The synthesis and ethylene reactivity of a new family of dinuclear Co2Br4 and Fe2Br4 complexes supported by binucleating macrocyclic bis(pyridine-diimine) (PDI) ligands that contain ...4,4″-R2-3,3″-o-terphenyl linkers (1, R = H; 2, R = Me; 3, R = iPr) are described. In the solid state, (1–3)M2Br4 (M = Zn, Fe, Co) adopt C s -skew-syn structures in which the (PDI)M planes are skewed 49–82° relative to each other and both middle rings of the o-terphenyl bridges are on the same side of the molecule. The metal–metal distances range between 5.7600(8) and 6.232(1) Å. In solution, (1)M2Br4 (M = Zn, Co, Fe) undergo a fluxional process that permutes the two inequivalent (PDI)M units, while (2)M2Br4 and (3)M2Br4 are static and adopt C s -symmetric structures similar to those observed in the solid state. Activation of (2)Fe2Br4 and (3)Fe2Br4 with MMAO-12 or triisobutylaluminum (TIBA) in the presence of ethylene generates catalysts that produce solid polyethylene (M w = 4500–280000 Da), which contrasts with the reported production of α-olefins by analogous mononuclear (PDI)FeCl2 catalysts. (3)Fe2Br4/TIBA and (3)Fe2Br4/MMAO-12 produce polyethylenes with broad molecular weight distributions (MWDs) due to chain transfer to Al. (3)Co2Br4/1000 TIBA and (3)Co2Br4/1000 MMAO-12 also produce polyethylenes with broad MWDs. However, in these cases chain transfer to Al is not operative and the broad MWDs result from multisite behavior.
Blazars are active galactic nuclei (AGN) with relativistic jets whose non-thermal radiation is extremely variable on various timescales
. This variability seems mostly random, although some ...quasi-periodic oscillations (QPOs), implying systematic processes, have been reported in blazars and other AGN. QPOs with timescales of days or hours are especially rare
in AGN and their nature is highly debated, explained by emitting plasma moving helically inside the jet
, plasma instabilities
or orbital motion in an accretion disc
. Here we report results of intense optical and γ-ray flux monitoring of BL Lacertae (BL Lac) during a dramatic outburst in 2020 (ref.
). BL Lac, the prototype of a subclass of blazars
, is powered by a 1.7 × 10
M
(ref.
) black hole in an elliptical galaxy (distance = 313 megaparsecs (ref.
)). Our observations show QPOs of optical flux and linear polarization, and γ-ray flux, with cycles as short as approximately 13 h during the highest state of the outburst. The QPO properties match the expectations of current-driven kink instabilities
near a recollimation shock about 5 parsecs (pc) from the black hole in the wake of an apparent superluminal feature moving down the jet. Such a kink is apparent in a microwave Very Long Baseline Array (VLBA) image.
The Fate of Nitrogen in Bioreactor Landfills Berge, N. D.; Reinhart, D. R.; Townsend, T. G.
Critical reviews in environmental science and technology,
07/2005, Letnik:
35, Številka:
4
Journal Article
Recenzirano
Although bioreactor landfills have many advantages associated with them, challenges remain, including the persistence of ammonia-nitrogen in the leachate. It has been suggested that ammonia-nitrogen ...is one of the most significant long-term pollution problem in landfills and is likely a parameter that will determine when landfill postclosure monitoring may end. The fate of nitrogen in bioreactor landfills is not well understood. As more landfills transition operation to bioreactors, more attention must be paid to how operating the landfill as a bioreactor may affect the fate of nitrogen. Processes such as sorption, volatilization, nitrification, denitrification, anaerobic ammonium oxidation, and dissimilatory nitrate reduction may all occur.
Phosphofructokinase from Bacillus stearothermophilus (BsPFK) is a 136 kDa homotetromeric enzyme. Binding of the substrate, fructose 6-phosphate (Fru-6-P), is allosterically regulated by the K-type ...inhibitor phosphoenolpyruvate (PEP). The allosteric coupling between the substrate and inhibitor is quantified by a standard coupling free energy that defines an equilibrium with the Fru-6-P-bound and PEP-bound complexes on one side and the apo form and ternary complex on the other. Methyl-transverse relaxation-optimized spectroscopy (Me-TROSY) nuclear magnetic resonance was employed to gain structural information about BsPFK in all four states of ligation relevant to the allosteric coupling. BsPFK was uniformly labeled with 15N and 2H and specifically labeled with δ-13CH3-isoleucine utilizing an isotopically labeled α-keto acid isoleucine precursor. Me-TROSY experiments were conducted on all four ligation states, and all 30 isoleucines, which are well dispersed throughout each subunit of the enzyme, are well-resolved in chemical shift correlation maps of 13C and 1H. Assignments for 17 isoleucines were determined through three-dimensional HMQC-NOESY experiments with U-15N,2H;Ileδ1-13CH3-BsPFK and complementary HNCA and HNCOCA experiments with U-2H,15N,13C-BsPFK. The assignments allowed for the mapping of resonances representing isoleucine residues to a previously determined X-ray crystallography structure. This analysis, performed for all four states of ligation, has allowed specific regions of the enzyme influenced by the binding of allosteric ligands and those involved in the propagation of the allosteric effect to be identified and distinguished from one another.
ABSTRACT
In 2022 the BL Lac object S4 0954+65 underwent a major variability phase, reaching its historical maximum brightness in the optical and γ-ray bands. We present optical photometric and ...polarimetric data acquired by the Whole Earth Blazar Telescope (WEBT) Collaboration from 2022 April 6 to July 6. Many episodes of unprecedented fast variability were detected, implying an upper limit to the size of the emitting region as low as $10^{-4}$ parsec. The WEBT data show rapid variability in both the degree and angle of polarization. We analyse different models to explain the polarization behaviour in the framework of a twisting jet model, which assumes that the long-term trend of the flux is produced by variations in the emitting region viewing angle. All the models can reproduce the average trend of the polarization degree, and can account for its general anticorrelation with the flux, but the dispersion of the data requires the presence of intrinsic mechanisms, such as turbulence, shocks, or magnetic reconnection. The WEBT optical data are compared to γ-ray data from the Fermi satellite. These are analysed with both fixed and adaptive binning procedures. We show that the strong correlation between optical and γ-ray data without measurable delay assumes different slopes in faint and high brightness states, and this is compatible with a scenario where in faint states we mainly see the imprint of the geometrical effects, while in bright states the synchrotron self-Compton process dominates.
This paper describes a multi-site and multi-decadal dataset of artificially drained agricultural fields in seven Midwest states and North Carolina, USA. Thirty-nine research sites provided data on ...three conservation practices for cropland with subsurface tile drainage: saturated buffers, controlled drainage, and drainage water recycling. These practices utilize vegetation and/or infrastructure to minimize off-site nutrient losses and retain water in the landscape. A total of 219 variables are reported, including 90 field measurement variables and 129 management operations and metadata. Key measurements include subsurface drain flow (206 site-years), nitrate-N load (154 site-years) and other water quality metrics, as well as agronomic, soil, climate, farm management and metadata records. Data are published at the USDA National Agricultural Library Ag Data Commons repository and are also available through an interactive website at Iowa State University. These multi-disciplinary data have large reuse potential by the scientific community as well as for design of drainage systems and implementation in the US and globally.
Microscopic simulation models are becoming increasingly important tools in modeling transport systems. A large number of models are used in many countries. The most difficult stage in the development ...and use of such models is the calibration and validation of the microscopic submodels such as the car-following and gap acceptance models. This difficulty results from the lack of suitable methods for adapting models to empirical data. Recent progress has been made in calibrating a number of microscopic traffic flow models. Ten very different models have been tested with data collected via Differential Global Positioning System-equipped cars on a test track in Japan. To calibrate the models, the data of the leading car are fed into the model under consideration, and the model is used to compute the headway time series of the following car. The deviations between the measured and the simulated headways are then used to calibrate and validate the models. The calibration results agree with earlier studies as there are errors of 12% to 17% for all models, and no model can be denoted to be the best. The differences between individual drivers are larger than the differences between different models. The validation process gives acceptable errors from 17% to 22%. But for special data sets with validation errors up to 60%, the calibration process has reached what is known as "overfitting": because of the adaptation to a particular situation, the models are not capable of generalizing to other situations.
Silica based glasses are commonly used as window material in applications which are subject to high velocity impacts. Thorough understanding of the response to shock loading in these materials is ...crucial to the development of new designs. Despite the lack of long range order in amorphous glasses, the structure can be described statistically by the random network model. Changes to the network structure alter the response to shock loading. Results indicate that in fused silica, substitution of boron as a network former does not have a large effect on the shock loading properties while modifying the network with sodium and calcium changes the dynamic response. These initial results suggest the potential of a predictive capability to determine the effects of other network substitutions.
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