3,991 research outputs found
Controlled DNA compaction within chromatin: the tail-bridging effect
We study the mechanism underlying the attraction between nucleosomes, the
fundamental packaging units of DNA inside the chromatin complex. We introduce a
simple model of the nucleosome, the eight-tail colloid, consisting of a charged
sphere with eight oppositely charged, flexible, grafted chains that represent
the terminal histone tails. We demonstrate that our complexes are attracted via
the formation of chain bridges and that this attraction can be tuned by
changing the fraction of charged monomers on the tails. This suggests a
physical mechanism of chromatin compaction where the degree of DNA condensation
can be controlled via biochemical means, namely the acetylation and
deacetylation of lysines in the histone tails.Comment: 4 pages, 5 figures, submitte
Temperature and orientation dependence of kinetic roughening during homoepitaxy: A quantitative x-ray-scattering study of Ag
URL:http://link.aps.org/doi/10.1103/PhysRevB.54.17938
DOI:10.1103/PhysRevB.54.17938Kinetic roughening during homoepitaxial growth was studied for Ag(111) and Ag(001). For Ag(111), from 150 to 500 K, the rms roughness exhibits a power law, σ∝tβ over nearly three decades in thickness. β≈1/2 at low temperatures, and there is an abrupt transition to smaller values above 300 K. In contrast, Ag(001) exhibits layer-by-layer growth with a significantly smaller β. These results are the first to establish the evolution of surface roughness quantitatively for a broad thickness and temperature range, as well as for the case where growth kinetics are dominated by a step-ledge diffusion barrier.Support is acknowledged from the University of Missouri Research Board, the NSF under Contract Nos. DMR-9202528 and DMR-9623827, and the Midwest Superconductivity Consortium ~MISCON! under DOE Grant No. DE-FG02-90ER45427. The SUNY X3 beamline is supported by the DOE under Contract No. DE-FG02-86ER45231, and the NSLS is supported by the DOE, Div. of Materials Sciences and Div. of Chemical Sciences. One of us
~W.C.E.! acknowledges support from the GAANN program of the U.S. Department of Education. We thank Ian Robinson for the Ag~111! crystal
Path integral Monte Carlo simulations of silicates
We investigate the thermal expansion of crystalline SiO in the --
cristobalite and the -quartz structure with path integral Monte Carlo
(PIMC) techniques. This simulation method allows to treat low-temperature
quantum effects properly. At temperatures below the Debye temperature, thermal
properties obtained with PIMC agree better with experimental results than those
obtained with classical Monte Carlo methods.Comment: 27 pages, 10 figures, Phys. Rev. B (in press
Rate- and State-Dependent Friction Law and Statistical Properties of Earthquakes
In order to clarify how the statistical properties of earthquakes depend on
the constitutive law characterizing the stick-slip dynamics, we make an
extensive numerical simulation of the one-dimensional spring-block model with
the rate- and state-dependent friction law. Both the magnitude distribution and
the recurrence-time distribution are studied with varying the constitutive
parameters characterizing the model. While a continuous spectrum of seismic
events from smaller to larger magnitudes is obtained, earthquakes described by
this model turn out to possess pronounced ``characteristic'' features.Comment: Minor revisions are made in the text and in the figures. Accepted for
publication in Europhys. Letter
Environmental barriers to communication for individuals with dysarthria
Poster Symposium: Disorders TrackpostprintThe 2010 Motor Speech Conference, Savannah, GA., 3-7 March 2010
Mitochondrial peroxiredoxin functions as crucial chaperone reservoir in Leishmania infantum
Cytosolic eukaryotic 2-Cys-peroxiredoxins have been widely reported to act as dual-function proteins, either detoxifying reactive oxygen species or acting as chaperones to prevent protein aggregation. Several stimuli, including peroxide-mediated sulfinic acid formation at the active site cysteine, have been proposed to trigger the chaperone activity. However, the mechanism underlying this activation and the extent to which the chaperone function is crucial under physiological conditions in vivo remained unknown. Here we demonstrate that in the vector-borne protozoan parasite Leishmania infantum, mitochondrial peroxiredoxin (Prx) exerts intrinsic ATP-independent chaperone activity, protecting a wide variety of different proteins against heat stress-mediated unfolding in vitro and in vivo. Activation of the chaperone function appears to be induced by temperature-mediated restructuring of the reduced decamers, promoting binding of unfolding client proteins in the center of Prx's ringlike structure. Client proteins are maintained in a folding-competent conformation until restoration of nonstress conditions, upon which they are released and transferred to ATP-dependent chaperones for refolding. Interference with client binding impairs parasite infectivity, providing compelling evidence for the in vivo importance of Prx's chaperone function. Our results suggest that reduced Prx provides a mitochondrial chaperone reservoir, which allows L. infantum to deal successfully with protein unfolding conditions during the transition from insect to the mammalian hosts and to generate viable parasites capable of perpetuating infection.We thank Frederico Silva for help with size-exclusion chromatography experiments, and Ana G. Gomes-Alves and Ricardo Silva for constructing the pSSU-PHLEO-infantum-MTS.His.THR-mTXNPx plasmid. This work was supported by National Institutes of Health Grant GM065318 (to U.J.) and Project "NORTE-07-0124-FEDER-000002-Host-Pathogen Interactions" cofunded by Programa Operacional Regional do Norte under the Quadro de Referencia Estrategico Nacional, through Fundo Europeu de Desenvolvimento Regional, and by the Portuguese Foundation for Science and Technology (FCT) (A.M.T.). F.T. and H.C. were supported by Portuguese FCT Fellowships SFRH/BD/70438/2010 and SFRH/BPD/80836/2011, respectively
Variational method for learning Quantum Channels via Stinespring Dilation on neutral atom systems
The state of a closed quantum system evolves under the
Schr\"{o}dinger equation, where the reversible evolution of the state is
described by the action of a unitary operator on the initial state
, i.e.\ . However,
realistic quantum systems interact with their environment, resulting in
non-reversible evolutions, described by Lindblad equations. The solution of
these equations give rise to quantum channels that describe the
evolution of density matrices according to , which
often results in decoherence and dephasing of the state. For many quantum
experiments, the time until which measurements can be done might be limited,
e.g. by experimental instability or technological constraints. However, further
evolution of the state may be of interest. For instance, to determine the
source of the decoherence and dephasing, or to identify the steady state of the
evolution. In this work, we introduce a method to approximate a given target
quantum channel by means of variationally approximating equivalent unitaries on
an extended system, invoking the Stinespring dilation theorem. We report on an
experimentally feasible method to extrapolate the quantum channel on discrete
time steps using only data on the first time steps. Our approach heavily relies
on the ability to spatially transport entangled qubits, which is unique to the
neutral atom quantum computing architecture. Furthermore, the method shows
promising predictive power for various non-trivial quantum channels. Lastly, a
quantitative analysis is performed between gate-based and pulse-based
variational quantum algorithms.Comment: 11 pages, 7 figure
The electronic structure of amorphous silica: A numerical study
We present a computational study of the electronic properties of amorphous
SiO2. The ionic configurations used are the ones generated by an earlier
molecular dynamics simulations in which the system was cooled with different
cooling rates from the liquid state to a glass, thus giving access to
glass-like configurations with different degrees of disorder [Phys. Rev. B 54,
15808 (1996)]. The electronic structure is described by a tight-binding
Hamiltonian. We study the influence of the degree of disorder on the density of
states, the localization properties, the optical absorption, the nature of
defects within the mobility gap, and on the fluctuations of the Madelung
potential, where the disorder manifests itself most prominently. The
experimentally observed mismatch between a photoconductivity threshold of 9 eV
and the onset of the optical absorption around 7 eV is interpreted by the
picture of eigenstates localized by potential energy fluctuations in a mobility
gap of approximately 9 eV and a density of states that exhibits valence and
conduction band tails which are, even in the absence of defects, deeply located
within the former band gap.Comment: 21 pages of Latex, 5 eps figure
Effect of Holstein phonons on the optical conductivity of gapped graphene
We study the optical conductivity of a doped graphene when a sublattice
symmetry breaking is occurred in the presence of the electron-phonon
interaction. Our study is based on the Kubo formula that is established upon
the retarded self-energy. We report new features of both the real and imaginary
parts of the quasiparticle self-energy in the presence of a gap opening. We
find an analytical expression for the renormalized Fermi velocity of massive
Dirac Fermions over broad ranges of electron densities, gap values and the
electron-phonon coupling constants. Finally we conclude that the inclusion of
the renormalized Fermi energy and the band gap effects are indeed crucial to
get reasonable feature for the optical conductivity.Comment: 12 pages, 4 figures. To appear in Eur. Phys. J.
Tunable Excitons in Biased Bilayer Graphene
Recent measurements have shown that a continuously tunable bandgap of up to
250 meV can be generated in biased bilayer graphene [Y. Zhang et al., Nature
459, 820 (2009)], opening up pathway for possible graphene-based nanoelectronic
and nanophotonic devices operating at room temperature. Here, we show that the
optical response of this system is dominated by bound excitons. The main
feature of the optical absorbance spectrum is determined by a single symmetric
peak arising from excitons, a profile that is markedly different from that of
an interband transition picture. Under laboratory conditions, the binding
energy of the excitons may be tuned with the external bias going from zero to
several tens of meV's. These novel strong excitonic behaviors result from a
peculiar, effective ``one-dimensional'' joint density of states and a
continuously-tunable bandgap in biased bilayer graphene. Moreover, we show that
the electronic structure (level degeneracy, optical selection rules, etc.) of
the bound excitons in a biased bilayer graphene is markedly different from that
of a two-dimensional hydrogen atom because of the pseudospin physics
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