141 research outputs found
Charge transport in bacteriorhodopsin monolayers: The contribution of conformational change to current-voltage characteristics
When moving from native to light activated bacteriorhodopsin, modification of
charge transport consisting of an increase of conductance is correlated to the
protein conformational change. A theoretical model based on a map of the
protein tertiary structure into a resistor network is implemented to account
for a sequential tunneling mechanism of charge transfer through neighbouring
amino acids. The model is validated by comparison with current-voltage
experiments. The predictability of the model is further tested on bovine
rhodopsin, a G-protein coupled receptor (GPCR) also sensitive to light. In this
case, results show an opposite behaviour with a decrease of conductance in the
presence of light.Comment: 6 pages, 4 figure
Duality and reciprocity of fluctuation-dissipation relations in conductors
By analogy with linear-response we formulate the duality and reciprocity
properties of current and voltage fluctuations expressed by Nyquist relations
including the intrinsic bandwidths of the respective fluctuations. For this
purpose we individuate total-number and drift-velocity fluctuations of carriers
inside a conductor as the microscopic sources of noise. The spectral densities
at low frequency of the current and voltage fluctuations and the respective
conductance and resistance are related in a mutual exclusive way to the
corresponding noise-source. The macroscopic variance of current and voltage
fluctuations are found to display a dual property via a plasma conductance that
admits a reciprocal plasma resistance. Analogously, the microscopic
noise-sources are found to obey a dual property and a reciprocity relation. The
formulation is carried out in the frame of the grand canonical (for current
noise) and canonical (for voltage noise) ensembles and results are derived
which are valid for classical as well as for degenerate statistics including
fractional exclusion statistics. The unifying theory so developed sheds new
light on the microscopic interpretation of dissipation and fluctuation
phenomena in conductors. In particular it is proven that, as a consequence of
the Pauli principle, for Fermions non-vanishing single-carrier velocity
fluctuations at zero temperature are responsible for diffusion but not for
current noise, which vanishes in this limit.Comment: 5 pages, 1 figur
Trapping-detrapping fluctuations in organic space-charge layers
A trapping-detrapping model is proposed for explaining the current
fluctuation behavior in organic semiconductors (polyacenes) operating under
current-injection conditions. The fraction of ionized traps obtained from the
current-voltage characteristics, is related to the relative current noise
spectral density at the trap-filling transition. The agreement between theory
and experiments validates the model and provides an estimate of the
concentration and energy level of deep traps
Charge transport in purple membrane monolayers: A sequential tunneling approach
Current voltage (I-V) characteristics in proteins can be sensitive to
conformational change induced by an external stimulus (photon, odour, etc.).
This sensitivity can be used in medical and industrial applications besides
shedding new light in the microscopic structure of biological materials. Here,
we show that a sequential tunneling model of carrier transfer between
neighbouring amino-acids in a single protein can be the basic mechanism
responsible of the electrical properties measured in a wide range of applied
potentials. We also show that such a strict correlation between the protein
structure and the electrical response can lead to a new generation of
nanobiosensors that mimic the sensorial activity of living species. To
demonstrate the potential usefulness of protein electrical properties, we
provide a microscopic interpretation of recent I-V experiments carried out in
bacteriorhodopsin at a nanoscale length.Comment: 4 pages, 4 figure
Fluctuation dissipation theorem and electrical noise revisited
The fluctuation dissipation theorem (FDT) is the basis for a microscopic
description of the interaction between electromagnetic radiation and matter.By
assuming the electromagnetic radiation in thermal equilibrium and the
interaction in the linear response regime, the theorem interrelates the
spontaneous fluctuations of microscopic variables with the kinetic coefficients
that are responsible for energy dissipation.In the quantum form provided by
Callen and Welton in their pioneer paper of 1951 for the case of conductors,
electrical noise detected at the terminals of a conductor was given in terms of
the spectral density of voltage fluctuations, , and was related
to the real part of its impedance, , by a simple relation.The
drawbacks of this relation concern with: (I) the appearance of a zero point
contribution which implies a divergence of the spectrum at increasing
frequencies; (ii) the lack of detailing the appropriate equivalent-circuit of
the impedance, (iii) the neglect of the Casimir effect associated with the
quantum interaction between zero-point energy and boundaries of the considered
physical system; (iv) the lack of identification of the microscopic noise
sources beyond the temperature model. These drawbacks do not allow to validate
the relation with experiments. By revisiting the FDT within a brief historical
survey, we shed new light on the existing drawbacks by providing further
properties of the theorem, focusing on the electrical noise of a two-terminal
sample under equilibrium conditions. Accordingly, we will discuss the duality
and reciprocity properties of the theorem, its applications to the ballistic
transport regime, to the case of vacuum and to the case of a photon gas.Comment: 16 pages, 8 figure
Beyond the Formulations of the Fluctuation Dissipation Theorem Given by Callen and Welton (1951) and Expanded by Kubo (1966)
The quantum formula of the fluctuation dissipation theorem (FDT) was given by Callen and
Welton in 1951 [1] for the case of conductors, and then expanded by Kubo in 1966 [2, 3].
The drawback of these quantum relations concerns with the appearance of a zero-point
contribution, hω/2 with h the reduced Planck constant and ω the angular frequency of the
considered photon, which implies a divergence of the fluctuation spectrum at increasing
frequencies. This divergence is responsible for a vacuum-catastrophe, to keep the
analogy with the well-known ultraviolet catastrophe of the classical black-body radiation
spectrum. As a consequence, the quantum formulation of the FDT as given by CallenWelton and Kubo introduces a Field Grand Challenge associated with the existence or
less of a vacuum-fluctuations catastrophe for the energy-density spectrum. Here we
propose a solution to this challenge by taking into account of the Casimir energy that, in
turns, is found to be responsible for a quantum correction of the Stefan-Boltzmann la
Generalized Gumbel distribution of current fluctuations in purple membrane monolayers
We investigate the nature of a class of probability density functions, say
G(a), with a the shape parameter, which generalizes the Gumbel distribution.
These functions appear in a model of charge transport, when applied to a
metal-insulator-metal structure, where the insulator is constituted by a
monolayer of bacteriorhodopsin. Current shows a sharp increase above about 3 V,
interpreted as the cross-over between direct and injection sequential-tunneling
regimes.
In particular, we show that, changing the bias value, the probability density
function changes its look from bimodal to unimodal. Actually, the bimodal
distributions can be resolved in at least a couple of functions with
different values of the shape parameter.Comment: 5 pages, 6 figure
The fundamental unit of quantum conductance and quantum diffusion for a gas of massive particles
By analogy with the fundamental quantum units of electrical conductance
and thermal conductance we
define a fundamental quantum unit of conductance, , and diffusion of a
massive gas of atomic particles, respectively given by with the Planck constant, the Boltzmann
constant, the absolute temperature, the unit charge and the mass of
the atomic gas particle that move balistically in a one dimensional medium of
length . The effect of scattering can be accounted for by introducing an
appropriate transmission probability in analogy with the quantum electrical
conductance model introduced by Landauer in 1957. For an electron gas
and $D_0 = 7.3 \times 10^{-3} \
m^2/s
Human olfactory receptor 17-40 as active part of a nanobiosensor: A microscopic investigation of its electrical properties
Increasing attention has been recently devoted to protein-based
nanobiosensors. The main reason is the huge number of possible technological
applications, going from drug detection to cancer early diagnosis. Their
operating model is based on the protein activation and the corresponding
conformational change, due to the capture of an external molecule, the
so-called ligand. Recent measurements, performed with different techniques on
human 17-40 olfactory receptor, evidenced a very narrow window of response in
respect of the odour concentration. This is a crucial point for understanding
whether the use of this olfactory receptor as sensitive part of a nanobiosensor
is a good choice. In this paper we investigate the topological and electrical
properties of the human olfactory receptor 17-40 with the objective of
providing a microscopic interpretation of available experiments. To this
purpose, we model the protein by means of a graph able to capture the mean
features of the 3D backbone structure. The graph is then associated with an
equivalent impedance network, able to evaluate the impedance spectra of the
olfactory receptor, in its native and activated state. We assume a topological
origin of the different protein electrical responses to different ligand
concentrations: In this perspective all the experimental data are collected and
interpreted satisfactorily within a unified scheme, also useful for application
to other proteins.Comment: 6 pages, 6 figures, DOI:10.1039/c1ra0002
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