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Biomedical subjects

J Klafter

Publications and source records attributed to J Klafter.

At least 37 records · Page 2Linked to original sources
PubMed2000

Atomic scale engines: cars and wheels.

We introduce a new approach to build microscopic engines on the atomic scale that move translationally or rotationally and can perform useful functions such as the pulling of a cargo. Characteristic of these engines is the possibility to determine dynamically the directionality of the motion. The approach is based on the transformation of the fed energy to directed motion through a dynamical competition between the intrinsic lengths of the moving object and the supporting carrier.

Journal Article↗
PubMed2000

One-dimensional stochastic Levy-lorentz gas

We introduce a Levy-Lorentz gas in which a light particle is scattered by static point scatterers arranged on a line. We investigate the case where the intervals between scatterers xi(i) are independent random variables identically distributed according to the probability density function &mgr;(xi) approximately xi(-(1+gamma)). We show that under certain conditions the mean square displacement of the particle obeys >/=Ct3-gamma for 1<gamma<2. This behavior is compatible with a renewal Levy walk scheme. We discuss the importance of rare events in the proper characterization of the diffusion process.

Journal Article↗
PubMed2000

From continuous time random walks to the fractional fokker-planck equation

We generalize the continuous time random walk (CTRW) to include the effect of space dependent jump probabilities. When the mean waiting time diverges we derive a fractional Fokker-Planck equation (FFPE). This equation describes anomalous diffusion in an external force field and close to thermal equilibrium. We discuss the domain of validity of the fractional kinetic equation. For the force free case we compare between the CTRW solution and that of the FFPE.

Journal Article↗
PubMed2000

Generalized model for dynamic percolation

We study the dynamics of a carrier, which performs a biased motion under the influence of an external field E-->, in an environment which is modeled by dynamic percolation and created by hard-core particles. The particles move randomly on a simple cubic lattice, constrained by hard-core exclusion, and they spontaneously annihilate and reappear at some prescribed rates. We determine the density profiles of the "environment" particles, as seen from the stationary moving carrier, and calculate its terminal velocity V(c) as the function of the applied field and other system parameters. For sufficiently small driving forces the force exerted on the carrier by the "environment" particles shows a viscouslike behavior. An analog Stokes formula for such dynamic percolative environments and the corresponding friction coefficient are derived. We show that the density profile of the environment particles is strongly inhomogeneous: In front of the stationary moving carrier the density is higher than the average density rho(s), while past the carrier the local density is lower than rho(s).

Journal Article↗
PubMed1999

Hierarchies and logarithmic oscillations in the temporal relaxation patterns of proteins and other complex systems.

Logarithmic oscillations superimposed on the temporal relaxation patterns of complex systems are considered from the standpoint of their hierarchical origin. We propose that a closer examination of experimental data should reveal logarithmic oscillations in systems that are characterized by a hierarchical structure of their dynamical degrees of freedom. On that footing, a new methodology of data analysis is proposed that may prove important for the dynamics of protein folding and of conformational fluctuations in proteins in which the relevant time scales of the dynamical evolution underlying the relaxation kinetics can be deduced from these oscillations.

Protein Conformation↗
PubMed1999

Escape from a fluctuating system: a master equation and trapping approach.

We present a general solution for the mean exit time in a system with on-site fluctuations between two configurations described by a master equation. The coupled configurations represent a spatially discretized version of an escape over a fluctuating barrier [C. R. Doering and J. C. Gadoua, Phys. Rev. Lett. 69, 2318 (1992)], and passage through modulating channels. Based on the general properties of the mean exit time, we obtain a simple solution for a coupled "birth" and "death" case that exhibits resonant activation. Within this exactly solvable model we derive analytically the optimal fluctuating rate, which is sensitive to the initial condition and scales as 1/n, where n is the system size. Our approach unifies a number of escape problems and points towards the generality of resonant activation.

Journal Article↗
PubMed1999

Sorting single events: mean arrival times of N random walkers.

Using a scaling approach we investigate the first passage time < mu(1,N)(r)> for the first out of N identical independently diffusing particles in ordered and disordered structures. For Euclidean spaces we obtain < mu(1,N)(r)> in terms of a series in (ln N)(-1), independent of dimension. In the case of disordered ramified fractals < mu(1,N)(r)> is expressed in terms of a series in (ln N)((1-d(l)(w))), where d(l)(w) describes how the mean topological distance evolves with time t. We propose a scaling behavior for the related quantity S(N)(t), the number of distinct sites visited by N particles. We verify our predictions by numerical simulations.

Journal Article↗