18,412 research outputs found
Stretching Homopolymers
Force induced stretching of polymers is important in a variety of contexts.
We have used theory and simulations to describe the response of homopolymers,
with monomers, to force () in good and poor solvents. In good solvents
and for {{sufficiently large}} we show, in accord with scaling predictions,
that the mean extension along the axis for small , and
(the Pincus regime) for intermediate values of . The
theoretical predictions for \la Z\ra as a function of are in excellent
agreement with simulations for N=100 and 1600. However, even with N=1600, the
expected Pincus regime is not observed due to the the breakdown of the
assumptions in the blob picture for finite . {{We predict the Pincus scaling
in a good solvent will be observed for }}. The force-dependent
structure factors for a polymer in a poor solvent show that there are a
hierarchy of structures, depending on the nature of the solvent. For a weakly
hydrophobic polymer, various structures (ideal conformations, self-avoiding
chains, globules, and rods) emerge on distinct length scales as is varied.
A strongly hydrophobic polymer remains globular as long as is less than a
critical value . Above , an abrupt first order transition to a
rod-like structure occurs. Our predictions can be tested using single molecule
experiments.Comment: 24 pages, 7 figure
Wavelet treatment of the intra-chain correlation functions of homopolymers in dilute solutions
Discrete wavelets are applied to parametrization of the intra-chain two-point
correlation functions of homopolymers in dilute solutions obtained from Monte
Carlo simulation. Several orthogonal and biorthogonal basis sets have been
investigated for use in the truncated wavelet approximation. Quality of the
approximation has been assessed by calculation of the scaling exponents
obtained from des Cloizeaux ansatz for the correlation functions of
homopolymers with different connectivities in a good solvent. The resulting
exponents are in a better agreement with those from the recent renormalisation
group calculations as compared to the data without the wavelet denoising. We
also discuss how the wavelet treatment improves the quality of data for
correlation functions from simulations of homopolymers at varied solvent
conditions and of heteropolymers.Comment: RevTeX, 19 pages, 7 PS figures. Accepted for publication in PR
Continuous Model for Homopolymers
We consider the model for the distribution of a long homopolymer in a
potential field. The typical shape of the polymer depends on the temperature
parameter. We show that at a critical value of the temperature the transition
occurs from a globular to an extended phase. For various values of the
temperature, including those at or near the critical value, we consider the
limiting behavior of the polymer when its size tends to infinity
Monte Carlo simulations of copolymers at homopolymer interfaces: Interfacial structure as a function of the copolymer density
By means of extensive Monte Carlo simulations of the bond fluctuation model,
we study the effect of adding AB diblock copolymers on the properties of an
interface between demixed homopolymer phases. The parameters are chosen such
that the homopolymers are strongly segregated, and the whole range of copolymer
concentrations in the two phase coexistence region is scanned. We compare the
``mushroom'' regime, in which copolymers are diluted and do not interact with
each other, with the ``wet brush'' regime, where copolymers overlap and
stretch, but are still swollen by the homopolymers. A ``dry brush'' regime is
never entered for our choice of chain lengths. ``Intrinsic'' profiles are
calculated using a block analysis method introduced by us in earlier work. We
discuss density profiles, orientational profiles and contact number profiles.
In general, the features of the profiles are similar at all copolymer
concentrations, however, the profiles in the concentrated regime are much
broader than in the dilute regime. The results compare well with
self-consistent field calculations.Comment: to appear in J. Chem. Phy
Structure and stability of helices in square-well homopolymers
Recently, it has been demonstrated [Magee et al., Phys. Rev. Lett. 96, 207802
(2006)] that isolated, square-well homopolymers can spontaneously break chiral
symmetry and freeze into helical structures at sufficiently low temperatures.
This behavior is interesting because the square-well homopolymer is itself
achiral. In this work, we use event-driven molecular dynamics, combined with an
optimized parallel tempering scheme, to study this polymer model over a wide
range of parameters. We examine the conditions where the helix structure is
stable and determine how the interaction parameters of the polymer govern the
details of the helix structure. The width of the square well (proportional to
lambda) is found to control the radius of the helix, which decreases with
increasing well width until the polymer forms a coiled sphere for sufficiently
large wells. The helices are found to be stable for only a window of molecular
weights. If the polymer is too short, the helix will not form. If the polymer
is too long, the helix is no longer the minimum energy structure, and other
folded structures will form. The size of this window is governed by the chain
stiffness, which in this model is a function of the ratio of the monomer size
to the bond length. Outside this window, the polymer still freezes into a
locked structure at low temperature, however, unless the chain is sufficiently
stiff, this structure will not be unique and is similar to a glassy state.Comment: Submitted to Physical Review
Side-chain and backbone ordering in Homopolymers
In order to study the relation between backbone and side chain ordering in
proteins, we have performed multicanonical simulations of deka-peptide chains
with various side groups. Glu10, Gln10, Asp10, Asn10, and Lys10 were selected
to cover a wide variety of possible interactions between the side chains of the
monomers. All homopolymers undergo helix-coil transitions. We found that
peptides with long side chains that are capable of hydrogen bonding, i.e.
Glu10, and Gln10, exhibit a second transition at lower temperatures connected
with side chain ordering. This occurs in gas phase as well as in solvent,
although the character of the side chain structure is different in each case.
However, in polymers with short side chains capable of hydrogen bonding, i.e.
Asp10 and Asn10, side chain ordering takes place over a wide temperature range
and exhibits no phase transition like character. Moreover, non-backbone
hydrogen bonds show enhanced formation and fluctuations already at the
helix-coil transition temperature, indicating competition between side chain
and backbone hydrogen bond formation. Again, these results are qualitatively
independent of the environment. Side chain ordering in Lys10, whose side groups
are long and polar, also takes place over a wide temperature range and exhibits
no phase transition like character in both environments. Reasons for the
observed chain length threshold and consequences from these results for protein
folding are discussed.Comment: 12 pages,11 figure
A graph theoretical analysis of the energy landscape of model polymers
In systems characterized by a rough potential energy landscape, local
energetic minima and saddles define a network of metastable states whose
topology strongly influences the dynamics. Changes in temperature, causing the
merging and splitting of metastable states, have non trivial effects on such
networks and must be taken into account. We do this by means of a recently
proposed renormalization procedure. This method is applied to analyze the
topology of the network of metastable states for different polypeptidic
sequences in a minimalistic polymer model. A smaller spectral dimension emerges
as a hallmark of stability of the global energy minimum and highlights a
non-obvious link between dynamic and thermodynamic properties.Comment: 15 pages, 15 figure
Monte Carlo Study of the Phase Structure of Compact Polymer Chains
We study the phase behavior of single homopolymers in a simple
hydrophobic/hydrophilic off-lattice model with sequence independent local
interactions. The specific heat is, not unexpectedly, found to exhibit a
pronounced peak well below the collapse temperature, signalling a possible
low-temperature phase transition. The system size dependence at this maximum is
investigated both with and without the local interactions, using chains with up
to 50 monomers. The size dependence is found to be weak. The specific heat
itself seems not to diverge. The homopolymer results are compared with those
for two non-uniform sequences. Our calculations are performed using the methods
of simulated and parallel tempering. The performances of these algorithms are
discussed, based on careful tests for a small system.Comment: 13 pages LaTeX, 6 Postscript figures, References adde
Diblock copolymers at a homopolymer-homopolymer-interface: a Monte Carlo simulation
The properties of diluted symmetric A-B diblock copolymers at the interface
between A and B homopolymer phases are studied by means of Monte Carlo (MC)
simulations of the bond fluctuation model. We calculate segment density
profiles as well as orientational properties of segments, of A and B blocks,
and of the whole chain. Our data support the picture of oriented ``dumbbells'',
which consist of mildly perturbed A and B Gaussian coils. The results are
compared to a self consistent field theory (SCFT) for single copolymer chains
at a homopolymer interface. We also discuss the number of interaction contacts
between monomers, which provide a measure for the ``active surface'' of
copolymers or homopolymers close to the interface
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