1,140 research outputs found
Evaluating the Applicability of the Fokker-Planck Equation in Polymer Translocation: A Brownian Dynamics Study
Brownian dynamics (BD) simulations are used to study the translocation
dynamics of a coarse-grained polymer through a cylindrical nanopore. We
consider the case of short polymers, with a polymer length, N, in the range
N=21-61. The rate of translocation is controlled by a tunable friction
coefficient, gamma_{0p}, for monomers inside the nanopore. In the case of
unforced translocation, the mean translocation time scales with polymer length
N as ~ (N-N_p)^alpha, where N_p is the average number of monomers in the
nanopore. The exponent approaches the value alpha=2 when the pore friction is
sufficiently high, in accord with the prediction for the case of the
quasi-static regime where pore friction dominates. In the case of forced
translocation, the polymer chain is stretched and compressed on the cis and
trans sides, respectively, for low gamma_{0p}. However, the chain approaches
conformational quasi-equilibrium for sufficiently large gamma_{0p}. In this
limit the observed scaling of with driving force and chain length
supports the FP prediction that is proportional to N/f_d for sufficiently
strong driving force. Monte Carlo simulations are used to calculate
translocation free energy functions for the system. The free energies are used
with the Fokker-Planck equation to calculate translocation time distributions.
At sufficiently high gamma_{0p}, the predicted distributions are in excellent
agreement with those calculated from the BD simulations. Thus, the FP equation
provides a valid description of translocation dynamics for sufficiently high
pore friction for the range of polymer lengths considered here. Increasing N
will require a corresponding increase in pore friction to maintain the validity
of the FP approach. Outside the regime of low N and high pore friction, the
polymer is out of equilibrium, and the FP approach is not valid.Comment: 13 pages, 11 figure
Polymer Translocation Dynamics in the Quasi-Static Limit
Monte Carlo (MC) simulations are used to study the dynamics of polymer
translocation through a nanopore in the limit where the translocation rate is
sufficiently slow that the polymer maintains a state of conformational
quasi-equilibrium. The system is modeled as a flexible hard-sphere chain that
translocates through a cylindrical hole in a hard flat wall. In some
calculations, the nanopore is connected at one end to a spherical cavity.
Translocation times are measured directly using MC dynamics simulations. For
sufficiently narrow pores, translocation is sufficiently slow that the mean
translocation time scales with polymer length N according to \propto
(N-N_p)^2, where N_p is the average number of monomers in the nanopore; this
scaling is an indication of a quasi-static regime in which polymer-nanopore
friction dominates. We use a multiple-histogram method to calculate the
variation of the free energy with Q, a coordinate used to quantify the degree
of translocation. The free energy functions are used with the Fokker-Planck
formalism to calculate translocation time distributions in the quasi-static
regime. These calculations also require a friction coefficient, characterized
by a quantity N_{eff}, the effective number of monomers whose dynamics are
affected by the confinement of the nanopore. This was determined by fixing the
mean of the theoretical distribution to that of the distribution obtained from
MC dynamics simulations. The theoretical distributions are in excellent
quantitative agreement with the distributions obtained directly by the MC
dynamics simulations for physically meaningful values of N_{eff}. The free
energy functions for narrow-pore systems exhibit oscillations with an amplitude
that is sensitive to the nanopore length. Generally, larger oscillation
amplitudes correspond to longer translocation times.Comment: 13 pages, 13 figure
Mode Repulsion and Mode Coupling in Random Lasers
We studied experimentally and theoretically the interaction of lasing modes
in random media. In a homogeneously broadened gain medium, cross gain
saturation leads to spatial repulsion of lasing modes. In an inhomogeneously
broadened gain medium, mode repulsion occurs in the spectral domain. Some
lasing modes are coupled through photon hopping or electron absorption and
reemission. Under pulsed pumping, weak coupling of two modes leads to
synchronization of their lasing action. Strong coupling of two lasing modes
results in anti-phased oscillations of their intensities.Comment: 13 pages, 4 figure
The effect of universal maternal antenatal iron supplementation on neurodevelopment in offspring : a systematic review and meta-analysis
Funding One author (CJ) was funded by the Ministry of Health, Government of Sri Lanka. Availability of data and materials Supporting data can be obtained from the corresponding author.Peer reviewedPublisher PD
Spontaneous Glioblastoma Spheroid Infiltration of Early-Stage Cerebral Organoids Models Brain Tumor Invasion.
Organoid methodology provides a platform for the ex vivo investigation of the cellular and molecular mechanisms underlying brain development and disease. The high-grade brain tumor glioblastoma multiforme (GBM) is considered a cancer of unmet clinical need, in part due to GBM cell infiltration into healthy brain parenchyma, making complete surgical resection improbable. Modeling the process of GBM invasion in real time is challenging as it requires both tumor and neural tissue compartments. Here, we demonstrate that human GBM spheroids possess the ability to spontaneously infiltrate early-stage cerebral organoids (eCOs). The resulting formation of hybrid organoids demonstrated an invasive tumor phenotype that was distinct from noncancerous adult neural progenitor (NP) spheroid incorporation into eCOs. These findings provide a basis for the modeling and quantification of the GBM infiltration process using a stem-cell-based organoid approach, and may be used for the identification of anti-GBM invasion strategies
Who needs a stapling device for haemorrhoidectomy, if one has the radiofrequency device?
Peer reviewedPublisher PD
Factors influencing the catalytic oxidation of benzyl alcohol using supported phosphine-capped gold nanoparticles
Open Access Article. This article is licensed under a Creative Commons Attribution 3.0 Unported Licence.Two phosphine-stabilised gold clusters, Au101(PPh3)21Cl5 and Au9(PPh3)8(NO3)3, were deposited and activated on anatase TiO2 and fumed SiO2. These catalysts showed an almost complete oxidation of benzyl alcohol (>90%) within 3 hours at 80 Ā°C and 3 bar O2 in methanol with a high substrate-to-metal molar ratio of 5800 and turn-over frequency of 0.65 sā1. Factors influencing catalytic activity were investigated, including metalāsupport interaction, effects of heat treatments, chemical composition of gold clusters, the size of gold nanoparticles and catalytic conditions. It was found that the anions present in gold clusters play a role in determining the catalytic activity in this reaction, with NO3ā diminishing the catalytic activity. High catalytic activity was attributed to the formation of large gold nanoparticles (>2 nm) that coincides with partial removal of ligands which occurs during heat treatment and catalysis. Selectivity towards the formation of methyl benzoate can be tuned by selection of the reaction temperature. The catalysts were characterised using transmission electron microscopy, UV-vis diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy
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