558 research outputs found
Dynamical Monte Carlo Study of Equilibrium Polymers : Static Properties
We report results of extensive Dynamical Monte Carlo investigations on
self-assembled Equilibrium Polymers (EP) without loops in good solvent. (This
is thought to provide a good model of giant surfactant micelles.) Using a novel
algorithm we are able to describe efficiently both static and dynamic
properties of systems in which the mean chain length \Lav is effectively
comparable to that of laboratory experiments (up to 5000 monomers, even at high
polymer densities). We sample up to scission energies of over
nearly three orders of magnitude in monomer density , and present a
detailed crossover study ranging from swollen EP chains in the dilute regime up
to dense molten systems. Confirming recent theoretical predictions, the
mean-chain length is found to scale as \Lav \propto \phi^\alpha \exp(\delta
E) where the exponents approach
and in the
dilute and semidilute limits respectively. The chain length distribution is
qualitatively well described in the dilute limit by the Schulz-Zimm
distribution \cN(s)\approx s^{\gamma-1} \exp(-s) where the scaling variable
is s=\gamma L/\Lav. The very large size of these simulations allows also an
accurate determination of the self-avoiding walk susceptibility exponent
. ....... Finite-size effects are discussed in
detail.Comment: 15 pages, 14 figures, LATE
The reversible polydisperse Parking Lot Model
We use a new version of the reversible Parking Lot Model to study the
compaction of vibrated polydisperse media. The particle sizes are distributed
according to a truncated power law. We introduce a self-consistent desorption
mechanism with a hierarchical initialization of the system. In this way, we
approach densities close to unity. The final density depends on the
polydispersity of the system as well as on the initialization and will reach a
maximum value for a certain exponent in the power law.Comment: 7 pages, Latex, 12 figure
Reactions at polymer interfaces: A Monte Carlo Simulation
Reactions at a strongly segregated interface of a symmetric binary polymer
blend are investigated via Monte Carlo simulations. End functionalized
homopolymers of different species interact at the interface instantaneously and
irreversibly to form diblock copolymers. The simulations, in the framework of
the bond fluctuation model, determine the time dependence of the copolymer
production in the initial and intermediate time regime for small reactant
concentration . The results are compared to
recent theories and simulation data of a simple reaction diffusion model. For
the reactant concentration accessible in the simulation, no linear growth of
the copolymer density is found in the initial regime, and a -law is
observed in the intermediate stage.Comment: to appear in Macromolecule
The impact of ENSO on Southern African rainfall in CMIP5 ocean atmosphere coupled climate models
We study the ability of 24 ocean atmosphere global coupled models from the Coupled Model Intercomparison Project 5 (CMIP5) to reproduce the teleconnections between El Niño Southern Oscillation (ENSO) and Southern African rainfall in austral summer using historical forced simulations, with a focus on the atmospheric dynamic associated with El Niño. Overestimations of summer rainfall occur over Southern Africa in all CMIP5 models. Abnormal westward extensions of ENSO patterns are a common feature of all CMIP5 models, while the warming of the Indian Ocean that happens during El Niño is not correctly reproduced. This could impact the teleconnection between ENSO and Southern African rainfall which is represented with mixed success in CMIP5 models. Large-scale anomalies of suppressed deep-convection over the tropical maritime continent and enhanced convection from the central to eastern Pacific are correctly simulated. However, regional biases occur above Africa and the Indian Ocean, particularly in the position of the deep convection anomalies associated with El Niño, which can lead to the wrong sign in rainfall anomalies in the northwest part of South Africa. From the near-surface to mid-troposphere, CMIP5 models underestimate the observed anomalous pattern of pressure occurring over Southern Africa that leads to dry conditions during El Niño years
Critical dynamics in thin films
Critical dynamics in film geometry is analyzed within the field-theoretical
approach. In particular we consider the case of purely relaxational dynamics
(Model A) and Dirichlet boundary conditions, corresponding to the so-called
ordinary surface universality class on both confining boundaries. The general
scaling properties for the linear response and correlation functions and for
dynamic Casimir forces are discussed. Within the Gaussian approximation we
determine the analytic expressions for the associated universal scaling
functions and study quantitatively in detail their qualitative features as well
as their various limiting behaviors close to the bulk critical point. In
addition we consider the effects of time-dependent fields on the
fluctuation-induced dynamic Casimir force and determine analytically the
corresponding universal scaling functions and their asymptotic behaviors for
two specific instances of instantaneous perturbations. The universal aspects of
nonlinear relaxation from an initially ordered state are also discussed
emphasizing the different crossovers that occur during this evolution. The
model considered is relevant to the critical dynamics of actual uniaxial
ferromagnetic films with symmetry-preserving conditions at the confining
surfaces and for Monte Carlo simulations of spin system with Glauber dynamics
and free boundary conditions.Comment: 64 pages, 21 figure
String-like Clusters and Cooperative Motion in a Model Glass-Forming Liquid
A large-scale molecular dynamics simulation is performed on a glass-forming
Lennard-Jones mixture to determine the nature of dynamical heterogeneities
which arise in this model fragile liquid. We observe that the most mobile
particles exhibit a cooperative motion in the form of string-like paths
(``strings'') whose mean length and radius of gyration increase as the liquid
is cooled. The length distribution of the strings is found to be similar to
that expected for the equilibrium polymerization of linear polymer chains.Comment: 6 pages of RevTex, 6 postscript figures, uses epsf.st
Identification of four families of yCCR4- and Mg(2+)-dependent endonuclease-related proteins in higher eukaryotes, and characterization of orthologs of yCCR4 with a conserved leucine-rich repeat essential for hCAF1/hPOP2 binding
BACKGROUND: The yeast yCCR4 factor belongs to the CCR4-NOT transcriptional regulatory complex, in which it interacts, through its leucine-rich repeat (LRR) motif with yPOP2. Recently, yCCR4 was shown to be a component of the major cytoplasmic mRNA deadenylase complex, and to contain a fold related to the Mg(2+)-dependent endonuclease core. RESULTS: Here, we report the identification of nineteen yCCR4-related proteins in eukaryotes (including yeast, plants and animals), which all contain the yCCR4 endonuclease-like fold, with highly conserved CCR4-specific residues. Phylogenetic and genomic analyses show that they form four distinct families, one of which contains the yCCR4 orthologs. The orthologs in animals possess a leucine-rich repeat domain. We show, using two-hybrid and far-Western assays, that the human member binds to the human yPOP2 homologs, i.e. hCAF1 and hPOP2, in a LRR-dependent manner. CONCLUSIONS: We have identified the mammalian orthologs of yCCR4 and have shown that the human member binds to the human yPOP2 homologs, thus strongly suggesting conservation of the CCR4-NOT complex from yeast to human. All members of the four identified yCCR4-related protein families show stricking conservation of the endonuclease-like catalytic motifs of the yCCR4 C-terminal domain and therefore constitute a new family of potential deadenylases in mammals
Madagascar corals track sea surface temperature variability in the Agulhas Current core region over the past 334 years
The Agulhas Current (AC) is the strongest western boundary current in the Southern Hemisphere and is key for weather and climate patterns, both regionally and globally. Its heat transfer into both the midlatitude South Indian Ocean and South Atlantic is of global significance. A new composite coral record (Ifaty and Tulear massive Porites corals), is linked to historical AC sea surface temperature (SST) instrumental data, showing robust correlations. The composite coral SST data start in 1660 and comprise 200 years more than the AC instrumental record. Numerical modelling exhibits that this new coral derived SST record is representative for the wider core region of the AC. AC SSTs variabilities show distinct cooling through the Little Ice Age and warming during the late 18th, 19th and 20th century, with significant decadal variability superimposed. Furthermore, the AC SSTs are teleconnected with the broad southern Indian and Atlantic Oceans, showing that the AC system is pivotal for inter-ocean heat exchange south of Africa
iPSC-derived neuronal models of PANK2-associated neurodegeneration reveal mitochondrial dysfunction contributing to early disease
Mutations in PANK2 lead to neurodegeneration with brain iron accumulation. PANK2 has a role in the biosynthesis of coenzyme A (CoA) from dietary vitamin B5, but the neuropathological mechanism and reasons for iron accumulation remain unknown. In this study, atypical patient-derived fibroblasts were reprogrammed into induced pluripotent stem cells (iPSCs) and subsequently differentiated into cortical neuronal cells for studying disease mechanisms in human neurons. We observed no changes in PANK2 expression between control and patient cells, but a reduction in protein levels was apparent in patient cells. CoA homeostasis and cellular iron handling were normal, mitochondrial function was affected; displaying activated NADH-related and inhibited FADH-related respiration, resulting in increased mitochondrial membrane potential. This led to increased reactive oxygen species generation and lipid peroxidation in patient-derived neurons. These data suggest that mitochondrial deficiency is an early feature of the disease process and can be explained by altered NADH/FADH substrate supply to oxidative phosphorylation. Intriguingly, iron chelation appeared to exacerbate the mitochondrial phenotype in both control and patient neuronal cells. This raises caution for the use iron chelation therapy in general when iron accumulation is absent
Exact Three Dimensional Casimir Force Amplitude, -function and Binder's Cumulant Ratio: Spherical Model Results
The three dimensional mean spherical model on a hypercubic lattice with a
film geometry under periodic boundary conditions is
considered in the presence of an external magnetic field . The universal
Casimir amplitude and the Binder's cumulant ratio are calculated
exactly and found to be and
A discussion on the relations
between the finite temperature -function, usually defined for quantum
systems, and the excess free energy (due to the finite-size contributions to
the free energy of the system) scaling function is presented. It is
demonstrated that the -function of the model equals 4/5 at the bulk critical
temperature . It is analytically shown that the excess free energy is a
monotonically increasing function of the temperature and of the magnetic
field in the vicinity of This property is supposed to hold for any
classical -dimensional model with a film geometry under periodic
boundary conditions when . An analytical evidence is also presented to
confirm that the Casimir force in the system is negative both below and in the
vicinity of the bulk critical temperature Comment: 12 pages revtex, one eps figure, submitted to Phys. Rev E A set of
references added with the text needed to incorporate them. Small changes in
the title and in the abstrac
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