27 research outputs found
Profile instabilities of the millisecond pulsar PSR J1022+1001
We present evidence that the integrated profiles of some millisecond pulsars
exhibit severe changes that are inconsistent with the moding phenomenon as
known from slowly rotating pulsars. We study these profile instabilities in
particular for PSR J1022+1001 and show that they occur smoothly, exhibiting
longer time constants than those associated with moding. In addition, the
profile changes of this pulsar seem to be associated with a relatively
narrow-band variation of the pulse shape. Only parts of the integrated profile
participate in this process which suggests that the origin of this phenomenon
is intrinsic to the pulsar magnetosphere and unrelated to the interstellar
medium. A polarization study rules out profile changes due to geometrical
effects produced by any sort of precession. However, changes are observed in
the circularly polarized radiation component. In total we identify four
recycled pulsars which also exhibit instabilities in the total power or
polarization profiles due to an unknown phenomenon (PSRs J1022+1001,
J1730-2304, B1821-24, J2145-0750).
The consequences for high precision pulsar timing are discussed in view of
the standard assumption that the integrated profiles of millisecond pulsars are
stable. As a result we present a new method to determine pulse times-of-arrival
that involves an adjustment of relative component amplitudes of the template
profile. Applying this method to PSR J1022+1001, we obtain an improved timing
solution with a proper motion measurement of -17 \pm 2 mas/yr in ecliptic
longitude. Assuming a distance to the pulsar as inferred from the dispersion
measure this corresponds to an one-dimensional space velocity of 50 km/s.Comment: 29 pages, 12 figures, accepted for publication in Ap
GBT Discovery of Two Binary Millisecond Pulsars in the Globular Cluster M30
We report the discovery of two binary millisecond pulsars in the
core-collapsed globular cluster M30 using the Green Bank Telescope (GBT) at 20
cm. PSR J2140-2310A (M30A) is an eclipsing 11-ms pulsar in a 4-hr circular
orbit and PSR J2140-23B (M30B) is a 13-ms pulsar in an as yet undetermined but
most likely highly eccentric (e>0.5) and relativistic orbit. Timing
observations of M30A with a 20-month baseline have provided precise
determinations of the pulsar's position (within 4" of the optical centroid of
the cluster), and spin and orbital parameters, which constrain the mass of the
companion star to be m_2 >~ 0.1Msun. The position of M30A is coincident with a
possible thermal X-ray point source found in archival Chandra data which is
most likely due to emission from hot polar caps on the neutron star. In
addition, there is a faint (V_555 ~ 23.8) star visible in archival HST F555W
data that may be the companion to the pulsar. Eclipses of the pulsed radio
emission from M30A by the ionized wind from the compact companion star show a
frequency dependent duration (\propto\nu^{-\alpha} with \alpha ~ 0.4-0.5) and
delay the pulse arrival times near eclipse ingress and egress by up to 2-3 ms.
Future observations of M30 may allow both the measurement of post-Keplerian
orbital parameters from M30B and the detection of new pulsars due to the
effects of strong diffractive scintillation.Comment: 10 pages, 6 figures, Submitted to ApJ. This version includes many
recommended modifications, an improved structure, a new author, and a
completely redone optical analysi
Erratum: 'A precise mass measurement of the intermediate-mass binary pulsar PSR J1802-2124' (2010, ApJ, 711, 764)
Figure 1 as originally published did not match its corresponding caption; although the image was meant to be updated, it was unintentionally left unchanged. The correct Figure 1 with its corresponding caption is shown below
Biochemical and Structural Characterization of Selective Allosteric Inhibitors of the Plasmodium falciparum Drug Target, Prolyl-tRNA-synthetase
Plasmodium falciparum (<i>Pf</i>) prolyl-tRNA
synthetase (ProRS) is one of the few chemical-genetically validated
drug targets for malaria, yet highly selective inhibitors have not
been described. In this paper, approximately 40,000 compounds were
screened to identify compounds that selectively inhibit <i>Pf</i>ProRS enzyme activity versus Homo sapiens (<i>Hs</i>) ProRS. X-ray crystallography structures were
solved for apo, as well as substrate- and inhibitor-bound forms of <i>Pf</i>ProRS. We identified two new inhibitors of <i>Pf</i>ProRS that bind outside the active site. These two allosteric inhibitors
showed >100 times specificity for <i>Pf</i>ProRS compared
to <i>Hs</i>ProRS, demonstrating this class of compounds
could overcome the toxicity related to <i>Hs</i>ProRS inhibition
by halofuginone and its analogues. Initial medicinal chemistry was
performed on one of the two compounds, guided by the cocrystallography
of the compound with <i>Pf</i>ProRS, and the results can
instruct future medicinal chemistry work to optimize these promising
new leads for drug development against malaria
Inhibitor-bound complexes of dihydrofolate reductase-thymidylate synthase from Babesia bovis
Structural characterization of the bifunctional enzyme dihydrofolate reductase-thymidylate synthase from B. bovis in the apo state and complexed with antifolate inhibitors in both enzymatic active sites is reported
Lysyl-tRNA synthetase as a drug target in malaria and cryptosporidiosis
Malaria and cryptosporidiosis, caused by apicomplexan parasites, remain major drivers of global child mortality. New drugs for the treatment of malaria and cryptosporidiosis, in particular, are of high priority; however, there are few chemically validated targets. The natural product cladosporin is active against blood- and liver-stage; Plasmodium falciparum; and; Cryptosporidium parvum; in cell-culture studies. Target deconvolution in; P. falciparum; has shown that cladosporin inhibits lysyl-tRNA synthetase (; Pf; KRS1). Here, we report the identification of a series of selective inhibitors of apicomplexan KRSs. Following a biochemical screen, a small-molecule hit was identified and then optimized by using a structure-based approach, supported by structures of both; Pf; KRS1 and; C. parvum; KRS (; Cp; KRS). In vivo proof of concept was established in an SCID mouse model of malaria, after oral administration (ED; 90; = 1.5 mg/kg, once a day for 4 d). Furthermore, we successfully identified an opportunity for pathogen hopping based on the structural homology between; Pf; KRS1 and; Cp; KRS. This series of compounds inhibit; Cp; KRS and; C. parvum; and; Cryptosporidium hominis; in culture, and our lead compound shows oral efficacy in two cryptosporidiosis mouse models. X-ray crystallography and molecular dynamics simulations have provided a model to rationalize the selectivity of our compounds for; Pf; KRS1 and; Cp; KRS vs. (human); Hs; KRS. Our work validates apicomplexan KRSs as promising targets for the development of drugs for malaria and cryptosporidiosis
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Ab initio structure solution of a proteolytic fragment using ARCIMBOLDO
Crystal structure determination requires solving the phase problem. This can be accomplished using ab initio direct methods for small molecules and macromolecules at resolutions higher than 1.2 Å, whereas macromolecular structure determination at lower resolution requires either molecular replacement using a homologous structure or experimental phases using a derivative such as covalent labeling (for example selenomethionine or mercury derivatization) or heavy-atom soaking (for example iodide ions). Here, a case is presented in which crystals were obtained from a 30.8 kDa protein sample and yielded a 1.6 Å resolution data set with a unit cell that could accommodate approximately 8 kDa of protein. Thus, it was unclear what had been crystallized. Molecular replacement with pieces of homologous proteins and attempts at iodide ion soaking failed to yield a solution. The crystals could not be reproduced. Sequence-independent molecular replacement using the structures available in the Protein Data Bank also failed to yield a solution. Ultimately, ab initio structure solution proved successful using the program ARCIMBOLDO, which identified two α-helical elements and yielded interpretable maps. The structure was the C-terminal dimerization domain of the intended target from Mycobacterium smegmatis. This structure is presented as a user-friendly test case in which an unknown protein fragment could be determined using ARCIMBOLDO