6,712 research outputs found
Atomistic mechanism of transmembrane helix association
Transmembrane helix association is a fundamental step in the folding of helical membrane proteins. The prototypical example of this association is formation of the glycophorin dimer. While its structure and stability have been well-characterized experimentally, the detailed assembly mechanism is harder to obtain. Here, we use all-atom simulations within phospholipid membrane to study glycophorin association. We find that initial association results in the formation of a non-native intermediate, separated by a significant free energy barrier from the dimer with a native binding interface. We have used transition-path sampling to determine the association mechanism. We find that the mechanism of the initial bimolecular association to form the intermediate state can be mediated by many possible contacts, but seems to be particularly favoured by formation of non-native contacts between the C-termini of the two helices. On the other hand, the contacts which are key to determining progression from the intermediate to the native state are those which define the native binding interface, reminiscent of the role played by native contacts in determining folding of globular proteins. As a check on the simulations, we have computed association and dissociation rates from the transition-path sampling. We obtain results in reasonable accord with available experimental data, after correcting for differences in native state stability. Our results yield an atomistic description of the mechanism for a simple prototype of helical membrane protein folding
Perception of nonnative tonal contrasts by Mandarin-English and English-Mandarin sequential bilinguals
This study examined the role of acquisition order and crosslinguistic similarity in influencing transfer at the initial stage of perceptually acquiring a tonal third language (L3). Perception of tones in Yoruba and Thai was tested in adult sequential bilinguals representing three different first (L1) and second language (L2) backgrounds: L1 Mandarin-L2 English (MEBs), L1 English-L2 Mandarin (EMBs), and L1 English-L2 intonational/non-tonal (EIBs). MEBs outperformed EMBs and EIBs in discriminating L3 tonal contrasts in both languages, while EMBs showed a small advantage over EIBs on Yoruba. All groups showed better overall discrimination in Thai than Yoruba, but group differences were more robust in Yoruba. MEBs’ and EMBs’ poor discrimination of certain L3 contrasts was further reflected in the L3 tones being perceived as similar to the same Mandarin tone; however, EIBs, with no knowledge of Mandarin, showed many of the same similarity judgments. These findings thus suggest that L1 tonal experience has a particularly facilitative effect in L3 tone perception, but there is also a facilitative effect of L2 tonal experience. Further, crosslinguistic perceptual similarity between L1/L2 and L3 tones, as well as acoustic similarity between different L3 tones, play a significant role at this early stage of L3 tone acquisition.Published versio
Measurement of 25Mg(p; gamma)26Al resonance strengths via gamma spectrometry
The COMPTEL instrument performed the first mapping of the 1.809 MeV photons
in the Galaxy, triggering considerable interest in determing the sources of
interstellar 26Al. The predicted 26Al is too low compared to the observation,
for a better understanding more accurate rates for the 25Mg(p; gamma)26Al
reaction are required. The 25Mg(p;gamma)26Al reaction has been investigated at
the resonances at Er= 745; 418; 374; 304 keV at Ruhr-Universitat-Bochum using a
Tandem accelerator and a 4piNaI detector. In addition the resonance at Er = 189
keV has been measured deep underground laboratory at Laboratori Nazionali del
Gran Sasso, exploiting the strong suppression of cosmic background. This low
resonance has been studied with the 400 kV LUNA accelerator and a HPGe
detector. The preliminary results of the resonance strengths will be reported.Comment: Accepted for publication in Journal of Physics
Star Formation, Radio Sources, Cooling X-ray Gas, and Galaxy Interactions in the Brightest Cluster Galaxy in 2A0335+096
We present deep emission-line imaging taken with the SOAR Optical Imaging
Camera of the brightest cluster galaxy (BCG) in the nearby (z=0.035) X-ray
cluster 2A0335+096. We analyze long-slit optical spectroscopy, archival VLA,
Chandra X-ray, and XMM UV data. 2A0335+096 is a bright, cool-core X-ray
cluster, once known as a cooling flow. Within the highly disturbed core
revealed by Chandra X-ray observations, 2A0335+096 hosts a highly structured
optical emission-line system. The redshift of the companion is within 100 km/s
of the BCG and has certainly interacted with the BCG, and is likely bound to
it. The comparison of optical and radio images shows curved filaments in
H-alpha emission surrounding the resolved radio source. The velocity structure
of the emission-line bar between the BCG nucleus and the companion galaxy
provides strong evidence for an interaction between the two in the last ~50
Myrs. The age of the radio source is similar to the interaction time, so this
interaction may have provoked an episode of radio activity. We estimate a star
formation rate of >7 solar mass/yr based on the Halpha and archival UV data, a
rate similar to, but somewhat lower than, the revised X-ray cooling rate of
10-30 solar masses/year estimated from XMM spectra by Peterson & workers. The
Halpha nebula is limited to a region of high X-ray surface brightness and cool
X-ray temperature. The detailed structures of H-alpha and X-ray gas differ. The
peak of the X-ray emission is not the peak of H-alpha emission, nor does it lie
in the BCG. The estimated age of the radio lobes and their interaction with the
optical emission-line gas, the estimated timescale for depletion and
accumulation of cold gas, and the dynamical time in the system are all similar,
suggesting a common trigger mechanism.Comment: Accepted AJ, July 2007 publication. Vol 134, p. 14-2
Dyson-Schwinger Equations - aspects of the pion
The contemporary use of Dyson-Schwinger equations in hadronic physics is
exemplified via applications to the calculation of pseudoscalar meson masses,
and inclusive deep inelastic scattering with a determination of the pion's
valence-quark distribution function.Comment: 4 pages. Contribution to the Proceedings of ``DPF 2000,'' the Meeting
of the Division of Particles and Fields of the American Physical Society,
August 9-12, 2000, Department of Physics, the Ohio State University,
Columbus, Ohi
Double Lobed Radio Quasars from the Sloan Digital Sky Survey
We have combined a sample of 44984 quasars, selected from the Sloan Digital
Sky Survey (SDSS) Data Release 3, with the FIRST radio survey. Using a novel
technique where the optical quasar position is matched to the complete radio
environment within 450", we are able to characterize the radio morphological
make-up of what is essentially an optically selected quasar sample, regardless
of whether the quasar (nucleus) itself has been detected in the radio. About
10% of the quasar population have radio cores brighter than 0.75 mJy at 1.4
GHz, and 1.7% have double lobed FR2-like radio morphologies. About 75% of the
FR2 sources have a radio core (> 0.75 mJy). A significant fraction (~40%) of
the FR2 quasars are bent by more than 10 degrees, indicating either
interactions of the radio plasma with the ICM or IGM. We found no evidence for
correlations with redshift among our FR2 quasars: radio lobe flux densities and
radio source diameters of the quasars have similar distributions at low (mean
0.77) and high (mean 2.09) redshifts. Using a smaller high reliability FR2
sample of 422 quasars and two comparison samples of radio-quiet and non-FR2
radio-loud quasars, matched in their redshift distributions, we constructed
composite optical spectra from the SDSS spectroscopic data. Based on these
spectra we can conclude that the FR2 quasars have stronger high-ionization
emission lines compared to both the radio quiet and non-FR2 radio loud sources.
This is consistent with the notion that the emission lines are brightened by
ongoing shock ionization of ambient gas in the quasar host as the radio source
expands.Comment: 20 pages, 10 figures - some of which have been reduced in quality /
size. Accepted for publication in the Astronomical Journa
Higher-twist contributions to the Structure Functions coming from 4-fermion operators
We evaluate the contribution of a class of higher-twist operators to the
lowest moment of the Structure Functions, by computing appropriate matrix
elements of six four-fermion operators in the quenched approximation. Their
perturbative renormalization constants and mixing coefficients are calculated
in the 't Hooft-Veltman scheme of dimensional regularization, using codes
written in the algebraic manipulation computer language FORM.Comment: Talk presented at LATTICE99(matrix elements), Pisa (Italy), June 29 -
July 3; 3 pages; to be published in Nucl. Phys. B (Proc. Suppl.
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