4,178 research outputs found
Export of honeybee prepromelittin in Escherichia coli depends on the membrane potential but does not depend on proteins secA and secY
Honeybee prepromelittin (70 amino acid residues), the precursor of an eukaryotic secretory protein, and a hybrid protein between prepromelittin and mouse dihydrofolate reductase (257 amino acid residues) were expressed in Escherichia coli and characterized with respect to their requirements for transport across the plasma membrane. Both precursor proteins are posttranslationally processed and exported into the periplasm, and they both depend on the membrane potential for this to occur. With respect to dependence on components of the export machinery, however, the two precursor proteins show striking differences: the small precursor protein prepromelittin does not require the function of proteins secA and secY; the large precursor protein prepromelittin-dihydrofolate reductase, on the other hand, depends on both components. The implications of these observations with respect to the mechanisms of protein export in E. coli and of protein import into the endoplasmic reticulum are discussed
State Dependence and Alternative Explanations for Consumer Inertia
For many consumer packaged goods products, researchers have documented a form of state dependence whereby consumers become "loyal" to products they have consumed in the past. That is, consumers behave as though there is a utility premium from continuing to purchase the same product as they have purchased in the past or, equivalently, there is a psychological cost to switching products. However, it has not been established that this form of state dependence can be identified in the presence of consumer heterogeneity of an unknown form. Most importantly, before this inertia can be given a structural interpretation and used in policy experiments such as counterfactual pricing exercises,alternative explanations which might give rise to similar consumer behavior must be ruled out. We develop a flexible model of heterogeneity which can be given a semi-parametric interpretation and rule out alternative explanations for positive state dependence such as autocorrelated choice errors, consumer search, or consumer learning.
Practical scheme for a light-induced gauge field in an atomic Bose gas
We propose a scheme to generate an Abelian gauge field in an atomic gas using
two crossed laser beams. If the internal atomic state follows adiabatically the
eigenstates of the atom-laser interaction, Berry's phase gives rise to a vector
potential that can nucleate vortices in a Bose gas. The present scheme operates
even for a large detuning with respect to the atomic resonance, making it
applicable to alkali-metal atoms without significant heating due to spontaneous
emission. We test the validity of the adiabatic approximation by integrating
the set of coupled Gross-Pitaevskii equations associated with the various
internal atomic states, and we show that the steady state of the interacting
gas indeed exhibits a vortex lattice, as expected from the adiabatic gauge
field.Comment: 4 pages, 3 figure
Ultra-High Energy Cosmic Rays and the GeV-TeV Diffuse Gamma-Ray Flux
Ultra-high energy cosmic ray protons accelerated in astrophysical objects
produce secondary electromagnetic cascades during propagation in the cosmic
microwave and infrared backgrounds. We show that such cascades can contribute
between ~1% and ~50% of the GeV-TeV diffuse photon flux measured by the EGRET
experiment. The GLAST satellite should have a good chance to discover this
flux.Comment: 4 pages, 5 figure
Variational methods with coupled Gaussian functions for Bose-Einstein condensates with long-range interactions. II. Applications
Bose-Einstein condensates with an attractive 1/r interaction and with
dipole-dipole interaction are investigated in the framework of the Gaussian
variational ansatz introduced by S. Rau, J. Main, and G. Wunner [Phys. Rev. A,
submitted]. We demonstrate that the method of coupled Gaussian wave packets is
a full-fledged alternative to direct numerical solutions of the
Gross-Pitaevskii equation, or even superior in that coupled Gaussians are
capable of producing both, stable and unstable states of the Gross-Pitaevskii
equation, and thus of giving access to yet unexplored regions of the space of
solutions of the Gross-Pitaevskii equation. As an alternative to numerical
solutions of the Bogoliubov-de Gennes equations, the stability of the
stationary condensate wave functions is investigated by analyzing the stability
properties of the dynamical equations of motion for the Gaussian variational
parameters in the local vicinity of the stationary fixed points. For
blood-cell-shaped dipolar condensates it is shown that on the route to collapse
the condensate passes through a pitchfork bifurcation, where the ground state
itself turns unstable, before it finally vanishes in a tangent bifurcation.Comment: 14 pages, 14 figures, submitted to Phys. Rev. A, some equations
correcte
Magnetic flux pumping in 3D nonlinear magnetohydrodynamic simulations
A self-regulating magnetic flux pumping mechanism in tokamaks that maintains
the core safety factor at , thus preventing sawteeth, is analyzed
in nonlinear 3D magnetohydrodynamic simulations using the M3D-C code. In
these simulations, the most important mechanism responsible for the flux
pumping is that a saturated quasi-interchange instability generates
an effective negative loop voltage in the plasma center via a dynamo effect. It
is shown that sawtoothing is prevented in the simulations if is
sufficiently high to provide the necessary drive for the
instability that generates the dynamo loop voltage. The necessary amount of
dynamo loop voltage is determined by the tendency of the current density
profile to centrally peak which, in our simulations, is controlled by the
peakedness of the applied heat source profile.Comment: submitted to Physics of Plasmas (23 pages, 15 Figures
Lorentz Violation for Photons and Ultra-High Energy Cosmic Rays
Lorentz symmetry breaking at very high energies may lead to photon dispersion
relations of the form omega^2=k^2+xi_n k^2(k/M_Pl)^n with new terms suppressed
by a power n of the Planck mass M_Pl. We show that first and second order terms
of size xi_1 > 10^(-14) and xi_2 < -10^(-6), respectively, would lead to a
photon component in cosmic rays above 10^(19) eV that should already have been
detected, if corresponding terms for electrons and positrons are significantly
smaller. This suggests that Lorentz invariance breakings suppressed up to
second order in the Planck scale are unlikely to be phenomenologically viable
for photons.Comment: 4 revtex pages, 3 postscript figures included, version published in
PR
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