2,539 research outputs found
Feedback control architecture & the bacterial chemotaxis network
Bacteria move towards favourable and away from toxic environments by changing their swimming pattern. This response is regulated by the chemotaxis signalling pathway, which has an important feature: it uses feedback to ‘reset’ (adapt) the bacterial sensing ability, which allows the bacteria to sense a range of background environmental changes. The role of this feedback has been studied extensively in the simple chemotaxis pathway of Escherichia coli. However it has been recently found that the majority of bacteria have multiple chemotaxis homologues of the E. coli proteins, resulting in more complex pathways. In this paper we investigate the configuration and role of feedback in Rhodobacter sphaeroides, a bacterium containing multiple homologues of the chemotaxis proteins found in E. coli. Multiple proteins could produce different possible feedback configurations, each having different chemotactic performance qualities and levels of robustness to variations and uncertainties in biological parameters and to intracellular noise. We develop four models corresponding to different feedback configurations. Using a series of carefully designed experiments we discriminate between these models and invalidate three of them. When these models are examined in terms of robustness to noise and parametric uncertainties, we find that the non-invalidated model is superior to the others. Moreover, it has a ‘cascade control’ feedback architecture which is used extensively in engineering to improve system performance, including robustness. Given that the majority of bacteria are known to have multiple chemotaxis pathways, in this paper we show that some feedback architectures allow them to have better performance than others. In particular, cascade control may be an important feature in achieving robust functionality in more complex signalling pathways and in improving their performance
A Thorium Metal-Organic Framework with Outstanding Thermal and Chemical Stability.
A new thorium metal-organic framework (MOF), Th(OBA)2 , where OBA is 4,4'-oxybis(benzoic) acid, has been synthesized hydrothermally in the presence of a range of nitrogen-donor coordination modulators. This Th-MOF, described herein as GWMOF-13, has been characterized by single-crystal and powder X-ray diffraction, as well as through a range of techniques including gas sorption, thermogravimetric analysis (TGA), solid-state UV/Vis and luminescence spectroscopy. Single-crystal X-ray diffraction analysis of GWMOF-13 reveals an interesting, high symmetry (cubic Ia 3 ‾ d) structure, which yields a novel srs-a topology. Most notably, TGA analysis of GWMOF-13 reveals framework stability to 525 °C, matching the thermal stability benchmarks of the UiO-66 series MOFs and zeolitic imidazolate frameworks (ZIFs), and setting a new standard for thermal stability in f-block based MOFs
A Comparison of Cosmological Hydrodynamic Codes
We present a detailed comparison of the simulation results of various
cosmological hydrodynamic codes. Starting with identical initial conditions
based on the Cold Dark Matter scenario for the growth of structure, we
integrate from redshift to to determine the physical state within
a representative volume of size where . Five
independent codes are compared: three of them Eulerian mesh based and two
variants of the Smooth Particle Hydrodynamics "SPH" Lagrangian approach. The
Eulerian codes were run at cells,
the SPH codes at and particles. Results were then rebinned
to a grid with the expectation that the rebinned data should converge,
by all techniques, to a common and correct result as . We
find that global averages of various physical quantities do, as expected, tend
to converge in the rebinned model, but that uncertainties in even primitive
quantities such as , persists
at the 3\%-17\% level after completion of very large simulations. The two SPH
codes and the two shock capturing Eulerian codes achieve comparable and
satisfactory accuracy for comparable computer time in their treatment of the
high density, high temperature regions as measured in the rebinned data; the
variance among the five codes (at highest resolution) for the mean temperature
(as weighted by ) is only 4.5\%. Overall the comparison allows us to
better estimate errors, it points to ways of improving this current generation
of hydrodynamic codes and of suiting their use to problems which exploit their
individually best features.Comment: 20p plaintex to appear in The Astrophysical Journal on July 20, 199
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