193 research outputs found
Structure of Polymer Brushes in Cylindrical Tubes: A Molecular Dynamics Simulation
Molecular Dynamics simulations of a coarse-grained bead-spring model of
flexible macromolecules tethered with one end to the surface of a cylindrical
pore are presented. Chain length and grafting density are varied
over a wide range and the crossover from ``mushroom'' to ``brush'' behavior is
studied for three pore diameters. The monomer density profile and the
distribution of the free chain ends are computed and compared to the
corresponding model of polymer brushes at flat substrates. It is found that
there exists a regime of and for large enough pore diameter where
the brush height in the pore exceeds the brush height on the flat substrate,
while for large enough and (and small enough pore diameters) the
opposite behavior occurs, i.e. the brush is compressed by confinement. These
findings are used to discuss the corresponding theories on polymer brushes at
concave substrates.Comment: 11 figure
Monte Carlo simulations reveal the straightening up of an end-grafted flexible chain with a rigid side chain
We have studied the conformational properties of a flexible end-grafted chain
(length ) with a rigid side chain (length ) by means of Monte Carlo
simulations. Depending on the lengths and and the branching site, ,
we observe a considerable straightening of the flexible backbone as quantified
via the gyration tensor. For , i.e. when attaching the side chain to the
free end of the flexible backbone, the effect was strongest
Structural Properties of Two-Dimensional Polymers
We present structural properties of two-dimensional polymers as far as they
can be described by percolation theory. The percolation threshold, critical
exponents and fractal dimensions of clusters are determined by computer
simulation and compared to the results of percolation theory. We also describe
the dependence of the typical cluster structures on the reaction rate.Comment: 7 pages, LaTeX with RevTeX and epsf styles and PostScript figures
included (uuencoded shell archive), TVP-93051
Recommended from our members
Survival and some physiological aspects of tissue cultured cells from Douglas-fir (Pseudotsuga menziesii Mirb. (Franco)) and a poplar hybrid after freezing to liquid nitrogen temperature
Cell aggregate size in both Douglas-fir and poplar suspension cultures was reduced by the addition of the chelator compounds EDTA and CDTA at concentrations under 100 ppm. Reduced cell aggregate size increased growth efficiency of suspension cultures of both species. Cell aggregates 550 j.z. or smaller in size were used in the cryogenic studies.
At room temperature, the cryoprotectants dimethyl sulfoxide (DMSO) and glycerol at concentrations above 5% were toxic to cell
survival; however when these were washed out with fresh media, the growth rate of both Douglas-fir and poplar suspension cultures was increased. The optimum cryoprotectant concentration was about 1.0%. At concentrations above 5% repeated washing with fresh media did not reduce the cellular cryoprotectant concentration and cellular injury or death may have resulted solely from the presence of cryoprotectants, even in the absence of freezing. Cryoprotectant influences observed were thought to be due to cell membrane changes allowing increased nutrient flow in the case of cell death. Cold conditioning or chilling callus at +4°C allowed higher concentrations of cryoprotectants to be added without killing cells. Suspension cultures showed that Douglas-fir entered the log phase of growth 7 days and poplar 3 days after innoculation. Cells were small and densely cytoplasmic during this time. For both species slow freezing of log-growth-phase cells at 1°C/mm was better than fast freezing; a two-step freezing protocol, cooling first to -40°C and then immersion into liquid nitrogen was successful; cooling to -40°C was more effective than cooling to higher temperatures; and fast thawing to +40°C was more effective than slow thawing in air. The cryoprotectants DMSO, glycerol and sucrose are effective alone and in combination. Most effective was a combination of all three; DMSO 5%/glycerol 1%/sucrose 4% for Douglas-fir and DMSO 5%/glycerol 5%/sucrose 4% for poplar. Douglas-fir and poplar cultures on thawing were tested for
viability-by assessing ability to reduce tetrazolium salts and ability to continue growth. Poplar cultures resumed growth after
approximately 5 weeks post-thaw culture, whereas for Douglas-fir over 80 days was required. Thawed and recultured poplar callus
differentiated into shoots and roots. Cold conditioned callus Of both Douglas-fir and poplar produced high TTC reduction values directly after thawing. Cold conditioned poplar callus frozen without cryoprotectantS or liquid responded better on thawing than when frozen with cryoprotectants in liquid media. Douglas-fir buds obtained from field trees in August or seedlings cold conditioned at +4°C for 8 weeks were frozen under various conditions, but no buds survived the freezing process. The development of techniques for successful freeze preservation requires a detailed empirical examination of; prefreezing
culture conditions, physiological state and size of cells, cryoprotectant type and concentration used, freezing rate and temperature frozen to, and thaw rate and post-thaw culture conditions
Large-scale Simulation of the Two-dimensional Kinetic Ising Model
We present Monte Carlo simulation results for the dynamical critical exponent
of the two-dimensional kinetic Ising model using a lattice of size spins. We used Glauber as well as Metropolis dynamics. The
-value of was calculated from the magnetization and energy
relaxation from an ordered state towards the equilibrium state at .Comment: 6 pages + 2 figures as separate uuencoded compressed tar file,
Postscipt also available at http://wwwcp.tphys.uni-heidelberg.de/papers
Dynamical temperature study for classical planar spin systems
In this micro-canonical simulation the temperature and also the specific heat
are determined as averages of expressions easy to implement. The XY-chain is
studied for a test. The second order transition on a cubic lattice and the
first order transition on an fcc lattice are analyzed in greater detail to have
a more severe test about the feasibility of this micro-canonical method.Comment: 9 pages in Latex(revtex), 7 PS-figure
The influence of the cylindrical shape of the nucleosomes and H1 defects on properties of chromatin
We present a model improving the two-angle model for interphase chromatin
(E2A model). This model takes into account the cylindrical shape of the histone
octamers, the H1 histones in front of the nucleosomes and the vertical distance
between the in and outgoing DNA strands. Factoring these chromatin features
in, one gets essential changes in the chromatin phase diagram: Not only the
shape of the excluded-volume borderline changes but also the vertical distance
has a dramatic influence on the forbidden area. Furthermore, we examined
the influence of H1 defects on the properties of the chromatin fiber. Thus we
present two possible strategies for chromatin compaction: The use of very dense
states in the phase diagram in the gaps in the excluded volume borderline or
missing H1 histones which can lead to very compact fibers. The chromatin fiber
might use both of these mechanisms to compact itself at least locally. Line
densities computed within the model coincident with the experimental values
Superdiffusion in a Model for Diffusion in a Molecularly Crowded Environment
We present a model for diffusion in a molecularly crowded environment. The
model consists of random barriers in percolation network. Random walks in the
presence of slowly moving barriers show normal diffusion for long times, but
anomalous diffusion at intermediate times. The effective exponents for square
distance versus time usually are below one at these intermediate times, but can
be also larger than one for high barrier concentrations. Thus we observe sub-
as well as super-diffusion in a crowded environment.Comment: 8 pages including 4 figure
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