352 research outputs found
Softening of the stiffness of bottlebrush polymers by mutual interaction
We study bottlebrush macromolecules in a good solvent by small-angle neutron
scattering (SANS), static light scattering (SLS), and dynamic light scattering
(DLS). These polymers consist of a linear backbone to which long side chains
are chemically grafted. The backbone contains about 1600 monomer units (weight
average) and every second monomer unit carries side-chains with ca. 60 monomer
units. The SLS- and SANS data extrapolated to infinite dilution lead to the
form factor of the polymer that can be described in terms of a worm-like chain
with a contour length of 380 nm and a persistence length of 17.5 nm. An
analysis of the DLS data confirm these model parameters. The scattering
intensities taken at finite concentration can be modeled using the polymer
reference interaction site model. It reveals a softening of the bottlebrush
polymers caused by their mutual interaction. We demonstrate that the
persistence decreases from 17.5 nm down to 5 nm upon increasing the
concentration from dilute solution to the highest concentration 40.59 g/l under
consideration. The observed softening of the chains is comparable to the
theoretically predicted decrease of the electrostatic persistence length of
linear polyelectrolyte chains at finite concentrations.Comment: 4 pages, 4 figure
Charge matters : mutations in Omicron variant favor binding to cells
Evidence is strengthening to suggest that the novel SARS-CoV-2 mutant Omicron, with its more than 60 mutations, will spread and dominate worldwide. Although the mutations in the spike protein are known, the molecular basis for why the additional mutations in the spike protein that have not previously occurred account for Omicron's higher infection potential, is not understood. We propose, based on chemical rational and molecular dynamics simulations, that the elevated occurrence of positively charged amino acids in certain domains of the spike protein (Delta: +4; Omicron: +5 vs. wild type) increases binding to cellular polyanionic receptors, such as heparan sulfate due to multivalent charge-charge interactions. This observation is a starting point for targeted drug development
Quantifying the Reversible Association of Thermosensitive Nanoparticles
Under many conditions, biomolecules and nanoparticles associate by means of
attractive bonds, due to hydrophobic attraction. Extracting the microscopic
association or dissociation rates from experimental data is complicated by the
dissociation events and by the sensitivity of the binding force to temperature
(T). Here we introduce a theoretical model that combined with light-scattering
experiments allows us to quantify these rates and the reversible binding energy
as a function of T. We apply this method to the reversible aggregation of
thermoresponsive polystyrene/poly(N-isopropylacrylamide) core-shell
nanoparticles, as a model system for biomolecules. We find that the binding
energy changes sharply with T, and relate this remarkable switchable behavior
to the hydrophobic-hydrophilic transition of the thermosensitive nanoparticles
Charge-induced conformational changes of dendrimers
We study the effect of chargeable monomers on the conformation of dendrimers
of low generation by computer simulations, employing bare Coulomb interactions.
The presence of the latter leads to an increase in size of the dendrimer due to
a combined effect of electrostatic repulsion and the presence of counterions
within the dendrimer, and also enhances a shell-like structure for the monomers
of different generations. In the resulting structures the bond-length between
monomers, especially near the center, will increase to facilitate a more
effective usage of space in the outer-regions of the dendrimer.Comment: 7 pages, 12 figure
Soft Interaction Between Dissolved Dendrimers: Theory and Experiment
Using small-angle neutron scattering and liquid integral equation theory, we
relate the structure factor of flexible dendrimers of 4th generation to their
average shape. The shape is measured as a radial density profile of monomers
belonging to a single dendrimer. From that, we derive an effective interaction
of Gaussian form between pairs of dendrimers and compute the structure factor
using the hypernetted chain approximation. Excellent agreement with the
corresponding experimental results is obtained, without the use of adjustable
parameters. The present analysis thus strongly supports the previous finding
that flexible dendrimers of low generation present fluctuating structures akin
to star polymers.Comment: 20 pages, 4 figures, submitted to Macromolecules on July 24, 200
Nonlinear response of dense colloidal suspensions under oscillatory shear: Mode-coupling theory and FT-rheology experiments
Using a combination of theory, experiment and simulation we investigate the
nonlinear response of dense colloidal suspensions to large amplitude
oscillatory shear flow. The time-dependent stress response is calculated using
a recently developed schematic mode-coupling-type theory describing colloidal
suspensions under externally applied flow. For finite strain amplitudes the
theory generates a nonlinear response, characterized by significant higher
harmonic contributions. An important feature of the theory is the prediction of
an ideal glass transition at sufficiently strong coupling, which is accompanied
by the discontinuous appearance of a dynamic yield stress. For the oscillatory
shear flow under consideration we find that the yield stress plays an important
role in determining the non linearity of the time-dependent stress response.
Our theoretical findings are strongly supported by both large amplitude
oscillatory (LAOS) experiments (with FT-rheology analysis) on suspensions of
thermosensitive core-shell particles dispersed in water and Brownian dynamics
simulations performed on a two-dimensional binary hard-disc mixture. In
particular, theory predicts nontrivial values of the exponents governing the
final decay of the storage and loss moduli as a function of strain amplitude
which are in excellent agreement with both simulation and experiment. A
consistent set of parameters in the presented schematic model achieves to
jointly describe linear moduli, nonlinear flow curves and large amplitude
oscillatory spectroscopy
Theory of Thermodynamic Stresses in Colloidal Dispersions at the Glass Transition
We discuss the nonlinear rheology of dense colloidal dispersions at the glass
transition. A first principles approach starting with interacting Brownian
particles in given arbitrary homogeneous (incompressible) flow neglecting
hydrodynamic interactions is sketched. It e.g. explains steady state flow
curves for finite shear rates measured in dense suspensions of thermosensitive
core-shell particles consisting of a polystyrene core and a crosslinked
poly(N-isopropylacrylamide)(PNIPAM) shell. The exponents of simple and
generalized Herschel Bulkley laws are computed for hard spheres.Comment: 3 pages, 1 figure; contribution to The XVth International Congress on
Rheology, August 3-8, 2008, Monterey, California; submitted to J. Rheo
Thermal Degradation of Adsorbed Bottle-Brush Macromolecules: Molecular Dynamics Simulation
The scission kinetics of bottle-brush molecules in solution and on an
adhesive substrate is modeled by means of Molecular Dynamics simulation with
Langevin thermostat. Our macromolecules comprise a long flexible polymer
backbone with segments, consisting of breakable bonds, along with two side
chains of length , tethered to each segment of the backbone. In agreement
with recent experiments and theoretical predictions, we find that bond cleavage
is significantly enhanced on a strongly attractive substrate even though the
chemical nature of the bonds remains thereby unchanged.
We find that the mean bond life time decreases upon adsorption by
more than an order of magnitude even for brush molecules with comparatively
short side chains $N=1 \div 4$. The distribution of scission probability along
the bonds of the backbone is found to be rather sensitive regarding the
interplay between length and grafting density of side chains. The life time
declines with growing contour length as ,
and with side chain length as . The probability
distribution of fragment lengths at different times agrees well with
experimental observations. The variation of the mean length of the
fragments with elapsed time confirms the notion of the thermal degradation
process as a first order reaction.Comment: 15 pages, 7 figure
Polysulfates block SARS-CoV-2 uptake through electrostatic interactions
Here we report that negatively charged polysulfates can bind to the spike protein of SARS-CoV-2 via electrostatic interactions. Using a plaque reduction assay, we compare inhibition of SARS-CoV-2 by heparin, pentosan sulfate, linear polyglycerol sulfate (LPGS) and hyperbranched polyglycerol sulfate (HPGS). Highly sulfated LPGS is the optimal inhibitor, with a half-maximal inhibitory concentration (IC50) of 67 μg/mL (approx.1.6 μM). This synthetic polysulfates exhibit more than 60-fold higher virus inhibitory activity than heparin (IC50: 4084μg/mL), along with much lower anticoagulant activity. Furthermore, in molecular dynamics simulations, we verified that LPGS can bind stronger to the spike protein than heparin, and that LPGS can interact even morewith the spike protein of the new N501Y and E484K variants. Our study demonstrates that the entry of SARS-CoV-2 into host cells can be blocked via electrostatic interaction, therefore LPGS can serve as a blueprint for the design of novel viral inhibitors of SARS-CoV-2
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