4,576 research outputs found
Protein connectivity in chemotaxis receptor complexes
The chemotaxis sensory system allows bacteria such as Escherichia coli to swim towards nutrients and away from repellents. The underlying pathway is remarkably sensitive in detecting chemical gradients over a wide range of ambient concentrations. Interactions among receptors, which are predominantly clustered at the cell poles, are crucial to this sensitivity. Although it has been suggested that the kinase CheA and the adapter protein CheW are integral for receptor connectivity, the exact coupling mechanism remains unclear. Here, we present a statistical-mechanics approach to model the receptor linkage mechanism itself, building on nanodisc and electron cryotomography experiments. Specifically, we investigate how the sensing behavior of mixed receptor clusters is affected by variations in the expression levels of CheA and CheW at a constant receptor density in the membrane. Our model compares favorably with dose-response curves from in vivo Förster resonance energy transfer (FRET) measurements, demonstrating that the receptor-methylation level has only minor effects on receptor cooperativity. Importantly, our model provides an explanation for the non-intuitive conclusion that the receptor cooperativity decreases with increasing levels of CheA, a core signaling protein associated with the receptors, whereas the receptor cooperativity increases with increasing levels of CheW, a key adapter protein. Finally, we propose an evolutionary advantage as explanation for the recently suggested CheW-only linker structures
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Integrating Green Infrastructure Into Stormwater Policy: Reliability, Watershed Management, and Environmental Psychology as Holistic Tools for Success
As cities continue to expand, the issues of flood control and urban water quality have become major modern sustainability challenges. Green infrastructure—the use of nature-based solutions to target, treat, and store stormwater at its source—has emerged as a possible solution. While green infrastructure does offer multiple benefits for urban users, its performance is also highly variable. This Article addresses a key gap in existing literature by explicitly addressing how uncertainty in environmental and anthropogenic factors affects green infrastructure performance and integration within the Clean Water Act’s municipal separate storm sewer (MS4) regulatory program
Dileptons in a coarse-grained transport approach
We calculate dilepton spectra in heavy-ion collisions using a coarse-graining
approach to the simulation of the created medium with the UrQMD transport
model. This enables the use of dilepton-production rates evaluated in
equilibrium quantum-field theory at finite temperatures and chemical
potentials.Comment: 4 pages, 2 figures, contribution to the proceedings of "The 15th
International Conference on Strangeness in Quark Matter" (SQM 2015), 06-11
July in Dubna, Russi
2000-times repeated imaging of strontium atoms in clock-magic tweezer arrays
We demonstrate single-atom resolved imaging with a survival probability of
and a fidelity of , enabling us to perform repeated
high-fidelity imaging of single atoms in tweezers for thousands of times. We
further observe lifetimes under laser cooling of more than seven minutes, an
order of magnitude longer than in previous tweezer studies. Experiments are
performed with strontium atoms in tweezer arrays, which is at
a magic wavelength for the clock transition. Tuning to this wavelength is
enabled by off-magic Sisyphus cooling on the intercombination line, which lets
us choose the tweezer wavelength almost arbitrarily. We find that a single not
retro-reflected cooling beam in the radial direction is sufficient for
mitigating recoil heating during imaging. Moreover, this cooling technique
yields temperatures below K, as measured by release and recapture.
Finally, we demonstrate clock-state resolved detection with average survival
probability of and average state detection fidelity of .
Our work paves the way for atom-by-atom assembly of large defect-free arrays of
alkaline-earth atoms, in which repeated interrogation of the clock transition
is an imminent possibility.Comment: 6 pages, 5 figures, 1 vide
A new proof of the Vorono\"i summation formula
We present a short alternative proof of the Vorono\"i summation formula which
plays an important role in Dirichlet's divisor problem and has recently found
an application in physics as a trace formula for a Schr\"odinger operator on a
non-compact quantum graph \mathfrak{G} [S. Egger n\'e Endres and F. Steiner, J.
Phys. A: Math. Theor. 44 (2011) 185202 (44pp)]. As a byproduct we give a new
proof of a non-trivial identity for a particular Lambert series which involves
the divisor function d(n) and is identical with the trace of the Euclidean wave
group of the Laplacian on the infinite graph \mathfrak{G}.Comment: Enlarged version of the published article J. Phys. A: Math. Theor. 44
(2011) 225302 (11pp
Mediation of Long Range Charge Transfer by Kondo Bound States
We present a theory of non-equilibrium long range charge transfer between
donor and acceptor centers in a model polymer mediated by magnetic exciton
(Kondo) bound states. Our model produces electron tunneling lengths easily
exceeding 10, as observed recently in DNA and organic charge transfer
systems. This long ranged tunneling is effective for weak to intermediate
donor-bridge coupling, and is enhanced both by weak to intermediate strength
Coulomb hole-electron attraction (through the orthogonality catastrophe) and by
coupling to local vibrational modes.Comment: Revised content (broadened scope, vibrations added), submitted to
Phys Rev Lett, added autho
Alkaline earth atoms in optical tweezers
We demonstrate single-shot imaging and narrow-line cooling of individual
alkaline earth atoms in optical tweezers; specifically, strontium-88 atoms
trapped in light. We achieve high-fidelity
single-atom-resolved imaging by detecting photons from the broad singlet
transition while cooling on the narrow intercombination line, and extend this
technique to highly uniform two-dimensional arrays of tweezers. Cooling
during imaging is based on a previously unobserved narrow-line Sisyphus
mechanism, which we predict to be applicable in a wide variety of experimental
situations. Further, we demonstrate optically resolved sideband cooling of a
single atom close to the motional ground state of a tweezer. Precise
determination of losses during imaging indicate that the branching ratio from
P to D is more than a factor of two larger than commonly
quoted, a discrepancy also predicted by our ab initio calculations. We also
measure the differential polarizability of the intercombination line in a
tweezer and achieve a magic-trapping configuration by tuning
the tweezer polarization from linear to elliptical. We present calculations, in
agreement with our results, which predict a magic crossing for linear
polarization at and a crossing independent of polarization
at 500.65(50)nm. Our results pave the way for a wide range of novel
experimental avenues based on individually controlled alkaline earth atoms in
tweezers -- from fundamental experiments in atomic physics to quantum
computing, simulation, and metrology implementations
Experimental constraints on the -ray strength function in Zr using partial cross sections of the Y(p,)Zr reaction
Partial cross sections of the Y(p,)Zr reaction have
been measured to investigate the -ray strength function in the
neutron-magic nucleus Zr. For five proton energies between
MeV and MeV, partial cross sections for the population of seven
discrete states in Zr have been determined by means of in-beam
-ray spectroscopy. Since these -ray transitions are dominantly
of character, the present measurement allows an access to the low-lying
dipole strength in Zr. A -ray strength function based on the
experimental data could be extracted, which is used to describe the total and
partial cross sections of this reaction by Hauser-Feshbach calculations
successfully. Significant differences with respect to previously measured
strength functions from photoabsorption data point towards deviations from the
Brink-Axel hypothesis relating the photo-excitation and de-excitation strength
functions.Comment: 5 pages, 5 figure
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