10,997 research outputs found
Bubbles, clusters and denaturation in genomic DNA: modeling, parametrization, efficient computation
The paper uses mesoscopic, non-linear lattice dynamics based
(Peyrard-Bishop-Dauxois, PBD) modeling to describe thermal properties of DNA
below and near the denaturation temperature. Computationally efficient notation
is introduced for the relevant statistical mechanics. Computed melting profiles
of long and short heterogeneous sequences are presented, using a recently
introduced reparametrization of the PBD model, and critically discussed. The
statistics of extended open bubbles and bound clusters is formulated and
results are presented for selected examples.Comment: to appear in a special issue of the Journal of Nonlinear Mathematical
Physics (ed. G. Gaeta
Toward the Jamming Threshold of Sphere Packings: Tunneled Crystals
We have discovered a new family of three-dimensional crystal sphere packings
that are strictly jammed (i.e., mechanically stable) and yet possess an
anomalously low density. This family constitutes an uncountably infinite number
of crystal packings that are subpackings of the densest crystal packings and
are characterized by a high concentration of self-avoiding "tunnels" (chains of
vacancies) that permeate the structures. The fundamental geometric
characteristics of these tunneled crystals command interest in their own right
and are described here in some detail. These include the lattice vectors (that
specify the packing configurations), coordination structure, Voronoi cells, and
density fluctuations. The tunneled crystals are not only candidate structures
for achieving the jamming threshold (lowest-density rigid packing), but may
have substantially broader significance for condensed matter physics and
materials science.Comment: 19 pages, 5 figure
Analysis of strain and stacking faults in single nanowires using Bragg coherent diffraction imaging
Coherent diffraction imaging (CDI) on Bragg reflections is a promising
technique for the study of three-dimensional (3D) composition and strain fields
in nanostructures, which can be recovered directly from the coherent
diffraction data recorded on single objects. In this article we report results
obtained for single homogeneous and heterogeneous nanowires with a diameter
smaller than 100 nm, for which we used CDI to retrieve information about
deformation and faults existing in these wires. The article also discusses the
influence of stacking faults, which can create artefacts during the
reconstruction of the nanowire shape and deformation.Comment: 18 pages, 6 figures Submitted to New Journal of Physic
Can one predict DNA Transcription Start Sites by studying bubbles?
It has been speculated that bubble formation of several base-pairs due to
thermal fluctuations is indicatory for biological active sites. Recent
evidence, based on experiments and molecular dynamics (MD) simulations using
the Peyrard-Bishop-Dauxois model, seems to point in this direction. However,
sufficiently large bubbles appear only seldom which makes an accurate
calculation difficult even for minimal models. In this letter, we introduce a
new method that is orders of magnitude faster than MD. Using this method we
show that the present evidence is unsubstantiated.Comment: 4 pages, 3 figures, accepted for publication in physical review
letter
Density-functional study of Cu atoms, monolayers, and coadsorbates on polar ZnO surfaces
The structure and electronic properties of single Cu atoms, copper monolayers
and thin copper films on the polar oxygen and zinc terminated surfaces of ZnO
are studied using periodic density-functional calculations. We find that the
binding energy of Cu atoms sensitively depends on how charge neutrality of the
polar surfaces is achieved. Bonding is very strong if the surfaces are
stabilized by an electronic mechanism which leads to partially filled surface
bands. As soon as the surface bands are filled (either by partial Cu coverage,
by coadsorbates, or by the formation of defects), the binding energy decreases
significantly. In this case, values very similar to those found for nonpolar
surfaces and for copper on finite ZnO clusters are obtained. Possible
implications of these observations concerning the growth mode of copper on
polar ZnO surfaces and their importance in catalysis are discussed.Comment: 6 pages with 2 postscript figures embedded. Uses REVTEX and epsf
macro
DNA Renaturation at the Water-Phenol Interface
We study DNA adsorption and renaturation in a water-phenol two-phase system,
with or without shaking. In very dilute solutions, single-stranded DNA is
adsorbed at the interface in a salt-dependent manner. At high salt
concentrations the adsorption is irreversible. The adsorption of the
single-stranded DNA is specific to phenol and relies on stacking and hydrogen
bonding. We establish the interfacial nature of a DNA renaturation at a high
salt concentration. In the absence of shaking, this reaction involves an
efficient surface diffusion of the single-stranded DNA chains. In the presence
of a vigorous shaking, the bimolecular rate of the reaction exceeds the
Smoluchowski limit for a three-dimensional diffusion-controlled reaction. DNA
renaturation in these conditions is known as the Phenol Emulsion Reassociation
Technique or PERT. Our results establish the interfacial nature of PERT. A
comparison of this interfacial reaction with other approaches shows that PERT
is the most efficient technique and reveals similarities between PERT and the
renaturation performed by single-stranded nucleic acid binding proteins. Our
results lead to a better understanding of the partitioning of nucleic acids in
two-phase systems, and should help design improved extraction procedures for
damaged nucleic acids. We present arguments in favor of a role of phenol and
water-phenol interface in prebiotic chemistry. The most efficient renaturation
reactions (in the presence of condensing agents or with PERT) occur in
heterogeneous systems. This reveals the limitations of homogeneous approaches
to the biochemistry of nucleic acids. We propose a heterogeneous approach to
overcome the limitations of the homogeneous viewpoint
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