14 research outputs found
Nanometer-Resolved Radio-Frequency Absorption and Heating in Biomembrane Hydration Layers
Radio-frequency (RF) electromagnetic
fields are readily absorbed
in biological matter and lead to dielectric heating. To understand
how RF radiation interacts with macromolecular structures and possibly
influences biological function, a quantitative description of dielectric
absorption and heating at nanometer resolution beyond the usual effective
medium approach is crucial. We report an exemplary multiscale theoretical
study for biomembranes that combines (i) atomistic simulations for
the spatially resolved absorption spectrum at a single planar DPPC
lipid bilayer immersed in water, (ii) calculation of the electric
field distribution in planar and spherical cell models, and (iii)
prediction of the nanometer resolved temperature profiles under steady
RF radiation. Our atomistic simulations show that the only 2 nm thick
lipid hydration layer strongly absorbs in a wide RF range between
10 MHz and 100 GHz. The absorption strength, however, strongly depends
on the direction of the incident wave. This requires modeling of the
electric field distribution using tensorial dielectric spectral functions.
For a spherical cell model, we find a strongly enhanced RF absorption
on an equatorial ring, which gives rise to temperature gradients inside
a single cell under radiation. Although absolute temperature elevation
is small under conditions of typical telecommunication usage, our
study points to hitherto neglected temperature gradient effects and
allows thermal RF effects to be predicted on an atomistically resolved
level. In addition to a refined physiological risk assessment of RF
fields, technological applications for controlling temperature profiles
in nanodevices are possible
Insight into the Molecular Mechanisms of Protein Stabilizing Osmolytes from Global Force-Field Variations
A prominent class of osmolytes that
are able to stabilize proteins
in their native fold consist of small highly water-soluble molecules
with a large dipole moment and hydrophobic groups attached to the
positively charged end of the molecule, for which we coin the term
dipolar/hydrophobic osmolytes. For TMAO, which is a prime member of
this class, we perform large-scale water-explicit MD simulations and
determine the bulk solution activity coefficient as well as the affinity
to a stretched polyglycine chain for varying TMAO dipolar strength
and hydrophobicity. Double optimization with respect to experimental
values for the activity coefficient and the polyglycine transfer free
energy is achieved. The resulting optimal TMAO force field shows excellent
transferability to different concentrations and also reproduces transfer
free energies of various amino acids, including the tryptophan anomaly,
for which TMAO acts as a denaturant. By globally analyzing the thermodynamic
and structural properties of suboptimal TMAO force fields, we identify
the frustration between dipolar and hydrophobic interactions as the
working mechanism and the design principle of dipolar/hydrophobic
osmolytes
Specific Ion Binding to Carboxylic Surface Groups and the pH Dependence of the Hofmeister Series
Ion binding to acidic groups is a
central mechanism for ion-specificity
of macromolecules and surfaces. Depending on pH, acidic groups are
either protonated or deprotonated and thus change not only charge
but also chemical structure with crucial implications for their interaction
with ions. In a two-step modeling approach, we first determine single-ion
surface interaction potentials for a few selected halide and alkali
ions at uncharged carboxyl (COOH) and charged carboxylate (COO<sup>â</sup>) surface groups from atomistic MD simulations with
explicit water. Care is taken to subtract the bare Coulomb contribution
due to the net charge of the carboxylate group and thereby to extract
the nonelectrostatic ionâsurface potential. Even at this stage,
pronounced ion-specific effects are observed and the ion surface affinity
strongly depends on whether the carboxyl group is protonated or not.
In the second step, the ion surface interaction potentials are used
in a PoissonâBoltzmann model to calculate the surface charge
and the potential distribution in the solution depending on salt type,
salt concentration, and solution pH in a self-consistent manner. Hofmeister
phase diagrams are derived on the basis of the long-ranged forces
between two carboxyl-functionalized surfaces. For cations we predict
direct, reversed, and altered Hofmeister series as a function of the
pH, qualitatively similar to recent experimental results for silica
surfaces. The Hofmeister series reversal for cations is rationalized
by a reversal of the single-cation affinity to the carboxyl group
depending on its protonation state: the deprotonated carboxylate (COO<sup>â</sup>) surface group interacts most favorably with small
cations such as Li<sup>+</sup> and Na<sup>+</sup>, whereas the protonated
carboxyl (COOH) surface group interacts most favorably with large
cations such as Cs<sup>+</sup> and thus acts similarly to a hydrophobic
surface group. Our results provide a general mechanism for the pH-dependent
reversal of the Hofmeister series due to the different specific ion
binding to protonated and deprotonated surface groups
Anionic and Cationic Hofmeister Effects on Hydrophobic and Hydrophilic Surfaces
Using a two-step modeling approach, we address the full
spectrum
of direct, reversed, and altered ionic sequences as the charge of
the ion, the charge of the surface, and the surface polarity are varied.
From solvent-explicit molecular dynamics simulations, we extract single-ion
surface interaction potentials for halide and alkali ions at hydrophilic
and hydrophobic surfaces. These are used within PoissonâBoltzmann
theory to calculate ion density and electrostatic potential distributions
at mixed polar/unpolar surfaces for varying surface charge. The resulting
interfacial tension increments agree quantitatively with experimental
data and capture the Hofmeister series, especially the anomaly of
lithium, which is difficult to obtain using continuum theory. Phase
diagrams that feature different Hofmeister series as a function of
surface charge, salt concentration, and surface polarity are constructed
from the long-range force between two surfaces interacting across
electrolyte solutions. Large anions such as iodide have a high hydrophobic
surface affinity and increase the effective charge magnitude on negatively
charged unpolar surfaces. Large cations such as cesium also have a
large hydrophobic surface affinity and thereby compensate an external
negative charge surface charge most efficiently, which explains the
well-known asymmetry between cations and anions. On the hydrophilic
surface, the size-dependence of the ion surface affinity is reversed,
explaining the Hofmeister series reversal when comparing hydrophobic
with hydrophilic surfaces
Viscous Friction of Hydrogen-Bonded Matter
Amontonsâ law successfully describes friction
between macroscopic
solid bodies for a wide range of velocities and normal forces. For
the diffusion and forced sliding of adhering or entangled macromolecules,
proteins, and biological complexes, temperature effects are invariably
important, and a similarly successful friction law at biological length
and velocity scales is missing. Hydrogen bonds (HBs) are key to the
specific binding of biomatter. Here we show that friction between
hydrogen-bonded matter obeys in the biologically relevant low-velocity
viscous regime a simple law: the friction force is proportional to
the number of HBs, the sliding velocity, and a friction coefficient
Îł<sub>HB</sub>. This law is deduced from atomistic molecular
dynamics simulations for short peptide chains that are laterally pulled
over planar hydroxylated substrates in the presence of water and holds
for widely different peptides, surface polarities, and applied normal
forces. The value of Îł<sub>HB</sub> is extrapolated from simulations
at sliding velocities in the range from <i>V</i> = 10<sup>â2</sup> to 100 m/s by mapping on a simple stochastic model
and turns out to be of the order of Îł<sub>HB</sub> â
10<sup>â8</sup> kg/s. The friction of a single HB thus amounts
to the Stokes friction of a sphere with an equivalent radius of roughly
1 ÎŒm moving in water. Cooperativity is pronounced: roughly three
HBs act collectively
Charged Surface-Active Impurities at Nanomolar Concentration Induce JonesâRay Effect
The
electrolyte surface tension exhibits a characteristic minimum
around a salt concentration of 1 mM for all ion types, known as the
JonesâRay effect. We show that a consistent description of
the experimental surface tension of salts, bases, and acids is possible
by assuming charged impurities in the water with a surface affinity
typical for surfactants. Comparison with experimental data yields
an impurity concentration in the nanomolar range, well below the typical
experimental detection limit. Our modeling reveals salt-screening
enhanced impurity adsorption as the mechanism behind the JonesâRay
effect: for very low salt concentration added salt screens theÂ
electrostatic repulsion between impurities at the surface, which dramatically
increases impurity adsorption and thereby reduces the surface tension
Peptide Desorption Kinetics from Single Molecule Force Spectroscopy Studies
We use a combined experimental/theoretical
approach to determine
the intrinsic monomeric desorption rate <i>k</i><sub>0</sub> of polytyrosine and polylysine homopeptides from flat surfaces.
To this end, single polypeptide molecules are covalently attached
to an AFM cantilever tip and desorbed from hydrophobic self-assembled
monolayers in two complementary experimental protocols. In the constant-pulling-velocity
protocol, the cantilever is moved at finite velocity away from the
surface and the distance at which the constant plateau force regime
ends and the polymer detaches is recorded. In the waiting-time protocol,
the cantilever is held at a fixed distance above the surface and the
time until the polymer detaches is recorded. The desorption plateau
force is varied between 10 and 90 pN, by systematically changing the
aqueous solvent quality via the addition of ethanol or salt. A simultaneous
fit of the experimental data from both protocols with simple two-state
kinetic polymer theory allows to unambiguously disentangle and determine
the model parameters corresponding to polymer contour length <i>L</i>, Kuhn length <i>a</i>, adsorption free energy
λ, and intrinsic monomeric desorption rate <i>k</i><sub>0</sub>. Crucial to our analysis is that a statistically significant
number of single-polymer desorption experiments are done with one
and the same single polymer molecule for different solvent qualities.
The surprisingly low value of about <i>k</i><sub>0</sub> â 10<sup>5</sup> Hz points to significant cooperativity in
the desorption process of single polymers
Ultralow Liquid/Solid Friction in Carbon Nanotubes: Comprehensive Theory for Alcohols, Alkanes, OMCTS, and Water
In this work, we perform a theoretical study of liquid
flow in
graphitic nanopores of different sizes and geometries. Molecular dynamics
flow simulations of different liquids (water, decane, ethanol, and
OMCTS) in carbon nanotubes (CNT) are shown to exhibit flow velocities
1â3 orders of magnitude higher than those predicted from the
continuum hydrodynamics framework and the no-slip boundary condition.
These results support previous experimental findings obtained by several
groups that reported exceptionally high liquid flow rates in CNT membranes.
The liquid/graphite friction coefficient is identified as the crucial
parameter for this fast mass transport in CNT. The friction coefficient
is found to be very sensitive to wall curvature: friction is independent
of confinement for liquids between flat graphene walls with zero curvature,
whereas it decreases with increasing positive curvature (liquid inside
CNT), and it increases with increasing negative curvature (liquid
outside CNT). Furthermore, we present a theoretical approximate expression
for the friction coefficient, which predicts qualitatively and semiquantitatively
its curvature dependent behavior. The proposed theoretical description,
which works well for different kinds of liquids (alcohols, alkanes,
and water), sheds light on the physical mechanisms at the origin of
the ultra low liquid/solid friction in CNT. In fact, it is due to
their perfectly ordered molecular structure and their atomically smooth
surface that carbon nanotubes are quasiperfect liquid conductors compared
to other membrane pores like nanochannels in amorphous silica
Particle Diffusion in Polymeric Hydrogels with Mixed Attractive and Repulsive Interactions
All
biogels are heterogeneous, consisting of functional groups
with different biophysical properties arrayed on spatially disordered
polymer networks. Nanoparticles diffusing in such biogels experience
a mixture of attractive and repulsive interactions. Here, we present
experimental and theoretical studies of charged particle diffusion
in gels with a random distribution of attractive and repulsive electrostatic
interaction sites inside the gel. In addition to interaction disorder,
we theoretically investigate the effect of spatial disorder of the
polymer network. Our coarse-grained simulations reveal that attractive
interactions primarily determine the diffusive behavior of the particles
in systems with mixed attractive and repulsive interactions. As a
consequence, charged particles of either sign are immobilized in mixed
cationic/anionic gels because they are trapped near oppositely charged
interaction sites, whereas neutral particles diffuse rapidly. Even
small fractions of oppositely charged interaction sites lead to strong
trapping of a charged particle. Translational diffusion coefficients
of charged probe molecules in gels consisting of mixed cationic and
anionic dextran polymers are determined by fluorescence correlation
spectroscopy and quantitatively confirm our theoretical predictions
Combination of MD Simulations with Two-State Kinetic Rate Modeling Elucidates the Chain Melting Transition of Phospholipid Bilayers for Different Hydration Levels
The
phase behavior of membrane lipids plays an important role in
the formation of functional domains in biological membranes and crucially
affects molecular transport through lipid layers, for instance, in
the skin. We investigate the thermotropic chain melting transition
from the ordered <i>L</i><sub>ÎČ</sub> phase to the
disordered <i>L</i><sub>α</sub> phase in membranes
composed of dipalmitoylphosphatidylcholine (DPPC) by atomistic molecular
dynamics simulations in which the membranes are subject to variable
heating rates. We find that the transition is initiated by a localized
nucleus and followed by the propagation of the phase boundary. A two-state
kinetic rate model allows characterizing the transition state in terms
of thermodynamic quantities such as transition state enthalpy and
entropy. The extrapolated equilibrium melting temperature increases
with reduced membrane hydration and thus in tendency reproduces the
experimentally observed dependence on dehydrating osmotic stress