472 research outputs found
Stretching and relaxation dynamics in double stranded DNA
We study numerically the mechanical stability and elasticity properties of
duplex DNA molecules within the frame of a network model incorporating
microscopic degrees of freedom related with the arrangement of the base pairs.
We pay special attention to the opening-closing dynamics of double-stranded DNA
molecules which are forced into non-equilibrium conformations. Mechanical
stress imposed at one terminal end of the DNA molecule brings it into a
partially opened configuration. We examine the subsequent relaxation dynamics
connected with energy exchange processes between the various degrees of freedom
and structural rearrangements leading to complete recombination to the
double-stranded conformation. The similarities and differences between the
relaxation dynamics for a planar ladder-like DNA molecule and a twisted one are
discussed in detail. In this way we show that the attainment of a
quasi-equilibrium regime proceeds faster in the case of the twisted DNA form
than for its thus less flexible ladder counterpart. Furthermore we find that
the velocity of the complete recombination of the DNA molecule is lower than
the velocity imposed by the forcing unit which is in compliance with the
experimental observations for the opening-closing cycle of DNA molecules.Comment: 21 pages, 9 figure
Konformationelle Vielfalt: Synthese eines spleißosomalen RNA-Konstruktes, NMR-Strukturen von Minigramicidin und einem molekularen Schalter
Probing the mechanical unzipping of DNA
A study of the micromechanical unzipping of DNA in the framework of the
Peyrard-Bishop-Dauxois model is presented. We introduce a Monte Carlo technique
that allows accurate determination of the dependence of the unzipping forces on
unzipping speed and temperature. Our findings agree quantitatively with
experimental results for homogeneous DNA, and for -phage DNA we
reproduce the recently obtained experimental force-temperature phase diagram.
Finally, we argue that there may be fundamental differences between {\em in
vivo} and {\em in vitro} DNA unzipping
Inferring DNA sequences from mechanical unzipping: an ideal-case study
We introduce and test a method to predict the sequence of DNA molecules from
in silico unzipping experiments. The method is based on Bayesian inference and
on the Viterbi decoding algorithm. The probability of misprediction decreases
exponentially with the number of unzippings, with a decay rate depending on the
applied force and the sequence content.Comment: Source as TeX file with ps figure
Bar-Halo Friction in Galaxies II: Metastability
It is well-established that strong bars rotating in dense halos generally
slow down as they lose angular momentum to the halo through dynamical friction.
Angular momentum exchanges between the bar and halo particles take place at
resonances. While some particles gain and others lose, friction arises when
there is an excess of gainers over losers. This imbalance results from the
generally decreasing numbers of particles with increasing angular momentum, and
friction can therefore be avoided if there is no gradient in the density of
particles across the major resonances. Here we show that anomalously weak
friction can occur for this reason if the pattern speed of the bar fluctuates
upwards. After such an event, the density of resonant halo particles has a
local inflexion created by the earlier exchanges, and bar slowdown can be
delayed for a long period; we describe this as a metastable state. We show that
this behavior in purely collisionless N-body simulations is far more likely to
occur in methods with adaptive resolution. We also show that the phenomenon
could arise in nature, since bar-driven gas inflow could easily raise the bar
pattern speed enough to reach the metastable state. Finally, we demonstrate
that mild external, or internal, perturbations quickly restore the usual
frictional drag, and it is unlikely therefore that a strong bar in a galaxy
having a dense halo could rotate for a long period without friction.Comment: 13 pages, 11 figures, to appear in Ap
A General Formula for Black Hole Gravitational Wave Kicks
Although the gravitational wave kick velocity in the orbital plane of
coalescing black holes has been understood for some time, apparently
conflicting formulae have been proposed for the dominant out-of-plane kick,
each a good fit to different data sets. This is important to resolve because it
is only the out-of-plane kicks that can reach more than 500 km/s and can thus
eject merged remnants from galaxies. Using a different ansatz for the
out-of-plane kick, we show that we can fit almost all existing data to better
than 5 %. This is good enough for any astrophysical calculation, and shows that
the previous apparent conflict was only because the two data sets explored
different aspects of the kick parameter space.Comment: 14 pages
The H I Environment Of The M101 Group
We present a wide (8. Degree-Sign 5 Multiplication-Sign 6. Degree-Sign 7, 1050 Multiplication-Sign 825 kpc), deep ({sigma}{sub N{sub H{sub {sub {sub i}}}}}10{sup 16.8}-10{sup 17.5} cm{sup -2}) neutral hydrogen (H I) map of the M101 galaxy group. We identify two new H I sources in the group environment, one an extremely low surface brightness (and hitherto unknown) dwarf galaxy, and the other a starless H I cloud, possibly primordial in origin. Our data show that M101\u27s extended H I envelope takes the form of a {approx}100 kpc long tidal loop or plume of H I extending to the southwest of the galaxy. The plume has an H I mass of {approx}10{sup 8} M{sub Sun} and a peak column density of N{sub H{sub i}}= 5 Multiplication-Sign 10{sup 17} cm{sup -2}, and while it rotates with the main body of M101, it shows kinematic peculiarities suggestive of a warp or flaring out of the rotation plane of the galaxy. We also find two new H I clouds near the plume with masses {approx}10{sup 7} M{sub Sun }, similar to H I clouds seen in the M81/M82 group, and likely also tidal in nature. Comparing to deep optical imaging of the M101 group, neithermore » the plume nor the clouds have any extended optical counterparts down to a limiting surface brightness of {mu}{sub B} = 29.5. We also trace H I at intermediate velocities between M101 and NGC 5474, strengthening the case for a recent interaction between the two galaxies. The kinematically complex H I structure in the M101 group, coupled with the optical morphology of M101 and its companions, suggests that the group is in a dynamically active state that is likely common for galaxies in group environments.« les
DNA unzipped under a constant force exhibits multiple metastable intermediates
Single molecule studies, at constant force, of the separation of
double-stranded DNA into two separated single strands may provide information
relevant to the dynamics of DNA replication. At constant applied force, theory
predicts that the unzipped length as a function of time is characterized by
jumps during which the strands separate rapidly, followed by long pauses where
the number of separated base pairs remains constant. Here, we report previously
uncharacterized observations of this striking behavior carried out on a number
of identical single molecules simultaneously. When several single lphage
molecules are subject to the same applied force, the pause positions are
reproducible in each. This reproducibility shows that the positions and
durations of the pauses in unzipping provide a sequence-dependent molecular
fingerprint. For small forces, the DNA remains in a partially unzipped state
for at least several hours. For larger forces, the separation is still
characterized by jumps and pauses, but the double-stranded DNA will completely
unzip in less than 30 min
Radiative Lifetimes of Single Excitons in Semiconductor Quantum Dots- Manifestation of the Spatial Coherence Effect
Using time correlated single photon counting combined with temperature
dependent diffraction limited confocal photoluminescence spectroscopy we
accurately determine, for the first time, the intrinsic radiative lifetime of
single excitons confined within semiconductor quantum dots. Their lifetime is
one (two) orders of magnitude longer than the intrinsic radiative lifetime of
single excitons confined in semiconductor quantum wires (wells) of comparable
confining dimensions. We quantitatively explain this long radiative time in
terms of the reduced spatial coherence between the confined exciton dipole
moment and the radiation electromagnetic field.Comment: 4 pages, 3 figure
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