516 research outputs found
The membrane-embedded segment of cytochrome b5 as studied by cross-linking with photoactivatable phospholipids
Vesicles were prepared from a 9:1 (mole/mol) mixture of dipalmitoyl phosphatidylcholine and the radioactively labeled phospholipids, 1-palmitoyl-2-ω -(m-diazirinophenoxy)undecanoyl-sn-glycero-3-phosphocholine (PC-I) or 1-palmitoyl-2-ω -(2-diazo-3,3,3-trifluropropionyloxy)lauroyl-sn- glycero-3-phosphocholine (PC-II). Rabbit liver cytochrome b5 was inserted into these vesicles spontaneously and the resulting vesicles containing the cytochrome b5 in the transferable form were photolyzed. Cytochrome b5 containing covalently cross-linked phospholipids was isolated by Sephadex LH-60 column chromatography using ethanol/formic acid as the solvent. Of the total radioactivity, 4.6% (PC-I) or 11.3% (PC-II) was linked to the protein; of the former, up to 51% was base-labile, while in the latter, 22% was base-labile. The sites of cross-linking of PC-I to the protein were investigated by fragmentation with trypsin, Staphylococcus aureas V8 protease, CNBr, and o-iodosobenzoic acid followed by Sephadex LH-60 chromatography and Edman sequencing (solid phase) of the appropriate fragments. The distribution of cross-linking was broad (Ser-104 to Met-130), showing a bell-shaped pattern with a significant peak at Ser-118. The labeling pattern is consistent with the previously proposed loop-back model for the membranous segment in the transferable form of cytochrome b5
Solutions to the ultradiscrete Toda molecule equation expressed as minimum weight flows of planar graphs
We define a function by means of the minimum weight flow on a planar graph
and prove that this function solves the ultradiscrete Toda molecule equation,
its B\"acklund transformation and the two dimensional Toda molecule equation.
The method we employ in the proof can be considered as fundamental to the
integrability of ultradiscrete soliton equations.Comment: 14 pages, 10 figures Added citations in v
Resonance Patterns of an Antidot Cluster: From Classical to Quantum Ballistics
We explain the experimentally observed Aharonov-Bohm (AB) resonance patterns
of an antidot cluster by means of quantum and classical simulations and Feynman
path integral theory. We demonstrate that the observed behavior of the AB
period signals the crossover from a low B regime which can be understood in
terms of electrons following classical orbits to an inherently quantum high B
regime where this classical picture and semiclassical theories based on it do
not apply.Comment: 5 pages revtex + 2 postscript figure
Electron-beam propagation in a two-dimensional electron gas
A quantum mechanical model based on a Green's function approach has been used
to calculate the transmission probability of electrons traversing a
two-dimensional electron gas injected and detected via mode-selective quantum
point contacts. Two-dimensional scattering potentials, back-scattering, and
temperature effects were included in order to compare the calculated results
with experimentally observed interference patterns. The results yield detailed
information about the distribution, size, and the energetic height of the
scattering potentials.Comment: 7 pages, 6 figure
Magnetic Quantum Dot: A Magnetic Transmission Barrier and Resonator
We study the ballistic edge-channel transport in quantum wires with a
magnetic quantum dot, which is formed by two different magnetic fields B^* and
B_0 inside and outside the dot, respectively. We find that the electron states
located near the dot and the scattering of edge channels by the dot strongly
depend on whether B^* is parallel or antiparallel to B_0. For parallel fields,
two-terminal conductance as a function of channel energy is quantized except
for resonances, while, for antiparallel fields, it is not quantized and all
channels can be completely reflected in some energy ranges. All these features
are attributed to the characteristic magnetic confinements caused by nonuniform
fields.Comment: 4 pages, 4 figures, to be published in Physical Review Letter
Andreev Reflection in Strong Magnetic Fields
We have studied the interplay of Andreev reflection and cyclotron motion of
quasiparticles at a superconductor-normal-metal interface with a strong
magnetic field applied parallel to the interface. Bound states are formed due
to the confinement introduced both by the external magnetic field and the
superconducting gap. These bound states are a coherent superposition of
electron and hole edge excitations similar to those realized in finite
quantum-Hall samples. We find the energy spectrum for these Andreev edge states
and calculate transport properties.Comment: 5 pages, 3 figures, RevTex, revised to include more detailed
discussion of currents and transpor
Conductance of a Semiconductor-Superconductor junction in high magnetic field
Conductance of a 2DEG-Superconductor (S) device in a high magnetic field
is studied: is calculated. When the cyclotron diameter in 2DEG is
larger than the width of the 2DEG-S surface then becomes nonmonotonous
function due to the Aharonov--Bohm type interference of quasiparticles at the
surface. At certain parameters of the junction the conductance oscillates with
.Comment: 4 pages, 3 figure
Inter edge Tunneling in Quantum Hall Line Junctions
We propose a scenario to understand the puzzling features of the recent
experiment by Kang and coworkers on tunneling between laterally coupled quantum
Hall liquids by modeling the system as a pair of coupled chiral Luttinger
liquid with a point contact tunneling center. We show that for filling factors
the effects of the Coulomb interactions move the system deep into
strong tunneling regime, by reducing the magnitude of the Luttinger parameter
, leading to the appearance of a zero-bias differential conductance peak of
magnitude at zero temperature. The abrupt appearance of the zero
bias peak as the filling factor is increased past a value ,
and its gradual disappearance thereafter can be understood as a crossover
controlled by the main energy scales of this system: the bias voltage , the
crossover scale , and the temperature . The low height of the zero bias
peak observed in the experiment, and its broad finite width,
can be understood naturally within this picture. Also, the abrupt reappearance
of the zero-bias peak for can be explained as an effect caused
by spin reversed electrons, \textit{i. e.} if the 2DEG is assumed to have a
small polarization near . We also predict that as the temperature is
lowered should decrease, and the width of zero-bias peak should become
wider. This picture also predicts the existence of similar zero bias peak in
the spin tunneling conductance near for .Comment: 17 pages, 8 figure
Normalization of Voltage-Sensitive Dye Signal with Functional Activity Measures
In general, signal amplitude in optical imaging is normalized using the
well-established ΔF/F method, where functional activity is divided by
the total fluorescent light flux. This measure is used both directly, as a
measure of population activity, and indirectly, to quantify spatial and
spatiotemporal activity patterns. Despite its ubiquitous use, the stability and
accuracy of this measure has not been validated for voltage-sensitive dye
imaging of mammalian neocortex in vivo. In this report, we find
that this normalization can introduce dynamic biases. In particular, the
ΔF/F is influenced by dye staining quality, and the ratio is also
unstable over the course of experiments. As methods to record and analyze
optical imaging signals become more precise, such biases can have an
increasingly pernicious impact on the accuracy of findings, especially in the
comparison of cytoarchitechtonic areas, in area-of-activation measurements, and
in plasticity or developmental experiments. These dynamic biases of the
ΔF/F method may, to an extent, be mitigated by a novel method of
normalization, ΔF/ΔFepileptiform. This normalization
uses as a reference the measured activity of epileptiform spikes elicited by
global disinhibition with bicuculline methiodide. Since this normalization is
based on a functional measure, i.e. the signal amplitude of
“hypersynchronized” bursts of activity in the cortical
network, it is less influenced by staining of non-functional elements. We
demonstrate that such a functional measure can better represent the amplitude of
population mass action, and discuss alternative functional normalizations based
on the amplitude of synchronized spontaneous sleep-like activity. These findings
demonstrate that the traditional ΔF/F normalization of voltage-sensitive
dye signals can introduce pernicious inaccuracies in the quantification of
neural population activity. They further suggest that normalization-independent
metrics such as waveform propagation patterns, oscillations in single detectors,
and phase relationships between detector pairs may better capture the biological
information which is obtained by high-sensitivity imaging
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