533 research outputs found
The Osmotic Coefficient of Rod-like Polyelectrolytes: Computer Simulation, Analytical Theory, and Experiment
The osmotic coefficient of solutions of rod-like polyelectrolytes is
considered by comparing current theoretical treatments and simulations to
recent experimental data. The discussion is restricted to the case of
monovalent counterions and dilute, salt-free solutions. The classical
Poisson-Boltzmann solution of the cell model correctly predicts a strong
decrease in the osmotic coefficient, but upon closer look systematically
overestimates its value. The contribution of ion-ion-correlations are
quantitatively studied by MD simulations and the recently proposed DHHC theory.
However, our comparison with experimental data obtained on synthetic,
stiff-chain polyelectrolytes shows that correlation effects can only partly
explain the discrepancy. A quantitative understanding thus requires theoretical
efforts beyond the restricted primitive model of electrolytes.Comment: 16 pages, 2 figure
Interaction of Charged Patchy Protein Models with Like Charged Polyelectrolyte Brushes
We study the adsorption of charged patchy particle models (CPPMs) on a thin
film of a like-charged and dense polyelectrolyte (PE) brush (of 50 monomers per
chain) by means of implicit-solvent, explicit-salt Langevin dynamics computer
simulations. Our previously introduced set of CPPMs embraces well-defined one-,
and two-patched spherical globules, each of the same net charge and (nanometer)
size, with mono- and multipole moments comparable to those of small globular
proteins. We focus on electrostatic effects on the adsorption far away from the
isoelectric point of typical proteins, i.e., where charge regulation plays no
role. Despite the same net charge of the brush and globule we observe large
binding affinities up to tens of the thermal energy, kT, which are enhanced by
decreasing salt concentration and increasing charge of the patch(es). Our
analysis of the distance-resolved potentials of mean force together with a
phenomenological description of all leading interaction contributions shows
that the attraction is strongest at the brush surface, driven by multipolar,
Born (self-energy), and counterion-release contributions, dominating locally
over the monopolar and steric repulsions.Comment: 16 pages, 8 figures, 2 table
Competitive adsorption of multiple proteins to nanoparticles: the Vroman effect revisited
Proteins adsorbed from the blood plasma change nanoparticles inter- actions with the surrounding biological environment. In general, the ad- sorption of multiple proteins has a non-monotonic time dependence, that is, proteins adsorbed at first may slowly be replaced by others. This “Vro- man effect” leads to a highly dynamic protein corona on nanoparticles that profoundly influences the immune response of the body. Thus, the temporal evolution of the corona must be taken into account when consid- ering applications of nanocarriers in, e.g., nanomedicine or drug delivery. Up to now, the Vroman effect is explained solely in terms of diffusion: Smaller proteins which diffuse faster are adsorbed first while larger ones, having a stronger interaction with the surface, are preferred at equilib- rium. Here we use dynamic density functional theory (DDFT) including steric and electrostatic interactions to provide a full model for the tem- poral evolution of the protein corona. In particular, we demonstrate that proper consideration of all interactions leads to Vroman-like adsorption signatures in widely different scenarios. Moreover, consideration of ener- getic terms predicts both competitive as well as co-operative adsorption. In this way, DDFT provides a reacher picture of the evolution of the dynamic protein coron
Structure factor and thermodynamics of rigid dendrimers in solution
The ''polymer reference interaction site model'' (PRISM) integral equation
theory is used to determine the structure factor of rigid dendrimers in
solution. The theory is quite successful in reproducing experimental structure
factors for various dendrimer concentrations. In addition, the structure factor
at vanishing scattering vector is calculated via the compressibility equation
using scaled particle theory and fundamental measure theory. The results as
predicted by both theories are systematically smaller than the experimental and
PRISM data for platelike dendrimers.Comment: 7 pages, 5 figures, submitte
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
Residual Stresses in Glasses
The history dependence of the glasses formed from flow-melted steady states
by a sudden cessation of the shear rate is studied in colloidal
suspensions, by molecular dynamics simulations, and mode-coupling theory. In an
ideal glass, stresses relax only partially, leaving behind a finite persistent
residual stress. For intermediate times, relaxation curves scale as a function
of , even though no flow is present. The macroscopic stress
evolution is connected to a length scale of residual liquefaction displayed by
microscopic mean-squared displacements. The theory describes this history
dependence of glasses sharing the same thermodynamic state variables, but
differing static properties.Comment: submitted to Physical Revie
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 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
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