1,429 research outputs found
Stochastic Dynamics of Electrical Membrane with Voltage-Dependent Ion Channel Fluctuations
Brownian ratchet like stochastic theory for the electrochemical membrane
system of Hodgkin-Huxley (HH) is developed. The system is characterized by a
continuous variable , representing mobile membrane charge density, and
a discrete variable representing ion channel conformational dynamics. A
Nernst-Planck-Nyquist-Johnson type equilibrium is obtained when multiple
conducting ions have a common reversal potential. Detailed balance yields a
previously unknown relation between the channel switching rates and membrane
capacitance, bypassing Eyring-type explicit treatment of gating charge
kinetics. From a molecular structural standpoint, membrane charge is a
more natural dynamic variable than potential ; our formalism treats
-dependent conformational transition rates as intrinsic
parameters. Therefore in principle, vs. is experimental
protocol dependent,e.g., different from voltage or charge clamping
measurements. For constant membrane capacitance per unit area and
neglecting membrane potential induced by gating charges, , and
HH's formalism is recovered. The presence of two types of ions, with different
channels and reversal potentials, gives rise to a nonequilibrium steady state
with positive entropy production . For rapidly fluctuating channels, an
expression for is obtained.Comment: 8 pages, two figure
From Structure to Function in Open Ionic Channels
We consider a simple working hypothesis that all permeation properties of
open ionic channels can be predicted by understanding electrodiffusion in fixed
structures, without invoking conformation changes, or changes in chemical
bonds. We know, of course, that ions can bind to specific protein structures,
and that this binding is not easily described by the traditional electrostatic
equations of physics textbooks, that describe average electric fields, the
so-called `mean field'. The question is which specific properties can be
explained just by mean field electrostatics and which cannot. I believe the
best way to uncover the specific chemical properties of channels is to invoke
them as little as possible, seeking to explain with mean field electrostatics
first. Then, when phenomena appear that cannot be described that way, by the
mean field alone, we turn to chemically specific explanations, seeking the
appropriate tools (of electrochemistry, Langevin, or molecular dynamics, for
example) to understand them. In this spirit, we turn now to the structure of
open ionic channels, apply the laws of electrodiffusion to them, and see how
many of their properties we can predict just that way.Comment: Nearly final version of publicatio
Kinetics and thermodynamics across single-file pores: solute permeability and rectified osmosis
We study the effects of solute interactions on osmotic transport through
pores. By extending single-file, single-species kinetic models to include
entrance of solute into membrane pores, we model the statistical mechanics of
competitive transport of two species across membrane pores. The results have
direct applications to water transport across biomembrane pores and particle
movement in zeolites, and can be extended to study ion channel transport.
Reflection coefficients, the reduction of osmotic fluxes measured using
different solutes, are computed in terms of the microscopic kinetic parameters.
We find that a reduction in solvent flow due to solute-pore interactions can be
modelled by a Langmuir adsorption isotherm. Osmosis experiments are discussed
and proposed. Special cases and Onsager relations are presented in the
Appendices.Comment: 15pp, 9 .eps figures. Accepted to J. Chem. Phys. 199
Ions in Fluctuating Channels: Transistors Alive
Ion channels are proteins with a hole down the middle embedded in cell
membranes. Membranes form insulating structures and the channels through them
allow and control the movement of charged particles, spherical ions, mostly
Na+, K+, Ca++, and Cl-. Membranes contain hundreds or thousands of types of
channels, fluctuating between open conducting, and closed insulating states.
Channels control an enormous range of biological function by opening and
closing in response to specific stimuli using mechanisms that are not yet
understood in physical language. Open channels conduct current of charged
particles following laws of Brownian movement of charged spheres rather like
the laws of electrodiffusion of quasi-particles in semiconductors. Open
channels select between similar ions using a combination of electrostatic and
'crowded charge' (Lennard-Jones) forces. The specific location of atoms and the
exact atomic structure of the channel protein seems much less important than
certain properties of the structure, namely the volume accessible to ions and
the effective density of fixed and polarization charge. There is no sign of
other chemical effects like delocalization of electron orbitals between ions
and the channel protein. Channels play a role in biology as important as
transistors in computers, and they use rather similar physics to perform part
of that role. Understanding their fluctuations awaits physical insight into the
source of the variance and mathematical analysis of the coupling of the
fluctuations to the other components and forces of the system.Comment: Revised version of earlier submission, as invited, refereed, and
published by journa
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