859 research outputs found
[FeFe]-Hydrogenases: recent developments and future perspectives
[FeFe]-Hydrogenases are the most efficient enzymes for catalytic hydrogen turnover. Their H2 production efficiency is hitherto unrivalled. However, functional details of the catalytic machinery and possible modes of application are discussed controversially. The incorporation of synthetically modified cofactors and utilization of semi-artificial enzymes only recently allowed us to shed light on key steps of the catalytic cycle. Herein, we summarize the essential findings regarding the redox chemistry of [FeFe]-hydrogenases and discuss their catalytic hydrogen turnover. We furthermore will give an outlook on potential research activities and exploit the utilization of synthetic cofactor mimics
Analysis of Kaon Production at SIS Energies
We analyse the production and propagation of pions and kaons in heavy-ion
reactions from 0.8 -- 1.8~AGeV within a coupled channel transport
approach including the kaon production channels and . Assuming the hyperon selfenergy to be 2/3 of the nucleon selfenergy
we find that all inclusive experimental spectra at SIS energies can be
reproduced reasonably well without any selfenergies for the kaons although a
slightly repulsive kaon potential cannot be excluded by the present data on
kaon spectra and flow.Comment: 19 pages, LaTeX, including 8 postscript figures, to appear in Nucl.
Phys.
The origin of human chromosome 2 analyzed by comparative chromosome mapping with a DNA microlibrary
Fluorescencein situ hybridization (FISH) of microlibraries established from distinct chromosome subregions can test the evolutionary conservation of chromosome bands as well as chromosomal rearrangements that occurred during primate evolution and will help to clarify phylogenetic relationships. We used a DNA library established by microdissection and microcloning from the entire long arm of human chromosome 2 for fluorescencein situ hybridization and comparative mapping of the chromosomes of human, great apes (Pan troglodytes, Pan paniscus, Gorilla gorilla, Pongo pygmaeus) and Old World monkeys (Macaca fuscata andCercopithecus aethiops). Inversions were found in the pericentric region of the primate chromosome 2p homologs in great apes, and the hybridization pattern demonstrates the known phylogenetically derived telomere fusion in the line that leads to human chromosome 2. The hybridization of the 2q microlibrary to chromosomes of Old World monkeys gave a different pattern from that in the gorilla and the orang-utan, but a pattern similar to that of chimpanzees. This suggests convergence of chromosomal rearrangements in different phylogenetic lines
Near-threshold Production in Heavy-ion Collisions
Within a hadronic transport model we study in detail contributions to kaon
yields and momentum spectra from various baryon (resonance)-baryon (resonance)
and interactions in heavy-ion collisions at beam energies near the
free-space kaon production threshold. It is found that the finite lifetime of
baryon resonances affects significantly the shape of kaon spectra, and the high
energy parts of the kaon spectra are dominated by kaons from processes. resonances are found to contribute
about 10\% to the kaon yield. Effects of boosting the Fermi momentum
distributions of the two colliding nuclei into their center of mass frame,
centrality of the reaction as well as the nuclear equation of state on kaon
yields and spectra are also discussed. Model calculations on ,
and spectra for the reaction of Au+Au at GeV are
compared with the experimental data from the KaoS collaboration.Comment: 18 pages, 11 figures available upon request. TAMU preprint #940403
Particle tracking in kaon electroproduction with cathode-charge sampling in multi-wire proportional chambers
Wire chambers are routinely operated as tracking detectors in magnetic
spectrometers at high-intensity continuous electron beams. Especially in
experiments studying reactions with small cross-sections the reaction yield is
limited by the background rate in the chambers. One way to determine the track
of a charged particle through a multi-wire proportional chamber (MWPC) is the
measurement of the charge distribution induced on its cathodes. In practical
applications of this read-out method, the algorithm to relate the measured
charge distribution to the avalanche position is an important factor for the
achievable position resolution and for the track reconstruction efficiency. An
algorithm was developed for operating two large-sized MWPCs in a strong
background environment with multiple-particle tracks. Resulting efficiencies
were determined as a function of the electron beam current and on the signal
amplitudes. Because of the different energy-losses of pions, kaons, and protons
in the momentum range of the spectrometer the efficiencies depend also on the
particle species
Low-energy cross section of the 7Be(p,g)8B solar fusion reaction from Coulomb dissociation of 8B
Final results from an exclusive measurement of the Coulomb breakup of 8B into
7Be+p at 254 A MeV are reported. Energy-differential Coulomb-breakup cross
sections are analyzed using a potential model of 8B and first-order
perturbation theory. The deduced astrophysical S_17 factors are in good
agreement with the most recent direct 7Be(p,gamma)8B measurements and follow
closely the energy dependence predicted by the cluster-model description of 8B
by Descouvemont. We extract a zero-energy S_17 factor of 20.6 +- 0.8 (stat) +-
1.2 (syst) eV b.Comment: 14 pages including 16 figures, LaTeX, accepted for publication in
Physical Review C. Minor changes in text and layou
Kaon versus Antikaon Production at SIS Energies
We analyse the production and propagation of kaons and antikaons in Ni + Ni
reactions from 0.8--1.85 GeV/u within a coupled channel transport approach
including the channels as well as and for
the antikaon absorption. Whereas the experimental spectra can be
reproduced without introducing any selfenergies for the mesons in Ni + Ni
collisions from 0.8 to 1.8 GeV/u, the yield is underestimated by a factor
of 5--7 at 1.66 and 1.85 GeV/u. However, introducing density dependent antikaon
masses as proposed by Kaplan and Nelson, the antikaon spectra can be reasonably
well described.Comment: 16 pages, LaTeX, plus 12 postscript figures, submitted to Nucl. Phys.
Measurement of the LT-asymmetry in \pi^0 electroproduction at the energy of the \Delta (1232) resonance
The reaction p(e,e'p)pi^0 has been studied at Q^2=0.2 (GeV/c)^2 in the region
of W=1232 MeV. From measurements left and right of q, cross section asymmetries
\rho_LT have been obtained in forward kinematics \rho_LT(\theta_\pi^0=20deg) =
(-11.68 +/- 2.36_stat +/- 2.36_sys)$ and backward kinematics
\rho_LT(\theta_\pi^0=160deg) =(12.18 +/- 0.27_stat +/- 0.82_sys). Multipole
ratios \Re(S_1+^* M_1+)/|M_1+|^2 and \Re(S_0+^* M_1+)/|M_1+|^2 were determined
in the framework of the MAID2003 model. The results are in agreement with older
data. The unusally strong negative \Re(S_0+^* M_1+)/|M_1+|^2 required to bring
also the result of Kalleicher et al. in accordance with the rest of the data is
almost excluded.Comment: 7 pages, 7 figures, 4 tables. Changed content. Accepted for
publication in EPJ
Nonequilibrium Weak Processes in Kaon Condensation I --- Reaction rate for the thermal kaon process ---
We investigate the thermal kaon process,in which kaons are thermally produced
via nucleon-nucleon collisions.This process is relevant to nonequilibrium
dynamics of kaon condensation inside neutron stars.The reaction rates for these
processes are calculated, and their temperature and density dependences are
compared with those of other reaction rates.It is shown that the thermal kaon
process is dominant over other relevant weak reactions throughout the
nonequilibrium process, such as the kaon-induced Urca and the modified Urca
reactions, and may control the entire evolution of the kaon condensate. The
characteristic role of the soft and hard kaons during the evolution is
explained, and implications for astrophysical phenomena are briefly discussed.Comment: 31 pages,incl.10 eps figures,RevTe
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