4,996 research outputs found
The Role of Non-native Interactions in the Folding of Knotted Proteins
Stochastic simulations of coarse-grained protein models are used to
investigate the propensity to form knots in early stages of protein folding.
The study is carried out comparatively for two homologous
carbamoyltransferases, a natively-knotted N-acetylornithine
carbamoyltransferase (AOTCase) and an unknotted ornithine carbamoyltransferase
(OTCase). In addition, two different sets of pairwise amino acid interactions
are considered: one promoting exclusively native interactions, and the other
additionally including non-native quasi-chemical and electrostatic
interactions. With the former model neither protein show a propensity to form
knots. With the additional non-native interactions, knotting propensity remains
negligible for the natively-unknotted OTCase while for AOTCase it is much
enhanced. Analysis of the trajectories suggests that the different entanglement
of the two transcarbamylases follows from the tendency of the C-terminal to
point away from (for OTCase) or approach and eventually thread (for AOTCase)
other regions of partly-folded protein. The analysis of the OTCase/AOTCase pair
clarifies that natively-knotted proteins can spontaneously knot during early
folding stages and that non-native sequence-dependent interactions are
important for promoting and disfavoring early knotting events.Comment: Accepted for publication on PLOS Computational Biolog
Study of psi' and chi_c decays as feed-down sources of J/psi hadro-production
The interpretation of the J/psi suppression patterns observed in nuclear
collisions, at CERN and RHIC, as a signature of the formation of a deconfined
phase of QCD matter, requires knowing which fractions of the measured J/psi
yields, in pp collisions, are due to decays of heavier charmonium states. From
a detailed analysis of the available mid-rapidity charmonium hadro-production
cross sections, or their ratios, we determine that the J/psi feed-down
contributions from psi' and chi_c decays are, respectively, (8.1 +/- 0.3) % and
(25 +/- 5) %. These proton-proton values are derived from global averages of
the proton-nucleus measurements, assuming that the charmonium states are
exponentially absorbed with the length of matter they traverse in the nuclear
targets.Comment: 12 pages, 4 figures, 3 table
The Effect of Interactions on the Conductance of Graphene Nanoribbons
We study the effects of the interaction between electrons and holes on the
conductance G of quasi-one-dimensional graphene systems.
We first consider as a benchmark the limit in which all interactions are
negligible, recovering the predictions of the tight-binding approximation for
the spectrum of the system, and the well-known result G=4 e^2/h for the lowest
conductance quantum. Then we consider an exactly solvable field theoretical
model in which the electro-magnetic interactions are effectively local.
Finally, we use the effective field theory formalism to develop an exactly
solvable model in which we also include the effect of non-local interactions.
We find that such interactions turn the nominally metallic armchair graphene
nanoribbon into a semi-conductor, while the short-range interactions lead to a
correction to the G=4 e^2/h formula.Comment: 9 pages, 1 figur
RR Lyrae in XSTPS: The halo density profile in the North Galactic Cap
We present a catalog of RR Lyrae stars (RRLs) observed by the Xuyi Schmidt
Telescope Photometric Survey (XDSS). The area we consider is located in the
North Galactic Cap, covering 376.75 sq deg at RA 150 deg and Dec
27 deg down to a magnitude limit of i 19. Using the
variability information afforded by the multi-epoch nature of our XDSS data,
combined with colors from the Sloan Digital Sky Survey, we are able to identify
candidate RRLs. We find 318 candidates, derive distances to them and estimate
the detection efficiency. The majority of our candidates have more than 12
observations and for these we are able to calculate periods. These also allows
us to estimate our contamination level, which we predict is between 30% to 40%.
Finally we use the sample to probe the halo density profile in the 9-49 kpc
range and find that it can be well fitted by a double power law. We find good
agreement between this model and the models derived for the South Galactic Cap
using the Watkins et al. (2009) and Sesar et al. (2010) RRL data-sets, after
accounting for possible contamination in our data-set from Sagittarius stream
members. We consider non-spherical double power law models of the halo density
profile and again find agreement with literature data-sets, although we have
limited power to constrain the flattening due to our small survey area. Much
tighter constraints will be placed by current and future wide-area surveys,
most notably ESA's astrometric Gaia mission. Our analysis demonstrates that
surveys with a limited number of epochs can effectively be mined for RRLs. Our
complete sample is provided as accompanying online material.Comment: 14 pages, ApJ (in press
Numerical Modelling Approaches for the Analysis of Earthquake Triggered Landslides
Different numerical approaches for the analysis of earthquake triggered landslides are studied. Improved simplified models are developed and their performance for evaluating the response of natural slopes subjected to earthquakes is studied. Using more sophisticated numerical techniques the influence of several factors such as the hydro mechanical behavior assumptions, 1D vs. 2D geometry, and the input motion are assessed. 2D computations have shown that the kinematics of the slope motion is largely affected by the geometry of the slope. As far as the soil behavior is concerned, the results indicate that introduction of progressive yielding in the soil model provides larger displacement, progressive pore-pressure generation and more diffuse deformation. For the input motion, the number of peaks and their amplitudes have been identified as being the mains factors for irreversible displacements
Exploring the Chiral Regime of QCD in the Interacting Instanton Liquid Model
Since contemporary lattice QCD calculations have highlighted the need to understand the dependence of QCD observables on the quark mass, this paper presents the framework to investigate this dependence in the context of the Interacting Instanton Liquid Model (IILM). By computing the nucleon and pion masses for a wide range of quark masses, we show that the IILM reproduces the existing lattice data for pion masses in the range 450 - 730 MeV. We also show that in the low-pion mass regime, the nucleon mass dependence in this model is consistent with O(p^4) Chiral Perturbation Theory and O(p^3) Baryon Chiral Perturbation theory. The nucleon sigma-term extracted from this analysis is in qualitative agreement with but slightly below the result from recent lattice calculations. To further explore this model in the chiral regime, we show that the spectral density of the Dirac operator and the three-point scalar correlation function agree with the behavior expected from chiral perturbation theory for two flavors. In the IILM, we identify a characteristic quark energy scale, m*= 80 MeV, which governs the zero mode zone and thus the scale for instanton mediated chiral symmetry breaking, and discuss its physical significance
Protótipo de um lisímetro de pesagem automático para estudos em casa de vegetação.
Um prototipo de lisimetro de pesagem automatico foi desenvolvido para medicoes do requerimento de agua das culturas. O sistema e operado com o auxilio de apenas um computador, na forma de placa, e basicamente, feito de um conjunto extenso metros de precisao ("strain gages"), um regulador de voltagem, um conversor de sinal analogico para digital e um conjunto de amplificadores operacionais. Os extenso metros transdutores foram colados, com adesivo epoxy, sobre uma lamina de aluminio e conectados em um circuito de "ponte de wheatstone" para a medicao de variaveis, tais como peso. Um sistema de aquisicao automatica de dados foi criado. Os principios, esboco, montagem e funcionamento da instrumentacao estao descritos. O sistema foi calibrado atraves de carregamento e descarregamento de pesos padrao pre-selecionados na plataforma de pesagem e testado e avaliado em plantas de milho e feijoeiro que foram cultivadas em vasos, em condicoes de casa de vegetacao, na EMBRAPA-CNPMS. Um programa de computador foi desenvolvido para adquirir os dados dos sensores, permitindo os registros de local, data, horario e dados para posicoes especificas da memoria e orientando o usuario para a entrada dos intervalos de tempo. Uma linguagem residente BASIC, contida no microprocessador, foi utilizada na programacao
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