70 research outputs found
Monte Carlo simulation of melting transition on DNA nanocompartment
DNA nanocompartment is a typical DNA-based machine whose function is
dependent of molecular collective effect. Fundamental properties of the device
have been addressed via electrochemical analysis, fluorescent microscopy, and
atomic force microscopy. Interesting and novel phenomena emerged during the
switching of the device. We have found that DNAs in this system exhibit a much
steep melting transition compared to ones in bulk solution or conventional DNA
array. To achieve an understanding to this discrepancy, we introduced DNA-DNA
interaction potential to the conventional Ising-like Zimm-Bragg theory and
Peyrard-Bishop model of DNA melting. To avoid unrealistic numerical calculation
caused by modification of the Peyrard-Bishop nonlinear Hamiltonian with the
DNA-DNA interaction, we established coarse-gained Monte Carlo recursion
relations by elucidation of five components of energy change during melting
transition. The result suggests that DNA-DNA interaction potential accounts for
the observed steep transition.Comment: 12 pages, 5 figure
Star polymers: From conformations to interactions to phase diagrams
We review recent progress achieved in the theoretical description of the
interactions, correlations, and phase behavior of concentrated solutions of
star polymers, sterically stabilized colloids, and micelles. We show that the
theoretical prediction of an ultrasoft, logarithmically diverging effective interaction
between the star centers, which has been confirmed by SANSexperiments
and computer simulations, lies in the core of a host of unusual
phenomena encountered in such systems. These include anomalous structure
factors, reentrant melting behavior, as well as a variety of exotic crystal
phases. Extensions to polydisperse stars and the role of many-body forces
are also discussed. A particular ‘mean-field’ character of star polymer fluids
is presented and it is shown that it manifests itself in the shape and
structure of sedimentation profiles of these systems.Здійснено огляд недавніх досягнень у теоретичному описі взаємодій, кореляцій і фазової поведінки концентрованих розчинів зіркових полімерів, просторово стійких колоїдів і міцел. Ми покажемо,
що теоретично передбачена надм’яка логарифмічно розбіжна ефективна взаємодія між центрами зірок, що була підтверджена SANS-експериментами і комп’ютерними симуляціями, потрапляє в множину незвичних явищ, які спостерігаються в таких системах. Сюди
відносяться аномальні структурні фактори, поведінка зворотнього
плавлення, множини екзотичних кристалічних фаз. Також обговорено узагальнення на випадок полідисперсних зірок і роль сил багатьох
тіл. Представлено особливу поведінку типу “cереднього поля” плинів
зіркових полімерів і показано, що вона проявляється у формі і структурі профілів осаджування цих систем
Counterion-mediated Electrostatic Interactions between Helical Molecules
We study the interaction of two cylinders with helical charge distribution
mediated by neutralizing counterions, by analyzing the separation as well as
the azimuthal angle dependence of the interaction force in the weak and strong
coupling limit. While the azimuthal dependence of the interaction in the weak
coupling limit is overall small and mostly negligible, the strong coupling
limit leads to qualitatively new features of the interaction, among others also
to an orientationally dependent optimal configuration that is driven by angular
dependence of the correlation attraction. We investigate the properties of this
azimuthal ordering in detail and compare it to existing results.Comment: 11 pages, 12 figure
Orientationally ordered aggregates of stiff polyelectrolytes in the presence of multivalent salt
Aggregation of stiff polyelectrolytes in solution and angle- and
distance-dependent potential of mean force between two like-charged rods are
studied in the presence of 3-valent salt using molecular dynamics simulations.
In the bulk solution, formation of long-lived metastable structures with
similarities to the raft-like structures of actin filaments is observed within
a range of salt concentration. The system finally goes to a state with lower
free energy in which finite-sized bundles of parallel polyelectrolytes form.
Preferred angle and interaction type between two like-charged rods at different
separations and salt concentrations are also studied, which shed some light on
the formation of orientationally ordered structures.Comment: 18 pages, 8 figures, accepted for publication in Soft Matte
Statistical mechanics of columnar DNA assemblies
Many physical systems can be mapped onto solved or "solvable" models of
magnetism. In this work, we have mapped the statistical mechanics of columnar
phases of ideally helical rigid DNA -- subject to the earlier found unusual,
frustrated pair potential [A. A. Kornyshev and S. Leikin, J. Chem. Phys. 107,
3656 (1997)] -- onto an exotic, unknown variant of the XY model on a fixed or
restructurable lattice. Here the role of the 'spin' is played by the azimuthal
orientation of the molecules. We have solved this model using a Hartree-Fock
approximation, ground state calculations, and finite temperature Monte Carlo
simulations. We have found peculiar spin order transitions, which may also be
accompanied by positional restructuring, from hexagonal to rhombohedric
lattices. Some of these have been experimentally observed in dense columnar
aggregates. Note that DNA columnar phases are of great interest in biophysical
research, not only because they are a useful in vitro tool for the study of DNA
condensation, but also since these structures have been detected in living
matter. Within the approximations made, our study provides insight into the
statistical mechanics of these systems.Comment: 19 pages, 18 figure
Torsional fluctuations in columnar DNA assemblies
In columnar assemblies of helical bio-molecules the azimuthal degrees of
freedom, i.e. rotations about the long axes of molecules, may be important in
determining the structure of the assemblies especially when the interaction
energy between neighbouring molecules explicitly depends on their relative
azimuthal orientations. For DNA this leads to a rich variety of mesophases for
columnar assemblies, each categorized by a specific azimuthal ordering. In a
preceding paper [A. Wynveen, D. J. Lee, and A. A. Kornyshev, Eur. Phys. J. E,
16, 303 (2005)] a statistical mechanical theory was developed for the
assemblies of torsionally rigid molecues in order to determine how thermal
fluctuations influence the structure of these mesophases. Here we extend this
theory by including torsional fluctuations of the molecules, where a DNA
molecule may twist about its long axis at the cost of torsional elastic energy.
Comparing this with the previous study, we find that inclusion of torsional
fluctuations further increases the density at which the transition between the
hexagonal structure and the predicted rhombic phase occurs and reduces the
level of distortion in the rhombic phase. As X-ray diffraction may probe the
2-D lattice structure of such assemblies and provide information concerning the
underlying interaction between molecules, we have also calculated correlation
functions for the azimuthal ordering which are manifest in an x-ray scattering
intensity profiles.Comment: 33 pages, 8 figure
Phase Transitions in a Two-Component Site-Bond Percolation Model
A method to treat a N-component percolation model as effective one component
model is presented by introducing a scaled control variable . In Monte
Carlo simulations on , , and simple cubic
lattices the percolation threshold in terms of is determined for N=2.
Phase transitions are reported in two limits for the bond existence
probabilities and . In the same limits, empirical formulas
for the percolation threshold as function of one
component-concentration, , are proposed. In the limit a new
site percolation threshold, , is reported.Comment: RevTeX, 5 pages, 5 eps-figure
Chiral Structure of F-actin Bundle Formed by Multivalent Counterions?
The mechanism of multivalent counterion-induced bundle formation by
filamentous actin (F-actin) is studied using a coarse-grained model and
molecular dynamics simulation. Real diameter size, helically ordered charge
distribution and twist rigidity of F-actin are taken into account in our model.
The attraction between parallel F-actins induced by multivalent counterions is
studied in detail and it is found that the maximum attraction occurs between
their closest charged domains. The model F-actins aggregate due to the
like-charge attraction and form closely packed bundles. Counterions are mostly
distributed in the narrowest gaps between neighboring F-actins inside the
bundles and the channels between three adjacent F-actins correspond to low
density of the counterions. Density of the counterions varies periodically with
a wave length comparable to the separation between consecutive G-actin monomers
along the actin polymers. Long-lived defects in the hexagonal order of F-actins
in the bundles are observed that their number increases with increasing the
bundles size. Combination of electrostatic interactions and twist rigidity has
been found not to change the symmetry of F-actin helical conformation from the
native 13/6 symmetry. Calculation of zero-temperature energy of hexagonally
ordered model F-actins with the charge of the counterions distributed as
columns of charge domains representing counterion charge density waves has
shown that helical symmetries commensurate with the hexagonal lattice
correspond to local minima of the energy of the system. The global minimum of
energy corresponds to 24/11 symmetry with the columns of charge domains
arranged in the narrowest gaps between the neighboring F-actins.Comment: 9 pages, 10 figures, Published online in Soft Matter journal:
http://pubs.rsc.org/en/content/articlelanding/2012/sm/c2sm07104
Rare isotope production in statistical multifragmentation
Producing rare isotopes through statistical multifragmentation is
investigated using the Mekjian method for exact solutions of the canonical
ensemble. Both the initial fragmentation and the the sequential decay are
modeled in such a way as to avoid Monte Carlo and thus provide yields for
arbitrarily scarce fragments. The importance of sequential decay, exact
particle-number conservation and the sensitivities to parameters such as
density and temperature are explored. Recent measurements of isotope ratios
from the fragmentation of different Sn isotopes are interpreted within this
picture.Comment: 10 eps figure
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