151 research outputs found
Clustering and coalescence from multiplicative noise: the Kraichnan ensemble
We study the dynamics of the two-point statistics of the Kraichnan ensemble
which describes the transport of a passive pollutant by a stochastic turbulent
flow characterized by scale invariant structure functions. The fundamental
equation of this problem consists in the Fokker-Planck equation for the
two-point correlation function of the density of particles performing spatially
correlated Brownian motions with scale invariant correlations. This problem is
equivalent to the stochastic motion of an effective particle driven by a
generic multiplicative noise. In this paper we propose an alternative and more
intuitive approach to the problem than the original one leading to the same
conclusions. The general features of this new approach make possible to fit it
to other more complex contexts.Comment: IOP-LaTeX, 17 pages J. Phys. A: Theor. Mat. 2008 in pres
Transport and fluctuation-dissipation relations in asymptotic and pre-asymptotic diffusion across channels with variable section
We study the asymptotic and pre-asymptotic diffusive properties of Brownian
particles in channels whose section varies periodically in space. The effective
diffusion coefficient is numerically determined by the
asymptotic behavior of the root mean square displacement in different
geometries, considering even cases of steep variations of the channel
boundaries. Moreover, we compared the numerical results to the predictions from
the various corrections proposed in the literature to the well known
Fick-Jacobs approximation. Building an effective one dimensional equation for
the longitudinal diffusion, we obtain an approximation for the effective
diffusion coefficient. Such a result goes beyond a perturbation approach, and
it is in good agreement with the actual values obtained by the numerical
simulations. We discuss also the pre-asymptotic diffusion which is observed up
to a crossover time whose value, in the presence of strong spatial variation of
the channel cross section, can be very large. In addition, we show how the
Einstein's relation between the mean drift induced by a small external field
and the mean square displacement of the unperturbed system is valid in both
asymptotic and pre-asymptotic regimes.Comment: RevTeX 4-1, 11 Pages, 11 pdf figure
Computational analysis of folding and mutation properties of C5 domain from Myosin binding protein C
Thermal folding Molecular Dynamics simulations of the domain C5 from Myosin
Binding Protein C were performed using a native-centric model to study the role
of three mutations related to Familial Hypertrophic Cardiomyopathy. Mutation of
Asn755 causes the largest shift of the folding temperature, and the residue is
located in the CFGA' beta-sheet featuring the highest Phi-values. The mutation
thus appears to reduce the thermodynamic stability in agreement with
experimental data. The mutations on Arg654 and Arg668, conversely, cause a
little change in the folding temperature and they reside in the low Phi-value
BDE beta-sheet, so that their pathologic role cannot be related to impairment
of the folding process but possibly to the binding with target molecules. As
the typical signature of Domain C5 is the presence of a longer and
destabilizing CD-loop with respect to the other Ig-like domains we completed
the work with a bioinformatic analysis of this loop showing a high density of
negative charge and low hydrophobicity. This indicates the CD-loop as a
natively unfolded sequence with a likely coupling between folding and ligand
binding.Comment: RevTeX, 10 pages, 9 eps-figure
Kinetics of self-induced aggregation in Brownian particles
We study a model of interacting random walkers that proposes a simple
mechanism for the emergence of cooperation in group of individuals. Each
individual, represented by a Brownian particle, experiences an interaction
produced by the local unbalance in the spatial distribution of the other
individuals. This interaction results in a nonlinear velocity driving the
particle trajectories in the direction of the nearest more crowded regions; the
competition among different aggregating centers generates nontrivial dynamical
regimes. Our simulations show that for sufficiently low randomness, the system
evolves through a coalescence behavior characterized by clusters of particles
growing with a power law in time. In addition, the typical scaling properties
of the general theory of stochastic aggregation processes are verified.Comment: RevTeX, 9 pages, 9 eps-figure
Mean Field Approach for a Statistical Mechanical Model of Proteins
We study the thermodynamical properties of a topology-based model proposed by
Galzitskaya and Finkelstein for the description of protein folding. We devise
and test three different mean-field approaches for the model, that simplify the
treatment without spoiling the description. The validity of the model and its
mean-field approximations is checked by applying them to the -hairpin
fragment of the immunoglobulin-binding protein (GB1) and making a comparison
with available experimental data and simulation results. Our results indicate
that this model is a rather simple and reasonably good tool for interpreting
folding experimental data, provided the parameters of the model are carefully
chosen. The mean-field approaches substantially recover all the relevant exact
results and represent reliable alternatives to the Monte Carlo simulations.Comment: RevTeX-4, 11 pages, 6 eps-figures, To Appear on J.Chem.Phy
Driven diffusion against electrostatic or effective energy barrier across Alpha-Hemolysin
We analyze the translocation of a charged particle across an Alpha-Hemolysin
(aHL) pore in the framework of a driven diffusion over an extended energy
barrier generated by the electrical charges of the aHL. A one-dimensional
electrostatic potential is extracted from the full 3D solution of the Poisson's
equation. We characterize the particle transport under the action of a constant
forcing by studying the statistics of the translocation time. We derive an
analytical expression of translocation time average that compares well with the
results from Brownian dynamic simulations of driven particles over the
electrostatic potential. Moreover, we show that the translocation time
distributions can be perfectly described by a simple theory which replaces the
true barrier by an equivalent structureless square barrier. Remarkably our
approach maintains its accuracy also for low-applied voltage regimes where the
usual inverse-Gaussian approximation fails. Finally we discuss how the
comparison between the simulated time distributions and their theoretical
prediction results to be greatly simplified when using the notion of the
empirical Laplace transform technique.Comment: RevTeX 4-1, 11 pages, 6 pdf figures, J. Chem. Phys. 2015 in pres
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