11,438 research outputs found
On the problem of mass-dependence of the two-point function of the real scalar free massive field on the light cone
We investigate the generally assumed inconsistency in light cone quantum
field theory that the restriction of a massive, real, scalar, free field to the
nullplane is independent of mass \cite{LKS}, but the
restriction of the two-point function depends on it (see, e.g., \cite{NakYam77,
Yam97}). We resolve this inconsistency by showing that the two-point function
has no canonical restriction to in the sense of distribution theory.
Only the so-called tame restriction of the two-point function exists which we
have introduced in \cite{Ull04sub}. Furthermore, we show that this tame
restriction is indeed independent of mass. Hence the inconsistency appears only
by the erroneous assumption that the two-point function would have a
(canonical) restriction to .Comment: 10 pages, 2 figure
Granular gases under extreme driving
We study inelastic gases in two dimensions using event-driven molecular
dynamics simulations. Our focus is the nature of the stationary state attained
by rare injection of large amounts of energy to balance the dissipation due to
collisions. We find that under such extreme driving, with the injection rate
much smaller than the collision rate, the velocity distribution has a power-law
high energy tail. The numerically measured exponent characterizing this tail is
in excellent agreement with predictions of kinetic theory over a wide range of
system parameters. We conclude that driving by rare but powerful energy
injection leads to a well-mixed gas and constitutes an alternative mechanism
for agitating granular matter. In this distinct nonequilibrium steady-state,
energy cascades from large to small scales. Our simulations also show that when
the injection rate is comparable with the collision rate, the velocity
distribution has a stretched exponential tail.Comment: 6 pages, 7 figures; new version contains 2 new figures and text
describing cascade
A single-molecule method for measuring fluorophore labeling yields for the study of membrane protein oligomerization in membranes
Membrane proteins are often observed as higher-order oligomers, and in some cases in multiple stoichiometric forms, raising the question of whether dynamic oligomerization can be linked to modulation of function. To better understand this potential regulatory mechanism, there is an ongoing effort to quantify equilibrium reactions of membrane protein oligomerization directly in membranes. Single-molecule photobleaching analysis is particularly useful for this as it provides a binary readout of fluorophores attached to protein subunits at dilute conditions. However, any quantification of stoichiometry also critically requires knowing the probability that a subunit is fluorescently labeled. Since labeling uncertainty is often unavoidable, we developed an approach to estimate labeling yields using the photobleaching probability distribution of an intrinsic dimeric control. By iterative fitting of an experimental dimeric photobleaching probability distribution to an expected dimer model, we estimate the fluorophore labeling yields and find agreement with direct measurements of labeling of the purified protein by UV-VIS absorbance before reconstitution. Using this labeling prediction, similar estimation methods are applied to determine the dissociation constant of reactive CLC-ec1 dimerization constructs without prior knowledge of the fluorophore labeling yield. Finally, we estimate the operational range of subunit labeling yields that allows for discrimination of monomer and dimer populations across the reactive range of mole fraction densities. Thus, our study maps out a practical method for quantifying fluorophore labeling directly from single-molecule photobleaching data, improving the ability to quantify reactive membrane protein stoichiometry in membranes
The quadrupole moment of slowly rotating fluid balls
In this paper we use the second order formalism of Hartle to study slowly and
rigidly rotating stars with focus on the quadrupole moment of the object. The
second order field equations for the interior fluid are solved numerically for
different classes of possible equations of state and these solutions are then
matched to a vacuum solution that includes the general asymptotically flat
axisymmetric metric to second order, using the Darmois-Israel procedure. For
these solutions we find that the quadrupole moment differs from that of the
Kerr metric, as has also been found for some equations of state in other
studies. Further we consider the post-Minkowskian limit analytically. In the
paper we also illustrate how the relativistic multipole moments can be
calculated from a complex gravitational potential.Comment: 13 pages, 5 figure
Equatorial symmetry/antisymmetry of stationary axisymmetric electrovac spacetimes
Two theorems are proved concerning how stationary axisymmetric electrovac
spacetimes that are equatorially symmetric or equatorially antisymmetric can be
characterized correctly in terms of the Ernst potentials \E and or in
terms of axis-data.Comment: 8 page
Analytical approach to semiconductor Bloch equations
Although semiconductor Bloch equations have been widely used for decades to
address ultrafast optical phenomena in semiconductors, they have a few
important drawbacks: (i) Coulomb terms between free electron-hole pairs require
Hartree-Fock treatment which, in its usual form, preserves excitonic poles but
loses biexcitonic resonances. (ii) Solving the resulting coupled differential
equations imposes heavy numerics which completely hide the physics. This can be
completely avoided if, instead of free electron-hole pairs, we use correlated
pairs, i.e., excitons. Their interactions are easy to handle through the
recently constructed composite-exciton many-body theory, which allows us to
\emph{analytically} obtain the time evolution of the polarization induced by a
laser pulse. This polarization comes from Coulomb interactions between virtual
excitons, but also from Coulomb-free fermion exchanges, which are dominant at
large detuning
Gi- and Gs-coupled GPCRs show different modes of G-protein binding.
More than two decades ago, the activation mechanism for the membrane-bound photoreceptor and prototypical G protein-coupled receptor (GPCR) rhodopsin was uncovered. Upon light-induced changes in ligand-receptor interaction, movement of specific transmembrane helices within the receptor opens a crevice at the cytoplasmic surface, allowing for coupling of heterotrimeric guanine nucleotide-binding proteins (G proteins). The general features of this activation mechanism are conserved across the GPCR superfamily. Nevertheless, GPCRs have selectivity for distinct G-protein family members, but the mechanism of selectivity remains elusive. Structures of GPCRs in complex with the stimulatory G protein, Gs, and an accessory nanobody to stabilize the complex have been reported, providing information on the intermolecular interactions. However, to reveal the structural selectivity filters, it will be necessary to determine GPCR-G protein structures involving other G-protein subtypes. In addition, it is important to obtain structures in the absence of a nanobody that may influence the structure. Here, we present a model for a rhodopsin-G protein complex derived from intermolecular distance constraints between the activated receptor and the inhibitory G protein, Gi, using electron paramagnetic resonance spectroscopy and spin-labeling methodologies. Molecular dynamics simulations demonstrated the overall stability of the modeled complex. In the rhodopsin-Gi complex, Gi engages rhodopsin in a manner distinct from previous GPCR-Gs structures, providing insight into specificity determinants
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