465 research outputs found
Osmotic force resisting chain insertion in a colloidal suspension
We consider the problem of inserting a stiff chain into a colloidal
suspension of particles that interact with it through excluded volume forces.
The free energy of insertion is associated with the work of creating a cavity
devoid of colloid and sufficiently large to accomodate the chain. The
corresponding work per unit length is the force that resists the entry of the
chain into the colloidal suspension. In the case of a hard sphere fluid, this
work can be calculated straightforwardly within the scaled particle theory; for
solutions of flexible polymers, on the other hand, we employ simple scaling
arguments. The forces computed in these ways are shown, for nanometer chain and
colloid diameters, to be of the order of tens of pN for solution volume
fraction for biophysical processes such as the ejection of DNA from viral
capsids into the cell cytoplasm.Comment: 16 pages,3 figures. Accepted for publication in European Physical
Journal
Electrostatic complexation of spheres and chains under elastic stress
We consider the complexation of highly charged semiflexible polyelectrolytes
with oppositely charged macroions. On the basis of scaling arguments we discuss
how the resulting complexes depend on the persistence length of the
polyelectrolyte, the salt concentration, and the sizes and charges of the chain
and the macroions. We study first the case of complexation with a single sphere
and calculate the wrapping length of the chain. We then extend our
considerations to complexes involving many wrapped spheres and study
cooperative effects. The mechanical properties of such a complex under an
external deformation are evaluated.Comment: 16 pages, submitted to J. Chem. Phy
What do emulsification failure and Bose-Einstein condensation have in common?
Ideal bosons and classical ring polymers formed via self-assembly, are known
to have the same partition function, and so analogous phase transitions. In
ring polymers, the analogue of Bose-Einstein condensation occurs when a ring
polymer of macroscopic size appears. We show that a transition of the same
general form occurs within a whole class of systems with self-assembly, and
illustrate it with the emulsification failure of a microemulsion phase of
water, oil and surfactant. As with Bose-Einstein condensation, the transition
occurs even in the absence of interactions.Comment: 7 pages, 1 figure, typeset with EUROTeX, uses epsfi
Structural Polymorphism of the Cytoskeleton: A Model of Linker-Assisted Filament Aggregation
The phase behavior of charged rods in the presence of inter-rod linkers is
studied theoretically as a model for the equilibrium behavior underlying the
organization of actin filaments by linker proteins in the cytoskeleton. The
presence of linkers in the solution modifies the effective inter-rod
interaction and can lead to inter-filament attraction. Depending on the
system's composition and physical properties such as linker binding energies,
filaments will either orient perpendicular or parallel to each other, leading
to network-like or bundled structures. We show that such a system can have one
of three generic phase diagrams, one dominated by bundles, another by networks,
and the third containing both bundle and network-like phases. The first two
diagrams can be found over a wide range of interaction energies, while the
third occurs only for a narrow range. These results provide theoretical
understanding of the classification of linker proteins as bundling proteins or
crosslinking proteins. In addition, they suggest possible mechanisms by which
the cell may control cytoskeletal morphology.Comment: 17 pages, 3 figure
Organized condensation of worm-like chains
We present results relevant to the equilibrium organization of DNA strands of
arbitrary length interacting with a spherical organizing center, suggestive of
DNA-histone complexation in nucleosomes. We obtain a rich phase diagram in
which a wrapping state is transformed into a complex multi-leafed, rosette
structure as the adhesion energy is reduced. The statistical mechanics of the
"melting" of a rosette can be mapped into an exactly soluble one-dimensional
many-body problem.Comment: 15 pages, 2 figures in a pdf fil
Charge reversal of colloidal particles
A theory is presented for the effective charge of colloidal particles in
suspensions containing multivalent counterions. It is shown that if colloids
are sufficiently strongly charged, the number of condensed multivalent
counterion can exceed the bare colloidal charge leading to charge reversal.
Charge renormalization in suspensions with multivalent counterions depends on a
subtle interplay between the solvation energies of the multivalent counterions
in the bulk and near the colloidal surface. We find that the effective charge
is {\it not} a monotonically decreasing function of the multivalent salt
concentration. Furthermore, contrary to the previous theories, it is found that
except at very low concentrations, monovalent salt hinders the charge reversal.
This conclusion is in agreement with the recent experiments and simulations
Why is the condensed phase of DNA preferred at higher temperature? DNA compaction in the presence of a multivalent cation
Upon the addition of multivalent cations, a giant DNA chain exhibits a large
discrete transition from an elongated coil into a folded compact state. We
performed single-chain observation of long DNAs in the presence of a
tetravalent cation (spermine), at various temperatures and monovalent salt
concentrations. We confirmed that the compact state is preferred at higher
temperatures and at lower monovalent salt concentrations. This result is
interpreted in terms of an increase in the net translational entropy of small
ions due to ionic exchange between higher and lower valence ions.Comment: 4pages,3figure
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