Iron-sulfur cluster-containing L-serine dehydratase from Peptostreptococcus asaccharolyticus: correlation of the cluster type with enzymatic activity.

AbstractInvestigations were performed with regard to the function of the iron—sulfur cluster of l-serine dehydratase from Peptostreptococcusasaccharolyticus, an enzyme which is novel in the class of deaminating hydro-lyases in that it lacks pyridoxal-5′-phosphate. Anaerobically purified l-serine dehydratase from P. asaccharolyticus revealed EPR spectra characteristic of a [3Fe4S]+ cluster constituting 1% of the total enzyme concentration. Upon incubation of the enzyme under air the intensity of the [3Fe4S]+ signal increased correlating with the loss of enzymatic activity. Addition of l-serine prevented this. Hence, active l-serine dehydratase probably contains a diamagnetic [4Fe4S]2+ cluster which is converted by oxidation and loss of one iron ion to a paramagnetic [3Fe4S]+ cluster, resulting in inactivation of the enzyme. In analogy to the mechanism elucidated for aconitase, it is proposed that l-serine is coordinated via its hydroxyl and carboxyl groups to the labile iron atom of the [4Fe4S]2+ cluster

Modular System for Shelves and Coasts (MOSSCO v1.0) - a flexible and multi-component framework for coupled coastal ocean ecosystem modelling

Shelf and coastal sea processes extend from the atmosphere through the water column and into the sea bed. These processes are driven by physical, chemical, and biological interactions at local scales, and they are influenced by transport and cross strong spatial gradients. The linkages between domains and many different processes are not adequately described in current model systems. Their limited integration level in part reflects lacking modularity and flexibility; this shortcoming hinders the exchange of data and model components and has historically imposed supremacy of specific physical driver models. We here present the Modular System for Shelves and Coasts (MOSSCO, http://www.mossco.de), a novel domain and process coupling system tailored---but not limited--- to the coupling challenges of and applications in the coastal ocean. MOSSCO builds on the existing coupling technology Earth System Modeling Framework and on the Framework for Aquatic Biogeochemical Models, thereby creating a unique level of modularity in both domain and process coupling; the new framework adds rich metadata, flexible scheduling, configurations that allow several tens of models to be coupled, and tested setups for coastal coupled applications. That way, MOSSCO addresses the technology needs of a growing marine coastal Earth System community that encompasses very different disciplines, numerical tools, and research questions.Comment: 30 pages, 6 figures, submitted to Geoscientific Model Development Discussion

Two-temperature coronal flow above a thin disk

We extended the disk corona model (Meyer & Meyer-Hofmeister 1994; Meyer, Liu, & Meyer-Hofmeister 2000a) to the inner region of galactic nuclei by including different temperatures in ions and electrons as well as Compton cooling. We found that the mass evaporation rate and hence the fraction of accretion energy released in the corona depend strongly on the rate of incoming mass flow from outer edge of the disk, a larger rate leading to more Compton cooling, less efficient evaporation and a weaker corona. We also found a strong dependence on the viscosity, higher viscosity leading to an enhanced mass flow in the corona and therefore more evaporation of gas from the disk below. If we take accretion rates in units of the Eddington rate our results become independent on the mass of the central black hole. The model predicts weaker contributions to the hard X-rays for objects with higher accretion rate like narrow-line Seyfert 1 galaxies (NLS1s), in agreement with observations. For luminous active galactic nuclei (AGN) strong Compton cooling in the innermost corona is so efficient that a large amount of additional heating is required to maintain the corona above the thin disk.Comment: 17 pages, 6 figures. ApJ accepte

Discovery of a Magnetic White Dwarf in the Symbiotic Binary Z Andromedae

We report the first result from our survey of rapid variability in symbiotic binaries: the discovery of a persistent oscillation at P=1682.6 +- 0.6 s in the optical emission from the prototype symbiotic, Z Andromedae. The oscillation was detected on all 8 occasions on which the source was observed over a timespan of nearly a year, making it the first such persistent periodic pulse found in a symbiotic binary. The amplitude was typically 2 - 5 mmag, and it was correlated with the optical brightness during a relatively small outburst of the system. The most natural explanation is that the oscillation arises from the rotation of an accreting, magnetic (B_S > 10^5 G) white dwarf. This discovery constrains the outburst mechanisms, since the oscillation emission region near the surface of the white dwarf was visible during the outburst.Comment: Accepted for publication in the Astrophysical Journal (6 pages, including 4 figures), LaTe

Weak Disorder in Fibonacci Sequences

We study how weak disorder affects the growth of the Fibonacci series. We introduce a family of stochastic sequences that grow by the normal Fibonacci recursion with probability 1-epsilon, but follow a different recursion rule with a small probability epsilon. We focus on the weak disorder limit and obtain the Lyapunov exponent, that characterizes the typical growth of the sequence elements, using perturbation theory. The limiting distribution for the ratio of consecutive sequence elements is obtained as well. A number of variations to the basic Fibonacci recursion including shift, doubling, and copying are considered.Comment: 4 pages, 2 figure

Structural investigation of K-feldspar KAlSi3O8 crystals by XRD and Raman spectroscopy: an application to petrological study of Luc Yen Pegmatites, Yen Bai Province, Vietnam

K-feldspars in pegmatites from Luc Yen gem mining area, Yen Bai province, Vietnam were studied by X-ray fluorescence, X-ray powder diffraction and the Raman spectroscopy. Chemical analysis determined the K-feldspars in the form of (K0:8909Na0:0388Ca0:002Pb0:0042Cs0:0024Rb0:0338)(Al0:9975Fe0:0053Ti0:0004)Si2:988O8. Both X-ray powder diffraction and Raman spectroscopy indicated Luc Yen K-feldspars as orthoclase phase. Together with the values of Al content of the T1 tetrahedral sites in orthoclase, it is understood that Luc Yen pegmatites are of young ages (Cenozoic) and shallow intrusive types

Fluorescent Silicon Clusters and Nanoparticles

The fluorescence of silicon clusters is reviewed. Atomic clusters of silicon have been at the focus of research for several decades because of the relevance of size effects for material properties, the importance of silicon in electronics and the potential applications in bio-medicine. To date numerous examples of nanostructured forms of fluorescent silicon have been reported. This article introduces the principles and underlying concepts relevant for fluorescence of nanostructured silicon such as excitation, energy relaxation, radiative and non-radiative decay pathways and surface passivation. Experimental methods for the production of silicon clusters are presented. The geometric and electronic properties are reviewed and the implications for the ability to emit fluorescence are discussed. Free and pure silicon clusters produced in molecular beams appear to have properties that are unfavourable for light emission. However, when passivated or embedded in a suitable host, they may emit fluorescence. The current available data show that both quantum confinement and localised transitions, often at the surface, are responsible for fluorescence. By building silicon clusters atom by atom, and by embedding them in shells atom by atom, new insights into the microscopic origins of fluorescence from nanoscale silicon can be expected.Comment: 5 figures, chapter in "Silicon Nanomaterials Sourcebook", editor Klaus D. Sattler, CRC Press, August 201

Detectability of dirty dust grains in brown dwarf atmospheres

Dust clouds influence the atmospheric structure of brown dwarfs, and they affect the heat transfer and change the gas-phase chemistry. However, the physics of their formation and evolution is not well understood. In this letter, we predict dust signatures and propose a potential observational test of the physics of dust formation in brown dwarf atmosphere based on the spectral features of the different solid components predicted by dust formation theory. A momentum method for the formation of dirty dust grains (nucleation, growth, evaporation, drift) is used in application to a static brown dwarf atmosphere structure to compute the dust grain properties, in particular the heterogeneous grain composition and the grain size. Effective medium and Mie theory are used to compute the extinction of these spherical grains. Dust formation results in grains whose composition differs from that of grains formed at equilibrium. Our kinetic model predicts that solid amorphous SiO2[s] (silica) is one of the most abundant solid component followed by amorphous MgSiO4[s] and MgSiO3[s], while SiO2[s] is absent in equilibrium models because it is a metastable solid. Solid amorphous SiO2[s] possesses a strong broad absorption feature centered at 8.7mum, while amorphous Mg2SiO4[s]/MgSiO3[s] absorb at 9.7mum beside other absorption features at longer wavelength. Those features at lambda < 15mum are detectable in absorption if grains are small (radius < 0.2mum) in the upper atmosphere as suggested by our model. We suggest that the detection of a feature at 8.7mum in deep infrared spectra could provide evidence for non-equilibrium dust formation that yields grains composed of metastable solids in brown dwarf atmospheres. This feature will shift towards 10mum and broaden if silicates (e.g. fosterite) are much more abundant.Comment: A&A Letter, accepte