Showing papers by "University of Stuttgart published in 1994"

Scalable concept for diode-pumped high-power solid-state lasers

Abstract: A new, scalable concept for diode-pumped high-power solid-state lasers is presented The basic idea of our approach is a very thin laser crystal disc with one face mounted on a heat sink This allows very high pump power densities without high temperature rises within the crystal Together with a flat-top pump-beam profile this geometry leads to an almost homogeneous and one-dimensional heat flux perpendicular to the surface This design dramatically reduces thermal distortions compared to conventional cooling schemes and is particularly suited for quasi-three-level systems which need high pump power densities Starting from the results obtained with a Ti:Sapphire-pumped Yb:YAG laser at various temperatures, the design was proved by operating a diode-pumped Yb:YAG laser with an output power of 44 W and a maximum slope efficiency of 68% From these first results we predict an exctracted cw power of 100 W at 300 K (140 W at 200 K) with high beam quality from a single longitudinally pumped Yb: YAG crystal with an active volume of 2 mm3 Compact diode-pumped solid-state lasers in the kilowatt range seem to be possible by increasing the pump-beam diameter and/or by using several crystal discs

Three-dimensional extension of non-linear shell formulation based on the enhanced assumed strain concept

Abstract: Conventional shell formulations, such as 3- or 5-parameter theories or even 6-parameter theories including the thickness change as extra parameter, require a condensation of the constitutive law in order to avoid a significant error due to the assumption of a linear displacement field across the thickness. This means that the normal stress in thickness direction has to either vanish or be constant. In general, these extra constraints cannot be satisfied explicitly or they lead to elaborate strain expressions. The main objective of the present study is to introduce directly a complete 3-D constitutive law without modification. Therefore, a 7-parameter theory is utilized which includes a linear varying thickness stretch as extra variable allowing also large strain effects

Positive feedback between acetylcholine and the neurotrophins nerve growth factor and brain-derived neurotrophic factor in the rat hippocampus.

Abstract: In the rat hippocampus, nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are synthesized by neurons in an activity-dependent manner. Glutamate receptor activation increases whereas GABAergic stimulation decreases NGF and BDNF mRNA levels. Here we demonstrate that NGF and BDNF mRNA and NGF protein are up-regulated in the rat hippocampus by the activation of muscarinic receptors. Conversely, NGF and BDNF enhance the release of acetylcholine (ACh) from rat hippocampal synaptosomes containing the nerve endings of the septal cholinergic neurons. NGF also rapidly increases the high-affinity choline transport into synaptosomes. The reciprocal regulation of ACh, NGF and BDNF in the hippocampus suggests a novel molecular framework by which the neurotrophins might influence synaptic plasticity.

Evolution of three-dimensional coherent structures in a flat-plate boundary layer

Abstract: An investigation is presented that analyses the energy flows that are connected to the dynamical behaviour of coherent structures in a transitional flat-plate boundary layer. Based on a mathematical description of the three-dimensional coherent structures of this flow as provided by the Karhunen–Loeve procedure, energy equations for the coherent structures are derived by Galerkin projection of the Navier–Stokes equations in vorticity transport formulation onto the corresponding basis of eigenfunctions. In a first step, the time-averaged energy balance – showing the energy flows that support the different coherent structures and thus maintain the fluctuations of the velocity field – is considered. In a second step, the instantaneous power budget is investigated for the particularly interesting case of a coherent structure providing a prime contribution to the characteristic spike events of the transitional boundary layer. As this structure shows a strong variation in energy, the question about which mechanisms cause these variations is addressed. Our results show that the occurrence of a spike must be attributed to an autonomous event and cannot be interpreted as just an epiphenomenon of the passage of a Λ-vortex.

Synthesis and Reactions of Optically Active Cyanohydrins

Abstract: Cyanohydrins have always held a place of importance both as technical products and as reagents in organic chemistry. It is surprising, therefore, that optically active Cyanohydrins have been extensively investigated and employed for syntheses relatively recently. This can be explained by the fact that only in the past few years have enzymatic methods made chiral Cyanohydrins readily available in high optical purity. Chiral Cyanohydrins are widespread in nature in the form of the respective glycosides and serve roughly 3000 plants and many insects as antifeedants. For the preparative organic chemist, this class of compounds offers an enormous synthetic potential for making other chiral compounds accessible. In a few instances, the pharmacological principle of a drug also incorporates a chiral cyanohydrin as constitutive structural element. In the development of novel, physiologically active compounds all possible stereo-isomers must be synthesized and investigated with respect to their activity range and the pathway of their metabolic transformations and/or degradation. The development of simple synthetic procedures for such compounds, which also entail a high degree of stereoselectivity, therefore has prime importance. To this end chiral Cyanohydrins may serve as stereochemically pure starting materials. In the present review, the following topics will be addressed: enantioselective addition of hydrogen cyanide (HCN), catalysed by the enzymes (R)-and (S)-oxynitrilase, to aldehydes and Ketones yielding (R) and (S) cyanohydrins, respectivity; enantioselective addition of HCN to aldehydes catalyzed by cyclic dipeptides; enantioselective esterification of racemic ocyanohydrins and enantioselective hydrolysis of cyanohydrin esters caytalyzed by lipases and esterases, reprectively; transformation of the nitrile groups of chiral cyanohydrins to provide optically active -hydroxycarboxylic acids, aldehydes, and ketones, as well as 2-amino alcohols; sulfonylation of the OH group of chiral cyanohydrins to furnish optically active -sulfonyloxynitriles which undergo SN2 displacement of the activated OH group yielding α-azido-,α -amino, and α-fluoronitriles with inverted configuration.

A new model for nucleate boiling heat transfer

Abstract: A new model to calculate heat transfer coefficients in nucleate boiling is presented. Heat transfer and fluid flow around a single bubble are investigated taking into account the influence of meniscus curvature, adhesion forces and interfacial thermal resistance on the thermodynamic equilibrium at the gas-liquid interface. The model requires only bubble site densities and departure diameters. Further quantities except the thermophysical properties are not needed. From the results bubble growth rates can be derived. As an example nucleate boiling heat transfer coefficients of R-114 were calculated. They agree with experimental values within the experimental accuracy.

SNNS (Stuttgart Neural Network Simulator)

Abstract: We here describe SNNS, a neural network simulator for Unix workstations that has been developed at the University of Stuttgart, Germany. Our network simulation environment is a tool to generate, train, test, and visualize artificial neural networks. The simulator consists of three major components: a simulator kernel that operates on the internal representation of the neural networks, a graphical user interface based on X-Windows to interactively create, modify and visualize neural nets, and a compiler to generate large neural networks from a high level network description language.

Three-dimensional image sensing by chromatic confocal microscopy

Abstract: In the image of a confocal microscope, only those parts of an object appear bright that are located in the focal plane of the objective. Because of an axial chromatic aberration deliberately introduced into the microscope objective, the location of the focal plane depends on the wavelength used. By using a white-light source and examining an object with a depth variation less than the axial range of the chromatic focus, we find that all parts of the object appear sharp and bright in the image, but according to its height they appear in different colors. A camera with black-and-white film sequentially combines, with three selected chromatic filters, intensity and tone of color of each object point. For each tone of color one can assign a height by using a calibration curve. This assignment could be made unequivocal by the selection of filters with adequate chromatic transmission.

Metabolism of homoacetogens

Abstract: Homoacetogenic bacteria are strictly anaerobic microorganisms that catalyze the formation of acetate from C1 units in their energy metabolism. Most of these organisms are able to grow at the expense of hydrogen plus CO2 as the sole energy source. Hydrogen then serves as the electron donor for CO2 reduction to acetate. The methyl group of acetate is formed from CO2 via formate and reduced C1 intermediates bound to tetrahydrofolate. The carboxyl group is derived from carbon monoxide, which is synthesized from CO2 by carbon monoxide dehydrogenase. The latter enzyme also catalyzes the formation of acetyl-CoA from the methyl group plus CO. Acetyl-CoA is then converted either to acetate in the catabolism or to cell carbon in the anabolism of the bacteria. The homoacetogens are very versatile anaerobes, which convert a variety of different substrates to acetate as the major end product.

Part-of-Speech Tagging with Neural Networks

Abstract: Text corpora which are tagged with part-of-speech information are useful in many areas of linguistic research. In this paper, a new part-of-speech tagging method based on neural networks (Net- Tagger) is presented and its performance is compared to that of a HMM-tagger and a trigram-based tagger. It is shown that the Net- Tagger performs as well as the trigram-based tagger and better than the HMM-tagger.

Tetrachloroethene metabolism of Dehalospirillum multivorans

Abstract: Dehalospirillum multivorans is a strictly anaerobic bacterium that is able to dechlorinate tetrachloroethene (perchloroethylene; PCE) via trichloroethene (TCE) to cis-1,2-dichloroethene (DCE) as part of its energy metabolism. The present communication describes some features of the dechlorination reaction in growing cultures, cell suspensions, and cell extracts of D. multivorans. Cell suspensions catalyzed the reductive dechlorination of PCE with pyruvate as electron donor at specific rates of up to 150 nmol (chloride released) min-1 (mg cell protein)-1 (300 μM PCE initially, pH 7.5, 25°C). The rate of dechlorination depended on the PCE concentration; concentrations higher than 300 μM inhibited dehalogenation. The temperature optimum was between 25 and 30°C; the pH optimum at about 7.5. Dehalogenation was sensitive to potential alternative electron acceptors such as fumarate or sulfur; nitrate or sulfate had no significant effect on PCE reduction. Propyl iodide (50 μM) almost completely inhibited the dehalogenation of PCE in cell suspensions. Cell extracts mediated the dehalogenation of PCE and of TCE with reduced methyl viologen as the electron donor at specific rates of up to 0.5 μmol (chloride released) min-1 (mg protein).-1 An abiotic reductive dehalogenation could be excluded since cell extracts heated for 10 min at 95°C were inactive. The PCE dehalogenase was recovered in the soluble cell fraction after ultracentrifugation. The enzyme was not inactivated by oxygen.

The origin of Jurassic reefs: Current research developments and results

Abstract: In order to elucidate the control of local, regional and global factors on occurrence, distribution and character of Jurassic reefs, reefal settings of Mid and Late Jurassic age from southwestern Germany, Iberia and Romania were compared in terms of their sedimentological (including diagenetic), palaeoecological, architectural, stratigraphic and sequential aspects. Upper Jurassic reefs of southern Germany are dominated by siliceous sponge—microbial crust automicritic to allomicritic mounds. During the Oxfordian these form small to large buildups, whereas during the Kimmeridgian they more frequently are but marginal parts of large grain-dominated massive buildups. Diagenesis of sponge facies is largely governed by the original composition and fabric of sediments. The latest Kimmeridgian and Tithonian spongiolite development is locally accompanied by coral facies, forming large reefs on spongiolitic topographic elevations or, more frequently, small meadows and patch reefs within bioclastic to oolitic shoal and apron sediments. New biostratigraphic results indicate a narrower time gap between Swabian and Franconian coral development than previously thought. Palynostratigraphy and mineralostratigraphy partly allow good stratigraphic resolution also in spongiolitic buildups, and even in dolomitised massive limestones.

Highlights from a seismic broadband array on Stromboli Volcano

Abstract: An array of nine three-component broadband seismometers was deployed in two different configurations on Stromboli volcano. The analysis of the seismic wavefield related to volcanic explosions revealed some observations which offer a completely new insight into the internal dynamics of a volcano. These new observations are restricted to the low-frequency range below 1 Hz and underline, therefore, the superiority of broadband recordings over conventional short-period observations. Surprisingly simple wavelets indicate an initially contracting source mechanism. Gas-jets, that could not be seen in a short-period seismic record at all, generate a clear dilatational wavelet in a broadband recording suggesting the same contracting source mechanism. The analysis of particle motion and seismic array techniques permits a location of the seismic source. We find low-frequency signals of 3s and 6s period that are not related to eruptions and do not share a common source with the eruption-related events. A video recording of visible volcanic activity at the crater region allows one to correlate precisely eruptive features with seismic signals.

Rostro-caudal patterning of receptor-expressing olfactory neurones in the rat nasal cavity

Abstract: The rostro-caudal extent of odorant receptor expression zones in the rat olfactory epithelium was analysed by means of in situ hybridization Three broad non-overlapping zones were identified that extended along almost the entire anterior-posterior axis; each zone was composed of several separate bands running anterior to posterior throughout the olfactory epithelium Super-imposed onto these broad zones was the expression area of a particular receptor subtype (OR37); it was restricted to a small region of the epithelial sheet with a high density of reactive neurones in the centre and declining numbers towards the periphery of the region A quantitative evaluation of the reactive cells revealed that, despite their different distribution patterns, all receptor subtypes were expressed in an equal number of neurones

Identification of a hydride-Meisenheimer complex as a metabolite of 2,4,6-trinitrotoluene by a Mycobacterium strain.

Abstract: A bacterial strain, Mycobacterium sp. strain HL 4-NT-1, enriched with 4-nitrotoluene as its sole source of nitrogen, was able to metabolize 2,4,6-trinitrotoluene under aerobic conditions. The dark red-brown metabolite, which accumulated in the culture fluid, was identified as a hydride-Meisenheimer complex by comparison with an authentic synthetic sample.

Three-dimensional analysis by a microlens-array confocal arrangement

Abstract: Scanning confocal microscopy is now well developed and applied. As an alternative to a laser spot to be scanned, parallel processing can be obtained when a two-dimensional structure is moved through the focal plane and a series of image sections is recorded. Surface topography is determined by analysis of the normalized intensity of the appropriate image points, i.e., a search of the intensity maximum leads to surface coordinates. With a high numerical aperture of the optical system, the half-width of I(z) is small, and the topography can be calculated with high accuracy. But with a high numerical aperture, only small object fields can be reproduced. As an alternative to the Nipkow disk for parallel processing, high-numerical-aperture microlenses are combined in an array. The reproducible object field is then limited by the size of the array and the number of lens and detector elements.

Olfactory neurones expressing distinct odorant receptor subtypes are spatially segregated in the nasal neuroepithelium.

Abstract: In situ hybridization techniques have been employed to explore the olfactory epithelium of the rat for the distribution of odorant receptor gene transcripts. We demonstrate that olfactory neurone subpopulations expressing distinct receptor subtypes are spatially segregated within the olfactory epithelium. A compartmentalization of the neuroepithelium into distinct expression zone is apparent, cells expressing a specific receptor are randomly distributed within a given zone. Structurally related receptor subtypes share a common distribution pattern.

Representing time in multimedia systems

Abstract: As multimedia systems integrate a variety of temporally interrelated media items, synchronization is an important issue in those systems. One part of synchronization is the representation of temporal information. Time models are needed to specify temporal interrelations. With the emerging interactive media, deterministic models are replaced by indeterministic models with more expressiveness. Therefore, this paper classifies and evaluates a selection of the most common existing models by their expressiveness. Additionally, a temporal model based on temporal operators is proposed providing high-level abstractions and a high degree of expressiveness for multimedia systems. >

Optimal Experimental Design for Parameter Estimation in Unstructured Growth Models

Abstract: A method of optimal experimental design for parameter estimation in unstructured growth models is presented. The approach is based on a method suggested by Munack (1991) for application in fed-batch processes. In a critical analysis of this method, special emphasis is given to the model validity, because unstructured growth models often are not valid under transient conditions. In consequence, a combined object function has been introduced, which considers model validity and the accuracy of the kinetic parameters to be estimated. The application of this method for fed-batch processes leads to satisfactory results. Investigations of different fed-batch strategies regarding model validity and the quality of parameter estimation are presented. In addition, an experimental verification has been performed with fermentations of the yeast Trichosporon cutaneum.

TNF receptors TR60 and TR80 can mediate apoptosis via induction of distinct signal pathways

Abstract: TNF membrane receptors are usually co-expressed in many tissues but their relative contribution to cellular TNF responses is for most situations unknown. In a TNF cytotoxicity model of KYM-1, a human rhabdomyosarcoma cell line, we recently demonstrated that each of the two TNFRs is on its own capable of inducing cell death. Here we show that both receptors are able to induce apoptosis, as revealed from a similar onset of DNA fragmentation and typical morphologic criteria. To obtain additional information about the signaling pathways involved in TR60- and TR80-induced programmed cell death, we have used a series of selective inhibitors of intracellular signaling molecules. The overall pattern emerging from these experiments provides strong evidence for distinct signal pathway usage of TR60 and TR80, indicating protein kinase(s)-mediated control of TR60 signaling and a tight linkage of TR80 to arachidonate metabolism. The subsequent establishment of KYM-1-derived cell lines that display TNFR selective resistance further supports a segregation of TR60 and TR80 signaling pathways for induction of apoptotic cell death. Moreover, these results demonstrate an independent control of the distinct signaling cascades used by TR60 and TR80. This allows a highly flexible regulation of a cellular TNF response in those cases in which both receptors contribute to overall TNF responsiveness.

STM investigations of PTCDA and PTCDI on graphite and MoS2. A systematic study of epitaxy and STM image contrast

Abstract: Monolayers of the organic molecules perylene- 3,4,9,10-tetra-carboxylic-dianhydride (PTCDA) and -di- imide (PTCDI) on graphite and MoS 2 have been imaged with scanning tunneling microscopy. The epitaxial growth of the two molecules is determined by the inter- molecular interaction but nearly independent of the sub- strate. On both substrates the STM image contrast in the submolecularly resolved images is dominated by the aro- matic perylene system whereas the polar oxygen and ni- trogen groups are invisible. The correlation of the ob- served inner structure of the molecules to their molecular structure allows us to compare our results with theoreti- cal considerations.

Electron-phonon quantum kinetics in pulse-excited semiconductors: Memory and renormalization effects.

Abstract: The carrier dynamics in photoexcited semiconductors is studied in a quantum kinetic approach based on the density-matrix formalism. Besides the memory effects related to the energy-time uncertainty, we discuss interference effects between different types of interactions describing the fact that a transition due to one interaction occurs between states, which are renormalized by other interactions. We first analyze the relaxation process in a one-band model, which allows us to concentrate on memory effects in the electron-phonon interaction. We then extend the model to a two-band semiconductor interacting with a short laser pulse, which is more realistic due to the explicit treatment of the carrier generation process. Here we discuss, in particular, the role of renormalization effects. It turns out that these effects reduce the broadening due to the non-Markovian dynamics and lead to distribution functions, which are more similar to the semiclassical case; the positions of the peaks, however, exhibit slight time-dependent shifts. On the other hand, phonon quantum beats in the decay of the interband polarization are increased by these renormalization effects.

General Interface Problems-II

Abstract: We study transmission problems for elliptic operators of order 2m with general boundary and interface conditions, introducing new covering conditions. This allows to prove solvability, regularity and asymptotics of solutions in weighted Sobolev spaces. We give some numerical examples for the location of the singular exponents.

A theoretical model of phase transitions in the human brain.

Abstract: An experiment using a multisensor SQUID (superconducting quantum interference device) array was performed by Kelso and colleagues (1992) which combined information from three different sources: perception, motor response, and brain signals. When an acoustic stimulus frequency is changed systematically, a spontaneous transition in coordination occurs at a critical frequency in both motor behavior and brain signals. Qualitatively analogous transitions are known for physical and biological systems such as changes in the coordination of human hand movements (Kelso 1981, 1984). In this paper we develop a theoretical model based on methods from the interdisciplinary field of synergetics (Haken 1983, 1987) and nonlinear oscillator theory that reproduces the main experimental features very well and suggests a formulation of a fundamental biophysical coupling.