Crystallization of Arthrobacter sp. strain 1C N-(1-D-carboxyethyl)-L-norvaline dehydrogenase and its complex with NAD+

The novel NAD+-linked opine dehydrogenase from a soil isolate Arthrobacter sp. strain 1C belongs to an enzyme superfamily whose members exhibit quite diverse substrate specificites. Crystals of this opine dehydrogenase, obtained in the presence or absence of co-factor and substrates, have been shown to diffract to beyond 1.8 ? resolution. X-ray precession photographs have established that the crystals belong to space group P21212, with cell parameters a = 104.9, b = 80.0, c = 45.5 ? and a single subunit in the asymmetric unit. The elucidation of the three-dimensional structure of this enzyme will provide a structural framework for this novel class of dehydrogenases to enable a comparison to be made with other enzyme families and also as the basis for mutagenesis experiments directed towards the production of natural and synthetic opine-type compounds containing two chiral centres.     

Thermogravimetric and EPR study of the solid-state reactions between zinc chromate and zinc oxalate

The formation of some catalytically active Cr(V) and Cr(III) species is studied when the progressive thermal reduction of zinc chromate is enhanced by the addition of solid zinc oxalate, the reducing agent CO being formed in situ, during thermal analysis.When compared with the decomposition of pure ZnCrO₄, Cr(VI) reduction, as followed by EPR, is similar concerning the nature of the intermediate Cr species formed: Cr(V), Cr(III) in a Cr₂O₃-like bulk phase and Cr(III) dispersed in ZnO. However, the temperature range of Cr₂O₃ formation, as well as the magnitude of the Cr(III)-Cr(III) interaction in such a phase is found to be strongly dependent on the amount of ZnO formed. On the other hand, after the Cr(V) species resulting from progressive oxygen release have disappeared, some other stable Cr(V) species are formed again at higher temperatures. Their formation may be related to a further reduction of the remaining Cr(VI) species by CO stemming from the oxalate.

---

Zusammenfassung Die Bildung einiger katalytisch aktiver Cr(V) und Cr(III) Species wird für den Fall untersucht, bei dem die progressive thermische Reduktion von Zinkchromat durch Zugabe von festem Zinkoxalat gefördert wird, wobei das Reduktionsmittel CO in situ, während der Thermoanalyse gebildet wird.Ein Vergleich mit der Zersetzung der reinen ZnCrO₄ ergibt die ÄhnIichkeit der durch EPR verfolgten Cr(VI)-Reduktion hinsichtlich der Beschaffenheit der gebildeten Cr Intermediärspecies: Cr(V), Cr(III) in einer Cr₂O₃-artigen Hauptphase und Cr(III) in ZnO dispergiert. Jedoch sind Temperaturbereich der »Cr₂O₃«-Bildung sowie der Umfang der Cr(III)-Cr(III) Wechselwirkung in einer solchen Phase stark von der Menge des gebildeten ZnO abhängig. Andererseits werden nach dem Verschwinden der durch die progressive Sauerstoff-Freisetzung entstandenen Cr(V)-Species wider einige andere stabile Cr(V)-Species bei höheren Temperaturen gebildet. Ihre Bildung mag mit einer weiteren Reduktion der noch vorhandenen Cr(VI)-Species durch aus dem Oxalat stammendes CO verbunden sein.

---

Cr(V) Cr(III) , , , , . , Cr(VI), ZnCrO₄ . , Cr : Cr(V) Cr(III) , Cr₂₃, Cr(III) ZnO. , «Cr₂₃», Cr(III)-Cr(III) . , , Cr(V) , Cr(V) . Cr(VI) , .

     

Laser marked shadowgraphy: a novel optical planar technique for the study of microbubbles and droplets

A novel combination of backlighting and glare point velocimetry and sizing (GPVS) is proposed to measure the size distribution of microbubbles (or microdroplets). This new technique, which we will call laser marked shadowgraphy, avoids sizing out-of-focus bubbles (or droplets) and the associated bias error. Compared to backlighting, this combination also improves the precision of the diameter measurement and allows void fraction measurements. Compared with GPVS, a more robust image processing is obtained. The applicability of the developed technique is demonstrated on a cloud of electrochemically generated hydrogen bubbles.     

Advantages of Odd Random Phase Multisine Electrochemical Impedance Measurements

Electrochemical impedance spectroscopy (EIS) is a powerful technique to study electrochemical processes and to perform screening tasks. Recently an integrated measuring and modeling methodology for EIS based on a multisine excitation signal was developed. A key issue in this methodology is the data analysis, allowing us to rapidly quantify the reliability of the measured data. In this paper, a comparison is made between classical single‐sine and the proposed multisine measurements on the same system. The fitting of the impedance data obtained by single‐or multisine excitation and using different weighting factors is also discussed. In addition to the advantages reported in earlier work, it is concluded that, of all investigated frequencies, the odd random phase multisine excitation yields the highest quality data in the shortest measurement time.     

Aspartate‐Based CXCR4 Chemokine Receptor Binding of Cross‐Bridged Tetraazamacrocyclic Copper(II) and Zinc(II) Complexes

The CXCR4 chemokine receptor is implicated in a number of diseases including HIV infection and cancer development and metastasis. Previous studies have demonstrated that configurationally restricted bis‐tetraazamacrocyclic metal complexes are high‐affinity CXCR4 antagonists. Here, we present the synthesis of Cu²⁺ and Zn²⁺ acetate complexes of six cross‐bridged tetraazamacrocycles to mimic their coordination interaction with the aspartate side chains known to bind them to CXCR4. X‐ray crystal structures for three new Cu²⁺ acetate complexes and two new Zn²⁺ acetate complexes demonstrate metal‐ion‐dependent differences in the mode of binding the acetate ligand concomitantly with the requisite cis‐V‐configured cross‐bridged tetraazamacrocyle. Concurrent density functional theory molecular modelling studies produced an energetic rationale for the unexpected [Zn(OAc)(H₂O)]⁺ coordination motif present in all of the Zn²⁺ cross‐bridged tetraazamacrocycle crystal structures, which differs from the chelating acetate [Zn(OAc)]⁺ structures of known unbridged and side‐bridged tetraazamacrocyclic Zn²⁺‐containing CXCR4 antagonists.     

Kinetics and mechanism of the thermal chlorination of chloroform in the gas phase

The gas‐phase thermal chlorination of CHCl₃ has been studied up to high conversions by photometry and gas chromatography in a conditioned static quartz reaction vessel between 573 and 635 K. The initial pressures of both CHCl₃ and Cl₂ ranged from about 10–100 Torr, and the initial total pressure was varied between about 30–190 Torr. The reaction is rather complex because the produced CCl₄ is not stable. The rate of consumption of Cl₂ therefore increases in the course of time. This acceleration is explained quantitatively in terms of a radical mechanism and its kinetic and thermodynamic parameters. This reaction model is based on a known model for the pyrolysis of CCl₄ to which only one reaction couple involving CHCl₃ has been added. Analyses of the rates of the homogeneous elementary steps show that the primary source of Cl atoms is the second‐order dissociation of Cl₂, which is rapidly superseded by a secondary source, the first‐order dissociation of the CCl₄ primary product. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 466–472, 2000     

Pyrolysis of 1,1,1-Trichloroethane in the absence and the presence of added HCl and/or CCl4

The title reaction has been studied in a static quartz reaction vessel between 587 and 658 K at pressures between 40 and 152 torr. The dehydrochlorination is the only significant reaction and is autoaccelerated by the produced HCl. Numerical modelling indicates that the Rice-Herzfeld mechanism, generally used for describing the pyrolysis of halogenated ethanes, has to be completed in the case of CC1₃CH₃ with additional transfer reactions converting “dead” radicals into chain carriers and vice-versa. The numerical simulation fits the experimental results, in the absence as well as in the presence of different amounts of added HCl. The dehydrochlorination is also accelerated by the addition of CCl₄, which can be explained in terms of additional elementary steps involving · CCl₃ radicals.     

Micro Raman spectroscopy used for the study of corrosion products on copper alloys: study of the chemical composition of artificial patinas used for restoration purposes

Studying the atmospheric corrosion of copper alloy artifacts is important to acquire a better knowledge about the condition of the object and its possible conservation and restoration. The nature of the formed product, e.g. sulfate, carbonate or chloride, depends on factors such as the amount of polluting elements or humidity but may also depend on the nature of the underlying aesthetic patina, applied by the artist. The composition of the patination solution and the method of patination will both influence the nature of this aesthetic patina, i.e. its chemical composition and morphology. However, although a lot of patination recipes exist, little is known about these patinas as far as composition, structure and ageing is concerned. Therefore, a combination of several surface analytical techniques is required for the complete characterization of patina layers. In this paper, Raman spectroscopy is used to study the characteristics of several patinas obtained on copper following different traditional recipes.     

Quantitative determination of the composition of nitrided layers on iron using AES

Within the framework of the study of industrial nitriding of steel, AES was chosen as the principle analysis technique. In order to characterise the nitrided layers quantitatively, reliable sensitivity factors were needed. For that purpose, different reference samples containing the pure γ′‐Fe₄N_1?x and ε‐Fe₂N_1?z phases were prepared by gaseous nitriding of pure iron. The characterisation of these references by means of electron probe microanalysis (EPMA) is discussed. The first sample contained a nitrided layer with large γ′‐Fe₄N_1?x grains (～30 µm) with 19.6 at.% nitrogen on top of an iron substrate. The second one contained an ε‐Fe₂N_1?z outer layer (～6 µm) with 26 at.% nitrogen, on a γ′‐Fe₄N_1?x layer (～4 µm) with 19.8 at.% nitrogen, created on top of an iron substrate. In this study, Fe LMM and N KLL Auger electron spectral lines were acquired on the pure γ′‐Fe₄N_1?x and ε‐Fe₂N_1?z phases of these two reference samples in order to calculate the sensitivity factors of iron and nitrogen. Different Auger intensities were considered and compared. It was decided to use the peak areas of the direct Auger electron spectral lines. The values of the sensitivity factors are 0.74 for iron and 0.33 for nitrogen. Finally, a set of three independent and well‐characterised samples containing the γ′‐Fe₄N_1?x and ε‐Fe₂N_1?z phases was used to validate the elaborated quantification procedure. Copyright © 2007 John Wiley & Sons, Ltd.