6,629 research outputs found
Fluctuations of Complex Networks: Electrical Properties of Single Protein Nanodevices
We present for the first time a complex network approach to the study of the
electrical properties of single protein devices. In particular, we consider an
electronic nanobiosensor based on a G-protein coupled receptor. By adopting a
coarse grain description, the protein is modeled as a complex network of
elementary impedances. The positions of the alpha-carbon atoms of each amino
acid are taken as the nodes of the network. The amino acids are assumed to
interact electrically among them. Consequently, a link is drawn between any
pair of nodes neighboring in space within a given distance and an elementary
impedance is associated with each link. The value of this impedance can be
related to the physical and chemical properties of the amino acid pair and to
their relative distance. Accordingly, the conformational changes of the
receptor induced by the capture of the ligand, are translated into a variation
of its electrical properties. Stochastic fluctuations in the value of the
elementary impedances of the network, which mimic different physical effects,
have also been considered. Preliminary results concerning the impedance
spectrum of the network and its fluctuations are presented and discussed for
different values of the model parameters.Comment: 16 Pages and 10 Figures published in SPIE Proceedings of the II
International Symposium on Fluctuation and Noise, Maspalomas,Gran
Canaria,Spain, 25-28 May 200
Modelization of Thermal Fluctuations in G Protein-Coupled Receptors
We simulate the electrical properties of a device realized by a G protein
coupled receptor (GPCR), embedded in its membrane and in contact with two
metallic electrodes through which an external voltage is applied. To this
purpose, recently, we have proposed a model based on a coarse graining
description, which describes the protein as a network of elementary impedances.
The network is built from the knowledge of the positions of the C-alpha atoms
of the amino acids, which represent the nodes of the network. Since the
elementary impedances are taken depending of the inter-nodes distance, the
conformational change of the receptor induced by the capture of the ligand
results in a variation of the network impedance. On the other hand, the
fluctuations of the atomic positions due to thermal motion imply an impedance
noise, whose level is crucial to the purpose of an electrical detection of the
ligand capture by the GPCR. Here, in particular, we address this issue by
presenting a computational study of the impedance noise due to thermal
fluctuations of the atomic positions within a rhodopsin molecule. In our model,
the C-alpha atoms are treated as independent, isotropic, harmonic oscillators,
with amplitude depending on the temperature and on the position within the
protein (alpha-helix or loop). The relative fluctuation of the impedance is
then calculated for different temperatures.Comment: 5 pages, 2 figures, Proceeding of the 18-th International Conference
on Fluctuations and Noise, 19-23 September 2005, Salamanca, Spain -minor
proofreadings
Nonequilibrium nuclear-electron spin dynamics in semiconductor quantum dots
We study the spin dynamics in charged quantum dots in the situation where the
resident electron is coupled to only about 200 nuclear spins and where the
electron spin splitting induced by the Overhauser field does not exceed
markedly the spectral broadening. The formation of a dynamical nuclear
polarization as well as its subsequent decay by the dipole-dipole interaction
is directly resolved in time. Because not limited by intrinsic nonlinearities,
almost complete nuclear polarization is achieved, even at elevated
temperatures. The data suggest a nonequilibrium mode of nuclear polarization,
distinctly different from the spin temperature concept exploited on bulk
semiconductorsComment: 5 pages, 4 figure
Nominally forbidden transitions in the interband optical spectrum of quantum dots
We calculate the excitonic optical absorption spectra of (In,Ga)As/GaAs
self-assembled quantum dots by adopting an atomistic pseudopotential approach
to the single-particle problem followed by a configuration-interaction approach
to the many-body problem. We find three types of allowed transitions that would
be naively expected to be forbidden. (i) Transitions that are parity forbidden
in simple effective mass models with infinite confining wells (e.g. 1S-2S,
1P-2P) but are possible by finite band-offsets and orbital-mixing effects; (ii)
light-hole--to--conduction transitions, enabled by the confinement of
light-hole states; and (iii) transitions that show and enhanced intensity due
to electron-hole configuration mixing with allowed transitions. We compare
these predictions with results of 8-band k.p calculations as well as recent
spectroscopic data. Transitions in (i) and (ii) explain recently observed
satellites of the allowed P-P transitions.Comment: Version published in Phys. Rev.
Factorization and Scaling in Hadronic Diffraction
In standard Regge theory with a pomeron intercept a(0)=1+\epsilon, the
contribution of the tripe-pomeron amplitude to the t=0 differential cross
section for single diffraction dissociation has the form d\sigma/dM^2(t=0) \sim
s^{2\epsilon}/(M^2)^{1+\epsilon}. For \epsilon>0, this form, which is based on
factorization, does not scale with energy. From an analysis of p-p and p-pbar
data from fixed target to collider energies, we find that such scaling actually
holds, signaling a breakdown of factorization. Phenomenologically, this result
can be obtained from a scaling law in diffraction, which is embedded in the
hypothesis of pomeron flux renormalization introduced to unitarize the triple
pomeron amplitude.Comment: 39 pages, Latex, 16 figure
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