13,911 research outputs found

    Comment on "Chain Length Scaling of Protein Folding Time", PRL 77, 5433 (1996)

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    In a recent Letter, Gutin, Abkevich, and Shakhnovich (GAS) reported on a series of dynamical Monte Carlo simulations on lattice models of proteins. Based on these highly simplified models, they found that four different potential energies lead to four different folding time scales tau_f, where tau_f scales with chain length as N^lambda (see, also, Refs. [2-4]), with lambda varying from 2.7 to 6.0. However, due to the lack of microscopic models of protein folding dynamics, the interpretation and origin of the data have remained somewhat speculative. It is the purpose of this Comment to point out that the application of a simple "mesoscopic" model (cond-mat/9512019, PRL 77, 2324, 1996) of protein folding provides a full account of the data presented in their paper. Moreover, we find a major qualitative disagreement with the argumentative interpretation of GAS. Including, the origin of the dynamics, and size of the critical folding nucleus.Comment: 1 page Revtex, 1 fig. upon request. Submitted to PR

    A Criterion That Determines Fast Folding of Proteins: A Model Study

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    We consider the statistical mechanics of a full set of two-dimensional protein-like heteropolymers, whose thermodynamics is characterized by the coil-to-globular (TΞT_\theta) and the folding (TfT_f) transition temperatures. For our model, the typical time scale for reaching the unique native conformation is shown to scale as τf∌F(M)exp⁥(σ/σ0)\tau_f\sim F(M)\exp(\sigma/\sigma_0), where σ=1−Tf/TΞ\sigma=1-T_f/T_\theta, MM is the number of residues, and F(M)F(M) scales algebraically with MM. We argue that TfT_f scales linearly with the inverse of entropy of low energy non-native states, whereas TΞT_\theta is almost independent of it. As σ→0\sigma\rightarrow 0, non-productive intermediates decrease, and the initial rapid collapse of the protein leads to structures resembling the native state. Based solely on {\it accessible} information, σ\sigma can be used to predict sequences that fold rapidly.Comment: 10 pages, latex, figures upon reques

    Quantum nondemolition measurements of a particle in electric and gravitational fields

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    In this work we obtain a nondemolition variable for the case in which a charged particle moves in the electric and gravitational fields of a spherical body. Afterwards we consider the continuous monitoring of this nondemolition parameter, and calculate along the ideas of the so called restricted path integral formalism, the corresponding propagator. Using these results the probabilities associated with the possible measurement outputs are evaluated. The limit of our results, as the resolution of the measuring device goes to zero, is analyzed, and the dependence of the corresponding propagator upon the strength of the electric and gravitational fields are commented. The role that mass plays in the corresponding results, and its possible connection with the equivalence principle at quantum level, are studied.Comment: Accepted in International Journal of Modern Physics D, 14 page

    The Generalized Parton Distributions program in Hall A at Jefferson Lab

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    Deep exclusive reactions at Jefferson Lab Hall A

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    International audienceRecent results on the Generalized Parton Distribution (GPD) programat Jefferson Lab (JLab) will be presented. The emphasis will be in the Hall A program aiming at measuring Q2−dependences of different terms of the Deeply Virtual Compton Scattering (DVCS) cross section. This is a fundamental step before one can extract GPD information from JLab DVCS data. The upcoming program in Hall A, using both a 6 GeV beam (2010) and a 11 GeV beam (≈ 2015) will also be described

    The GPD experimental program at Jefferson Lab

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    International audienceRecent results on the Generalized Parton Distribution (GPD) program at Jefferson Lab (JLab) will be presented. The emphasis will be in the Hall A program aiming at measuring Q2−dependences of different terms of the Deeply Virtual Compton Scattering (DVCS) cross section. This is a fundamental step before one can extract GPD information from JLab DVCS data. The upcoming program in Hall A, using both a 6 GeV beam (≈ 2010) and a 11 GeV beam (≈ 2015) will also be described

    Spontaneous patterns in coherently driven polariton microcavities

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    We consider a polariton microcavity resonantly driven by two external lasers which simultaneously pump both lower and upper polariton branches at normal incidence. In this setup, we study the occurrence of instabilities of the pump-only solutions towards the spontaneous formation of patterns. Their appearance is a consequence of the spontaneous symmetry breaking of translational and rotational invariance due to interaction induced parametric scattering. We observe the evolution between diverse patterns which can be classified as single-pump, where parametric scattering occurs at the same energy as one of the pumps, and as two-pump, where scattering occurs at a different energy. For two-pump instabilities, stripe and chequerboard patterns become the dominant steady-state solutions because cubic parametric scattering processes are forbidden. This contrasts with the single-pump case, where hexagonal patterns are the most common arrangements. We study the possibility of controlling the evolution between different patterns. Our results are obtained within a linear stability analysis and are confirmed by finite size full numerical calculations.Comment: 15 pages, 9 figure

    Low-distortion slow light using two absorption resonances

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    We consider group delay and broadening using two strongly absorbing and widely spaced resonances. We derive relations which show that very large pulse bandwidths coupled with large group delays and small broadening can be achieved. Unlike single resonance systems, the dispersive broadening dominates the absorptive broadening which leads to a dramatic increase in the possible group delay. We show that the double resonance systems are excellent candidates for realizing all-optical delay lines. We report on an experiment which achieved up to 50 pulse delays with 40% broadening.Comment: 4 pages 4 figure
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