4,784 research outputs found
Problems Affecting Labor
Much experimental work has been devoted in comparing the folding behavior of proteins sharing the same fold but different sequence. The recent design of proteins displaying very high sequence identities but different 3D structure allows the unique opportunity to address the protein-folding problem from a complementary perspective. Here we explored by â-value analysis the pathways of folding of three different heteromorphic pairs, displaying increasingly high-sequence identity (namely, 30%, 77%, and 88%), but different structures called G A (a 3-α helix fold) and G B (an α/ÎČ fold). The analysis, based on 132 site-directed mutants, is fully consistent with the idea that protein topology is committed very early along the pathway of folding. Furthermore, data reveals that when folding approaches a perfect two-state scenario, as in the case of the G A domains, the structural features of the transition state appear very robust to changes in sequence composition. On the other hand, when folding is more complex and multistate, as for the G Bs, there are alternative nuclei or accessible pathways that can be alternatively stabilized by altering the primary structure. The implications of our results in the light of previous work on the folding of different members belonging to the same protein family are discussed
Low-temperature nucleation in a kinetic Ising model under different stochastic dynamics with local energy barriers
Using both analytical and simulational methods, we study low-temperature
nucleation rates in kinetic Ising lattice-gas models that evolve under two
different Arrhenius dynamics that interpose between the Ising states a
transition state representing a local energy barrier. The two dynamics are the
transition-state approximation [T. Ala-Nissila, J. Kjoll, and S. C. Ying, Phys.
Rev. B 46, 846 (1992)] and the one-step dynamic [H. C. Kang and W. H. Weinberg,
J. Chem. Phys. 90, 2824 (1989)]. Even though they both obey detailed balance
and are here applied to a situation that does not conserve the order parameter,
we find significant differences between the nucleation rates observed with the
two dynamics, and between them and the standard Glauber dynamic [R. J. Glauber,
J. Math. Phys. 4, 294 (1963)], which does not contain transition states. Our
results show that great care must be exercised when devising kinetic Monte
Carlo transition rates for specific physical or chemical systems.Comment: 14 pages RevTex, 6 embedded figures. Minor revisions. J. Chem. Phys.,
in pres
The Single-Photon Router
We have embedded an artificial atom, a superconducting "transmon" qubit, in
an open transmission line and investigated the strong scattering of incident
microwave photons ( GHz). When an input coherent state, with an average
photon number is on resonance with the artificial atom, we observe
extinction of up to 90% in the forward propagating field. We use two-tone
spectroscopy to study scattering from excited states and we observe
electromagnetically induced transparency (EIT). We then use EIT to make a
single-photon router, where we can control to what output port an incoming
signal is delivered. The maximum on-off ratio is around 90% with a rise and
fall time on the order of nanoseconds, consistent with theoretical
expectations. The router can easily be extended to have multiple output ports
and it can be viewed as a rudimentary quantum node, an important step towards
building quantum information networks.Comment: 5 pages, 3 figure
Philosophy and updating of the asteroid photometric catalogue
The Asteroid Photometric Catalogue now contains photometric lightcurves for 584 asteroids. We discuss some of the guiding principles behind it. This concerns both observers who offer input to it and users of the product
Are pinholes the cause of excess current in superconducting tunnel junctions? A study of Andreev current in highly resistive junctions
In highly resistive superconducting tunnel junctions, excess subgap current
is usually observed and is often attributed to microscopic "pinholes" in the
tunnel barrier. We have studied the subgap current in
superconductor-insulator-superconductor (SIS) and
superconductor-insulator-normal-metal (SIN) junctions. In Al/AlOx/Al junctions,
we observed a decrease of 2 orders of magnitude in the current upon the
transition from the SIS to the SIN regime, where it then matched theory. In
Al/AlOx/Cu junctions, we also observed generic features of coherent diffusive
Andreev transport in a junction with a homogenous barrier. We use the
quasiclassical Keldysh-Green function theory to quantify single- and
two-particle tunneling and find good agreement over 2 orders of magnitude in
transparency. We argue that our observations rule out pinholes as the origin of
the excess current.Comment: 4 pages, 4 figure
Energy bursts in fiber bundle models of composite materials
As a model of composite materials, a bundle of many fibers with
stochastically distributed breaking thresholds for the individual fibers is
considered. The bundle is loaded until complete failure to capture the failure
scenario of composite materials under external load. The fibers are assumed to
share the load equally, and to obey Hookean elasticity right up to the breaking
point. We determine the distribution of bursts in which an amount of energy
is released. The energy distribution follows asymptotically a universal power
law , for any statistical distribution of fiber strengths. A similar
power law dependence is found in some experimental acoustic emission studies of
loaded composite materials.Comment: 5 pages, 4 fig
Evaluation of New Density Functional with Account of van der Waals Forces by Use of Experimental H2 Physisorption Data on Cu(111)
Detailed experimental data for physisorption potential-energy curves of H2 on
low-indexed faces of Cu challenge theory. Recently, density-functional theory
has been developed to also account for nonlocal correlation effects, including
van der Waals forces. We show that one functional, denoted vdW-DF2, gives a
potential-energy curve promisingly close to the experiment-derived
physisorptionenergy curve. The comparison also gives indications for further
improvements of the functionals
Giant Cross Kerr Effect for Propagating Microwaves Induced by an Artificial Atom
We have investigated the cross Kerr phase shift of propagating microwave
fields strongly coupled to an artificial atom. The artificial atom is a
superconducting transmon qubit in an open transmission line. We demonstrate
average phase shifts of 11 degrees per photon between two coherent microwave
fields both at the single-photon level. At high control power, we observe phase
shifts up to 30 degrees. Our results provide an important step towards quantum
gates with propagating photons in the microwave regime.Comment: 5 pages, 4 figure
Site determination and thermally assisted tunneling in homogenous nucleation
A combined low-temperature scanning tunneling microscopy and density
functional theory study on the binding and diffusion of copper monomers,
dimers, and trimers adsorbed on Cu(111) is presented. Whereas atoms in trimers
are found in fcc sites only, monomers as well as atoms in dimers can occupy the
stable fcc as well as the metastable hcp site. In fact the dimer fcc-hcp
configuration was found to be only 1.3 meV less favorable with respect to the
fcc-fcc configuration. This enables a confined intra-cell dimer motion, which
at temperatures below 5 K is dominated by thermally assisted tunneling.Comment: 4 pages, 4 figure
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