40,410 research outputs found
Synthesis, solution stability, and crystal structure of aza-thia macrocyclic complexes of silver(I).
Synthesis of ω-Oxo Amino Acids and trans-5-Substituted Proline Derivatives using Cross-methathesis of Unsaturated Amino Acids
A range of 7-oxo, 8-oxo and 9-oxo amino acids, analogues of 8-oxo-2-aminodecanoic acid, one of the key components of the cyclic tetrapeptide apicidin, has been prepared by a three-step process involving copper-catalysed allylation of serine-, aspartic acid- and glutamic acid-derived organozinc reagents, followed by cross-metathesis of the resulting terminal alkenes with unsaturated ketones, and hydrogenation. The intermediate 7-oxo-5- enones underwent a highly diastereoselective (dr ≥96:4) acid-catalyzed aza-Michael reaction to give trans-2,5-disubstituted pyrrolidines, 5-substituted proline derivatives. The azaMichael reaction was first observed when the starting enones were allowed to stand in solution in deuterochloroform, but can be efficiently promoted by catalytic amounts of dry HC
The Impact of Isospin Breaking on the Distribution of Transition Probabilities
In the present paper we investigate the effect of symmetry breaking in the
statistical distributions of reduced transition amplitudes and reduced
transition probabilities. These quantities are easier to access experimentally
than the components of the eigenvectors and were measured by Adams et al. for
the electromagnetic transitions in ^{26}Al. We focus on isospin symmetry
breaking described by a matrix model where both, the Hamiltonian and the
electromagnetic operator, break the symmetry. The results show that for partial
isospin conservation, the statistical distribution of the reduced transition
probability can considerably deviate from the Porter-Thomas distribution.Comment: 16 pages, 8 figures, submitted to PR
Metastability and the Casimir Effect in Micromechanical Systems
Electrostatic and Casimir interactions limit the range of positional
stability of electrostatically-actuated or capacitively-coupled mechanical
devices. We investigate this range experimentally for a generic system
consisting of a doubly-clamped Au suspended beam, capacitively-coupled to an
adjacent stationary electrode. The mechanical properties of the beam, both in
the linear and nonlinear regimes, are monitored as the attractive forces are
increased to the point of instability. There "pull-in" occurs, resulting in
permanent adhesion between the electrodes. We investigate, experimentally and
theoretically, the position-dependent lifetimes of the free state (existing
prior to pull-in). We find that the data cannot be accounted for by simple
theory; the discrepancy may be reflective of internal structural instabilities
within the metal electrodes.Comment: RevTex, 4 pages, 4 figure
Analyzing powers in inclusive pion production at high energy and the nucleon spin structure
Analyzing powers in inclusive pion production in high energy transversely
polarized proton-proton collisions are studied theoretically in the framework
of the quark recombination model. Calculations by assuming the SU(6)
spin-flavor symmetry for the nucleon structure disagree with the experiments.
We solve this difficulty by taking into account the %We overcome this
difficulty by taking into account the realistic spin distribution functions of
the nucleon, which differs from the SU(6) expectation at large , %but
coincides with a perturbative QCD constraint on the ratio of the unpolarized
valence distributions, as . We also discuss the kaon spin
asymmetry and find in the polarized proton-proton
collisions at large .Comment: 13 pages, 4 figures, late
Giant viscosity enhancement in a spin-polarized Fermi liquid
The viscosity is measured for a Fermi liquid, a dilute He-He mixture,
under extremely high magnetic field/temperature conditions ( T, mK). The spin splitting energy is substantially greater than
the Fermi energy ; as a consequence the polarization tends to unity
and s-wave quasiparticle scattering is suppressed for . Using a
novel composite vibrating-wire viscometer an enhancement of the viscosity is
observed by a factor of more than 500 over its low-field value. Good agreement
is found between the measured viscosity and theoretical predictions based upon
a -matrix formalism.Comment: 4 pages, 4 figure
Desiccation survival in an Antarctic nematode: molecular analysis using expressed sequenced tags
<p>Abstract</p> <p>Background</p> <p>Nematodes are the dominant soil animals in Antarctic Dry Valleys and are capable of surviving desiccation and freezing in an anhydrobiotic state. Genes induced by desiccation stress have been successfully enumerated in nematodes; however we have little knowledge of gene regulation by Antarctic nematodes which can survive multiple environmental stresses. To address this problem we investigated the genetic responses of a nematode species, <it>Plectus murrayi</it>, that is capable of tolerating Antarctic environmental extremes, in particular desiccation and freezing. In this study, we provide the first insight into the desiccation induced transcriptome of an Antarctic nematode through cDNA library construction and suppressive subtractive hybridization.</p> <p>Results</p> <p>We obtained 2,486 expressed sequence tags (ESTs) from 2,586 clones derived from the cDNA library of desiccated <it>P. murrayi</it>. The 2,486 ESTs formed 1,387 putative unique transcripts of which 523 (38%) had matches in the model-nematode <it>Caenorhabditis elegans</it>, 107 (7%) in nematodes other than <it>C. elegans</it>, 153 (11%) in non-nematode organisms and 605 (44%) had no significant match to any sequences in the current databases. The 1,387 unique transcripts were functionally classified by using Gene Ontology (GO) hierarchy and the Kyoto Encyclopedia of Genes and Genomes (KEGG) database. The results indicate that the transcriptome contains a group of transcripts from diverse functional areas. The subtractive library of desiccated nematodes showed 80 transcripts differentially expressed during desiccation stress, of which 28% were metabolism related, 19% were involved in environmental information processing, 28% involved in genetic information processing and 21% were novel transcripts. Expression profiling of 14 selected genes by quantitative Real-time PCR showed 9 genes significantly up-regulated, 3 down-regulated and 2 continuously expressed in response to desiccation.</p> <p>Conclusion</p> <p>The establishment of a desiccation EST collection for <it>Plectus murrayi</it>, a useful model in assessing the structural, physiological, biochemical and genetic aspects of multiple stress tolerance, is an important step in understanding the genome level response of this nematode to desiccation stress. The type of transcript analysis performed in this study sets the foundation for more detailed functional and genome level analyses of the genes involved in desiccation tolerance in nematodes.</p
Azimuthal Correlations with High-pT Multi-Hadron Cluster Triggers in Au+Au Collisions at sqrt(sNN) = 200 GeV
Di-hadron correlation measurements have been used to probe di-jet production
in collisions at RHIC. A strong suppression of the away-side high-pT yield in
these measurements is direct evidence that high-pT partons lose energy as they
traverse the strongly interacting medium. However, since the momentum of the
trigger particle is not a good measure of the jet energy, azimuthal di-hadron
correlations have limited sensitivity to the shape of the fragmentation
function. We explore the possibility to better constrain the initial parton
energy by using clusters of multiple high-pT hadrons in a narrow cone as the
'trigger particle' in the azimuthal correlation analysis. We present first
results from this analysis of multi-hadron triggered correlated yields in Au+Au
collisions at sqrt(sNN) = 200 GeV from STAR. The results are compared to Pythia
calculations, and the implications for energy loss and jet fragmentation are
discussed.Comment: 5 pages, 4 figures, submitted to the proceedings of the 24th Winter
Workshop on Nuclear Dynamic
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