7 research outputs found
Solubility of Cellulose in Supercritical Water Studied by Molecular Dynamics Simulations
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<sup>ā3</sup>) 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 Ć 4 Ć
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 waterāwater
bonding in low density supercritical water whereas the opposite occurred
in ambient and high density supercritical water
Hydration-Dependent Dynamical Modes in Xyloglucan from Molecular Dynamics Simulation of <sup>13</sup>C NMR Relaxation Times and Their Distributions
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 <sup>13</sup>C NMR longitudinal relaxation times (<i>T</i><sub>1</sub>) 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 waterāchain
and chaināchain 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
An Ultrastrong Nanofibrillar Biomaterial: The Strength of Single Cellulose Nanofibrils Revealed via Sonication-Induced Fragmentation
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 3ā6
GPa. The strength values estimated for the cellulose nanofibrils in
the present study are comparable with those of commercially available
multiwalled carbon nanotubes
Hydration-Dependent Dynamical Modes in Xyloglucan from Molecular Dynamics Simulation of <sup>13</sup>C NMR Relaxation Times and Their Distributions
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 <sup>13</sup>C NMR longitudinal relaxation times (<i>T</i><sub>1</sub>) 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 waterāchain
and chaināchain 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
Molecular Dynamics Simulations of MembraneāSugar Interactions
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-<i>sn</i>-glycero-3-phosphocholine
(DMPC) and TRH by means of molecular dynamics (MD) computer simulations.
Ten different TRH concentrations were studied in the range <i>w</i><sub>TRH</sub> = 0ā0.20 (w/w). The potential of
mean force (PMF) for DMPC bilayerāTRH 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 membraneāsugar interactions
Translational Entropy and Dispersion Energy Jointly Drive the Adsorption of Urea to Cellulose
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 celluloseāurea dispersion energy and
the translational entropy gain of water are the main factors that
drive the adsorption of urea on cellulose
Toward Improved Understanding of the Interactions between Poorly Soluble Drugs and Cellulose Nanofibers
Cellulose
nanofibers (CNFs) have interesting physicochemical and colloidal properties that have been
recently exploited in novel drug-delivery systems for tailored release
of poorly soluble drugs. The morphology and release kinetics of such
drug-delivery systems heavily relied on the drugāCNF interactions;
however, in-depth understanding of the interactions was lacking. Herein,
the interactions between a poorly soluble model drug molecule, furosemide,
and cationic cellulose nanofibers with two different degrees of substitution
are studied by sorption experiments, Fourier transform infrared spectroscopy,
and molecular dynamics (MD) simulation. Both MD simulations and experimental
results confirmed the spontaneous sorption of drug onto CNF. Simulations
further showed that adsorption occurred by the flat aryl ring of furosemide.
The spontaneous sorption was commensurate with large entropy gains
as a result of release of surface-bound water. Association between
furosemide molecules furthermore enabled surface precipitation as
indicated by both simulations and experiments. Finally, sorption was
also found not to be driven by charge neutralization, between positive
CNF surface charges and the furosemide negative charge, so that surface
area is the single most important parameter determining the amount
of sorbed drug. An optimized CNFāfurosemide drug-delivery vehicle
thus needs to have a maximized specific surface area irrespective
of the surface charge with which it is achieved. The findings also
provide important insights into the design principles of CNF-based
filters suitable for removal of poorly soluble drugs from wastewater