688 research outputs found
Chirality and Protein Folding
There are several simple criteria of folding to a native state in model
proteins. One of them involves crossing of a threshold value of the RMSD
distance away from the native state. Another checks whether all native contacts
are established, i.e. whether the interacting amino acids come closer than some
characteristic distance. We use Go-like models of proteins and show that such
simple criteria may prompt one to declare folding even though fragments of the
resulting conformations have a wrong sense of chirality. We propose that a
better condition of folding should augment the simple criteria with the
requirement that most of the local values of the chirality should be nearly
native. The kinetic discrepancy between the simple and compound criteria can be
substantially reduced in the Go-like models by providing the Hamiltonian with a
term which favors native values of the local chirality. We study the effects of
this term as a function of its amplitude and compare it to other models such as
with the side groups and with the angle-dependent potentials.Comment: To be published in a special issue of J. Phys.: Cond. Mat. (Bedlewo
Workshop
Effects of confinement and crowding on folding of model proteins
We perform molecular dynamics simulations for a simple coarse-grained model
of crambin placed inside of a softly repulsive sphere of radius R. The
confinement makes folding at the optimal temperature slower and affects the
folding scenarios, but both effects are not dramatic. The influence of crowding
on folding are studied by placing several identical proteins within the sphere,
denaturing them, and then by monitoring refolding. If the interactions between
the proteins are dominated by the excluded volume effects, the net folding
times are essentially like for a single protein. An introduction of
inter-proteinic attractive contacts hinders folding when the strength of the
attraction exceeds about a half of the value of the strength of the single
protein contacts. The bigger the strength of the attraction, the more likely is
the occurrence of aggregation and misfolding
Experimental Limits on Primordial Black Hole Dark Matter from the First Two Years of Kepler Data
We present the analysis on our new limits of the dark matter (DM) halo
consisting of primordial black holes (PBHs) or massive compact halo objects
(MACHOs). We present a search of the first two years of publicly available
Kepler mission data for potential signatures of gravitational microlensing
caused by these objects, as well as an extensive analysis of the astrophysical
sources of background error. These include variable stars, flare events, and
comets or asteroids which are moving through the Kepler field. We discuss the
potential of detecting comets using the Kepler lightcurves, presenting
measurements of two known comets and one unidentified object, most likely an
asteroid or comet. After removing the background events with statistical cuts,
we find no microlensing candidates. We therefore present our Monte Carlo
efficiency calculation in order to constrain the PBH DM with masses in the
range of 2 x 10^-9 solar masses to 10^-7 solar masses. We find that PBHs in
this mass range cannot make up the entirety of the DM, thus closing a full
order of magnitude in the allowed mass range for PBH DM.Comment: 12 pages, 6 figure
Doping effects of Co, Ni, and Cu in FeTe0.65Se0.35 single crystals
The resistivity, magnetoresistance, and magnetic susceptibility are measured
in single crystals of FeTe0.65Se0.35 with Cu, Ni, and Co substitutions for Fe.
The crystals are grown by Bridgman's method. The resistivity measurements show
that superconductivity disappears with the rate which correlates with the
nominal valence of the impurity. From magnetoresistance we evaluate doping
effect on the basic superconducting parameters, such as upper critical field
and coherence length. We find indications that doping leads to two component
superconducting behavior, possibly because of local charge depression around
impurities.Comment: 4 pages, 4 figures, 1 table, Proceedings of the XV-th National School
"Hundred Years of Superconductivity", Kazimierz Dolny, October 9-13, 201
Response approach to the squeezed-limit bispectrum: application to the correlation of quasar and Lyman- forest power spectrum
The squeezed-limit bispectrum, which is generated by nonlinear gravitational
evolution as well as inflationary physics, measures the correlation of three
wavenumbers, in the configuration where one wavenumber is much smaller than the
other two. Since the squeezed-limit bispectrum encodes the impact of a
large-scale fluctuation on the small-scale power spectrum, it can be understood
as how the small-scale power spectrum "responds" to the large-scale
fluctuation. Viewed in this way, the squeezed-limit bispectrum can be
calculated using the response approach even in the cases which do not submit to
perturbative treatment. To illustrate this point, we apply this approach to the
cross-correlation between the large-scale quasar density field and small-scale
Lyman- forest flux power spectrum. In particular, using separate
universe simulations which implement changes in the large-scale density,
velocity gradient, and primordial power spectrum amplitude, we measure how the
Lyman- forest flux power spectrum responds to the local,
long-wavelength quasar overdensity, and equivalently their squeezed-limit
bispectrum. We perform a Fisher forecast for the ability of future experiments
to constrain local non-Gaussianity using the bispectrum of quasars and the
Lyman- forest. Combining with quasar and Lyman- forest power
spectra to constrain the biases, we find that for DESI the expected
constraint is . Ability for DESI to measure
through this channel is limited primarily by the aliasing and
instrumental noise of the Lyman- forest flux power spectrum. The
combination of response approach and separate universe simulations provides a
novel technique to explore the constraints from the squeezed-limit bispectrum
between different observables.Comment: 20 pages, 4 figures; matches JCAP accepted versio
Molecular jamming - the cystine slipknot mechanical clamp in all-atom simulations
A recent survey of 17 134 proteins has identified a new class of proteins
which are expected to yield stretching induced force-peaks in the range of 1
nN. Such high force peaks should be due to forcing of a slip-loop through a
cystine ring, i.e. by generating a cystine slipknot. The survey has been
performed in a simple coarse grained model. Here, we perform all-atom steered
molecular dynamics simulations on 15 cystine knot proteins and determine their
resistance to stretching. In agreement with previous studies within a coarse
grained structure based model, the level of resistance is found to be
substantially higher than in proteins in which the mechanical clamp operates
through shear. The large stretching forces arise through formation of the
cystine slipknot mechanical clamp and the resulting steric jamming. We
elucidate the workings of such a clamp in an atomic detail. We also study the
behavior of five top strength proteins with the shear-based mechanostability in
which no jamming is involved. We show that in the atomic model, the jamming
state is relieved by moving one amino acid at a time and there is a choice in
the selection of the amino acid that advances the first. In contrast, the
coarse grained model also allows for a simultaneous passage of two amino acids
Delineation of the Native Basin in Continuum Models of Proteins
We propose two approaches for determining the native basins in off-lattice
models of proteins. The first of them is based on exploring the saddle points
on selected trajectories emerging from the native state. In the second
approach, the basin size can be determined by monitoring random distortions in
the shape of the protein around the native state. Both techniques yield the
similar results. As a byproduct, a simple method to determine the folding
temperature is obtained.Comment: REVTeX, 6 pages, 5 EPS figure
Force-induced unfolding of a homopolymer on fractal lattice: exact results vs. mean field predictions
We study the force-induced unfolding of a homopolymer on the three
dimensional Sierpinski gasket. The polymer is subject to a contact energy
between nearest neighbour sites not consecutive along the chain and to a
stretching force. The hierarchical nature of the lattice we consider allows for
an exact treatment which yields the phase diagram and the critical behaviour.
We show that for this model mean field predictions are not correct, in
particular in the exact phase diagram there is {\em not} a low temperature
reentrance and we find that the force induced unfolding transition below the
theta temperature is second order.Comment: 15 pages, 5 eps figure
Rate Determining Factors in Protein Model Structures
Previous research has shown a strong correlation of protein folding rates to
the native state geometry, yet a complete explanation for this dependence is
still lacking. Here we study the rate-geometry relationship with a simple
statistical physics model, and focus on two classes of model geometries,
representing ideal parallel and antiparallel structures. We find that the
logarithm of the rate shows an almost perfect linear correlation with the
"absolute contact order", but the slope depends on the particular class
considered. We discuss these findings in the light of experimental results.Comment: 4 pages, 2 figure
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