999 research outputs found
Biochemistry and Occurrence of O-Demethylation in Plant Metabolism
Demethylases play a pivitol role in numerous biological processes from covalent histone modification and DNA repair to specialized metabolism in plants and microorganisms. Enzymes that catalyze O- and N-demethylation include 2-oxoglutarate (2OG)/Fe(II)-dependent dioxygenases, cytochromes P450, Rieske-domain proteins and flavin adenine dinucleotide (FAD)-dependent oxidases. Proposed mechanisms for demethylation by 2OG/Fe(II)-dependent enzymes involve hydroxylation at the O- or N-linked methyl group followed by formaldehyde elimination. Members of this enzyme family catalyze a wide variety of reactions in diverse plant metabolic pathways. Recently, we showed that 2OG/Fe(II)-dependent dioxygenases catalyze the unique O-demethylation steps of morphine biosynthesis in opium poppy, which provides a rational basis for the widespread occurrence of demethylases in benzylisoquinoline alkaloid metabolism
Nucleation and cluster formation in low-density nucleonic matter: A mechanism for ternary fission
Ternary fission yields in the reaction 241Pu(nth,f) are calculated using a
new model which assumes a nucleation-time moderated chemical equilibrium in the
low density matter which constitutes the neck region of the scissioning system.
The temperature, density, proton fraction and fission time required to fit the
experimental data are derived and discussed. A reasonably good fit to the
experimental data is obtained. This model provides a natural explanation for
the observed yields of heavier isotopes relative to those of the lighter
isotopes, the observation of low proton yields relative to 2H and 3H yields and
the non-observation of 3He, all features which are shared by similar thermal
neutron induced and spontaneous fissioning systems.Comment: 6 pages, 3 figure
Spin-zero anomaly in the magnetic quantum oscillations of a two-dimensional metal
We report on an anomalous behavior of the spin-splitting zeros in the de
Haas-van Alphen (dHvA) signal of a quasi-two-dimensional organic
superconductor. The zeros as well as the angular dependence of the amplitude of
the second harmonic deviate remarkably from the standard Lifshitz-Kosevich (LK)
prediction. In contrast, the angular dependence of the fundamental dHvA
amplitude as well as the spin-splitting zeros of the Shubnikov-de Haas signal
follow the LK theory. We can explain this behavior by small chemical-potential
oscillations and find a very good agreement between theory and experiment. A
detailed wave-shape analysis of the dHvA signal corroborates the existence of
an oscillating chemical potential
Isocaling and the Symmetry Energy in the Multifragmentation Regime of Heavy Ion Collisions
The ratio of the symmetry energy coefficient to temperature, , in
Fermi energy heavy ion collisions, has been experimentally extracted as a
function of the fragment atomic number using isoscaling parameters and the
variance of the isotope distributions. The extracted values have been compared
to the results of calculations made with an Antisymmetrized Molecular Dynamics
(AMD) model employing a statistical decay code to account for deexcitation of
excited primary fragments. The experimental values are in good agreement with
the values calculated but are significantly different from those characterizing
the yields of the primary AMD fragments.Comment: 12 pages, 6 figure
Terahertz frequency standard based on three-photon coherent population trapping
A scheme for a THz frequency standard based on three-photon coherent
population trapping in stored ions is proposed. Assuming the propagation
directions of the three lasers obey the phase matching condition, we show that
stability of few 10 at one second can be reached with a precision
limited by power broadening to in the less favorable case. The
referenced THz signal can be propagated over long distances, the useful
information being carried by the relative frequency of the three optical
photons.Comment: article soumis a PRL le 21 mars 2007, accepte le 10 mai, version 2
(24/05/2007
Novel technique to extract experimental symmetry free energy information of nuclear matter
A new method of accessing information on the symmetry free energy from yields
of fragments produced in Fermi-energy heavy-ion collisions is proposed.
Furthermore, by means of quantum fluctuation analysis techniques, correlations
between extracted symmetry free-energy coefficients with temperature and
density were studied. The obtained results are consistent with those of
commonly used isoscaling techniques.Comment: 6 pages, 3 figures Heavy-ion nuclear reactions at Fermi energies,
Nuclear equation of State, Fragmentatio
Critical behavior of the isotope yield distributions in the Multifragmentation Regime of Heavy Ion Reactions
Isotope yields have been analyzed within the framework of a Modified Fisher
Model to study the power law yield distribution of isotopes in the
multifragmentation regime. Using the ratio of the mass dependent symmetry
energy coefficient relative to the temperature, , extracted in
previous work and that of the pairing term, , extracted from this
work, and assuming that both reflect secondary decay processes, the
experimentally observed isotope yields have been corrected for these effects.
For a given I = N - Z value, the corrected yields of isotopes relative to the
yield of show a power law distribution, , in the mass range of and the distributions are
almost identical for the different reactions studied. The observed power law
distributions change systematically when I of the isotopes changes and the
extracted value decreases from 3.9 to 1.0 as I increases from -1 to 3.
These observations are well reproduced by a simple de-excitation model, which
the power law distribution of the primary isotopes is determined to
, suggesting that the disassembling system at the
time of the fragment formation is indeed at or very near the critical point.Comment: 5 pages, 5 figure
Temperature measurements of fusion plasmas produced by petawatt laser-irradiated D2-3He or CD4-3He clustering gases
Two different methods have been employed to determine the plasma temperature
in a laser-cluster fusion experiment on the Texas Petawatt laser. In the first,
the temperature was derived from time-of-flight data of deuterium ions ejected
from exploding D2 or CD4 clusters. In the second, the temperature was measured
from the ratio of the rates of two different nuclear fusion reactions occurring
in the plasma at the same time: D(d, 3He)n and 3He(d, p)4He. The temperatures
determined by these two methods agree well, which indicates that: i) The ion
energy distribution is not significantly distorted when ions travel in the
disassembling plasma; ii) The kinetic energy of deuterium ions, especially the
hottest part responsible for nuclear fusion, is well described by a
near-Maxwellian distribution.Comment: 13 pages, 4 figure
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