7,987 research outputs found
Lipid Ion Channels
The interpretation electrical phenomena in biomembranes is usually based on
the assumption that the experimentally found discrete ion conduction events are
due to a particular class of proteins called ion channels while the lipid
membrane is considered being an inert electrical insulator. The particular
protein structure is thought to be related to ion specificity, specific
recognition of drugs by receptors and to macroscopic phenomena as nerve pulse
propagation. However, lipid membranes in their chain melting regime are known
to be highly permeable to ions, water and small molecules, and are therefore
not always inert. In voltage-clamp experiments one finds quantized conduction
events through protein-free membranes in their melting regime similar to or
even undistinguishable from those attributed to proteins. This constitutes a
conceptual problem for the interpretation of electrophysiological data obtained
from biological membrane preparations. Here, we review the experimental
evidence for lipid ion channels, their properties and the physical chemistry
underlying their creation. We introduce into the thermodynamic theory of
membrane fluctuations from which the lipid channels originate. Furthermore, we
demonstrate how the appearance of lipid channels can be influenced by the
alteration of the thermodynamic variables (temperature, pressure, tension,
chemical potentials) in a coherent description that is free of parameters. This
description leads to pores that display dwell times closely coupled to the
fluctuation lifetime via the fluctuation-dissipation theorem. Drugs as
anesthetics and neurotransmitters are shown to influence the channel likelihood
and their lifetimes in a predictable manner. We also discuss the role of
proteins in influencing the likelihood of lipid channel formation.Comment: Revie
Atomistic Hydrodynamics and the Dynamical Hydrophobic Effect in Porous Graphene
Mirroring their role in electrical and optical physics, two-dimensional
crystals are emerging as novel platforms for fluid separations and water
desalination, which are hydrodynamic processes that occur in nanoscale
environments. For numerical simulation to play a predictive and descriptive
role, one must have theoretically sound methods that span orders of magnitude
in physical scales, from the atomistic motions of particles inside the channels
to the large-scale hydrodynamic gradients that drive transport. Here, we use
constraint dynamics to derive a nonequilibrium molecular dynamics method for
simulating steady-state mass flow of a fluid moving through the nanoscopic
spaces of a porous solid. After validating our method on a model system, we use
it to study the hydrophobic effect of water moving through pores of
electrically doped single-layer graphene. The trend in permeability that we
calculate does not follow the hydrophobicity of the membrane, but is instead
governed by a crossover between two competing molecular transport mechanisms.Comment: 6 pages, 3 figure
Nanoscale Carbon Greatly Enhances Mobility of a Highly Viscous Ionic Liquid
Ability to encapsulate molecules is one of the outstanding features of
nanotubes. The encapsulation alters physical and chemical properties of both
nanotubes and guest species. The latter normally form a separate phase,
exhibiting drastically different behavior compared to bulk. Ionic liquids (ILs)
and apolar carbon nanotubes (CNTs) are disparate objects; nevertheless, their
interaction leads to spontaneous CNT filling with ILs. Moreover, ionic
diffusion of highly viscous ILs can increase 5-fold inside CNTs, approaching
that of molecular liquids, even though the confined IL phase still contains
exclusively ions. We exemplify these unusual effects by computer simulation on
a highly hydrophilic, electrostatically structured, and immobile
1-ethyl-3-methylimidazolium chloride, [C2C1IM][Cl]. Self-diffusion constants
and energetic properties provide microscopic interpretation of the observed
phenomena. Governed by internal energy and entropy rather than external work,
the kinetics of CNT filling is characterized in detail. The significant growth
of the IL mobility induced by nanoscale carbon promises important advances in
electricity storage devices
Driving force of water entry into hydrophobic channels of carbon nanotubes: entropy or energy?
Spontaneous entry of water molecules inside single-wall carbon nanotubes
(SWCNTs) has been confirmed by both simulations and experiments. Using
molecular dynamics simulations, we have studied the thermodynamics of filling
of a (6,6) carbon nanotube in a temperature range from 273 to 353 K and with
different strengths of the nanotube-water interaction. From explicit energy and
entropy calculations using the two-phase thermodynamics method, we have
presented a thermodynamic understanding of the filling behaviour of a nanotube.
We show that both the energy and the entropy of transfer decrease with
increasing temperature. On the other hand, scaling down the attractive part of
the carbon-oxygen interaction results in increased energy of transfer while the
entropy of transfer increases slowly with decreasing the interaction strength.
Our results indicate that both energy and entropy favour water entry into (6,6)
SWCNTs. Our results are compared with those of several recent studies of water
entry into carbon nanotubes.Comment: 18 pages, 5 figures, Molecular Simulation, 201
Sub-diffusion and population dynamics of water confined in soft environments
We have studied by Molecular Dynamics computer simulations the dynamics of
water confined in ionic surfactants phases, ranging from well ordered lamellar
structures to micelles at low and high water loading, respectively. We have
analysed in depth the main dynamical features in terms of mean squared
displacements and intermediate scattering functions, and found clear evidences
of sub-diffusive behaviour. We have identified water molecules lying at the
charged interface with the hydrophobic confining matrix as the main responsible
for this unusual feature, and provided a comprehensive picture for dynamics
based on a very precise analysis of life times at the interface. We conclude by
providing, for the first time to our knowledge, a unique framework for
rationalising the existence of important dynamical heterogeneities in fluids
absorbed in soft confining environments
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