72 research outputs found
Tight coupling in thermal Brownian motors
We study analytically a thermal Brownian motor model and calculate exactly
the Onsager coefficients. We show how the reciprocity relation holds and that
the determinant of the Onsager matrix vanishes. Such condition implies that the
device is built with tight coupling. This explains why Carnot's efficiency can
be achieved in the limit of infinitely slow velocities. We also prove that the
efficiency at maximum power has the maximum possible value, which corresponds
to the Curzon-Alhborn bound. Finally, we discuss the model acting as a Brownian
refrigerator
Measuring the energy landscape roughness and the transition state location of biomolecules using single molecule mechanical unfolding experiments
Single molecule mechanical unfolding experiments are beginning to provide
profiles of the complex energy landscape of biomolecules. In order to obtain
reliable estimates of the energy landscape characteristics it is necessary to
combine the experimental measurements with sound theoretical models and
simulations. Here, we show how by using temperature as a variable in mechanical
unfolding of biomolecules in laser optical tweezer or AFM experiments the
roughness of the energy landscape can be measured without making any
assumptions about the underlying reaction oordinate. The efficacy of the
formalism is illustrated by reviewing experimental results that have directly
measured roughness in a protein-protein complex. The roughness model can also
be used to interpret experiments on forced-unfolding of proteins in which
temperature is varied. Estimates of other aspects of the energy landscape such
as free energy barriers or the transition state (TS) locations could depend on
the precise model used to analyze the experimental data. We illustrate the
inherent difficulties in obtaining the transition state location from loading
rate or force-dependent unfolding rates. Because the transition state moves as
the force or the loading rate is varied it is in general difficult to invert
the experimental data unless the curvature at the top of the one dimensional
free energy profile is large, i.e the barrier is sharp. The independence of the
TS location on force holds good only for brittle or hard biomolecules whereas
the TS location changes considerably if the molecule is soft or plastic. We
also comment on the usefulness of extension of the molecule as a surrogate
reaction coordinate especially in the context of force-quench refolding of
proteins and RNA.Comment: 44 pages, 7 figure
Scale-free energy dissipation and dynamic phase transition in stochastic sandpiles
We study numerically scaling properties of the distribution of cumulative
energy dissipated in an avalanche and the dynamic phase transition in a
stochastic directed cellular automaton [B. Tadi\'c and D. Dhar, Phys. Rev.
Lett. {\bf 79}, 1519 (1997)] in d=1+1 dimensions. In the critical steady state
occurring for the probability of toppling = 0.70548, the
dissipated energy distribution exhibits scaling behavior with new scaling
exponents and D_E for slope and cut-off energy, respectively,
indicating that the sandpile surface is a fractal. In contrast to avalanche
exponents, the energy exponents appear to be p- dependent in the region
, however the product remains universal. We
estimate the roughness exponent of the transverse section of the pile as . Critical exponents characterizing the dynamic phase transition
at are obtained by direct simulation and scaling analysis of the
survival probability distribution and the average outflow current. The
transition belongs to a new universality class with the critical exponents
, and , with apparent violation of hyperscaling. Generalized hyperscaling
relation leads to , where is the exponent governed by the ultimate survival
probability
Spontaneous Oscillations of Collective Molecular Motors
We analyze a simple stochastic model to describe motor molecules which
cooperate in large groups and present a physical mechanism which can lead to
oscillatory motion if the motors are elastically coupled to their environment.
Beyond a critical fuel concentration, the non-moving state of the system
becomes unstable with respect to a mode with angular frequency omega. We
present a perturbative description of the system near the instability and
demonstrate that oscillation frequencies are determined by the typical
timescales of the motors.Comment: 11 pages, Revtex, 4 pages Figure
Islands of conformational stability for Filopodia
Filopodia are long, thin protrusions formed when bundles of fibers grow outwardly from a cell surface while remaining closed in a membrane tube. We study the subtle issue of the mechanical stability of such filopodia and how this depends on the deformation of the membrane that arises when the fiber bundle adopts a helical configuration. We calculate the ground state conformation of such filopodia, taking into account the steric interaction between the membrane and the enclosed semiflexible fiber bundle. For typical filopodia we find that a minimum number of fibers is required for filopodium stability. Our calculation elucidates how experimentally observed filopodia can obviate the classical Euler buckling condition and remain stable up to several tens of . We briefly discuss how experimental observation of the results obtained in this work for the helical-like deformations of enclosing membrane tubes in filopodia could possibly be observed in the acrosomal reactions of the sea cucumber Thyone, and the horseshoe crab Limulus. Any realistic future theories for filopodium stability are likely to rely on an accurate treatment of such steric effects, as analysed in this work
Energetics of rocked inhomogeneous ratchets
We study the efficiency of frictional thermal ratchets driven by finite
frequency driving force and in contact with a heat bath. The efficiency
exhibits varied behavior with driving frequency. Both nonmonotonic and
monotonic behavior have been observed. In particular the magnitude of
efficiency in finite frequency regime may be more than the efficiency in the
adiabatic regime. This is our central result for rocked ratchets. We also show
that for the simple potential we have chosen, the presence of only spatial
asymmetry (homogeneous system) or only frictional ratchet (symmetric potential
profile), the adiabatic efficiency is always more than in the nonadiabatic
case.Comment: 5 figure
Hopping motion of lattice gases through nonsymmetric potentials under strong bias conditions
The hopping motion of lattice gases through potentials without
mirror-reflection symmetry is investigated under various bias conditions. The
model of 2 particles on a ring with 4 sites is solved explicitly; the resulting
current in a sawtooth potential is discussed. The current of lattice gases in
extended systems consisting of periodic repetitions of segments with sawtooth
potentials is studied for different concentrations and values of the bias.
Rectification effects are observed, similar to the single-particle case. A
mean-field approximation for the current in the case of strong bias acting
against the highest barriers in the system is made and compared with numerical
simulations. The particle-vacancy symmetry of the model is discussed.Comment: 8 pages (incl. 6 eps figures); RevTeX 3.
Breaking of general rotational symmetries by multi-dimensional classical ratchets
We demonstrate that a particle driven by a set of spatially uncorrelated,
independent colored noise forces in a bounded, multidimensional potential
exhibits rotations that are independent of the initial conditions. We calculate
the particle currents in terms of the noise statistics and the potential
asymmetries by deriving an n-dimensional Fokker-Planck equation in the small
correlation time limit. We analyze a variety of flow patterns for various
potential structures, generating various combinations of laminar and rotational
flows.Comment: Accepted, Physical Review
Congruent evolution of genetic and environmental robustness in microRNA
Genetic robustness, the preservation of an optimal phenotype in the face of
mutations, is critical to the understanding of evolution as phenotypically
expressed genetic variation is the fuel of natural selection. The origin of
genetic robustness, whether it evolves directly by natural selection or it is a
correlated byproduct of other phenotypic traits, is, however, unresolved.
Examining microRNA (miRNA) genes of several eukaryotic species, Borenstein and
Ruppin (Borenstein et al. 2006, PNAS 103: 6593), showed that the structure of
miRNA precursor stem-loops exhibits significantly increased mutational
robustness in comparison with a sample of random RNA sequences with the same
stem-loop structure. The observed robustness was found to be uncorrelated with
traditional measures of environmental robustness -- implying that miRNA
sequences show evidence of the direct evolution of genetic robustness. These
findings are surprising as theoretical results indicate that the direct
evolution of robustness requires high mutation rates and/or large effective
population sizes only found among RNA viruses, not multicellular eukaryotes.
Introducing a novel measure of environmental robustness based on the
equilibrium thermodynamic ensemble of secondary structures of the miRNA
precursor sequences we demonstrate that the biophysics of RNA folding, induces
a high level of correlation between genetic (mutational) and environmental
(thermodynamic) robustness, as expected from the theory of plastogenetic
congruence introduced by Ancel and Fontana (Ancel et al. 2000, J. Exp. Zool.
288: 242). In light of theoretical considerations we believe that this
correlation strongly suggests that genetic robustness observed in miRNA
sequences is the byproduct of selection for environmental robustness.Comment: Accepted for publication in Mol. Biol. Evol. Supplemental Information
available as a separate pdf file from
http://angel.elte.hu:/~ssolo/miRNA_supp_mat.pd
Reversible quantum Brownian heat engines for electrons
Brownian heat engines use local temperature gradients in asymmetric
potentials to move particles against an external force. The energy efficiency
of such machines is generally limited by irreversible heat flow carried by
particles that make contact with different heat baths. Here we show that, by
using a suitably chosen energy filter, electrons can be transferred reversibly
between reservoirs that have different temperatures and electrochemical
potentials. We apply this result to propose heat engines based on mesoscopic
semiconductor ratchets, which can quasistatically operate arbitrarily close to
Carnot efficiency.Comment: Physical Review Letters, accepted (July 2002
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