7,139 research outputs found
The size of the nucleosome
The structural origin of the size of the 11 nm nucleosomal disc is addressed.
On the nanometer length-scale the organization of DNA as chromatin in the
chromosomes involves a coiling of DNA around the histone core of the
nucleosome. We suggest that the size of the nucleosome core particle is
dictated by the fulfillment of two criteria: One is optimizing the volume
fraction of the DNA double helix; this requirement for close-packing has its
root in optimizing atomic and molecular interactions. The other criterion being
that of having a zero strain-twist coupling; being a zero-twist structure is a
necessity when allowing for transient tensile stresses during the
reorganization of DNA, e.g., during the reposition, or sliding, of a nucleosome
along the DNA double helix. The mathematical model we apply is based on a
tubular description of double helices assuming hard walls. When the base-pairs
of the linker-DNA is included the estimate of the size of an ideal nucleosome
is in close agreement with the experimental numbers. Interestingly, the size of
the nucleosome is shown to be a consequence of intrinsic properties of the DNA
double helix.Comment: 11 pages, 5 figures; v2: minor modification
Current dependence of grain boundary magnetoresistance in La_0.67Ca_0.33MnO_3 films
We prepared epitaxial ferromagnetic manganite films on bicrystal substrates
by pulsed laser ablation. Their low- and high-field magnetoresistance (MR) was
measured as a function of magnetic field, temperature and current. At low
temperatures hysteretic changes in resistivity up to 70% due to switching of
magnetic domains at the coercitive field are observed. The strongly non-ohmic
behavior of the current-voltage leads to a complete suppression of the MR
effect at high bias currents with the identical current dependence at low and
high magnetic fields. We discuss the data in view of tunneling and mesoscale
magnetic transport models and propose an explicit dependence of the spin
polarization on the applied current in the grain boundary region.Comment: 5 pages, to appear in J. Appl. Phy
Calculation of fragmentation functions in two-hadron semi-inclusive processes
We investigate the properties of interference fragmentation functions arising
from the emission of two leading hadrons inside the same jet for inclusive
lepton-nucleon deep-inelastic scattering. Using an extended spectator model for
the mechanism of the hadronization, we give a complete calculation and
numerical estimates for the examples of a proton-pion pair produced with
invariant mass on the Roper resonance, and of two pions produced with invariant
mass close to the mass. We discuss azimuthal angular dependence of the
leading order cross section to point up favourable conditions for extracting
transversity from experimental data.Comment: 5 pages, 3 figures in .eps format, AIP and epsfig styles included, to
appear in proceedings of "Second Workshop on Physics with an Electron
Polarized Light Ion Collider", MIT, Sept. 14-16, 200
Ultrabroadband single-cycle terahertz pulses with peak fields of 300 kV cm from a metallic spintronic emitter
To explore the capabilities of metallic spintronic thin-film stacks as a
source of intense and broadband terahertz electromagnetic fields, we excite a
W/CoFeB/Pt trilayer on a large-area glass substrate (diameter of 7.5 cm) by a
femtosecond laser pulse (energy 5.5 mJ, duration 40 fs, wavelength 800 nm).
After focusing, the emitted terahertz pulse is measured to have a duration of
230 fs, a peak field of 300 kV cm and an energy of 5 nJ. In particular,
the waveform exhibits a gapless spectrum extending from 1 to 10 THz at 10% of
amplitude maximum, thereby facilitating nonlinear control over matter in this
difficult-to-reach frequency range and on the sub-picosecond time scale.Comment: 7 pages, 4 figure
MaGICC-WDM: the effects of warm dark matter in hydrodynamical simulations of disc galaxy formation
We study the effect of warm dark matter (WDM) on hydrodynamic simulations of
galaxy formation as part of the Making Galaxies in a Cosmological Context
(MaGICC) project. We simulate three different galaxies using three WDM
candidates of 1, 2 and 5 keV and compare results with pure cold dark matter
simulations. WDM slightly reduces star formation and produces less centrally
concentrated stellar profiles. These effects are most evident for the 1 keV
candidate but almost disappear for keV. All simulations
form similar stellar discs independent of WDM particle mass. In particular, the
disc scale length does not change when WDM is considered. The reduced amount of
star formation in the case of 1 keV particles is due to the effects of WDM on
merging satellites which are on average less concentrated and less gas rich.
The altered satellites cause a reduced starburst during mergers because they
trigger weaker disc instabilities in the main galaxy. Nevertheless we show that
disc galaxy evolution is much more sensitive to stellar feedback than it is to
WDM candidate mass. Overall we find that WDM, especially when restricted to
current observational constraints ( keV), has a minor
impact on disc galaxy formation.Comment: 13 pages, 9 figures, 2 tables; minor clarifications added in results
section, conclusions unchanged; accepted for publication in MNRA
Stimulation induced variability of pulse plethysmography does not discriminate responsiveness to intubation
Background. Hypnotic depth but not haemodynamic response to painful stimulation can be measured with various EEG-based anaesthesia monitors. We evaluated the variation of pulse plethysmography amplitude induced by an electrical tetanic stimulus (PPG variation) as a potential measure for analgesia and predictor of haemodynamic responsiveness during general anaesthesia. Methods. Ninety-five patients, ASA I or II, were randomly assigned to five groups [Group 1: bispectral index (BIS) (range) 40-50, effect site remifentanil concentration 1 ng mlâ1;Group 2: BIS 40-50, remifentanil 2 ng mlâ1; Group 3: BIS 40-50, remifentanil 4 ng mlâ1; Group 4: BIS 25-35, remifentanil 2 ng mlâ1; Group 5: BIS 55-65, remifentanil 2 ng mlâ1]. A 60 mA tetanic stimulus was applied for 5 s on the ulnar nerve. From the digitized pulse oximeter wave recorded on a laptop computer, linear and non-linear parameters of PPG variation during the 60 s period after stimulation were computed. The haemodynamic response to subsequent orotracheal intubation was recorded. The PPG variation was compared between groups and between responders and non-responders to intubation (anova). Variables independently predicting the response were determined by logistic regression. Results. The probability of a response to tracheal intubation was 0.77, 0.47, 0.05, 0.18 and 0.52 in Groups 1-5, respectively (P<0.03). The PPG variability was significantly higher in responders than in non-responders but it did not improve the prediction of the response to tracheal intubation based on BIS level and effect site remifentanil concentration. Conclusion. Tetanic stimulation induced PPG variation does not reflect the analgesic state in a wide clinical range of surgical anaesthesi
Inertial amplification of continuous structures: Large band gaps from small masses
Wave motion in a continuous elastic rod with a periodically attached
inertial-amplification mechanism is investigated. The mechanism has properties
similar to an "inerter" typically used in vehicle suspensions, however here it
is constructed and utilized in a manner that alters the intrinsic properties of
a continuous structure. The elastodynamic band structure of the hybrid
rod-mechanism structure yields band gaps that are exceedingly wide and deep
when compared to what can be obtained using standard local resonators, while
still being low in frequency. With this concept, a large band gap may be
realized with as much as twenty times less added mass compared to what is
needed in a standard local resonator configuration. The emerging inertially
enhanced continuous structure also exhibits unique qualitative features in its
dispersion curves. These include the existence of a characteristic double-peak
in the attenuation constant profile within gaps and the possibility of
coalescence of two neighbouring gaps creating a large contiguous gap.Comment: Manuscript is under review for journal publicatio
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