548 research outputs found
Molecular Mechanism for Inhibition of G Protein-Coupled Receptor Kinase 2 by a Selective RNA Aptamer
SummaryCardiovascular homeostasis is maintained in part by the rapid desensitization of activated heptahelical receptors that have been phosphorylated by G protein-coupled receptor kinase 2 (GRK2). However, during chronic heart failure GRK2 is upregulated and believed to contribute to disease progression. We have determined crystallographic structures of GRK2 bound to an RNA aptamer that potently and selectively inhibits kinase activity. Key to the mechanism of inhibition is the positioning of an adenine nucleotide into the ATP-binding pocket and interactions with the basic αF-αG loop region of the GRK2 kinase domain. Constraints imposed on the RNA by the terminal stem of the aptamer also play a role. These results highlight how a high-affinity aptamer can be used to selectively trap a novel conformational state of a protein kinase
Extinction of the N=20 neutron-shell closure for 32Mg examined by direct mass measurements
The 'island of inversion' around Mg is one of the most important
paradigm for studying the disappearance of the stabilizing 'magic' of a shell
closure. We present the first Penning-trap mass measurements of the exotic
nuclides Na and Mg, which allow a precise determination of
the empirical shell gap for Mg. The new value of 1.10(3) MeV is the
lowest observed shell gap for any nuclide with a canonical magic number.Comment: 6 pages, 4 figures, submitted to Physical Review
Trapped-ion decay spectroscopy towards the determination of ground-state components of double-beta decay matrix elements
A new technique has been developed at TRIUMF's TITAN facility to perform
in-trap decay spectroscopy. The aim of this technique is to eventually measure
weak electron capture branching ratios (ECBRs) and by this to consequently
determine GT matrix elements of decaying nuclei. These branching
ratios provide important input to the theoretical description of these decays.
The feasibility and power of the technique is demonstrated by measuring the
ECBR of Cs.Comment: 9 pages, 9 figure
Precision mass measurements of magnesium isotopes and implications on the validity of the Isobaric Mass Multiplet Equation
If the mass excess of neutron-deficient nuclei and their neutron-rich mirror
partners are both known, it can be shown that deviations of the Isobaric Mass
Multiplet Equation (IMME) in the form of a cubic term can be probed. Such a
cubic term was probed by using the atomic mass of neutron-rich magnesium
isotopes measured using the TITAN Penning trap and the recently measured
proton-separation energies of Cl and Ar. The atomic mass of
Mg was found to be within 1.6 of the value stated in the Atomic
Mass Evaluation. The atomic masses of Mg were measured to be both
within 1, while being 8 and 34 times more precise, respectively. Using
the Mg mass excess and previous measurements of Cl we uncovered a
cubic coefficient of = 28(7) keV, which is the largest known cubic
coefficient of the IMME. This departure, however, could also be caused by
experimental data with unknown systematic errors. Hence there is a need to
confirm the mass excess of S and the one-neutron separation energy of
Cl, which have both come from a single measurement. Finally, our results
were compared to ab initio calculations from the valence-space in-medium
similarity renormalization group, resulting in a good agreement.Comment: 7 pages, 3 figure
Breakdown of the Isobaric Multiplet Mass Equation for the A = 20 and 21 Multiplets
Using the Penning trap mass spectrometer TITAN, we performed the first direct
mass measurements of 20,21Mg, isotopes that are the most proton-rich members of
the A = 20 and A = 21 isospin multiplets. These measurements were possible
through the use of a unique ion-guide laser ion source, a development that
suppressed isobaric contamination by six orders of magnitude. Compared to the
latest atomic mass evaluation, we find that the mass of 21Mg is in good
agreement but that the mass of 20Mg deviates by 3{\sigma}. These measurements
reduce the uncertainties in the masses of 20,21Mg by 15 and 22 times,
respectively, resulting in a significant departure from the expected behavior
of the isobaric multiplet mass equation in both the A = 20 and A = 21
multiplets. This presents a challenge to shell model calculations using either
the isospin non-conserving USDA/B Hamiltonians or isospin non-conserving
interactions based on chiral two- and three-nucleon forces.Comment: 5 pages, 2 figure
Direct Measurement of the Key e c. m.=456 keV Resonance in the Astrophysical Ne 19 (p,γ) Na 20 Reaction and Its Relevance for Explosive Binary Systems
We have performed a direct measurement of the Ne19(p,γ)Na20 reaction in inverse kinematics using a beam of radioactive Ne19. The key astrophysical resonance in the Ne19+p system has been definitely measured for the first time at Ec.m.=456-2+5 keV with an associated strength of 17-5+7 meV. The present results are in agreement with resonance strength upper limits set by previous direct measurements, as well as resonance energies inferred from precision (He3, t) charge exchange reactions. However, both the energy and strength of the 456 keV resonance disagree with a recent indirect study of the Ne19(d, n)Na20 reaction. In particular, the new Ne19(p,γ)Na20 reaction rate is found to be factors of ∼8 and ∼5 lower than the most recent evaluation over the temperature range of oxygen-neon novae and astrophysical x-ray bursts, respectively. Nevertheless, we find that the Ne19(p,γ)Na20 reaction is likely to proceed fast enough to significantly reduce the flux of F19 in nova ejecta and does not create a bottleneck in the breakout from the hot CNO cycles into the rp process
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