Study of surface diffusion by Langevin Dynamics simulation

We investigate the dynamic properties of Brownian interacting particles subject to a two-dimensional periodic potential. By employing the Langevin dynamics simulation, we calculate the collective diffusion coefficient in different situations corresponding to different densities. On the other hand, our numerical studies show that the collective diffusion coefficient depends not only on the shape of adsorbed potential but also on the coupling between particles.We investigate the dynamic properties of Brownian interacting particles subject to a two-dimensional periodic potential. By employing the Langevin dynamics simulation, we calculate the collective diffusion coefficient in different situations corresponding to different densities. On the other hand, our numerical studies show that the collective diffusion coefficient depends not only on the shape of adsorbed potential but also on the coupling between particles

Diffusion in Inhomogeneous Systems: Self-Consistent Random Phase Approximation

The classical diffusion of particles in an inhomogeneous periodic system is studied employing the Fokker-Planck equation. The full width at half-maximum (fwhm) of the quasielastic peak in the dynamic structure factorS(q,ω) is calculated numerically by the matrix continued fraction method up to large values of the momentum transfer covering several Brillouin zones. It is shown that fwhm exhibits strong oscillations with the scattering wave-vector q as it has been observed in β-Ag 2 S by the mean of neutron scattering.The classical diffusion of particles in an inhomogeneous periodic system is studied employing the Fokker-Planck equation. The full width at half-maximum (fwhm) of the quasielastic peak in the dynamic structure factorS(q,ω) is calculated numerically by the matrix continued fraction method up to large values of the momentum transfer covering several Brillouin zones. It is shown that fwhm exhibits strong oscillations with the scattering wave-vector q as it has been observed in β-Ag 2 S by the mean of neutron scattering

Fokker Planck Dynamic in a Periodic Triple-Well Potential

In this work we present a general theory for diffusion mechanism of Brownian particle submitted to a symmetric and periodic triple-well potential (Fig 1). The kinetics description is done with a Fokker-Planck equation (F.P.E). The F.P.E is resolved numerically using the Matrix Continued Fraction Method (M.C.F.M).In order to calculate some important correlation functions. The half-with of the quasi-elastic line λ(q) of dynamic structure factor S (q,w) is studied in the high friction regime and low temperature for different structure of potential with a fixed barrier potential The result show that the half with λ(q) present the same aspect for different values of the ratio of two potential barriers Δ (Δ=V1/V2), except for Δ≈1 for which λ(q) is a cosine function.In this work we present a general theory for diffusion mechanism of Brownian particle submitted to a symmetric and periodic triple-well potential (Fig 1). The kinetics description is done with a Fokker-Planck equation (F.P.E). The F.P.E is resolved numerically using the Matrix Continued Fraction Method (M.C.F.M).In order to calculate some important correlation functions. The half-with of the quasi-elastic line λ(q) of dynamic structure factor S (q,w) is studied in the high friction regime and low temperature for different structure of potential with a fixed barrier potential The result show that the half with λ(q) present the same aspect for different values of the ratio of two potential barriers Δ (Δ=V1/V2), except for Δ≈1 for which λ(q) is a cosine function

Diffusion in symmetric metastable periodic potential: the Fokker-Planck equation

We investigate the dynamic properties of Brownian particle subject to a metastable periodic potential. By employing the Fokker-Planck equation, which we solve numerically by the matrix continued fraction method, we have calculated the full width at half-maximum (FWHM) λ(q) of the quasi-elastic peak of the dynamic structure factor, for large range of values of the wave-vectors q and for different temperature. Our results show that at low temperature and for different values of the ratio of the barriers ∆=V2/V1 the diffusion process can be described by a simple jump motion with the jump length equal to a/2 for ∆=1 and equal a for 0≤∆≤1/2. While for the other cases, the diffusion process consists of a superposition of both of them. At high temperature, the Fokker-Planck equation describes a diffusion process, which has some characteristic of jump and liquid-like regimes.We investigate the dynamic properties of Brownian particle subject to a metastable periodic potential. By employing the Fokker-Planck equation, which we solve numerically by the matrix continued fraction method, we have calculated the full width at half-maximum (FWHM) λ(q) of the quasi-elastic peak of the dynamic structure factor, for large range of values of the wave-vectors q and for different temperature. Our results show that at low temperature and for different values of the ratio of the barriers ∆=V2/V1 the diffusion process can be described by a simple jump motion with the jump length equal to a/2 for ∆=1 and equal a for 0≤∆≤1/2. While for the other cases, the diffusion process consists of a superposition of both of them. At high temperature, the Fokker-Planck equation describes a diffusion process, which has some characteristic of jump and liquid-like regimes

Anomalous Behavior of the Diffusion Coefficient in Interacting Adsorbates

Langevin simulations provide an effective way to study collective effects of Brownian particles immersed in a two-dimensional periodic potential. In this paper, we concentrate essentially on the behaviour of the tracer (DTr) and bulk (DB) diffusion coefficients as function of friction. Our simulations show that in the high friction limit, the two physical quantities DTr and DB present qualitatively the same behaviour, for both coupled and decoupled substrate potentials. However, for the low friction regime, and especially for the coupled potential case, an anomalous diffusion behaviour is found. We also found that in the case of weak dynamical coupling between the ad-particles and the substrate, the exponents are not universal and rather depend on the potentials. Moreover, changes in the inter-particle potentials may reverse the behaviour to a normal one

Fokker-plank dynamic in a general periodic potential shape

A general theory for diffusion mechanism of Brownian particle submitted to a bistable and metastable periodic potential is presented. It is based on the description of the kinetics in the framework of Fokker-Planck equation (F.P.E). A formal solution of the F.P.E is obtained using the Matrix Continued Fraction Method (M.C.F.M), which gives an expression for the relevant correlation function. In particular, the half width of the quasi-elastic peak of the dynamical structure factor S(q, w) and the diffusion coefficient are fully studied in a wide range of physical parameters. A comparison between the results of system in a bistable and those in a metastable potential is presented and analysed.A general theory for diffusion mechanism of Brownian particle submitted to a bistable and metastable periodic potential is presented. It is based on the description of the kinetics in the framework of Fokker-Planck equation (F.P.E). A formal solution of the F.P.E is obtained using the Matrix Continued Fraction Method (M.C.F.M), which gives an expression for the relevant correlation function. In particular, the half width of the quasi-elastic peak of the dynamical structure factor S(q, w) and the diffusion coefficient are fully studied in a wide range of physical parameters. A comparison between the results of system in a bistable and those in a metastable potential is presented and analysed

Array-induced collective transport in the Brownian motion of coupled nonlinear oscillator systems

Brownian motion of an array of harmonically coupled particles subject to a periodic substrate potential and driven by an external bias is investigated. In the linear response limit (small bias), the coupling between particles may enhance the diffusion process, depending on the competition between the harmonic chain and the substrate potential. An analytical formula of the diffusion rate for the single-particle case is also obtained. In the nonlinear response regime, the moving kink may become phase-locked to its radiated phonon waves, hence the mobility of the chain may decrease as one increases the external force.Comment: 4 figures, to appear in Phys. Rev.

p21WAF1/CIP1 Upregulation through the Stress Granule-Associated Protein CUGBP1 Confers Resistance to Bortezomib-Mediated Apoptosis

p21(WAF1/CIP1) is a well known cyclin-dependent kinase inhibitor induced by various stress stimuli. Depending on the stress applied, p21 upregulation can either promote apoptosis or prevent against apoptotic injury. The stress-mediated induction of p21 involves not only its transcriptional activation but also its posttranscriptional regulation, mainly through stabilization of p21 mRNA levels. We have previously reported that the proteasome inhibitor MG132 induces the stabilization of p21 mRNA, which correlates with the formation of cytoplasmic RNA stress granules. The mechanism underlying p21 mRNA stabilization, however, remains unknown.We identified the stress granules component CUGBP1 as a factor required for p21 mRNA stabilization following treatment with bortezomib ( =  PS-341/Velcade). This peptide boronate inhibitor of the 26S proteasome is very efficient for the treatment of myelomas and other hematological tumors. However, solid tumors are sometimes refractory to bortezomib treatment. We found that depleting CUGBP1 in cancer cells prevents bortezomib-mediated p21 upregulation. FISH experiments combined to mRNA stability assays show that this effect is largely due to a mistargeting of p21 mRNA in stress granules leading to its degradation. Altering the expression of p21 itself, either by depleting CUGBP1 or p21, promotes bortezomib-mediated apoptosis.We propose that one key mechanism by which apoptosis is inhibited upon treatment with chemotherapeutic drugs might involve upregulation of the p21 protein through CUGBP1

Fragile Mental Retardation Protein Interacts with the RNA-Binding Protein Caprin1 in Neuronal RiboNucleoProtein Complexes

Fragile X syndrome is caused by the absence of the Fragile X Mental Retardation Protein (FMRP), an RNA-binding protein. FMRP is associated with messenger RiboNucleoParticles (mRNPs) present in polyribosomes and its absence in neurons leads to alteration in synaptic plasticity as a result of translation regulation defects. The molecular mechanisms by which FMRP plays a role in translation regulation remain elusive. Using immunoprecipitation approaches with monoclonal Ab7G1-1 and a new generation of chicken antibodies, we identified Caprin1 as a novel FMRP-cellular partner. In vivo and in vitro evidence show that Caprin1 interacts with FMRP at the level of the translation machinery as well as in trafficking neuronal granules. As an RNA-binding protein, Caprin1 has in common with FMRP at least two RNA targets that have been identified as CaMKIIα and Map1b mRNAs. In view of the new concept that FMRP species bind to RNA regardless of known structural motifs, we propose that protein interactors might modulate FMRP functions