591 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
What drives the translocation of stiff chains?
We study the dynamics of the passage of a stiff chain through a pore into a
cell containing particles that bind reversibly to it. Using Brownian Molecular
Dynamics simulations we investigate the mean-first-passage time as a function
of the length of the chain inside, for different concentrations of binding
particles. As a consequence of the interactions with these particles, the chain
experiences a net force along its length whose calculated value from the
simulations accounts for the velocity at which it enters the cell. This force
can in turn be obtained from the solution of a generalized diffusion equation
incorporating an effective Langmuir adsorption free energy for the chain plus
binding particles. These results suggest a role of binding particles in the
translocation process which is in general quite different from that of a
Brownian ratchet. Furthermore, non-equilibrium effects contribute significantly
to the dynamics, \emph{e.g.}, the chain often enters the cell faster than
particle binding can be saturated, resulting in a force several times smaller
than the equilibrium value.Comment: 7 pages, 4 figure
Attraction Between Like-Charged Walls: Short-Ranged Simulations Using Local Molecular Field Theory
Effective attraction between like-charged walls mediated by counterions is
studied using local molecular field (LMF) theory. Monte Carlo simulations of
the "mimic system'' given by LMF theory, with short-ranged "Coulomb core"
interactions in an effective single particle potential incorporating a
mean-field average of the long-ranged Coulomb interactions, provide a direct
test of the theory, and are in excellent agreement with more complex
simulations of the full Coulomb system by Moreira and Netz [Eur. Phys. J. E 8,
33 (2002)]. A simple, generally-applicable criterion to determine the
consistency parameter sigma_{min} needed for accurate use of the LMF theory is
presented
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
Using low energy medical cyclotrons to produce 99mTc - Technetium
This article was retracted on 05 February 2014This paper refers to work in progress, addressing the global trouble in delivering 99mTc to Nuclear Medicine Departments, Aiming to develop an efficient, safe and economical way to directly produce Technetium 99metastable (99mTc) using lowenergy - so-called “medical” - cyclotrons. The present delivery strategy has intrinsic limitations because it is not only based on old nuclear reactors, but also limits the weekly agenda workflow. Our approach is distinct, and is based on the broad distribution network of the low energy cyclotrons and the accessibility of Molybdenum 100 (100Mo) as the target material, so the system here presented, is not based on the use of Nuclear Reactors and highly enriched (or even low enriched) Uranium 235 (235U), but entirely complying with the current international trends and directives, concerning the need to reduce the use of this potential highly critical target material. The direct production technique is based on the nuclear reaction 100Mo(p,2n)99mTc whose production yields have already been widely documented. The 99mTc is produced in a routine, reliable and efficient manner that, remaining always flexible, entirely blends with established protocols.info:eu-repo/semantics/publishedVersio
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
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