494 research outputs found
Cloning and characterization of a gene encoding the major surface protein of the bacterial endosymbiont Wolbachia pipientis
The maternally inherited intracellular symbiont Wolbachia pipientis is well known for inducing a variety of reproductive abnormalities in the diverse arthropod hosts it infects. It has been implicated in causing cytoplasmic incompatibility, parthenogenesis, and the feminization of genetic males in different hosts. The molecular mechanisms by which this fastidious intracellular bacterium causes these reproductive and developmental abnormalities have not yet been determined. In this paper, we report on (i) the purification of one of the most abundantly expressed Wolbachia proteins from infected Drosophila eggs and (ii) the subsequent cloning and characterization of the gene (wsp) that encodes it. The functionality of the wsp promoter region was also successfully tested in Escherichia coli. Comparison of sequences of this gene from different strains of Wolbachia revealed a high level of variability. This sequence variation correlated with the ability of certain Wolbachia strains to induce or rescue the cytoplasmic incompatibility phenotype in infected insects. As such, this gene will be a very useful tool for Wolbachia strain typing and phylogenetic analysis, as well as understanding the molecular basis of the interaction of Wolbachia with its host
FIBâSEM and ToFâSIMS Analysis of HighâTemperature PEM Fuel Cell Electrodes
The phosphoric acid (PA) distribution in the electrodes is a crucial factor for the performance of high-temperature polymer electrolyte fuel cells (HT-PEM FCs). Therefore, understanding and optimizing the electrolyte distribution is vital to maximizing power output and achieving low degradation. Although challenging, tracking the PA in nanometer-sized pores is essential because most active sites in the commonly used carbon black-supported catalysts are located in pores below 1 ”m. For this study, a cell is operated at 200 mA cmâ2 for 5 days. After this break-in period, the cathode is separated from the membrane electrode assembly and subsequently investigated by cryogenic focused ion beam scanning electron microscopy (cryo FIB-SEM) coupled with energy-dispersive X-ray spectroscopy (EDX) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). PA is located on the surface and in the bulk of the cathode catalyst layer. In addition, the PA distribution can be successfully linked to the gas diffusion electrode morphology and the binder distribution. The PA preferably invades nanometer-sized pores and is uniformly distributed in the catalyst layer
Dynamics of allosteric transitions in GroEL
The chaperonin GroEL-GroES, a machine which helps some proteins to fold,
cycles through a number of allosteric states, the state, with high affinity
for substrate proteins (SPs), the ATP-bound state, and the
() complex. Structures are known for each
of these states. Here, we use a self-organized polymer (SOP) model for the
GroEL allosteric states and a general structure-based technique to simulate the
dynamics of allosteric transitions in two subunits of GroEL and the heptamer.
The transition, in which the apical domains undergo counter-clockwise
motion, is mediated by a multiple salt-bridge switch mechanism, in which a
series of salt-bridges break and form. The initial event in the transition, during which GroEL rotates clockwise, involves a
spectacular outside-in movement of helices K and L that results in K80-D359
salt-bridge formation. In both the transitions there is considerable
heterogeneity in the transition pathways. The transition state ensembles (TSEs)
connecting the , , and states are broad with the the
TSE for the transition being more plastic than the TSE. The results suggest that GroEL functions as a
force-transmitting device in which forces of about (5-30) pN may act on the SP
during the reaction cycle.Comment: 32 pages, 10 figures (Longer version than the one published
Rate equations for Coulomb blockade with ferromagnetic leads
We present a density-matrix rate-equation approach to sequential tunneling
through a metal particle weakly coupled to ferromagnetic leads. The
density-matrix description is able to deal with correlations between degenerate
many-electron states that the standard rate equation formalism in terms of
occupation probabilities cannot describe. Our formalism is valid for an
arbitrary number of electrons on the dot, for an arbitrary angle between the
polarization directions of the leads, and with or without spin-orbit scattering
on the metal particle. Interestingly, we find that the density-matrix
description may be necessary even for metal particles with unpolarized leads if
three or more single-electron levels contribute to the transport current and
electron-electron interactions in the metal particle are described by the
`universal interaction Hamiltonian'.Comment: 10 pages, 4 figures, REVTeX
Efficient data processing and quantum phenomena: Single-particle systems
We study the relation between the acquisition and analysis of data and
quantum theory using a probabilistic and deterministic model for photon
polarizers. We introduce criteria for efficient processing of data and then use
these criteria to demonstrate that efficient processing of the data contained
in single events is equivalent to the observation that Malus' law holds. A
strictly deterministic process that also yields Malus' law is analyzed in
detail. We present a performance analysis of the probabilistic and
deterministic model of the photon polarizer. The latter is an adaptive
dynamical system that has primitive learning capabilities. This additional
feature has recently been shown to be sufficient to perform event-by-event
simulations of interference phenomena, without using concepts of wave
mechanics. We illustrate this by presenting results for a system of two chained
Mach-Zehnder interferometers, suggesting that systems that perform efficient
data processing and have learning capability are able to exhibit behavior that
is usually attributed to quantum systems only.Comment: http://www.compphys.net/dl
Tunneling broadening of vibrational sidebands in molecular transistors
Transport through molecular quantum dots coupled to a single vibration mode
is studied in the case with strong coupling to the leads. We use an expansion
in the correlation between electrons on the molecule and electrons in the leads
and show that the tunneling broadening is strongly suppressed by the
combination of the Pauli principle and the quantization of the oscillator. As a
consequence the first Frank-Condon step is sharper than the higher order ones,
and its width, when compared to the bare tunneling strength, is reduced by the
overlap between the groundstates of the displaced and the non-displaced
oscillator.Comment: 8 pages, 3 figures. PRB, in pres
Hard x ray spectroscopy and imaging by a reflection zone plate in the presence of astigmatism
The feasibility of an off axis x ray reflection zone plate to perform wavelength dispersive spectroscopy, on axis point focusing, and two dimensional imaging is demonstrated by means of one and the same diffractive optical element DOE at a synchrotron radiation facility. The resolving power varies between 30 and 400 in the range of 7.6 keV to 9.0 keV, with its maximum at the design energy of 8.3 keV. This result is verified using an adjustable entrance slit, by which horizontal H and vertical V focusing to 0.85 amp; 956;m H and 1.29 amp; 956;m V is obtained near the sagittal focal plane of the astigmatic configuration. An angular and axial scan proves an accessible field of view of at least 0.6 arcmin 0.8 arcmin and a focal depth of plus minus 0.86 mm. Supported by the grating efficiency of around 17.5 and a very short pulse elongation, future precision x ray fluorescence and absorption studies of transition metals at their K edge on an ultrashort timescale could benefit from our finding
Photon-Phonon-assisted tunneling through a single-molecular quantum dot
Based on exactly mapping of a many-body electron-phonon interaction problem
onto a one-body problem, we apply the well-established nonequilibrium Green
function technique to solve the time-dependent phonon-assisted tunneling at low
temperature through a single-molecular quantum dot connected to two leads,
which is subject to a microwave irradiation field. It is found that in the
presence of the electron-phonon interaction and the microwave irradiation
field, the time-average transmission and the nonlinear differential conductance
display additional peaks due to pure photon absorption or emission processes
and photon-absorption-assisted phonon emission processes. The variation of the
time-average current with frequency of the microwave irradiation field is also
studied.Comment: 9 pages, 6 figures, submitted to Phys. Rev. B. accepted by Phys. Rev.
Spin states of the first four holes in a silicon nanowire quantum dot
We report measurements on a silicon nanowire quantum dot with a clarity that
allows for a complete understanding of the spin states of the first four holes.
First, we show control of the hole number down to one. Detailed measurements at
perpendicular magnetic fields reveal the Zeeman splitting of a single hole in
silicon. We are able to determine the ground-state spin configuration for one
to four holes occupying the quantum dot and find a spin filling with
alternating spin-down and spin-up holes, which is confirmed by
magnetospectroscopy up to 9T. Additionally, a so far inexplicable feature in
single-charge quantum dots in many materials systems is analyzed in detail. We
observe excitations of the zero-hole ground-state energy of the quantum dot,
which cannot correspond to electronic or Zeeman states. We show that the most
likely explanation is acoustic phonon emission to a cavity between the two
contacts to the nanowire.Comment: 24 pages, 8 figures, both including supporting informatio
Conductance Peak Height Correlations for a Coulomb-Blockaded Quantum Dot in a Weak Magnetic Field
We consider statistical correlations between the heights of conductance peaks
corresponding to two different levels in a Coulomb-blockaded quantum dot.
Correlations exist for two peaks at the same magnetic field if the field does
not fully break time-reversal symmetry as well as for peaks at different values
of a magnetic field that fully breaks time-reversal symmetry. Our results are
also relevant to Coulomb-blockade conductance peak height statistics in the
presence of weak spin-orbit coupling in a chaotic quantum dot.Comment: 5 pages, 3 figures, REVTeX 4, accepted for publication in Phys. Rev.
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