23,191 research outputs found
Ultra-high permeable phenine nanotube membranes for water desalination
Nanopore desalination technology hinges on high water-permeable membranes
which, at the same time, block ions efficiently. In this study, we consider a
recently synthesized [Science 363, 151-155 (2019)] phenine nanotube (PNT) for
water desalination applications. Using both equilibrium and non-equilibrium
molecular dynamics simulations, we show that the PNT membrane completely
rejects salts, but permeates water at a rate which is an order-of-magnitude
higher than that of all the membranes used for water filtration. We provide the
microscopic mechanisms of salt rejection and fast water-transport by
calculating the free-energy landscapes and electrostatic potential profiles. A
collective diffusion model accurately predicts the water permeability obtained
from the simulations over a wide range of pressure gradients. We propose a
method to calculate the osmotic water permeability from the equilibrium
simulation data and find that it is very high for the PNT membrane. These
remarkable properties of PNT can be applied in various nanofluidic
applications, such as ion-selective channels, ionic transistors, sensing,
molecular sieving, and blue energy harvesting.Comment: 23 pages, 5 figure
Calculating Single-Channel Permeability and Conductance from Transition Paths
Permeability and conductance are the major transport properties of membrane channels, quantifying the rate of channel crossing by the solute. It is highly desirable to calculate these quantities in all-atom molecular dynamics simulations. When the solute crossing rate is low, however, direct methods would require prohibitively long simulations, and one thus typically adopts alternative strategies based on the free energy of single solute along the channel. Here we present a new method to calculate the crossing rate by initiating unbiased trajectories in which the solute is released at the free energy barrier. In this method, the total time the solute spends in the barrier region during a channel crossing (transition path) is used to determine the kinetic rate. Our method achieves a significantly higher statistical accuracy than the classical reactive flux method, especially for diffusive barrier crossing. Our test on ion permeation through a carbon nanotube verifies that the method correctly predicts the crossing rate and reproduces the spontaneous crossing events as in long equilibrium simulations. The rigorous and efficient method here will be valuable for quantitatively connecting simulations to experimental measurement of membrane channels
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
- …