Showing papers by "Technical University of Berlin published in 1990"

Nonribosomal Biosynthesis of Peptide Antibiotics

Abstract: Peptide antibiotics are known to contain non-protein amino acids, D-amino acids, hydroxy acids, and other unusual constituents. In addition they may be modified by N-methylation and cyclization reactions. Their biosynthetic origin has been connected in many cases to an enzymatic system referred to as the 'thiotemplate multienzymic mechanism'. This mechanism includes the activation of the constituent residues as adenylates on the enzymic template, the acylation of specific template thiol groups, epimerization or N-methylation at this thioester stage, and polymerization in the sequence directed by the multienzymic structure with the aid of 4'-phosphopantetheine as a cofactor, including possible cyclization or terminal modification reactions. The reaction sequences leading to gramicidin S, tyrocidine, cyclosporine, bacitracin, polymyxin, actinomycin, enniatin, beauvericin, delta-(L-alpha-aminoadipyl)-L-cysteinyl-D-valine and linear gramicidin are discussed. The structures of the multienzymes, their genetic organization, the biological functions of these peptides and results on related systems are discussed.

Optical Properties of ZnSe Epilayers and Films

Abstract: Caracterisation optique et physique de ZnSe. Etude des macles et des deformations thermoelastiques d'une couche ZnSe sur un substrat. Etude des excitons libres dans des films de ZnSe, des impuretes et defauts dans la couche de ZnSe epitaxique, et de leurs proprietes optiques non lineaires

Graph Problems Related to Gate Matrix Layout and PLA Folding

Abstract: Graph Problems Related to Gate Matrix Layout and PLA Folding. This paper gives a survey on graph problems occuring in linear VLSI layout architectures such as gate matrix layout, folding of programmable logic arrays, and Weinberger arrays. These include a variety of mostly independently investigated graph problems such as augmentation of a given graph to an interval graph with small clique size, node search of graphs, matching problems with side constraints, and other. We discuss implications of graph theoretic results for the VLSI layout problems and survey new research directions. New results presented include NP-hardness of gate matrix layout on chordal graphs, efficient algorithms for trees, cographs, and certain chordal graphs, Lagrangean relaxation and approximation algorithms based on on-line interval graph augmentation.

On the regularization of projection methods for solving III-posed problems

Abstract: In this paper we consider a class of regularization methods for a discretized version of an operator equation (which includes the case that the problem is ill-posed) with approximately given right-hand side. We propose an a priori- as well as an a posteriori parameter choice method which is similar to the discrepancy principle of Ivanov-Morozov. From results on fractional powers of selfadjoint operators we obtain convergence rates, which are (in many cases) the same for both parameter choices.

Line shapes of intersubband and excitonic recombination in quantum wells: Influence of final-state interaction, statistical broadening, and momentum conservation

Abstract: A realistic and comprehensive theory of line shapes for spontaneous recombination of two-dimensional carriers in quantum-well (QW) structures is developed. Starting from the line shape for intersubband recombination, which takes into account the QW density of states and the thermal carrier distribution function, the impact of momentum (non)conservation on the luminescence line shape is considered. Then Lorentzian broadening due to the finite lifetime of the final states and Gaussian broadening due to statistical fluctuations of quantum-well eigenenergies characteristic, e.g., of interface roughness is incorporated. The effects of Coulomb interaction of the charge carriers at low densities are considered quantitatively, including both excitonic bound states and excitonic enhancement above the two-dimensional band gap. For a case study, GaAs QW luminescence line shapes are investigated. The line-shape evaluation definitely proves that recombination is excitonic at any temperature up to 300 K in contradiction to previous assumptions of some other authors. At low temperatures all lines consist, on a first view, of unresolved doublets which are very close in energy. The high-energy component is identified as the free-electron--heavy-hole exciton. Momentum is not found and does not need to be conserved in the free-exciton recombination process at low temperatures. At high temperatures, momentum conservationmore » is found to be reestablished: Momentum conservation is understood to depend on the relative amplitude of interface-roughness-induced lateral potential fluctuations as compared to the thermal energy of the excitons.« less

An experimental investigation of an incompressible turbulent boundary layer in the vicinity of separation

Abstract: The following is an investigation into the effects of small changes in the static pressure distribution on the development of an axisymmetric, incompressible, turbulent boundary layer with incipient separation. The pressure distribution was closely controlled to study three cases, in which the skin friction was either approximately zero, slightly negative, or slightly positive along a fixed length. Mean flow and turbulence structure in air were measured using pulsed-wire and hot-wire anemometry.These measurements show characteristic properties of steady turbulent boundary layers both on the verge of separation and with a long, shallow separation bubble. There is an asymptotic velocity defect law near separation. A linear relationship between χW, the wall value of the reverse-flow parameter, and the form parameter H12 suggests the importance of χW in characterizing the boundary layer. The occurrence of the first reverse-flow events coincides with the vanishing of the logarithmic law, the asymptotic mean velocity profile, and a sudden drop in the values of the skewness SW and the flatness FW of the skin friction. This implies that the presence of instantaneous reverse flow is associated with a complete change in the nature of the near-wall flow, well upstream of mean separation. As the three cases were investigated in a single test section under closely controlled conditions with the same experimental techniques, this data set is well suited to a sensitivity study. It is possible to show the effect of small changes in the upstream pressure gradient on the separation region and to distinguish the effect of mean reverse flow from that of the adverse pressure gradient. This effect of the reverse flow is displayed most clearly in a plateau in near the wall and in unusual behaviour of the skewness and the flatness profiles over the inner half of the boundary layer.

Functional and structural analysis of photosystem II core complexes from spinach with high oxygen evolution capacity.

Abstract: Oxygen-evolving photosystem II core complexes were prepared from spinach by solubilizing photosystem II membrane fragments with dodecyl-β-d-maltoside. The core complexes consist of the intrinsic 47-kDa, 43-kDa, D1 and D2 polypeptides, the two subunits, of cytochrome b559 and the extrinsic 33-kDa protein. In the presence of 50 mM CaCl2 they exhibit a high oxygen evolution rate of 1.3 ± 0.2 mmol O2· mg chloropyll−1· h−1 with either 2,6-dichloro-benzouinone or K3[Fe(CN)6] as acceptor. Electron micrographs of these complexes reveal an obtuse triangular structure in when viewed from the top, measuring 15.3 nm on one side and 10.6 nm on the other two sides. An average height of 7.3 nm was determined from the side view position. These data are in good agreement with previously reported dimensions for photosystem II core complexes [Irrgang, K.-D., Boekema, E. J., Vater, J. and Renger, G. (1988) Eur. J. Biochem. 178, 209–217]. In contrast to previous reports the extrinsic 33-kDa subunit could be resolved for the first time. It appears as a small protrusion when the complex is viewed from the side and seems to cover the lumenal side of the core complex appearing as a disk with a thickness of 1.5–3.3 nm.

Image charges in semiconductor quantum wells: Effect on exciton binding energy

Abstract: Binding energies of excitons in a quantum-well structure are calculated including fully the effects of image charges, finite barriers, the z correlation of electrons and holes, and anisotropic hole masses. The influence of discontinuous masses and discontinuous dielectric constants across the interfaces is evaluated in detail: While the mass difference becomes important only when the excitonic wave function penetrates into the barrier, the image charges appreciably modify the Coulomb interaction and therefore influence the exciton binding energy even at well widths larger than the exciton Bohr radius. Results for technologically important, particular material systems are presented.

Turn-Taking: A Critical Analysis of the Research Tradition

Abstract: In the following we present a radical critique of the assumptions, concepts, methods, statistics and interpretation of data, and theories that have characterized the recent research tradition concerned with turn-taking. The principal representative of this tradition is the “simplest systematics” of Sacks, Schegloff, and Jefferson (1974). Attempts to describe the generalizable properties of turn-taking have quite inappropriately and unsuccessfully been limited for the most part to formal approaches that have deliberately excluded considerations of conversational content and purpose. We start instead from the assumption that the ultimate criterion for the success of a conversation is not “the smooth interchange of speaking turns” (Cutler & Pearson, 1986, p. 139) or any other prescriptive ideal, but the fulfillment of the purposes entertained by two or more interlocutors. Our approach is that of a psychology of language use based on Buhler (1927; 1934/1982) and Rommetveit (1974). The emphasis is deliberately placed on social aspects of language as means of communication.

A Temporal Terminological Logic

Abstract: An attempt is made to integrate three well-known formalisms of knowledge representation: terminological logic in the tradition of KL-ONE, the temporal logic of Shoham, and Allen's interval calculus. Drawing on each of these sources, a temporal terminological logic is proposed which combines structural with temporal abstraction. A formal semantics is provided, and some hints are given for exploring the computational properties of reasoning in the formalism.

From Graph Grammars to High Level Replacement Systems

Abstract: The algebraic approach to graph grammars — well-known in the literature for several types of graphs and structures — is extended to include several new types of replacement systems, especially the replacement of algebraic specifications which were recently introduced for a rule-based approach to modular system design.

Reconstitution of CF0F1 into liposomes using a new reconstitution procedure.

Abstract: The H+-ATPase (ATP synthase) from chloroplasts was isolated, purified and reconstituted into phosphatidylcholine/phosphatidic-acid liposomes. Liposomes prepared by reverse-phase evaporation were treated with various amounts of Triton X-100 and protein incorporation was studied at each step of the solubilization process. After detergent removal by SM2-Biobeads, the activities of the resulting proteoliposomes were measured indicating that the most efficient reconstitution was obtained by insertion of the protein into preformed, detergent-saturated liposomes. The conditions for the reconstitution were optimized with regard to ATP synthesis driven by an artificially generated ΔpH/ΔΨ. An important benefit of the new reconstituted CF0F1 liposomes is the finding that the rate of ATP synthesis remains constant up to 10 s, indicating a low basal membrane permeability.

Pan-African reworked early/middle Proterozoic crust in NE Africa west of the Nile: Sr and Nd isotope evidence

Abstract: Whole-rock Rb-Sr studies on six suites of highly metamorphosed basement rocks from southern Egypt and northern Sudan, west of the River Nile, yield Pan-African ages (562–918 Ma): these ages are interpreted as reset. Orthogneisses give Nd model ages of 1600 to 2600 Ma for their calc-alkaline plutonic precursors. A gneiss derived from sedimentary precursors gave a mean crustal residence age of 2200 Ma. A series of I-type, late-tectonic, granitoids were intruded during a period of uplift, erosion, and wrench faulting at 560–620 Ma. Geochemical characteristics and Nd model ages (1200–1700 Ma) indicate that these rocks were derived from a mixed source of juvenile Pan-African mantle material and older continental crust of mainly early/middle Proterozoic age. Negative ɛNd t , values for granitoid samples (–18.9 to –5.3) show that the Pan-African episode in NE Africa, west of the Nile, involved considerable reworking of pre-existing crust.

Systematics and evolution within the Compositae, seen with the eyes of a chemist.

Abstract: The broad knowledge of the chemistry of theCompositae allows the discussion of its relevance for the systematics and evolution within the family. Furthermore a separation into subfamilies can be supported by the observed differences in the distribution of the main constituents in the tribes.

Purification and properties of two oxidoreductases catalyzing the enantioselective reduction of diacetyl and other diketones from baker's yeast.

Abstract: The NADPH-linked diacetyl reductase system from the cytosolic fraction of Saccharomyces cerevisiae has been resolved into two oxidoreductases catalyzing irreversibly the enantioselective reduction of diacetyl (2,3-butanedione) to (S)- and (R)-acetoin (3-hydroxy-2-butanone) [so-called (S)- and (R)-diacetyl reductases] (EC 1.1.1.5) which have been isolated to apparent electrophoretical purity. The clean-up procedures comprising streptomycin sulfate treatment, Sephadex G-25 filtration, DEAE-Sepharose CL-6B column chromatography, affinity chromatography on Matrex Gel Red A and Superose 6 prep grade filtration led to 120-fold and 368-fold purifications, respectively. The relative molecular mass of the (R)-diacetyl reductase, estimated by means of HPLC filtration on Zorbax GF 250 and sodium dodecyl sulfate/polyacrylamide gel electrophoresis, was 36,000. The (R)-enzyme was most active at pH 6.4 and accepted in addition to diacetyl C5-, C6-2,3-diketones, 1,2-cyclohexanedione, 2-oxo aldehydes and short-chain 2- and 3-oxo esters as substrates. The enzyme was characterized by high enantioselectivity and regiospecificity. The Km values for diacetyl and 2,3-pentanedione were determined as 2.0 mM. The Mr of the (S)-diacetyl reductase was determined as 75,000 by means of HPLC filtration of Zorbax GF 250. The enzyme decomposed into subunits of Mr 48,000 and 24,000 on sodium dodecyl sulfate/polyacrylamide gel electrophoresis. The optimum pH was 6.9. The purified (S)-enzyme reduced stereospecifically a broad spectrum of substrates, comprising 2,3-, 2,4- and 2,5-diketones, 2-oxo aldehydes, 1,2-cyclohexanedione and methyl ketones as well as 3-, 4- and 5-oxo esters. The 2,3- and 2,4-diketones are transformed to the corresponding (S)-2-hydroxy ketones; 2,5-hexanedione, however, was reduced to (S,S)-2,5-hexanediol. The Km values for diacetyl and 2,3-pentanedione were estimated as 2.3 and 1.5 mM, respectively. Further characterization of the (S)-diacetyl reductase revealed that it is identical with the so-called '(S)-enzyme', involved in the enantioselective reduction of 3-, 4- and 5-oxo esters in baker's yeast.

Evaluation of the Titertek-Enterobac-Automated System (TTE-AS) for identification of members of the family Enterobacteriaceae.

Abstract: The Titertek-Enterobac-Automated System (TTE-AS, Flow Laboratories, Meckenheim, FRG) was evaluated for identification of members of the family of Enterobacteriaceae using a total of 816 bacterial strains obtained from stock cultures and clinical specimens. A comparison of the results of the TTE-AS kit with conventional methods was conducted. Identifications were classified as correct, not identified (two or more taxa possible, identification score less than 80%, supplementary tests for final identification were not performed), and incorrect. Correct identification results were further subdivided according to the level of identification by species or biotype: (a) greater than or equal to 98%, (b) 90% to 97%, (c) 80% to 89%. When compared with conventional identification results, the TTE-AS system correctly identified 95.3% (778 of 816 strains), 90.0% (734 strains) belonging to category (a), 3.4% (28 strains) belonging to category (b), and 2.0% (16 strains) belonging to category (c). For 18 (2.2%) of the remaining strains, the system generated the correct identification result, but with an identification level less than 80% (low discrimination results). 11 strains (1.3%) were misidentified and the system failed to generate identification results with 9 strains (1.1%). Reactions within TTE-AS were reproducible with an estimated probability of erroneous test results of 1.7%.

Influence of electric field on a first-order smectic-A-ferroelectric-smectic-C liquid-crystal phase transition: A field-induced critical point.

Abstract: The observation of a new critical point in liquid-crystal phase transitions is reported. Detailed tilt-angle, polarization, and susceptibility measurements of a ferroelectric liquid-crystal compound in the presence of an applied dc electric field show that a first-order phase transition between the polarized smectic-A phase and the ferroelectric-smectic-C phase terminates at a critical point in the temperature field plane. A liquid-crystal sample exhibiting a second-order smectic-A--ferroelectric-smectic-C transition is studied by similar measurements demonstrating a clearly different behavior to the first-order compound. A simple Landau description of the new critical point is given.

Tutorial Introduction to the Algebraic Approach of Graph Grammars Based on Double and Single Pushouts

Abstract: The gluing construction on which the algebraic notion of a derivation is based operationally provides a simple and intuitive understanding of graph rewriting. Inheriting the powerful toolbox of category theory, its abstract version as a (single resp. double) pushout leads to highly compact and elegant proofs especially for the basic constructions of sequential and parallel independent derivations as well as for concurrent and amalgamated productions respectively.

The shape invariant of triangles and trigonometry in two-point homogeneous spaces

Abstract: We define a fourth basic invariant, which, besides the lengths of the three sides of a triangle, determines a triangle in the complex and quaternion projective spaces ℂP n and ℍP n (n≥2) uniquely up to isometry. We give inequalities describing the exact range of the four basic invariants. We express the angular invariants of a triangle with our basic invariants, giving a new completely elementary proof of the laws of trigonometry. As a corollary we derive a large number of congruence theorems. Finally we get, in exactly the same way, the corresponding results for triangles in the complex and quaternion hyperbolic spaces ℂH n and ℍH n (n≥2).

Physiology and genetics of methylotrophic bacteria

Abstract: Methylotrophic bacteria comprise a broad range of obligate aerobic microorganisms, which are able to proliferate on (a number of) compounds lacking carbon-carbon bonds. This contribution will essentially be limited to those organisms that are able to utilize methanol and will cover the physiological, biochemical and genetic aspects of this still diverse group of organisms. In recent years much progress has been made in the biochemical and genetic characterization of pathways and the knowledge of specific reactions involved in methanol catabolism. Only a few of the genetic loci hitherto found have been matched by biochemical experiments through the isolation or demonstration of specific gene products. Conversely, several factors have been identified by biochemical means and were shown to be involved in the methanol dehydrogenase reaction or subsequent electron transfer. For the majority of these components, their genetic loci are unknown. A comprehensive treatise on the regulation and molecular mechanism of methanol oxidation is therefore presented, followed by the data that have become available through the use of genetic analysis. The assemblage of methanol dehydrogenase enzyme, the role of pyrrolo-quinoline quinone, the involvement of accessory factors, the evident translocation of all these components to the periplasm and the dedicated link to the electron transport chain are now accepted and well studied phenomena in a few selected facultative methylotrophs. Metabolic regulation of gene expression, efficiency of energy conservation and the question whether universal rules apply to methylotrophs in general, have so far been given less attention. In order to expand these studies to less well known methylotrophic species initial results concerning such area as genetic mapping, the molecular characterization of specific genes and extrachromosomal genetics will also pass in review.