1,391 research outputs found
Phosphorothioate Anti-sense Oligonucleotides: The Kinetics and Mechanism of the Generation of the Sulfurising Agent from Phenylacetyl Disulfide (PADS)
The synthesis of phosphorothioate oligonucleotides is often accomplished in the pharmaceutical industry by the sulfurisation of the nucleotide-phosphite using phenylacetyl disulfide (PADS) which has an optimal combination of properties. This is best achieved by an initial âageingâ of PADS for 48 hrs in acetonitrile with 3-picoline to generate polysulfides. The initial base-catalysed degradation of PADS occurs by an E1cB-type elimination to generate a ketene and acyldisulfide anion. Proton abstraction to reversibly generate a carbanion is demonstrated by H/D exchange, the rate of which is greatly increased by electron-withdrawing substituents in the aromatic ring of PADS. The ketene can be trapped intramolecularly by an o-allyl group. The disulfide anion generated subsequently attacks unreacted PADS on sulfur to give polysulfides, the active sulfurising agent. The rate of degradation of PADS is decreased by less basic substituted pyridines and is only first order in PADS indicating that the rate-limiting step is formation of the disulfide anion from the carbanion
Designing pseudocubic perovskites with enhanced nanoscale polarization
A crystal-chemical framework has been proposed for the design of pseudocubic perovskites with
nanoscale ferroelectric order, and its applicability has been demonstrated using a series of represen-
tative solid solutions that combined ferroelectric (K
0.5
Bi
0.5
TiO
3
, BaTiO
3
, and PbTiO
3
) and antifer-
roelectric (Nd-substituted BiFeO
3
) end members. The pseudocubic structures obtained in these
systems exhibited distortions that were coherent on a scale ranging from sub-nanometer to tens of
nanometers, but, in all cases, the macroscopic distortion remained unresolvable even if using high-
resolution X-ray powder diffraction. Different coherence lengths for the local atomic displacements
account for the distinctly different dielectric, ferroelectric, and electromechanical properties exhib-
ited by the samples. The guidelines identified provide a rationale for chemically tuning the coher-
ence length to obtain the desired functional response
Effective temperatures and radii of planet-hosting stars from IR photometry
In this paper we present and analyse determinations of effective temperatures
of planet-hosting stars using infrared (IR) photometry. One of our goals is the
comparison with spectroscopic temperatures to evaluate the presence of
systematic effects that could alter the determination of metal abundances. To
estimate the stellar temperatures we have followed a new approach based on
fitting the observed 2MASS IR photometry with accurately calibrated synthetic
photometry. Special care has been put in evaluating all sources of possible
errors and incorporating them in the analysis. A comparison of our temperature
determinations with spectroscopic temperatures published by different groups
reveals the presence of no systematic trends and a scatter compatible with the
quoted uncertainties of 0.5-1.3%. This mutual agreement strengthens the results
of both the spectroscopic and IR photometry analyses. Comparisons with other
photometric temperature calibrations, generally with poorer performances, are
also presented. In addition, the method employed of fitting IR photometry
naturally yields determinations of the stellar semi-angular diameters, which,
when combined with the distances, results in estimations of the stellar radii
with remarkable accuracies of ~2-4%. A comparison with the only star in the
sample with an empirically determined radius (HD 209458 -- from transit
photometry) indicates excellent agreement.Comment: 4 pages, 1 figure, accepted for publication as a letter in A&
Blogging in the physics classroom: A research-based approach to shaping students' attitudes towards physics
Even though there has been a tremendous amount of research done in how to
help students learn physics, students are still coming away missing a crucial
piece of the puzzle: why bother with physics? Students learn fundamental laws
and how to calculate, but come out of a general physics course without a deep
understanding of how physics has transformed the world around them. In other
words, they get the "how" but not the "why". Studies have shown that students
leave introductory physics courses almost universally with decreased
expectations and with a more negative attitude. This paper will detail an
experiment to address this problem: a course weblog or "blog" which discusses
real-world applications of physics and engages students in discussion and
thinking outside of class. Specifically, students' attitudes towards the value
of physics and its applicability to the real-world were probed using a
26-question Likert scale survey over the course of four semesters in an
introductory physics course at a comprehensive Jesuit university. We found that
students who did not participate in the blog study generally exhibited a
deterioration in attitude towards physics as seen previously. However, students
who read, commented, and were involved with the blog maintained their initially
positive attitudes towards physics. Student response to the blog was
overwhelmingly positive, with students claiming that the blog made the things
we studied in the classroom come alive for them and seem much more relevant.Comment: 20 pages, 6 figure
Nanoscale polar heterogeneities and branching Bi-displacement directions in K0.5Bi0.5TiO3
K0.5Bi0.5TiO3 (KBT)âone of the few perovskite-like ferroelectric compounds with room-temperature tetragonal symmetryâdiffers from other members of its family (BaTiO3 and PbTiO3) by the presence of a disordered mixture of K and Bi on cuboctahedral sites. This disorder is expected to affect local atomic displacements and their response to an applied electric field. We have derived nanoscale atomistic models of KBT by refining atomic coordinates to simultaneously fit neutron/X-ray total scattering and extended X-ray absorption fine-structure data. Both Bi and Ti ions were found to be offset relative to their respective oxygen cages in the high-temperature cubic phase; in contrast, the coordination environment of K remained relatively undistorted. In the cubic structure, Bi displacements prefer the âš100â© directions and the probability density distribution of Bi features six well-separated sites; a similar preference exists for the much smaller Ti displacements, although the split sites for Ti could not be resolved. The cation displacements are correlated, yielding polar nanoregions, whereas on average, the structure appears as cubic. The cubic â tetragonal phase transition involves both order/disorder and displacive mechanisms. A qualitative change in the form of the Bi probability density distribution occurs in the tetragonal phase on cooling to room temperature because Bi displacements âbranch offâ to âš111â© directions. This change, which preserves the average symmetry, is accompanied by the development of nanoscale polar heterogeneities that exhibit significant deviations of their polarization vectors from the average polar axis
Hypoxia PET/CT and Colorectal Cancer : A Case Report
Funding: Funding for the pilot study was provided by the Colorectal Study Fund, a NHSG endowment fund number: NER 11482.Peer reviewedPublisher PD
The formation and habitability of terrestrial planets in the presence of hot jupiters
`Hot jupiters,' giant planets with orbits very close to their parent stars,
are thought to form farther away and migrate inward via interactions with a
massive gas disk. If a giant planet forms and migrates quickly, the
planetesimal population has time to re-generate in the lifetime of the disk and
terrestrial planets may form (Armitage 2003). We present results of simulations
of terrestrial planet formation in the presence of hot jupiters, broadly
defined as having orbital radii <= 0.5 AU. We show that terrestrial planets
similar to those in the Solar System can form around stars with hot jupiters,
and can have water contents equal to or higher than the Earth's. For small
orbital radii of hot jupiters (e.g. 0.15, 0.25 AU) potentially habitable
planets can form, but for semi-major axes of 0.5 AU or greater their formation
is suppressed. We show that the presence of an outer giant planet such as
Jupiter does not enhance the water content of the terrestrial planets, but
rather decreases their formation and water delivery timescales. We speculate
that asteroid belts may exist interior to the terrestrial planets in systems
with hot jupiters.Comment: 5 pages, 2 color figures in emulate ApJ style submitted to Icaru
Decisions to consent for autopsy after stillbirth: Aboriginal and Torres Strait Islander womenâs experiences
Abstract
Background: The stillbirth rate for Australian Aboriginal and Torres Strait Islander infants is twice that for non-Indigenous infants. Autopsy is the gold standard for fetal investigation, however, parental consent is low. There is little research investigating the drivers of parentsâ decision-making for autopsy after stillbirth.
Aims: The current study explored the reasons why Aboriginal and Torres Strait Islander women did or did not give permission to autopsy after stillbirth.
Materials and Methods: Five Aboriginal and/or Torres Strait Islander women participated in semi-structured interviews. Thematic analysis was conducted within a phenomenological framework.
Results: Five themes were identified as reasons for giving permission â to find out why baby died; confirm diagnosis; understand future risk; help others; and doubt about maternal causes. Four themes were identified as reasons for declining permission â not asked in a sensitive manner; not enough time to think; distress about the autopsy procedure; and unwilling to agree. There was a lack of acceptability of the lengthy timeframe for the availability of autopsy results as families usually wait between three and nine months. This lengthy waiting period negatively impacted upon familiesâ health and wellbeing.
Conclusions: It is important for health professionals to understand the factors that parents consider when giving permission for autopsy after stillbirth. It is hoped that an increase in autopsy rate will enhance the understanding of the causes of stillbirth and ultimately decrease the stillbirth rate for Aboriginal and Torres Strait Islander families
Elemental abundance differences in the 16 Cygni binary system: a signature of gas giant planet formation?
The atmospheric parameters of the components of the 16Cygni binary system, in
which the secondary has a gas giant planet detected, are measured accurately
using high quality observational data. Abundances relative to solar are
obtained for 25 elements with a mean error of 0.023 dex. The fact that 16CygA
has about four times more lithium than 16CygB is normal considering the
slightly different masses of the stars. The abundance patterns of 16CygA and B,
relative to iron, are typical of that observed in most of the so-called solar
twin stars, with the exception of the heavy elements (Z>30), which can,
however, be explained by Galactic chemical evolution. Differential (A-B)
abundances are measured with even higher precision (0.018 dex, on average). We
find that 16CygA is more metal-rich than 16CygB by 0.041+/-0.007 dex. On an
element-to-element basis, no correlation between the A-B abundance differences
and dust condensation temperature (Tc) is detected. Based on these results, we
conclude that if the process of planet formation around 16CygB is responsible
for the observed abundance pattern, the formation of gas giants produces a
constant downwards shift in the photospheric abundance of metals, without a Tc
correlation. The latter would be produced by the formation of terrestrial
planets instead, as suggested by other recent works on precise elemental
abundances. Nevertheless, a scenario consistent with these observations
requires the convective envelopes of 1 Msun stars to reach their present-day
sizes about three times quicker than predicted by standard stellar evolution
models.Comment: ApJ, in pres
Searching for the signatures of terrestial planets in solar analogs
We present a fully differential chemical abundance analysis using very
high-resolution (R >~ 85,000) and very high signal-to-noise (S/N~800 on
average) HARPS and UVES spectra of 7 solar twins and 95 solar analogs, 24 are
planet hosts and 71 are stars without detected planets. The whole sample of
solar analogs provide very accurate Galactic chemical evolution trends in the
metalliciy range -0.3<[Fe/H]<0.5. Solar twins with and without planets show
similar mean abundance ratios. We have also analysed a sub-sample of 28 solar
analogs, 14 planet hosts and 14 stars without known planets, with spectra at
S/N~850 on average, in the metallicity range 0.14<[Fe/H]<0.36 and find the same
abundance pattern for both samples of stars with and without planets. This
result does not depend on either the planet mass, from 7 Earth masses to 17.4
Jupiter masses, or the orbital period of the planets, from 3 to 4300 days. In
addition, we have derived the slope of the abundance ratios as a function of
the condensation temperature for each star and again find similar distributions
of the slopes for both stars with and without planets. In particular, the peaks
of these two distributions are placed at a similar value but with opposite sign
as that expected from a possible signature of terrestial planets. In
particular, two of the planetary systems in this sample, containing each of
them a Super-Earth like planet, show slope values very close to these peaks
which may suggest that these abundance patterns are not related to the presence
of terrestial planets.Comment: Accepted for publication in The Astrophysical Journa
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