2,007 research outputs found
Microbial transformation of elements: the case of arsenic and selenium
Microbial activity is responsible for the transformation of at least one third of the elements in the periodic table. These transformations are the result of assimilatory, dissimilatory, or detoxification processes and form the cornerstones of many biogeochemical cycles. Arsenic and selenium are two elements whose roles in microbial ecology have only recently been recognized. Known as “essential toxins”, they are required in trace amounts for growth and metabolism but are toxic at elevated concentrations. Arsenic is used as an osmolite in some marine organisms while selenium is required as selenocysteine (i.e. the twenty-first amino acid) or as a ligand to metal in some enzymes (e.g. FeNiSe hydrogenase). Arsenic resistance involves a small-molecularweight arsenate reductase (ArsC). The use of arsenic and selenium oxyanions for energy is widespread in prokaryotes with representative organisms from the Crenarchaeota, thermophilic bacteria, low and high G+C gram-positive bacteria, and Proteobacteria. Recent studies have shown that both elements are actively cycled and play a significant role in carbon mineralization in certain environments. The occurrence of multiple mechanisms involving different enzymes for arsenic and selenium transformation indicates several different evolutionary pathways (e.g. convergence and lateral gene transfer) and underscores the environmental significance and selective impact in microbial evolution of these two elements
Comparison of Simulator Wear Measured by Gravimetric vs Optical Surface Methods for Two Million Cycles
Understanding wear mechanisms are key for better implants
Critical to the success of the simulation
Small amount of metal wear can have catastrophic effects in the patient such as heavy metal poisoning or deterioration of the bone/implant interface leading to implant failure
Difficult to measure in heavy hard-on-hard implants (metal-on-metal or ceramic-on-ceramic)
May have only fractions of a milligram of wear on a 200 g component
At the limit of detection of even high-end balances when the component is 200 g and the change in weight is on the order of 0.000 1 grams
Here we compare the standard gravimetric wear estimate with
A non-contact 3D optical profiling method at each weighing stop
A coordinate measuring machine (CMM) at the beginning and end of the ru
Altitude e coordenadas geográficas na estimativa da temperatura mínima média decendial do ar no Estado do Rio Grande do Sul.
Single Proton Knock-Out Reactions from 24,25,26F
The cross sections of the single proton knock-out reactions from 24F, 25F,
and 26F on a 12C target were measured at energies of about 50 MeV/nucleon.
Ground state populations of 6.6+-.9 mb, 3.8+-0.6 mb for the reactions
12C(24F,23O) and 12C(25F,24O) were extracted, respectively. The data were
compared to calculations based on the many-body shell model and the eikonal
theory. In the reaction 12C(26F,25O) the particle instability of 25O was
confirmed
Numerical simulation of exciton dynamics in Cu2O at ultra low temperatures within a potential trap
We have studied theoretically the relaxation behaviour of excitons in cuprous
oxide (Cu2O) at ultra low temperatures when excitons are confined within a
potential trap by solving numerically the Boltzmann equation. As relaxation
processes, we have included in this paper deformation potential phonon
scattering, radiative and non-radiative decay and Auger decay. The relaxation
kinetics has been analysed for temperatures in the range between 0.3K and 5K.
Under the action of deformation potential phonon scattering only, we find for
temperatures above 0.5K that the excitons reach local equilibrium with the
lattice i.e. that the effective local temperature is coming down to bath
temperature, while below 0.5K a non-thermal energy distribution remains.
Interestingly, for all temperatures the global spatial distribution of excitons
does not reach the equilibrium distribution, but stays at a much higher
effective temperature. If we include further a finite lifetime of the excitons
and the two-particle Auger decay, we find that both the local and the global
effective temperature are not coming down to bath temperature. In the first
case we find a Bose-Einstein condensation (BEC) to occur for all temperatures
in the investigated range. Comparing our results with the thermal equilibrium
case, we find that BEC occurs for a significantly higher number of excitons in
the trap. This effect could be related to the higher global temperature, which
requires an increased number of excitons within the trap to observe the BEC. In
case of Auger decay, we do not find at any temperature a BEC due to the heating
of the exciton gas
Detection of orbital and superhump periods in Nova V2574 Ophiuchi (2004)
We present the results of 37 nights of CCD unfiltered photometry of nova
V2574 Oph (2004) from 2004 and 2005. We find two periods of 0.14164 d (~3.40 h)
and 0.14773 d (~3.55 h) in the 2005 data. The 2004 data show variability on a
similar timescale, but no coherent periodicity was found. We suggest that the
longer periodicity is the orbital period of the underlying binary system and
that the shorter period represents a negative superhump. The 3.40 h period is
about 4% shorter than the orbital period and obeys the relation between
superhump period deficit and binary period. The detection of superhumps in the
light curve is evidence of the presence of a precessing accretion disk in this
binary system shortly after the nova outburst. From the maximum magnitude -
rate of decline relation, we estimate the decay rate t_2 = 17+/-4 d and a
maximum absolute visual magnitude of M_Vmax = -7.7+/-1.7 mag.Comment: 6 pages, 6 figures, 2 .sty files, AJ accepted, minor change to one of
reference
Excitonic Photoluminescence in Semiconductor Quantum Wells: Plasma versus Excitons
Time-resolved photoluminescence spectra after nonresonant excitation show a
distinct 1s resonance, independent of the existence of bound excitons. A
microscopic analysis identifies excitonic and electron-hole plasma
contributions. For low temperatures and low densities the excitonic emission is
extremely sensitive to even minute optically active exciton populations making
it possible to extract a phase diagram for incoherent excitonic populations.Comment: 9 pages, 4 figure
Beta-delayed proton emission in the 100Sn region
Beta-delayed proton emission from nuclides in the neighborhood of 100Sn was
studied at the National Superconducting Cyclotron Laboratory. The nuclei were
produced by fragmentation of a 120 MeV/nucleon 112Sn primary beam on a Be
target. Beam purification was provided by the A1900 Fragment Separator and the
Radio Frequency Fragment Separator. The fragments of interest were identified
and their decay was studied with the NSCL Beta Counting System (BCS) in
conjunction with the Segmented Germanium Array (SeGA). The nuclei 96Cd, 98Ing,
98Inm and 99In were identified as beta-delayed proton emitters, with branching
ratios bp = 5.5(40)%, 5.5+3 -2%, 19(2)% and 0.9(4)%, respectively. The bp for
89Ru, 91,92Rh, 93Pd and 95Ag were deduced for the first time with bp = 3+1.9
-1.7%, 1.3(5)%, 1.9(1)%, 7.5(5)% and 2.5(3)%, respectively. The bp = 22(1)% for
101Sn was deduced with higher precision than previously reported. The impact of
the newly measured bp values on the composition of the type-I X-ray burst ashes
was studied.Comment: 15 pages, 14 Figures, 4 Table
Simplicity of eigenvalues in Anderson-type models
We show almost sure simplicity of eigenvalues for several models of
Anderson-type random Schr\"odinger operators, extending methods introduced by
Simon for the discrete Anderson model. These methods work throughout the
spectrum and are not restricted to the localization regime. We establish
general criteria for the simplicity of eigenvalues which can be interpreted as
separately excluding the absence of local and global symmetries, respectively.
The criteria are applied to Anderson models with matrix-valued potential as
well as with single-site potentials supported on a finite box.Comment: 20 page
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Interplay between DNA sequence and negative superhelicity drives R-loop structures.
R-loops are abundant three-stranded nucleic-acid structures that form in cis during transcription. Experimental evidence suggests that R-loop formation is affected by DNA sequence and topology. However, the exact manner by which these factors interact to determine R-loop susceptibility is unclear. To investigate this, we developed a statistical mechanical equilibrium model of R-loop formation in superhelical DNA. In this model, the energy involved in forming an R-loop includes four terms-junctional and base-pairing energies and energies associated with superhelicity and with the torsional winding of the displaced DNA single strand around the RNA:DNA hybrid. This model shows that the significant energy barrier imposed by the formation of junctions can be overcome in two ways. First, base-pairing energy can favor RNA:DNA over DNA:DNA duplexes in favorable sequences. Second, R-loops, by absorbing negative superhelicity, partially or fully relax the rest of the DNA domain, thereby returning it to a lower energy state. In vitro transcription assays confirmed that R-loops cause plasmid relaxation and that negative superhelicity is required for R-loops to form, even in a favorable region. Single-molecule R-loop footprinting following in vitro transcription showed a strong agreement between theoretical predictions and experimental mapping of stable R-loop positions and further revealed the impact of DNA topology on the R-loop distribution landscape. Our results clarify the interplay between base sequence and DNA superhelicity in controlling R-loop stability. They also reveal R-loops as powerful and reversible topology sinks that cells may use to nonenzymatically relieve superhelical stress during transcription
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