Isomerenspezifische Bestimmung der Polychlordibenzodioxine (PCDD) und Polychlordibenzofurane (PCDF)

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Isomer-specific determination of the polychlorinated dibenzodioxins (PCDD) and polychlorinated dibenzofurans (PCDF)

     

Laterally patterned high mobility two-dimensional electron gases obtained by overgrowth of focused ion beam implanted Al1−xGaxAs

The use of focused ion beam implantation doping of an inverted GaAs/Al_1−xGa_xAs heterostructure during a growth interruption allows for the lateral modulation of the heterostructure doping. Hence, laterally patterned two dimensional electron gases (2DEGs) are obtained with no further processing steps required. We have performed the direct writing of a 2DEG with a Hall-bar pattern, such that only the application of ohmic contacts was necessary and the sample surface remained unharmed otherwise. The 2DEG has an electron density of 3.6×10¹¹ cm⁻² and an electron mobility of 4.8×10⁵ cm²/V s, as determined by magnetotransport measurements. A conventional mesa-etched Hall-bar with almost identical electronic properties has also been studied. Different behaviour of the longitudinal as well as the transversal magnetoresistance for the two Hall-bars is observed and can be concluded to be due to a different confinement potential.     

Heterologous Expression,Biosynthetic Studies,and Ecological Function of the Selective Gq‐Signaling Inhibitor FR900359

The cyclic depsipeptide FR900359 (FR), isolated from the tropical plant Ardisia crenata, is a strong and selective inhibitor of Gq proteins, making it an indispensable pharmacological tool to study Gq‐related processes, as well as a promising drug candidate. Gq inhibition is a novel mode of action for defense chemicals and crucial for the ecological function of FR, as shown by in vivo experiments in mice, its affinity to insect Gq proteins, and insect toxicity studies. The uncultured endosymbiont of A. crenata was sequenced, revealing the FR nonribosomal peptide synthetase (frs) gene cluster. We here provide a detailed model of FR biosynthesis, supported by in vitro enzymatic and bioinformatic studies, and the novel analogue AC‐1, which demonstrates the flexibility of the FR starter condensation domains. Finally, expression of the frs genes in E. coli led to heterologous FR production in a cultivable, bacterial host for the first time.     

Reaktionen von Undecacarbonyl(acetonitril)trieisen mit Alkinethern

Reactions of Undecacarbonyl(acetonitrile)triiron with Alkyne Ethers (CO)₁₁(CH₃CN) 1 reacts with the alkyne ethers H₃C? C?C? OC₂H₅ 2a , H? C?C? OC₂H₅ 2b , H₃C? O? CH₂? C?C? CH₂? O? CH₃, 2c and H₃C? O? C(CH₃)H? C?C? C(CH₃)H? O? CH₃ 2d forming different cluster products depending on the substituents and the reaction conditions. The product obtained with 2a is the bisalkylidyne cluster Fe₃(CO)₉(m?₃-C? CH₃)(m?₃-C? OC₂H₅) 3 which results from the cleavage of the carbon carbon triple bond. The alkyne 2b however yields the vinylidene cluster Fe₃(CO)₁₀(m?₃-η²-C? C(H)OC₂H₅) 4 by 1,2 proton shift. The alkyne clusters Fe₃(CO)₁₀(m?₃-η²-C? C(H)OC₂H₅) 4 by 1,2 proton shift. The alkyne clusters Fe₃(CO)₁₀(m?₃-η²- H₃ C? O? CH₂? C?C? CH₂? O? CH₃) 6 and Fe₃(CO)₉(m?-η²-H₃C? O? CH₂? C?C? CH₂? O? CH₃) 7 are the isolated products obtained from 2c . Thermolysis of 7 results in the formation of the dinuclear butatrien complex Fe₂(CO)₆ (H₂C? C? C? CH₂) 8a . The analogous compound Fe₂(CO)₆[H(H₃C)C ? C ? C ? C(CH₃)H] 8b is the only product of 2d and 1 . The structures of 4, 5 , and 6 have been determined by crystal structure determinations.     

Zur Kinetik der Spinellbildung von β-Ga2O3 mit zweiwertigen Oxiden. I. Die Festkörperreaktion von β-Ga2O3 mit NiO

The solid state reaction in a sandwich type diffusion couple of NiO and β-Ga₂O₃ has been investigated between 1240 and 1550°C in air and inert gas atmosphere.Optical microscope and X-ray methods are used to identify the reaction product, which is a singlephased spinel of the general formula $\text{Ni}{}_{\mathrm{1?y}}\text{Ga}{}_{\mathrm{<mtext>2+</mtext><mtext>2y</mtext><mtext>3</mtext>}}\text{O}{}_4$. The homogeneity range of the spinel phase was investigated by X-ray methods; there is a high solubility of β-Ga₂O₃ in the spinel lattice.The growth of the thickness of the reaction layer follows a parabolic rate law, and therefore a diffusion process must be rate determining. The activation energy of the rate controlling step is 82 kcal/mole.Pt-marker experiments are not sufficient for determining the reaction mechanism. Investigations with an electron-probe microanalyzer, an connection with a modified marker technique, resulted in a Wagner-mechanism of counterdiffusion of cations for formation of nickel-gallium spinel; the total amount in Ga³⁺ ions is lost for spinel formation before there is an appreciable solubility of gallium in NiO.     

Synthesis,Biological Evaluation,and Crystallographic Studies of Extended Guanine‐Based (lin‐Benzoguanine) Inhibitors for tRNA‐Guanine Transglycosylase (TGT)

This paper describes the rational design, synthesis, and biological evaluation of a new generation of inhibitors of the bacterial enzyme tRNA‐guanine transglycosylase (TGT), which has been identified as a new target in the fight against bacillary dysentery (Shigellosis). The enzyme catalyzes the exchange of guanine in the anticodon wobble position of tRNA by the modified base preQ₁, a guanine derivative, according to a ping‐pong mechanism involving a covalent TGT‐tRNA intermediate (Fig. 2). Based on computer modeling (Fig. 3), lin‐benzoguanine (6‐aminoimidazol[4,5‐g]quinazolin‐8(7H)‐one ( 2 )) was selected as an extended central scaffold, to form up to seven in‐plane intermolecular H‐bonds with the protein while sandwiching between Tyr106 and Met260. Versatile synthetic protocols were developed for the synthesis of 2 , and derivatives with phenyl, benzyl, and 2‐phenylethyl side chains (i.e., 16, 17a , and 12a, 12b, 13, 17 , resp.) to reach into the lipophilic pocket lined by Val282, Val45, and Leu68 (Schemes 1–3). To account for the limited solubility of the new ligands and in consequence of a recently developed detailed understanding of the mechanism of TGT catalysis (Fig. 2), the enzyme kinetic assay was completely redesigned, providing competitive (K_ic) and uncompetitive (K_iu) inhibition constants with respect to tRNA binding by TGT. The modifications of the various parameters in the new assay are described in detail. Binding affinities of the new inhibitors were found to be in the single‐digit micromolar range (K_ic values, Fig. 8). Decoration of the lin‐benzoguanine scaffold with lipophilic residues only gave a modest improvement in biological activity which was explained on structural grounds with the help of four crystal structures (Fig. 10) obtained by soaking the protein with inhibitors 2 and 12a – 12c . Both biochemical and biostructural analyses reported in this paper provide a fertile basis for the development of more potent future generations of TGT inhibitors.     

Counting formulas for glued lattices

A tolerance relation of a lattice L, i.e., a reflexive and symmetric relation of L which is compatible with join and meet, is called glued if covering blocks of have nonempty intersection. For a lattice L with a glued tolerance relation we prove a formula counting the number of elements of L with exactly k lower (upper) covers. Moreover, we prove similar formulas for incidence structures and graphs and we give a new proof of Dilworth's covering theorem.     

Determinants and a unified approach to estimating resolvents of operators in operator ideals of Riesz type lp

An operator idealA is said to be of Riesz type l_p (0A(E), where E is a complex Banach space, is a Riesz operator with absolutely p-summable eigenvalues. The main purpose of this paper is to give a unified approach to the problem of estimating resolvents of operators T that belong to an arbitrary operator ideal of Riesz type l_p. The method used is based on a representation of the resolvent of T²ⁿ (n>p) in the weak operator topology by characteristic determinants. The same method is used to derive estimates of Fredholm minors. The results obtained extend, generalize, and simplify results of Markus, König, and Engelbrecht and Grobler, and verify a conjecture of König. Finally, the resolvent estimates are applied to establish sufficient conditions for the completeness of principal elements.