3,729 research outputs found
Monte Carlo simulations of copolymers at homopolymer interfaces: Interfacial structure as a function of the copolymer density
By means of extensive Monte Carlo simulations of the bond fluctuation model,
we study the effect of adding AB diblock copolymers on the properties of an
interface between demixed homopolymer phases. The parameters are chosen such
that the homopolymers are strongly segregated, and the whole range of copolymer
concentrations in the two phase coexistence region is scanned. We compare the
``mushroom'' regime, in which copolymers are diluted and do not interact with
each other, with the ``wet brush'' regime, where copolymers overlap and
stretch, but are still swollen by the homopolymers. A ``dry brush'' regime is
never entered for our choice of chain lengths. ``Intrinsic'' profiles are
calculated using a block analysis method introduced by us in earlier work. We
discuss density profiles, orientational profiles and contact number profiles.
In general, the features of the profiles are similar at all copolymer
concentrations, however, the profiles in the concentrated regime are much
broader than in the dilute regime. The results compare well with
self-consistent field calculations.Comment: to appear in J. Chem. Phy
Export of honeybee prepromelittin in Escherichia coli depends on the membrane potential but does not depend on proteins secA and secY
Honeybee prepromelittin (70 amino acid residues), the precursor of an eukaryotic secretory protein, and a hybrid protein between prepromelittin and mouse dihydrofolate reductase (257 amino acid residues) were expressed in Escherichia coli and characterized with respect to their requirements for transport across the plasma membrane. Both precursor proteins are posttranslationally processed and exported into the periplasm, and they both depend on the membrane potential for this to occur. With respect to dependence on components of the export machinery, however, the two precursor proteins show striking differences: the small precursor protein prepromelittin does not require the function of proteins secA and secY; the large precursor protein prepromelittin-dihydrofolate reductase, on the other hand, depends on both components. The implications of these observations with respect to the mechanisms of protein export in E. coli and of protein import into the endoplasmic reticulum are discussed
Ab-Initio Calculation of the Metal-Insulator Transition in Sodium rings and chains and in mixed Sodium-Lithium systems
We study how the Mott metal-insulator transition (MIT) is influenced when we
deal with electrons with different angular momenta. For lithium we found an
essential effect when we include -orbitals in the description of the Hilbert
space. We apply quantum-chemical methods to sodium rings and chains in order to
investigate the analogue of a MIT, and how it is influenced by periodic and
open boundaries. By changing the interatomic distance we analyse the character
of the many-body wavefunction and the charge gap. In the second part we mimic a
behaviour found in the ionic Hubbard model, where a transition from a band to a
Mott insulator occurs. For that purpose we perform calculations for mixed
sodium-lithium rings. In addition, we examine the question of bond alternation
for the pure sodium system and the mixed sodium-lithium system, in order to
determine under which conditions a Peierls distortion occurs.Comment: 8 pages, 7 figures, accepted Eur. J. Phys.
Effect of long range forces on the interfacial profiles in thin binary polymer films
We study the effect of surface fields on the interfacial properties of a
binary polymer melt confined between two parallel walls. Each wall attracts a
different component of the blend by a non-retarded van der Waals potential. An
interface which runs parallel to the surfaces is stabilized in the center of
the film. Using extensive Monte Carlo simulations we study the interfacial
properties as a function of the film thickness, the strength of the surface
forces and the lateral size over which the profiles across the film are
averaged. We find evidence for capillary wave broadening of the apparent
interfacial profiles. However, the apparent interfacial width cannot be
described quantitatively by a simple logarithmic dependence on the film
thickness. The Monte Carlo simulations reveal that the surface fields give rise
to an additional reduction of the intrinsic interfacial width and an increase
of the effective interfacial tension upon decreasing the film thickness. These
modifications of the intrinsic interfacial properties are confirmed by
self-consistent field calculations. Taking account of the thickness dependence
of the intrinsic interfacial properties and the capillary wave broadening, we
can describe our simulation results quantitatively.Comment: to appear in J.Chem.Phy
Ribonucleoparticle-independent transport of proteins into mammalian microsomes
There are at least two different mechanisms for the transport of secretory proteins into the mammalian endoplasmic reticulum. Both mechanisms depend on the presence of a signal peptide on the respective precursor protein and involve a signal peptide receptor on the cis-side and signal peptidase on the trans-side of the membrane. Furthermore, both mechanisms involve a membrane component with a cytoplasmically exposed sulfhydryl. The decisive feature of the precursor protein with respect to which of the two mechanisms is used is the chain length of the polypeptide. The critical size seems to be around 70 amino acid residues (including the signal peptide). The one mechanism is used by precursor proteins larger than about 70 amino acid residues and involves two cytosolic ribonucleoparticles and their receptors on the microsomal surface. The other one is used by small precursor proteins and relies on the mature part within the precursor molecule and a cytosolic ATPase
Theory of nuclear excitation by electron capture for heavy ions
We investigate the resonant process of nuclear excitation by electron
capture, in which a continuum electron is captured into a bound state of an ion
with the simultaneous excitation of the nucleus. In order to derive the cross
section a Feshbach projection operator formalism is introduced. Nuclear states
and transitions are described by a nuclear collective model and making use of
experimental data. Transition rates and total cross sections for NEEC followed
by the radiative decay of the excited nucleus are calculated for various heavy
ion collision systems
Genetic diversity of honeybee colonies predicts gut bacterial diversity of individual colony members.
The gut microbiota of social bees is relatively simple and dominated by a set of core taxa found consistently in individuals around the world. Yet, variation remains, and can affect host health. We characterised individual- and regional-scale variation in honeybee (Apis mellifera) gut microbiota from 64 colonies in North-West England by sequencing the V4 region of the 16S rRNA gene, and asked whether microbiota were influenced by host genotype and landscape composition. We also characterised the genotypes of individual bees and the land cover surrounding each colony. The literature-defined core taxa dominated across the region despite the varied environments. However, there was variation in the relative abundance of core taxa, and colony membership explained much of this variation. Individuals from more genetically diverse colonies had more diverse microbiotas, but individual genetic diversity did not influence gut microbial diversity. There were weak trends for colonies in more similar landscapes to have more similar microbiota, and for bees from more urban landscapes to have less diverse microbiota. To our knowledge, this is the first report for any species that the gut bacterial communities of individuals are influenced by the genotypes of others in the population. This article is protected by copyright. All rights reserved
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
Intrinsic profiles and capillary waves at homopolymer interfaces: a Monte Carlo study
A popular concept which describes the structure of polymer interfaces by
``intrinsic profiles'' centered around a two dimensional surface, the ``local
interface position'', is tested by extensive Monte Carlo simulations of
interfaces between demixed homopolymer phases in symmetric binary (AB)
homopolymer blends, using the bond fluctuation model. The simulations are done
in an LxLxD geometry. The interface is forced to run parallel to the LxL planes
by imposing periodic boundary conditions in these directions and fixed boundary
conditions in the D direction, with one side favoring A and the other side
favoring B. Intrinsic profiles are calculated as a function of the ``coarse
graining length'' B by splitting the system into columns of size BxBxD and
averaging in each column over profiles relative to the local interface
position. The results are compared to predictions of the self-consistent field
theory. It is shown that the coarse graining length can be chosen such that the
interfacial width matches that of the self-consistent field profiles, and that
for this choice of B the ``intrinsic'' profiles compare well with the
theoretical predictions.Comment: to appear in Phys. Rev.
Spinodal Decomposition in a Binary Polymer Mixture: Dynamic Self Consistent Field Theory and Monte Carlo Simulations
We investigate how the dynamics of a single chain influences the kinetics of
early stage phase separation in a symmetric binary polymer mixture. We consider
quenches from the disordered phase into the region of spinodal instability. On
a mean field level we approach this problem with two methods: a dynamical
extension of the self consistent field theory for Gaussian chains, with the
density variables evolving in time, and the method of the external potential
dynamics where the effective external fields are propagated in time. Different
wave vector dependencies of the kinetic coefficient are taken into account.
These early stages of spinodal decomposition are also studied through Monte
Carlo simulations employing the bond fluctuation model that maps the chains --
in our case with 64 effective segments -- on a coarse grained lattice. The
results obtained through self consistent field calculations and Monte Carlo
simulations can be compared because the time, length, and temperature scales
are mapped onto each other through the diffusion constant, the chain extension,
and the energy of mixing. The quantitative comparison of the relaxation rate of
the global structure factor shows that a kinetic coefficient according to the
Rouse model gives a much better agreement than a local, i.e. wave vector
independent, kinetic factor. Including fluctuations in the self consistent
field calculations leads to a shorter time span of spinodal behaviour and a
reduction of the relaxation rate for smaller wave vectors and prevents the
relaxation rate from becoming negative for larger values of the wave vector.
This is also in agreement with the simulation results.Comment: Phys.Rev.E in prin
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