Showing papers by "Bielefeld University published in 1992"

Multicanonical ensemble: A new approach to simulate first-order phase transitions.

Abstract: Relying on the recent proposed multicanonical algorithm, we present a numerical simulation of the first-order phase transition in the 2D 10-state Potts model on lattices up to sizes 100\ifmmode\times\else\texttimes\fi{}100. It is demonstrated that the new algorithm lacks an exponentially fast increase of the tunneling time between metastable states as a function of the linear size L of the system. Instead, the tunneling time diverges approximately proportional to ${\mathit{L}}^{2.65}$. On our largest lattice we gain more than 2 orders of magnitude as compared to a standard heat-bath algorithm. As a first physical application we report a high-precision computation of the interfacial free energy per unit area.

The complexity of reconstructing trees from qualitative characters and subtrees

Abstract: In taxonomy and other branches of classification it is useful to know when tree-like classifications on overlapping sets of labels can be consistently combined into a parent tree. This paper considers the computation complexity of this problem. Recognizing when a consistent parent tree exists is shown to be intractable (NP-complete) for sets of unrooted trees, even when each tree in the set classifies just four labels. Consequently determining the compatibility of qualitative characters and partial binary characters is, in general, also NP-complete. However for sets of rooted trees an algorithm is described which constructs the “strict consensus tree” of all consistent parent trees (when they exist) in polynomial time. The related question of recognizing when a set of subtrees uniquely defines a parent tree is also considered, and a simple necessary and sufficient condition is described for rooted trees.

Internally contracted multiconfiguration-reference configuration interaction calculations for excited states

Abstract: The calculation of electronically excited states with the internally contracted multiconfiguration-reference configuration interaction (CMRCI) method is discussed A straightforward method, in which contracted functions for all states are included in the basis, is shown to be very accurate and stable even in cases of narrow avoided crossings However, the expense strongly increases with the number of states A new method is proposed, which employs different contracted basis sets for each state, and in which eigensolutions of the Hamiltonian are found using an approximate projection operator technique The computational effort for this method scales only linearly with the number of states The two methods are compared for various applications

Photodissociation of water in the first absorption band: A prototype for dissociation on a repulsive potential energy surface

Abstract: The photodissociation of water in the first absorption band, H{sub 2}O(X) + {Dirac_h}{omega} {yields} H{sub 2}O(A{sup 1}B{sub 1}) {yields} H({sup 2}S) + OH({sup 2}II), is a prototype of fast and direct bond rupture in an excited electronic state. It has been investigated from several perspectives-absorption spectrum, final state distributions of the products, dissociation of vibrationally excited states, isotope effects, and emission spectroscopy. The availability of a calculated potential energy surface for the A state, including all three internal degrees of freedom, allows comparison of all experimental data with the results of rigorous quantum mechanical calculations without any fitting parameters or simplifying model assumptions. As the result of the confluence of ab initio electronic structure theory, dynamical theory, and experiment, water is probably the best studied and best understood polyatomic photodissociation system. In this article we review the joint experimental and theoretical advances which make water a unique system for studying molecular dynamics in excited electronic states. We focus our attention especially on the interrelation between the various perspectives and the correlation with the characteristic features of the upper-state potential energy surface. 80 refs., 14 figs.

H2-forming methylenetetrahydromethanopterin dehydrogenase, a novel type of hydrogenase without iron-sulfur clusters in methanogenic archaea

Abstract: A novel hydrogenase has recently been found in methanogenic archaea. It catalyzes the reversible dehydrogenation of methylenetetrahydromethanopterin (CH2 = H4MPT) to methenyltetrahydromethanopterin (CH identical to H4MPT+) and H2 and was therefore named H2-forming methylenetetrahydromethanopterin dehydrogenase. The hydrogenase, which is composed of only one polypeptide with an apparent molecular mass of 43 kDa, does not mediate the reduction of viologen dyes with either H2 or CH2 = H4MPT. We report here that the purified enzyme from Methanobacterium thermoautotrophicum exhibits the following other unique properties: (a) the colorless protein with a specific activity of 2000 U/mg (Vmax) did not contain iron-sulfur clusters, nickel, or flavins; (b) the activity was not inhibited by carbon monoxide, acetylene, nitrite, cyanide, or azide; (c) the enzyme did not catalyze an isotopic exchange between 3H2 and 1H+; (d) the enzyme catalyzed the reduction of CH identical to H4MPT+ with 3H2 generating [methylene-3H]CH2 = H4MPT; and (e) the primary structure contained at most four conserved cysteines as revealed by a comparison of the DNA-deduced amino acid sequence of the proteins from M. thermoautotrophicum and Methanopyrus kandleri. None of the four cysteines were closely spaced as would be indicative for a (NiFe) hydrogenase or a ferredoxin-type iron-sulfur protein. Properties of the H2-forming methylenetetrahydromethanopterin dehydrogenase from Methanobacterium wolfei are also described indicating that the enzyme from this methanogenic archaeon is very similar to the enzyme from M. thermoautotrophicum with respect both to molecular and catalytic properties.

Naphthalene degradation via salicylate and gentisate by Rhodococcus sp. strain B4.

Abstract: Rhodococcus sp. strain B4, isolated from a soil sample contaminated with polycyclic aromatic hydrocarbons, grows with naphthalene as the sole source of carbon and energy. Salicylate and gentisate were identified as intermediates in the catabolism of naphthalene. In contrast to the well-studied catabolic pathway encoded by the NAH7 plasmid of Pseudomonas putida, salicylate does not induce the genes of the naphthalene-degradative pathway in Rhodococcus sp. strain B4. The key enzymes of naphthalene degradation in Rhodococcus sp. strain B4 have unusual cofactor requirements. The 1,2-dihydroxynaphthalene oxygenase activity depends on NADH and the salicylate 5-hydroxylase requires NADPH, ATP, and coenzyme A.

Production rate of hard thermal photons and screening of quark mass singularity

Abstract: The emission of hard real photons from a quark-gluon plasma at high temperatures is studied. At lowest order the QCD processes for quark-antiquark annihilation\((q\bar q \to \gamma g)\) and for Compton scattering (qg→γq) lead to logarithmically divergent thermal production rates for massless quarks. By applying recently developed resummation methods of finite temperature perturbation theory we show that effects due to Landau damping on the exchanged quark provide finite rates. Comparisons with previous estimates and phenomenological implications are discussed.

Genetics of xanthan production in Xanthomonas campestris: the xanA and xanB genes are involved in UDP-glucose and GDP-mannose biosynthesis.

Abstract: The nucleotide sequence of a 3.4-kb EcoRI-PstI DNA fragment of Xanthomonas campestris pv. campestris revealed two open reading frames, which were designated xanA and xanB. The genes xanA and xanB encode proteins of 448 amino acids (molecular weight of 48,919) and 466 amino acids (molecular weight of 50,873), respectively. These genes were identified by analyzing insertion mutants which were known to be involved in xanthan production. Specific tests for the activities of enzymes involved in the biosynthesis of UDP-glucose and GDP-mannose indicated that the xanA gene product was involved in the biosynthesis of both glucose 1-phosphate and mannose 1-phosphate. The deduced amino acid sequence of xanB showed a significant degree of homology (59%) to the phosphomannose isomerase of Pseudomonas aeruginosa, a key enzyme in the biosynthesis of alginate. Moreover, biochemical analysis and complementation experiments with the Escherichia coli manA fragment revealed that xanB encoded a bifunctional enzyme, phosphomannose isomerase-GDP-mannose pyrophosphorylase.

Transgenic plants containing the phosphinothricin-N-acetyltransferase gene metabolize the herbicide L-phosphinothricin (glufosinate) differently from untransformed plants.

Abstract: l-Phosphinothricin (l-Pt)-resistant plants were constructed by introducing a modified phosphinothricin-N-acetyl-transferase gene (pat) via Agrobacterium-mediated gene transfer into tobacco (Nicotiana tabacum L), and via direct gene transfer into carrot (Daucus carota L). The metabolism of l-Pt was studied in these transgenic, Pt-resistant plants, as well as in the untransformed species. The degradation of l-Pt, 14C-labeled specifically at different C-atoms, was analysed by measuring the release of 14CO2 and by separating the labeled degradation products on thin-layer-chromatography plates. In untransformed tobacco and carrot plants, l-Pt was deaminated to form its corresponding oxo acid 4-methylphosphinico-2-oxo-butanoic acid (PPO), which subsequently was decarboxylated to form 3-methylphosphinico-propanoic acid (MPP). This compound was stable in plants. A third metabolite remained unidentified. The l-Pt was rapidly N-acetylated in herbicide-resistant tobacco and carrot plants, indicating that the degradation pathway of l-Pt into PPO and MPP was blocked. The N-acetylated product, l-N-acetyl-Pt remained stable with regard to degradation, but was found to exist in a second modified form. In addition, there was a pH-dependent, reversible change in the mobility of l-N-acetyl-Pt thin-layer during chromatography.

Social desirability scales as moderator and suppressor variables

Abstract: This study concerned the effectiveness of social desirability (SD) as a moderator or suppressor variable. Three SD scales were employed: the Edwards SDS, the Marlowe‐Crowne SDS, and a new scale tha...

Implicit personality theory and the five-factor model.

Abstract: The Big Five have not only been identified in ratings of knowledgeable informants, but also in ratings of strangers, in co-occurrence likelihood ratings of traits, in semantic similarity ratings of trait pairs, and in prototypicality ratings of acts for traits. This article describes the shared and distinctive characteristics of correlations among trait ratings and implicit personality theory and reviews studies that compare the structure of memory-based ratings and of on-line behavior counts. Three hypotheses suggested in the literature to account for these correlations are delineated and discussed: an accurate reflection hypothesis, a distortion hypothesis, and an overlap hypothesis. It is concluded that the distortion hypothesis, has been discredited and that an overlap model best accounts for the available evidence. This implies that traits are real and accurately perceived, provided that the judges have the necessary information.

MacLane homology and topological Hochschild homology

Abstract: The topological Hochschild homology of a discrete ring is shown to agree with the MacLane homology of that ring.

Convergence of sequential and asynchronous nonlinear paracontractions

Abstract: We establish the convergence of sequential and asynchronous iteration schemes for nonlinear paracontracting operators acting in finite dimensional spaces. Applications to the solution of linear systems of equations with convex constraints are outlined. A first generalization of one of our convergence results to an infinite pool of asymptotically paracontracting operators is also presented.

Classroom cultures from a social constructivist's perspective

Abstract: Using examples from related case studies, the possible consequences of a social constructivist perspective for the teaching and learning of mathematics are discussed.

Differential responses of two types of electroreceptive afferents to signal distortions may permit capacitance measurement in a weakly electric fish, Gnathonemus petersii

Abstract: Gnathonemus petersii discriminates between ohmic and capacitive objects. To investigate the sensory basis of this discrimination we recorded from primary afferents that innervate either A or B mormyromast sensory cells. Modified and natural electric organ discharges were used as stimuli. In both A and B fibres frequencies below the peak-power frequency (3.8 to 4.5 kHz) of the electric organ discharge caused minimal first-spike latencies and a maximum number of spikes. A fibres did not discriminate phase-shifted stimuli, whereas B fibres responded significantly with a decrease in first-spike latency if the phase shift was only — 1°. In both A and B fibres an amplitude increase caused a decrease in spike latency and an increase in spike number; an amplitude decrease had the reverse effect. If stimulated with quasi-natural electric organ discharges distorted by capacitive objects, the responses of A fibres decreased with increasing signal distortion. In contrast, the responses of B fibres increased until amplitude effects began to dominate. Gnathonemus may use the physiological differences between A and B fibres to detect and discriminate between capacitive and purely ohmic objects.

Cloning and sequence analysis of genes for dehalogenation of 4-chlorobenzoate from Arthrobacter sp. strain SU.

Abstract: Strains of Arthrobacter catalyze a hydrolytic dehalogenation of 4-chlorobenzoate (4-CBA) to p-hydroxybenzoate. The reaction requires ATP and coenzyme A (CoA), indicating activation of the substrate via a thioester, like that reported for Pseudomonas sp. strain CBS3 (J. D. Scholten, K.-H. Chang, P. C. Babbit, H. Charest, M. Sylvestre, and D. Dunaway-Mariano, Science 253:182-185, 1991). The dehalogenase genes of Arthrobacter sp. strain SU were cloned and expressed in Escherichia coli. Analyses of deletions indicate that dehalogenation depends on three open reading frames (ORFs) which are organized in an operon. There is extensive sequence homology to corresponding gene products in Pseudomonas sp. strain CBS3, suggesting that ORF1 and ORF2 encode a 4-CBA-CoA-ligase and a 4-CBA-CoA dehalogenase, respectively. ORF3 possibly represents a thioesterase, although no homology to the enzyme from Pseudomonas sp. strain CBS3 exists.

Recurrence properties of Lorentz lattice gas cellular automata

Abstract: Recurrence properties of a point particle moving on a regular lattice randomly occupied with scatterers are studied for strictly deterministic, nondeterministic, and purely random scattering rules.

Direct Methods of the Calculus of Variations

Abstract: In our previous chapter we showed that the determination of the extrema, such as the minima, of a function, is central to the calculus of variations. An extreme value problem in its most general form can be understood to be the following.

A tungsten-containing active formylmethanofuran dehydrogenase in the thermophilic archaeon Methanobacterium wolfei.

Abstract: Methanobacterium wolfei is a thermophilic methanogenic archaeon which requires tungsten or molybdenum for growth. We have found that the orgainsm contains two formylmethanofuran dehydrogenases, one of which is a tungsten enzymes. Indirect evidence indicated that the other formylmethanofuran dehydrogenase is a molydbenum enzyme. The tungsten enzyme was purified and characterized. The native enzyme had an apparent molecular mass of 130kDa. SDS/PAGE revealed a composition of three subunits of apparent molecular mass 35, 51 and 64kDa, the N terminalamino acid sequences of two of which were determined. 0.3–0.4 mol tungsten/mol enzyme was found but no moybdenum. The pterin cofactor was identified as molybdopterin guanine dincleotide. The purified enzymes exhibited a specific activity of 8.3 μmol · min−1· mg protein−1 and an apparent km for formylmethanofuran and methylviologen of 13μM and 0.4mM, respectively. The optimum temperature for activity was 65°C. At 40–60°C, the rate increased with a Q10 of 1.9; the activation energy of the reaction was 45kJ/mol. The enzyme was found to require potassium ions for thermostability. The oxygen-sensitive enzyme was not inactivated by cyanide.

Paradoxical Effects of Praise and Criticism on Perceived Ability

Abstract: In this chapter the effects of praise and criticism are considered from an attributional perspective. It is shown that—when analyzed from this perspective—praise and criticism may have consequences that seem nothing less than paradoxical as compared to a reinforcement standpoint, i.e. the point of view that ascribes to positive and negative verbal feedback corresponding positive and negative behavioral, emotional, and motivational effects. Empirical evidence presented shows that praise can lead to the inference that the other person evaluates the recipient's ability as low, while criticism can lead to the inference that the recipient's ability was estimated as high. It is also shown that evaluative feedback can have paradoxical effects on the recipient's self-perception of ability. Finally, some implications of the research are spelled out for the social psychology of the classroom and for the development of the self-concept.

Benzoanellated centropolyquinanes, 11. Synthesis of tribenzotriquinacene and some centro-substituted derivatives

Abstract: The syntheses of tribenzotriquinacene (1a) and five centro-substituted derivatives, 1b-1e and 1g, as well as of the related diindan 13 are reported. The three-step synthetic sequences include the reduction of suitably substituted 2-benz-hydryl-1,3-indandiones 3 to the corresponding 1,3-indandiols 4 and the twofold cyclodehydration of the latter to close two additional five-membered rings at a time. Although the yields of the cyclodehydration step 4 · 1 are only low to moderate (10-33%), the overall approach allows the preparation of 5-50-gram amounts of the centropolyindans in most cases by starting from simple 1,3-indandiones 2a-2c. This includes the new synthesis of the parent tribenzotriquinacene (1a). The related Cs-symmetrical diindan, 4b,9,9a,10-tetrahydroindeno[1,2-a]indene (13), has been prepared in high yield by using the same cyclodehydration technique. Scope and limitations of the double cyclodehydration strategy are described concerning the synthesis of 1,3-indandiones with bulky substituents at C-2 and the cyclization of 1,3-indandiols with an aptitude to undergo heterolytic cleavage of an exocyclic C-2-C- bond, in particular 4f. The course of the reduction of the 2,2-disubstituted 1,3-indandiones with lithium aluminum hydride is discussed on the basis of the stereochemistry of the product 1,3-indandiols.