182 research outputs found
Polymer ejection from strong spherical confinement
We examine the ejection of an initially strongly confined flexible polymer
from a spherical capsid through a nanoscale pore. We use molecular dynamics for
unprecedentedly high initial monomer densities. We show that the time for an
individual monomer to eject grows exponentially with the number of ejected
monomers. By measurements of the force at the pore we show this dependence to
be a consequence of the excess free energy of the polymer due to confinement
growing exponentially with the number of monomers initially inside the capsid.
This growth relates closely to the divergence of mixing energy in the
Flory-Huggins theory at large concentration. We show that the pressure inside
the capsid driving the ejection dominates the process that is characterized by
the ejection time growing linearly with the lengths of different polymers.
Waiting time profiles would indicate that the superlinear dependence obtained
for polymers amenable to computer simulations results from a finite-size effect
due to the final retraction of polymers' tails from capsids.Comment: 6 pages, 9 figures, accepted for publication in Phys. Rev. E,
increased readability from previous versio
Chaperone-assisted translocation of flexible polymers in three dimensions
Polymer translocation through a nanometer-scale pore assisted by chaperones
binding to the polymer is a process encountered in vivo for proteins. Studying
the relevant models by computer simulations is computationally demanding.
Accordingly, previous studies are either for stiff polymers in three dimensions
or flexible polymers in two dimensions. Here, we study chaperone-assisted
translocation of flexible polymers in three dimensions using Langevin dynamics.
We show that differences in binding mechanisms, more specifically, whether a
chaperone can bind to a single or multiple sites on the polymer, lead to
substantial differences in translocation dynamics in three dimensions. We show
that the single-binding mode leads to dynamics that is very much like that in
the constant-force driven translocation and accordingly mainly determined by
tension propagation on the cis side. We obtain for the
exponent for the scaling of the translocation time with polymer length. This
fairly low value can be explained by the additional friction due to binding
particles. The multiple-site binding leads to translocation whose dynamics is
mainly determined by the trans side. For this process we obtain . This value can be explained by our derivation of for
constant-bias translocation, where translocated polymer segments form a globule
on the trans side. Our results pave the way for understanding and utilizing
chaperone-assisted translocation where variations in microscopic details lead
to rich variations in the emerging dynamics.Comment: 10 pages, 12 figure
Dynamics of polymer ejection from capsid
Polymer ejection from a capsid through a nanoscale pore is an important
biological process with relevance to modern biotechnology. Here, we study
generic capsid ejection using Langevin dynamics. We show that even when the
ejection takes place within the drift-dominated region there is a very high
probability for the ejection process not to be completed. Introducing a small
aligning force at the pore entrance enhances ejection dramatically. Such a pore
asymmetry is a candidate for a mechanism by which a viral ejection is
completed. By detailed high-resolution simulations we show that such capsid
ejection is an out-of-equilibrium process that shares many common features with
the much studied driven polymer translocation through a pore in a wall or a
membrane. We find that the escape times scale with polymer length, . We show that for the pore without the asymmetry the previous
predictions corroborated by Monte Carlo simulations do not hold. For the pore
with the asymmetry the scaling exponent varies with the initial monomer density
(monomers per capsid volume) inside the capsid. For very low densities
the polymer is only weakly confined by the capsid, and we
measure , which is close to obtained for polymer
translocation. At intermediate densities the scaling exponents
and for and , respectively. These scalings are in
accord with a crude derivation for the lower limit . For the
asymmetrical pore precise scaling breaks down, when the density exceeds the
value for complete confinement by the capsid, . The
high-resolution data show that the capsid ejection for both pores, analogously
to polymer translocation, can be characterized as a multiplicative stochastic
process that is dominated by small-scale transitions.Comment: 10 pages, 6 figure
Critical evaluation of the computational methods used in the forced polymer translocation
In forced polymer translocation, the average translocation time, ,
scales with respect to pore force, , and polymer length, , as . We demonstrate that an artifact in Metropolis Monte Carlo
method resulting in breakage of the force scaling with large may be
responsible for some of the controversies between different computationally
obtained results and also between computational and experimental results. Using
Langevin dynamics simulations we show that the scaling exponent is not universal, but depends on . Moreover, we show that forced
translocation can be described by a relatively simple force balance argument
and to arise solely from the initial polymer configuration
Event distributions of polymer translocation
We present event distributions for the polymer translocation obtained by
extensive Langevin dynamics simulations. Such distributions have not been
reported previously and they provide new understanding of the stochastic
characteristics of the process. We extract at a high length scale resolution
distributions of polymer segments that continuously traverse through a
nanoscale pore. The obtained log-normal distributions together with the
characteristics of polymer translocation suggest that it is describable as a
multiplicative stochastic process. In spite of its clear out-of-equilibrium
nature the forced translocation is surprisingly similar to the unforced case.
We find forms for the distributions almost unaltered with a common cut-off
length. We show that the individual short-segment and short-time movements
inside the pore give the scaling relations and for the polymer translocation.Comment: Second revision. 7 pages, 8 figure
Superconducting NbN microstrip detectors
Superconducting NbN strip transmission line counters and coupling circuits were processed on silicon wafers using thin film techniques, and they were characterized with several methods to verify the design principles. The stripline circuits, designed using microwave design rules, were simulated using a circuit design tool enhanced to include modelling of the superconducting lines. The strips, etched out of the 282 nm thick top NbN film with resistivity 284 µ?cm at 20 K, have critical temperatures in the range 12 to 13 K and a critical current density approximately Jc(0) = 3.3·105 A/cm2. The linearized heat transfer coefficient between the strip and the substrate is approximately 1.1·105 W/(m2K) and the healing length is about 1.6 µm between 3 and 5 K temperatures. Traversing 5 MeV a-particles caused the strips to quench. No events due to electrons could be detected in agreement with the predicted signal amplitude which is below the noise threshold of our wideband circuitry. The strip bias current and hence the signal amplitude were limited due to a microbridge at the isolator step of the impedance transformer
Dynamics of forced biopolymer translocation
We present results from our simulations of biopolymer translocation in a
solvent which explain the main experimental findings. The forced translocation
can be described by simple force balance arguments for the relevant range of
pore potentials in experiments and biological systems. Scaling of translocation
time with polymer length varies with pore force and friction. Hydrodynamics
affects this scaling and significantly reduces translocation times.Comment: Published in:
http://www.iop.org/EJ/article/0295-5075/85/5/58006/epl_85_5_58006.htm
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