1,755 research outputs found
Accuracy of SUPREX (Stability of Unpurified Proteins from Rates of H/D Exchange) and MALDI Mass Spectrometry-Derived Protein Unfolding Free Energies Determined Under Non-EX2 Exchange Conditions
Described here is the impact of so-called non-EX2 exchange behavior on the accuracy of protein unfolding free energies (i.e., ΔGu values) and m values (i.e.,-δΔGu/δ[denaturant] values) determined by an H/D exchange and mass spectrometry-based technique termed stability of unpurified proteins from rates of H/D exchange (SUPREX). Both experimental and theoretical results on a model protein, ubiquitin, reveal that reasonably accurate thermodynamic parameters for its folding reaction can be determined by SUPREX even when H/D exchange data is collected in a non-EX2 regime. Not surprisingly, the theoretical results reported here on a series of hypothetical protein systems with a wide range of biophysical properties show that the accuracy of SUPREX-derived ΔGu and m values is compromised for many proteins when analyses are performed at high pH (e.g., pH 9) and for selected proteins with specific biophysical parameters (e.g., slow folding rates) when analyses are performed at lower pH. Of more significance is that the experimental and theoretical results reveal a means by which problems with non-EX2 exchange behavior can be detected in the SUPREX experiment without prior knowledge of the protein’s biophysical properties. The results of this work also reveal that such problems with non-EX2 exchange behavior can generally be minimized if appropriate H/D exchange times are employed in the SUPREX experiment to yield SUPREX curve transition midpoints at chemical denaturant concentrations less than 2 M
First scattered light images of debris disks around HD 53143 and HD 139664
We present the first scattered light images of debris disks around a K star
(HD 53143) and an F star (HD 139664) using the coronagraphic mode of the
Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST). With
ages 0.3 - 1 Gyr, these are among the oldest optically detected debris disks.
HD 53143, viewed ~45 degrees from edge-on, does not show radial variation in
disk structure and has width >55 AU. HD 139664 is seen close to edge-on and has
belt-like morphology with a dust peak 83 AU from the star and a distinct outer
boundary at 109 AU. We discuss evidence for significant diversity in the radial
architecture of debris disks that appears unconnected to stellar spectral type
or age. HD 139664 and possibly the solar system belong in a category of narrow
belts 20-30 AU wide. HD 53143 represents a class of wide disk architecture with
characteristic width >50 AU.Comment: 7 pages, 3 figure
A Ring of Warm Dust in the HD 32297 Debris Disk
We report the detection of a ring of warm dust in the edge-on disk
surrounding HD 32297 with the Gemini-N/MICHELLE mid-infrared imager. Our
N'-band image shows elongated structure consistent with the orientation of the
scattered-light disk. The Fnu(11.2 um) = 49.9+/-2.1 mJy flux is significantly
above the 28.2+/-0.6 mJy photosphere. Subtraction of the stellar point spread
function reveals a bilobed structure with peaks 0.5"-0.6" from the star. An
analysis of the stellar component of the SED suggests a spectral type later
than A0, in contrast to commonly cited literature values. We fit
three-dimensional, single-size grain models of an optically thin dust ring to
our image and the SED using a Markov chain Monte Carlo algorithm in a Bayesian
framework. The best-fit effective grain sizes are submicron, suggesting the
same dust population is responsible for the bulk of the scattered light. The
inner boundary of the warm dust is located 0.5"-0.7" (~65 AU) from the star,
which is approximately cospatial with the outer boundary of the scattered-light
asymmetry inward of 0.5". The addition of a separate component of larger,
cooler grains that provide a portion of the 60 um flux improves both the
fidelity of the model fit and consistency with the slopes of the
scattered-light brightness profiles. Previous indirect estimates of the stellar
age (~30 Myr) indicate the dust is composed of debris. The peak vertical
optical depths in our models (~0.3-1 x 1e-2) imply that grain-grain collisions
likely play a significant role in dust dynamics and evolution. Submicron grains
can survive radiation pressure blow-out if they are icy and porous. Similarly,
the inferred warm temperatures (130-200 K) suggest that ice sublimation may
play a role in truncating the inner disk.Comment: ApJ accepted, 8 pages, 4 figure
Speckle Statistics in Adaptively Corrected Images
(abridged) Imaging observations are generally affected by a fluctuating
background of speckles, a particular problem when detecting faint stellar
companions at small angular separations. Knowing the distribution of the
speckle intensities at a given location in the image plane is important for
understanding the noise limits of companion detection. The speckle noise limit
in a long-exposure image is characterized by the intensity variance and the
speckle lifetime. In this paper we address the former quantity through the
distribution function of speckle intensity. Previous theoretical work has
predicted a form for this distribution function at a single location in the
image plane. We developed a fast readout mode to take short exposures of
stellar images corrected by adaptive optics at the ground-based UCO/Lick
Observatory, with integration times of 5 ms and a time between successive
frames of 14.5 ms ( m). These observations temporally
oversample and spatially Nyquist sample the observed speckle patterns. We show,
for various locations in the image plane, the observed distribution of speckle
intensities is consistent with the predicted form. Additionally, we demonstrate
a method by which and can be mapped over the image plane. As the
quantity is proportional to the PSF of the telescope free of random
atmospheric aberrations, this method can be used for PSF calibration and
reconstruction.Comment: 7 pages, 4 figures, ApJ accepte
The AU Microscopii Debris Disk: Multiwavelength Imaging and Modeling
(abridged) Debris disks around main sequence stars are produced by the
erosion and evaporation of unseen parent bodies. AU Microscopii (GJ 803) is a
compelling object to study in the context of disk evolution across different
spectral types, as it is an M dwarf whose near edge-on disk may be directly
compared to that of its A5V sibling beta Pic. We resolve the disk from 8-60 AU
in the near-IR JHK' bands at high resolution with the Keck II telescope and
adaptive optics, and develop a novel data reduction technique for the removal
of the stellar point spread function. The point source detection sensitivity in
the disk midplane is more than a magnitude less sensitive than regions away
from the disk for some radii. We measure a blue color across the near-IR bands,
and confirm the presence of substructure in the inner disk. Some of the
structural features exhibit wavelength-dependent positions. The disk
architecture and characteristics of grain composition are inferred through
modeling. We approach the modeling of the dust distribution in a manner that
complements previous work. Using a Monte Carlo radiative transfer code, we
compare a relatively simple model of the distribution of porous grains to a
broad data set, simultaneously fitting to midplane surface brightness profiles
and the spectral energy distribution. Our model confirms that the large-scale
architecture of the disk is consistent with detailed models of steady-state
grain dynamics. Here, a belt of parent bodies from 35-40 AU is responsible for
producing dust that is then swept outward by the stellar wind and radiation
pressures. We infer the presence of very small grains in the outer region, down
to sizes of ~0.05 micron. These sizes are consistent with stellar mass-loss
rates Mdot_* << 10^2 Mdot_sun.Comment: ApJ accepted, 56 pages, preprint style. Version in emulateapj with
high-resolution figures available at http://tinyurl.com/y6ent
Recommended from our members
Particulate matter and risk of parkinson disease in a large prospective study of women
Background: Exposure to air pollution has been implicated in a number of adverse health outcomes and the effect of particulate matter (PM) on the brain is beginning to be recognized. Yet, no prospective study has examined the association between PM and risk of Parkinson Disease. Thus, our goal was assess if exposure to particulate matter air pollution is related to risk of Parkinson’s disease (PD) in the Nurses’ Health Study (NHS), a large prospective cohort of women. Methods: Cumulative average exposure to different size fractions of PM up to 2 years before the onset of PD, was estimated using a spatio-temporal model by linking each individual’s places of residence throughout the study with location-specific air pollution levels. We prospectively followed 115,767 women in the NHS, identified 508 incident PD cases and used multivariable Cox proportional hazards models to estimate the risk of PD associated with each size fraction of PM independently. Results: In models adjusted for age in months, smoking, region, population density, caffeine and ibuprofen intake, we observed no statistically significant associations between exposure to air pollution and PD risk. The relative risk (RR) comparing the top quartile to the bottom quartile of PM exposure was 0.99 (95% Confidence Intervals (CI): 0.84,1.16) for PM10 (≤10 microns in diameter), 1.08 (95% CI: 0.81, 1.45) for PM2.5 (≤2.5 microns in diameter), and 0.92 (95% CI: 0.71, 1.19) for PM10–2.5 (2.5 to 10 microns in diameter). Conclusions: In this study, we found no evidence that exposure to air pollution is a risk factor for PD
Quantum Computing in Molecular Magnets
Shor and Grover demonstrated that a quantum computer can outperform any
classical computer in factoring numbers and in searching a database by
exploiting the parallelism of quantum mechanics. Whereas Shor's algorithm
requires both superposition and entanglement of a many-particle system, the
superposition of single-particle quantum states is sufficient for Grover's
algorithm. Recently, the latter has been successfully implemented using Rydberg
atoms. Here we propose an implementation of Grover's algorithm that uses
molecular magnets, which are solid-state systems with a large spin; their spin
eigenstates make them natural candidates for single-particle systems. We show
theoretically that molecular magnets can be used to build dense and efficient
memory devices based on the Grover algorithm. In particular, one single crystal
can serve as a storage unit of a dynamic random access memory device. Fast
electron spin resonance pulses can be used to decode and read out stored
numbers of up to 10^5, with access times as short as 10^{-10} seconds. We show
that our proposal should be feasible using the molecular magnets Fe8 and Mn12.Comment: 13 pages, 2 figures, PDF, version published in Nature, typos
correcte
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