2,638 research outputs found
The Regularizing Capacity of Metabolic Networks
Despite their topological complexity almost all functional properties of
metabolic networks can be derived from steady-state dynamics. Indeed, many
theoretical investigations (like flux-balance analysis) rely on extracting
function from steady states. This leads to the interesting question, how
metabolic networks avoid complex dynamics and maintain a steady-state behavior.
Here, we expose metabolic network topologies to binary dynamics generated by
simple local rules. We find that the networks' response is highly specific:
Complex dynamics are systematically reduced on metabolic networks compared to
randomized networks with identical degree sequences. Already small topological
modifications substantially enhance the capacity of a network to host complex
dynamic behavior and thus reduce its regularizing potential. This exceptionally
pronounced regularization of dynamics encoded in the topology may explain, why
steady-state behavior is ubiquitous in metabolism.Comment: 6 pages, 4 figure
Uniqueness properties of the Kerr metric
We obtain a geometrical condition on vacuum, stationary, asymptotically flat
spacetimes which is necessary and sufficient for the spacetime to be locally
isometric to Kerr. Namely, we prove a theorem stating that an asymptotically
flat, stationary, vacuum spacetime such that the so-called Killing form is an
eigenvector of the self-dual Weyl tensor must be locally isometric to Kerr.
Asymptotic flatness is a fundamental hypothesis of the theorem, as we
demonstrate by writing down the family of metrics obtained when this
requirement is dropped. This result indicates why the Kerr metric plays such an
important role in general relativity. It may also be of interest in order to
extend the uniqueness theorems of black holes to the non-connected and to the
non-analytic case.Comment: 30 pages, LaTeX, submitted to Classical and Quantum Gravit
A spacetime characterization of the Kerr metric
We obtain a characterization of the Kerr metric among stationary,
asymptotically flat, vacuum spacetimes, which extends the characterization in
terms of the Simon tensor (defined only in the manifold of trajectories) to the
whole spacetime. More precisely, we define a three index tensor on any
spacetime with a Killing field, which vanishes identically for Kerr and which
coincides in the strictly stationary region with the Simon tensor when
projected down into the manifold of trajectories. We prove that a stationary
asymptotically flat vacuum spacetime with vanishing spacetime Simon tensor is
locally isometric to Kerr. A geometrical interpretation of this
characterization in terms of the Weyl tensor is also given. Namely, a
stationary, asymptotically flat vacuum spacetime such that each principal null
direction of the Killing form is a repeated principal null direction of the
Weyl tensor is locally isometric to Kerr.Comment: 23 pages, No figures, LaTeX, to appear in Classical and Quantum
Gravit
Novel P-in-N Si-Sensor technology for high resolution and high repetition-rate experiments at accelerator facilities
Linear array detectors with high spatial resolution and MHz frame-rates are essential for high-rate experiments at accelerator facilities. KALYPSO, a line array detector with 1024 pixels operating over 1 Mfps has been developed. To improve the spatial resolution and sensitivity at different wavelengths, novel p-in-n Si microstrip sensors based on have been developed with a pitch of 25 micrometer. The efficiency of the sensor has been improved with the use of anti reflecting coating layers optimized for near infrared, visible and near ultraviolet. In this contribution the detector system and the sensors will be presented
Crossover between Thermally Assisted and Pure Quantum Tunneling in Molecular Magnet Mn12-Acetate
The crossover between thermally assisted and pure quantum tunneling has been
studied in single crystals of high spin (S=10) uniaxial molecular magnet Mn12
using micro-Hall-effect magnetometry. Magnetic hysteresis and relaxation
experiments have been used to investigate the energy levels that determine the
magnetization reversal as a function of magnetic field and temperature. These
experiments demonstrate that the crossover occurs in a narrow (0.1 K) or broad
(1 K) temperature interval depending on the magnitude of the field transverse
to the anisotropy axis.Comment: 5 pages, 4 figure
Observation of mesoscopic crystalline structures in a two-dimensional Rydberg gas
The ability to control and tune interactions in ultracold atomic gases has
paved the way towards the realization of new phases of matter. Whereas
experiments have so far achieved a high degree of control over short-ranged
interactions, the realization of long-range interactions would open up a whole
new realm of many-body physics and has become a central focus of research.
Rydberg atoms are very well-suited to achieve this goal, as the van der Waals
forces between them are many orders of magnitude larger than for ground state
atoms. Consequently, the mere laser excitation of ultracold gases can cause
strongly correlated many-body states to emerge directly when atoms are
transferred to Rydberg states. A key example are quantum crystals, composed of
coherent superpositions of different spatially ordered configurations of
collective excitations. Here we report on the direct measurement of strong
correlations in a laser excited two-dimensional atomic Mott insulator using
high-resolution, in-situ Rydberg atom imaging. The observations reveal the
emergence of spatially ordered excitation patterns in the high-density
components of the prepared many-body state. They have random orientation, but
well defined geometry, forming mesoscopic crystals of collective excitations
delocalised throughout the gas. Our experiment demonstrates the potential of
Rydberg gases to realise exotic phases of matter, thereby laying the basis for
quantum simulations of long-range interacting quantum magnets.Comment: 10 pages, 7 figure
Chemical Genetic Analysis and Functional Characterization of Staphylococcal Wall Teichoic Acid 2-Epimerases Reveals Unconventional Antibiotic Drug Targets
Here we describe a chemical biology strategy performed in Staphylococcus aureus and Staphylococcus epidermidis to identify MnaA, a 2-epimerase that we demonstrate interconverts UDP-GlcNAc and UDP-ManNAc to modulate substrate levels of TarO and TarA wall teichoic acid (WTA) biosynthesis enzymes. Genetic inactivation of mnaA results in complete loss of WTA and dramatic in vitro β-lactam hypersensitivity in methicillin-resistant S. aureus (MRSA) and S. epidermidis (MRSE). Likewise, the β-lactam antibiotic imipenem exhibits restored bactericidal activity against mnaA mutants in vitro and concomitant efficacy against 2-epimerase defective strains in a mouse thigh model of MRSA and MRSE infection. Interestingly, whereas MnaA serves as the sole 2-epimerase required for WTA biosynthesis in S. epidermidis, MnaA and Cap5P provide compensatory WTA functional roles in S. aureus. We also demonstrate that MnaA and other enzymes of WTA biosynthesis are required for biofilm formation in MRSA and MRSE. We further determine the 1.9Å crystal structure of S. aureus MnaA and identify critical residues for enzymatic dimerization, stability, and substrate binding. Finally, the natural product antibiotic tunicamycin is shown to physically bind MnaA and Cap5P and inhibit 2-epimerase activity, demonstrating that it inhibits a previously unanticipated step in WTA biosynthesis. In summary, MnaA serves as a new Staphylococcal antibiotic target with cognate inhibitors predicted to possess dual therapeutic benefit: as combination agents to restore β-lactam efficacy against MRSA and MRSE and as non-bioactive prophylactic agents to prevent Staphylococcal biofilm formation.publishe
Action Spectroscopy on Dense Samples of Photosynthetic Reaction Centers of Rhodobacter sphaeroides WT Based on Nanosecond Laser-Flash 13C Photo-CIDNP MAS NMR
Photochemically induced dynamic nuclear polarization magic-angle spinning nuclear magnetic resonance (photo-CIDNP MAS NMR) allows for the investigation of the electronic structure of the photochemical machinery of photosynthetic reaction centers (RCs) at atomic resolution. For such experiments, either continuous radiation from white xenon lamps or green laser pulses are applied to optically dense samples. In order to explore their optical properties, optically thick samples of isolated and quinone-removed RCs of the purple bacteria of Rhodobacter sphaeroides wild type are studied by nanosecond laser-flash 13C photo-CIDNP MAS NMR using excitation wavelengths between 720 and 940Â nm. Action spectra of both the transient nuclear polarization as well as the nuclear hyperpolarization, remaining in the electronic ground state at the end of the photocycle, are obtained. It is shown that the signal intensity is limited by the amount of accessible RCs and that the different mechanisms of the photo-CIDNP production rely on the same photophysical origin, which is the photocycle induced by one single photon
A randomized double-blind placebo-controlled trial to investigate the effects of nasal calcitonin on bone microarchitecture measured by high-resolution peripheral quantitative computerized tomography in postmenopausal women — Study protocol
<p>Abstract</p> <p>Background</p> <p>Bone microarchitecture is a significant determinant of bone strength. So far, the assessment of bone microarchitecture has required bone biopsies, limiting its utilization in clinical practice to one single skeletal site. With the advance of high-resolution imaging techniques, non-invasive in vivo measurement of bone microarchitecture has recently become possible. This provides an opportunity to efficiently assess the effects of anti-osteoporotic therapies on bone microarchitecture. We therefore designed a protocol to investigate the effects of nasal salmon calcitonin, an inhibitor of osteoclast activity, on bone microarchitecture in postmenopausal women, comparing weight bearing and non-weight bearing skeletal sites.</p> <p>Methods</p> <p>One hundred postmenopausal women will be included in a randomized, placebo-controlled, double-blind trial comparing the effect of nasal salmon calcitonin (200 UI/day) to placebo over two years. Bone microarchitecture at the distal radius and distal tibia will be determined yearly by high-resolution peripheral quantitative computerized tomography (p-QCT) with a voxel size of 82 μm and an irradiation of less than 5 μSv. Serum markers of bone resorption and bone formation will be measured every 6 months. Safety and compliance will be assessed. Primary endpoint is the change in bone microarchitecture; secondary endpoint is the change in markers of bone turnover.</p> <p>Hypothesis</p> <p>The present study should provide new information on the mode of action of nasal calcitonin. We hypothezise that - compared to placebo - calcitonin impacts on microstructural parameters, with a possible difference between weight bearing and non-weight bearing bones.</p> <p>Trial Registration</p> <p>ClinicalTrials.gov NCT00372099</p
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