48 research outputs found
XMM-Newton survey of the low-metallicity open cluster NGC 2516
We present the first results of an XMM-Newton EPIC survey of NGC 2516, a
southern low-metallicity open cluster with an age close to the Pleiades. The
attained limiting sensitivity is of ~ 2.4 10^-15 erg sec^-1} cm^-2 in the
0.1--4.0 keV bandpass. This has been achieved by summing the data of the MOS
and PN cameras of two distinct observations for a total exposure time of ~ 33
ks and by analyzing the summed data set with the wavelet detection algorithm
developed at Osservatorio Astronomico di Palermo (OAPA) that has yielded over
200 X-ray detections. Using data just from a single exposure or from a single
camera would have reduced by a factor 2--4 our limiting sensitivity and would
have resulted in 25--40% less X-ray detections. To date, 129 detections have as
counterparts one or more of the 540 photometrically selected cluster members in
the surveyed region, for a total of 147 likely detected members, with unique
identification in 112 cases. We derive the X-ray luminosity functions (XLF) of
NGC 2516 members of different spectral types and compare them with those of the
more metal rich, approximately coeval Pleiades cluster, finding the NGC 2516
photometrically selected dG and dK stars less luminous than the Pleiades. The
XLFs of the NGC 2516 and of the Pleiades dM stars are indistinguishable. We
compare the XMM-Newton results with those recently obtained with Chandra.Comment: 8 pages, 5 figures (in 11 postscript files
Fermi-LAT Study of Gamma-ray Emission in the Direction of Supernova Remnant W49B
We present an analysis of the gamma-ray data obtained with the Large Area
Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope in the direction of
SNR W49B (G43.3-0.2). A bright unresolved gamma-ray source detected at a
significance of 38 sigma is found to coincide with SNR W49B. The energy
spectrum in the 0.2-200 GeV range gradually steepens toward high energies. The
luminosity is estimated to be 1.5x10^{36} (D/8 kpc)^2 erg s^-1 in this energy
range. There is no indication that the gamma-ray emission comes from a pulsar.
Assuming that the SNR shell is the site of gamma-ray production, the observed
spectrum can be explained either by the decay of neutral pi mesons produced
through the proton-proton collisions or by electron bremsstrahlung. The
calculated energy density of relativistic particles responsible for the LAT
flux is estimated to be remarkably large, U_{e,p}>10^4 eV cm^-3, for either
gamma-ray production mechanism.Comment: 9 pages, 10 figure
The Synthesis, Photophysical Characterization, and XâRay Structure Analysis of Two Polymorphs of 4,4âČâDiacetylstilbene
A palladium(II) acetateâcatalyzed synthesis of 1 that utilizes the novel triazene 1â{4â[(E)âmorpholinâ4âyldiazenyl]phenyl}ethanone as a synthon is described. The room temperature absorption spectra of 1 in various solvents exhibited a ÏâÏ* transition in the range of 330â350â
nm. Compound 1 was observed to be luminescent, with roomâtemperature solution and solidâstate emission spectra that exhibited maxima in the range 400â500â
nm. All roomâtemperature absorption and emission spectra exhibited some degree of vibrational structure. The emission spectrum of 1 at 77â
K in propanenitrile glass was broad and featureless with a maximum at 447â
nm. Compound 1 crystallized as a yellow and colorless polymorph. XâRay structure analyses of both of these polymorphs and 1â{4â[(E)âmorpholinâ4âyldiazenyl]phenyl}ethanone are presented
Retrospective analysis of natural products provides insights for future discovery trends
Understanding of the capacity of the natural world to produce secondary metabolites is important to a broad range of fields, including drug discovery, ecology, biosynthesis, and chemical biology, among others. Both the absolute number and the rate of discovery of natural products have increased significantly in recent years. However, there is a perception and concern that the fundamental novelty of these discoveries is decreasing relative to previously known natural products. This study presents a quantitative examination of the field from the perspective of both number of compounds and compound novelty using a dataset of all published microbial and marine-derived natural products. This analysis aimed to explore a number of key questions, such as how the rate of discovery of new natural products has changed over the past decades, how the average natural product structural novelty has changed as a function of time, whether exploring novel taxonomic space affords an advantage in terms of novel compound discovery, and whether it is possible to estimate how close we are to having described all of the chemical space covered by natural products. Our analyses demonstrate that most natural products being published today bear structural similarity to previously published compounds, and that the range of scaffolds readily accessible from nature is limited. However, the analysis also shows that the field continues to discover appreciable numbers of natural products with no structural precedent. Together, these results suggest that the development of innovative discovery methods will continue to yield compounds with unique structural and biological properties
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A Strategy for Direct Chemical Activation of the Retinoblastoma Protein.
The retinoblastoma (Rb) tumor suppressor protein negatively regulates cell proliferation by binding and inhibiting E2F transcription factors. Rb inactivation occurs in cancer cells upon cyclin-dependent kinase (Cdk) phosphorylation, which induces E2F release and activation of cell cycle genes. We present a strategy for activating phosphorylated Rb with molecules that bind Rb directly and enhance affinity for E2F. We developed a fluorescence polarization assay that can detect the effect of exogenous compounds on modulating affinity of Rb for the E2F transactivation domain. We found that a peptide capable of disrupting the compact inactive Rb conformation increases affinity of the repressive Rb-E2F complex. Our results demonstrate the feasibility of discovering novel molecules that target the cell cycle and proliferation through directly targeting Rb rather than upstream kinase activity
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A Strategy for Direct Chemical Activation of the Retinoblastoma Protein.
The retinoblastoma (Rb) tumor suppressor protein negatively regulates cell proliferation by binding and inhibiting E2F transcription factors. Rb inactivation occurs in cancer cells upon cyclin-dependent kinase (Cdk) phosphorylation, which induces E2F release and activation of cell cycle genes. We present a strategy for activating phosphorylated Rb with molecules that bind Rb directly and enhance affinity for E2F. We developed a fluorescence polarization assay that can detect the effect of exogenous compounds on modulating affinity of Rb for the E2F transactivation domain. We found that a peptide capable of disrupting the compact inactive Rb conformation increases affinity of the repressive Rb-E2F complex. Our results demonstrate the feasibility of discovering novel molecules that target the cell cycle and proliferation through directly targeting Rb rather than upstream kinase activity