose in Supercritical Water Studied by Molecular Dynamics Simulations2015In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 119, no 13, 4739-4748 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 23.
    Wang, Yan
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Bergenstråhle-Wohlert, Malin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Tu, Yaoquan
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Berglund, Lars A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Swelling and dimensional stability of xyloglucan/montmorillonite nanocomposites in moist conditions from molecular dynamics simulations2017In: Computational Materials Science, ISSN 0927-0256, Vol. 128, 191-197 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 24.
    Wang, Yan
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Bergenstråhle-Wohlert, Malin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Kochumalayil, Joby J.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Berglund, Lars A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Tu, Yaoquan
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Molecular Adhesion at Clay Nanocomposite Interfaces Depends on Counterion Hydration-Molecular Dynamics Simulation of Montmorillonite/Xyloglucan2015In: Biomacromolecules, ISSN 1525-7797, E-ISSN 1526-4602, Vol. 16, no 1, 257-265 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 25.
    Wang, Yan
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Bergenstråhle-Wohlert, Malin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Tu, Yaoquan
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Hydration and dimensional stability of the intercalated galleries in xyloglucan/montmorillonite nanocomposites studied by molecular dynamics simulationsManuscript (preprint) (Other academic)
    Abstract [en]

    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.

  • 26.
    Wang, Yan
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Bergenstråhle-Wohlert, Malin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Tu, Yaoquan
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Molecular mechanisms for the adhesion of chitin and chitosan to montmorillonite clayManuscript (preprint) (Other academic)
    Abstract [en]

    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.

  • 27.
    Wang, Yan
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Bergenstråhle-Wohlert, Malin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Tu, Yaoquan
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Molecular mechanisms for the adhesion of chitin and chitosan to montmorillonite clay2015In: RSC Advances, ISSN 2046-2069, Vol. 5, no 67, 54580-54588 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 28.
    Wang, Yan
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Berglund, Lars A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Tu, Yaoquan
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Molecular dynamics simulation of strong interaction mechanisms at wet interfaces in clay-polysaccharide nanocomposites2014In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 2, no 25, 9541-9547 p.Article in journal (Refereed)
    Abstract [en]

    Bio-composites comprised of the polysaccharide xyloglucan (XG) and montmorillonite (MTM) clay has potential as a 'green' replacement of conventional petroleum-derived polymers in the packaging industry. These materials have been shown to possess excellent material properties, even in high relative humidity. Although interfacial interaction between XG and MTM, and the molecular structure of XG can be identified as key parameters for the complex formation process and the resulting tensile properties, these properties are usually difficult to address using experimental methods. Here we use molecular dynamics (MD) simulations to study the adsorption of fully atomistic models of both native and chemically modified XG to MTM clay surfaces in explicit water. We show that the driving force for adsorption is a favorable change in enthalpy, and furthermore that native XG adsorbs stronger than modified XG. This highlights the importance of molecular structure details to molecular adhesion. The present study provides insights into the molecular scale adsorption mechanisms and can therefore help in designing routes for further improvements of bio-composite materials.

  • 29.
    Wang, Yan
    et al.
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Zhang, Qiong
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Tu, Yaoquan
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Ågren, Hans
    KTH, School of Biotechnology (BIO), Theoretical Chemistry and Biology.
    Berglund, Lars
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites.
    Jose, Joby Kochumalayil
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Biocomposites.
    Molecular dynamic simulations of xyloglucan adsorbed onto Na-montmorillonite clay: Exploration of interaction mechanisms and conformational properties2013In: Abstract of Papers of the American Chemical Society, ISSN 0065-7727, Vol. 246, 342-POLY- p.Article in journal (Other academic)
  • 30.
    Wohlert, Jakob
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
    Atomistic computer simulations of lipid bilayers2006Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Computer simulation has become an important tool for the study of biomolecular systems. This thesis deals with molecular dynamics simulations of one-component lipid bilayers, which may serve as models for biological membranes.

    The main scientific contributions are:

    • It is possible to analyze the electrostatic contribution to the surface tension at a lipid-water interface in terms of dipole-dipole interactions between lipid headgroup shielded by a dielectric medium (water). The interaction can be divided into two parts. The in-plane components of the dipoles give rise to a positive, i.e. contractive

    contribution to the surface tension, albeit rather short ranged due to them being fluctuating dipoles. The normal components give rise to a negative, i.e. expansive contribution that will dominate the interaction at large distances.

    • Simulated membrane areas are extremely sensitive to details, especially the treatment of long-range electrostatic interactions. When cut-offs are used for the electrostatics, the exact definition of charge groups play an important role. Furthermore, using Ewald summation for the long-range interactions seems to have an overall stabilizing effect, and the area becomes less sensitive to other factors, such as system size and hydration.

    • Using atomistic simulations it is possible to study formation and evolution of a hydrophilic trans-membrane pore in detail. Free energy of pore nucleation and expansion can be calculated using potentials of mean constraint force. The resulting free energy profile shows no local maximum between the intact and pre-pore states, contrary to what is suggested by experiments.

    • The present force field reproduces even the slowest dynamics in the lipid chains, as reflected in NMR relaxation rates. Furthermore, since the simulated system was relatively small, the experimentally observed variation of relaxation rates with Larmor frequency cannot be explained by large scale collective dynamics, or it would not have shown up in the simulation.

    • Lipid lateral diffusion can be studied in detail on all relevant time scales by molecular dynamics. Using simple assumptions, the different diffusion coefficients measured on short and long times respectively can be connected in an analytic expression that fit calculated mean square displacements on timescales ranging from picoseconds to hundreds of nanoseconds.

  • 31.
    Wohlert, Jakob
    KTH, Superseded Departments, Physics.
    Electrostatics in lipid membranes: a computer simulation study2004Licentiate thesis, comprehensive summary (Other scientific)
  • 32.
    Wohlert, Jakob
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Vapor Pressures and Heats of Sublimation of Crystalline beta-Cellobiose from Classical Molecular Dynamics Simulations with Quantum Mechanical Corrections2014In: Journal of Physical Chemistry B, ISSN 1520-6106, E-ISSN 1520-5207, Vol. 118, no 20, 5365-5373 p.Article in journal (Refereed)
    Abstract [en]

    In this paper, we report the calculation of the enthalpy of sublimation, Delta H-sub, as a function of temperature of crystalline beta-cellobiose from molecular dynamics (MD) simulations, using two popular carbohydrate force fields. Together with the entropy difference between the solid and the vapor, AS, evaluated at atmospheric pressure, Delta H-sub gives the vapor pressure of cellobiose over the solid phase as a function of T. It is found that when quantum mechanical corrections to the enthalpy calculated from the distribution of normal modes is applied both force fields give Delta H-sub close to experiments. The entropy change, Delta S, which is calculated within a harmonic approximation becomes too small, leading to vapor pressures that are too low. These findings are relevant to MD simulations of crystalline carbohydrates in general, e.g., native cellulose.

  • 33.
    Wohlert, Jakob
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Bergenstråhle-Wohlert, Malin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Berglund, Lars A.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology. KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Deformation of cellulose nanocrystals: entropy, internal energy and temperature dependence2012In: Cellulose (London), ISSN 0969-0239, E-ISSN 1572-882X, Vol. 19, no 6, 1821-1836 p.Article in journal (Refereed)
    Abstract [en]

    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.

  • 34.
    Wohlert, Jakob
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Berglund, Lars A.
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    A Coarse-Grained Model for Molecular Dynamics Simulations of Native Cellulose2011In: Journal of Chemical Theory and Computation, ISSN 1549-9618, E-ISSN 1549-9626, Vol. 7, no 3, 753-760 p.Article in journal (Refereed)
    Abstract [en]

    We have constructed a coarse-grained model of crystalline cellulose to be used in molecular dynamics simulations. Using cellobiose from the recently published MARTINI coarse-grained force field for carbohydrates [Lopez, C. A. et al. J. Chem. Theory Comput. 2009, 5, 3195-3210] as a starting point, we have reparameterized the nonbonded interactions to reproduce the partitioning free energies between water and cyclohexane for a series of cellooligomers, cellobiose through cellopentaose. By extrapolating the model to longer cellooligomers, and by assigning special cellulose cellulose nonbonded interactions, we obtain a model which gives a stable, ordered structure in water that closely resembles the crystal structure of cellulose I beta. Furthermore, the resulting model is compatible with an existing coarse-grained force field for proteins. This is demonstrated by a simulation of the motion of the carbohydrate-binding domain of the fungal cellulase Cel7A from Trichoderma reesei on a crystalline cellulose surface. The diffusion coefficient at room temperature is calculated at D-1 = 3.1 x 10(-11) cm(2) s(-1), which is in good agreement with experimental numbers.

  • 35.
    Wohlert, Jakob
    et al.
    KTH, School of Engineering Sciences (SCI), Physics.
    den Otter, W. K.
    Edholm, Olof
    KTH, School of Engineering Sciences (SCI), Physics.
    Briels, W.J.
    Free energy of a trans-membrane pore calculated from atomistic molecular dynamics simulations2006In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 124, 154905- p.Article in journal (Refereed)
    Abstract [en]

    Atomistic molecular dynamics simulations of a lipid bilayer were performed to calculate the free energy of a trans-membrane pore as a function of its radius. The free energy was calculated as a function of a reaction coordinate using a potential of mean constraint force. The pore radius was then calculated from the reaction coordinate using Monte Carlo particle insertions. The main characteristics of the free energy that comes out of the simulations are a quadratic shape for a radius less than about 0.3 nm, a linear shape for larger radii than this, and a rather abrupt change without local minima or maxima between the two regions. In the outer region, a line tension can be calculated, which is consistent with the experimentally measured values. Further, this line tension can be rationalized and understood in terms of the energetic cost for deforming a part of the lipid bilayer into a hydrophilic pore. The region with small radii can be described and understood in terms of statistical mechanics of density fluctuations. In the region of crossover between a quadratic and linear free energy there was some hysteresis associated with filling and evacuation of the pore with water. The metastable prepore state hypothesized to interpret the experiments was not observed in this region.

  • 36.
    Wohlert, Jakob
    et al.
    KTH, Superseded Departments, Physics.
    Edholm, Olle
    KTH, Superseded Departments, Physics.
    The Range and Shielding of Dipole-Dipole Interactions in Phospholipid Bilay2004In: Biophysical Journal, ISSN 0006-3495, E-ISSN 1542-0086, Vol. 87, 2433- p.Article in journal (Refereed)
  • 37.
    Wohlert, Jakob
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
    Edholm, Olle
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Theoretical Biological Physics.
    The range and shielding of dipole-dipole interactions in phospholipid bilayers2004In: Biophysical Journal, ISSN 0006-3495, E-ISSN 1542-0086, Vol. 87, no 4, 2433-2445 p.Article in journal (Refereed)
    Abstract [en]

    In molecular dynamics simulations of lipid bilayers, the structure is sensitive to the precise treatment of electrostatics. The dipole-dipole interactions between headgroup dipoles are not long-ranged, but the area per lipid and, through it, other properties of the bilayer are very sensitive to the detailed balance between the perpendicular and in-plane components of the headgroup dipoles. This is affected by the detailed properties of the cutoff scheme or if long-range interactions are included by Ewald or particle-mesh Ewald techniques. Interaction between the in-plane components of the headgroup dipoles is attractive and decays as the inverse sixth power of distance. The interaction is screened by the square of a dielectric permittivity close to the value for water. Interaction between the components perpendicular to the membrane plane is repulsive and decays as the inverse third power of distance. These interactions are screened by a dielectric permittivity of the order 10. Thus, despite the perpendicular components being much smaller in magnitude than the in-plane components, they will dominate the interaction energies at large distances.

  • 38.
    Wohlert, Jakob
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Edholm, Olof
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Dynamics in atomistic simulations of phospholipid membranes: Nuclear magnetic resonance relaxation rates and lateral diffusion2006In: Journal of chemical physics, ISSN 0021-9606, Vol. 125, 204703- p.Article in journal (Refereed)
    Abstract [en]

    It is shown that a long, near microsecond, atomistic simulation can shed some light upon the dynamical processes occurring in a lipid bilayer. The analysis focuses on reorientational dynamics of the chains and lateral diffusion of lipids. It is shown that the reorientational correlation functions exhibits an algebraic decay (rather than exponential) for several orders of magnitude in time. The calculated nuclear magnetic resonance relaxation rates agree with experiments for carbons at the C7 position while there are some differences for C3. Lateral diffusion can be divided into two stages. In a first stage occurring at short times, t < 5 ns, the center of mass of the lipid moves due to conformational changes of the chains while the headgroup position remains relatively fixed. In this stage, the center of mass can move up to similar to 0.8 nm. The fitted short-time diffusion coefficient is D-1=13x10(-7) cm(2) s(-1) On a longer time scale, the diffusion coefficient becomes D-2=0.79x10(-7) cm(2) s(-1).

  • 39.
    Wohlert, Jakob
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Centres, Wallenberg Wood Science Center.
    Schnupf, Udo
    Brady, John W.
    Free energy surfaces for the interaction of D-glucose with planar aromatic groups in aqueous solution2010In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 133, no 15, 155103- p.Article in journal (Refereed)
    Abstract [en]

    Multidimensional potentials of mean force for the interactions in aqueous solution of both anomers of D-glucopyranose with two planar aromatic molecules, indole and para-methyl-phenol, have been calculated using molecular dynamics simulations with umbrella sampling and were subsequently used to estimate binding free energies. Indole and para-methyl-phenol serve as models for the side chains of the amino acids tryptophan and tyrosine, respectively. In all cases, a weak affinity between the glucose molecules and the flat aromatic surfaces was found. The global minimum for these interactions was found to be for the case when the pseudoplanar face of beta-D-glucopyranose is stacked against the planar surfaces of the aromatic residues. The calculated binding free energies are in good agreement with both experiment and previous simulations. The multidimensional free energy maps suggest a mechanism that could lend kinetic stability to the complexes formed by sugars bound to sugar-binding proteins. (C) 2010 American Institute of Physics.

  • 40.
    Wohlert, Jakob
    et al.
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    Tolonen, Lasse K.
    Bergenstråhle-Wohlert, Malin
    KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology.
    A simple model for cellulose solubility in supercritical water2015In: Nordic Pulp & Paper Research Journal, ISSN 0283-2631, Vol. 30, no 1, 14-19 p.Article in journal (Refereed)
    Abstract [en]

    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.

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