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.     

Optical vs. direct sorption and swelling measurements for the study of stiff-chain polymer-penetrant interactions

This work involves interferometric ‘optical thickness’ and refractive index measurements performed in an optical thickness meter (OTM), on supported cellulose acetate (CA) films equilibrated with various activities of methylene chloride (MC) vapor. The relevant equilibrium sorption and volume swelling isotherms were determined by application of the Claussius-Mossotti equation on the assumption that these films swell unidimensionally along the thickness direction, and were compared with corresponding direct equilibrium sorption (weight gain), elongation and thickness dilation measurements on similar free films performed in a vacuum sorption/swelling apparatus (VSA) and complemented with refractive index data. Combined elongation and thickness dilation data from the VSA showed that free glass-cast CA films exhibit pronounced swelling anisotropy. The said anisotropy, although it cannot be completely eliminated, by conditioning at high degrees of swelling, does not appear to affect the extent of volume swelling significantly, thus permitting quantitative comparison of sorption and swelling isotherms determined by the VSA and the OTM. Such comparison showed satisfactory agreement between these two sets of results up to an MC uptake of ca. 0.4 gMC/cm³ of dry CA corresponding to a degree of swelling of ca. 0.2. Increasing discrepancies are observed at higher MC concentrations, which are attributable to breakdown of the assumption used that the supported films swell unidimensionally along the thickness direction. The present CA-MC volume swelling data exhibit the negative deviation from volume additivity on mixing typical glassy polymers.     

Application of surface complexation modeling to the reactivity of iron(II) with nitroaromatic and oxime carbamate contaminants in aqueous TiO2 suspensions

This study reports on the application of surface complexation modeling to interpret observed kinetic trends for Fe(II) redox reactions with model nitroaromatic (4-chloronitrobenzene) and oxime carbamate (oxamyl) contaminants in aqueous TiO(2(s)) suspensions. Pseudo-first-order rate constants for reduction of the two probe contaminants (k(red), s(-1)) vary by several orders of magnitude with changing conditions (100-500 microM Fe(II), 0-15 g L(-1) TiO(2(s)), pH 2-9), but the relationship between reaction rates and Fe(II) speciation differs considerably for the two contaminants. For oxamyl, k(red) measurements are most strongly correlated with the volumetric total adsorbed Fe(II) concentration (moles Fe(II) adsorbed per liter of TiO(2(s)) suspension), whereas k(red) measurements for 4-chloronitrobenzene are proportional to the concentration of the hydrolyzed Fe(II) surface complex (equivalent TiOFe(II)OH(0)). The differing trends demonstrate that Fe(II) redox reactivity at the aqueous/TiO(2(s)) interface is influenced, in part, by specific molecular interactions with the target oxidant. Results are also geochemically relevant in that they demonstrate unambiguously that mononuclear Fe(II)-metal (hydr)oxide surface complexes are sufficiently reactive species to reduce nitroaromatic contaminants, an issue that remained open following earlier studies in Fe(III) (hydr)oxide suspensions because structural Fe(II) species are simultaneously present in such systems because of interfacial Fe(II)-to-Fe(III) electron transfer processes that occur on Fe(II) adsorption.     

Diffusion induced phase separation with crystallizable nylons. II. Relation to final membrane morphology

Intermediate stages during membrane formation by means of immersion precipitation were studied by cryo-substitution for the system nylon 4,6, formic acid and water. The presence, nature and size of solid particles was determined as a function of time and of the distance from the interface. The spherulitic nature of these particles was confirmed by staining the samples. It was shown that at a relatively low nucleation density the concentration profile in the film was hardly influenced by a starting phase separation process, while in a situation with a relatively high number of nuclei per volume concentration patterns must be considerable altered.     

Investigation of the kinetics of aquation of the 1:2 complex between Cr and nitrilotriacetic acid

Aquation of the 1:2 complex between Cr^III and nitrilotriacetic acid (NTA) was monitored using a combination of capillary electrophoresis (CE), ultraviolet–visible (UV–vis) spectrophotometry, and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy. To our knowledge, this is the first published report of the use of either CE or ATR-FTIR to monitor the kinetics of ligand exchange reactions of Cr^III–aminocarboxylate complexes. The aquation products were identified as the 1:1 Cr^III complex with NTA and “free” NTA. The 1:1 complex dimerized to form a 2:2 complex in a slower subsequent reaction step. Rates of disappearance of the 1:2 complex were first-order under all experimental conditions. First-order rate constants for aquation, k_obs (h⁻¹), measured using all three techniques were similar at equivalent pH conditions, and with values reported previously in the literature. Measured k_obs values exhibited a complicated pH dependence with three distinct regions: (i) at pH < 6.5, k_obs values increased with decreasing pH, (ii) between pH 6.5 and 8.0, k_obs values were relatively constant, and (iii) at 8.0 < pH < 10.0, k_obs increased with increasing pH and then leveled off. A kinetic model incorporating five distinct aquation pathways was successfully employed to model the pH dependence of k_obs from 0.0 < pH < 10.0. These results show that CE and ATR-FTIR can be used as tools for better understanding ligand exchange processes occurring in aqueous solution.     

Zum Bildungsmechanismus von asymmetrischen Zelluloseazetatmembranen

Zusammenfassung Der Bildungsmechanismus von asymmetrischen Zelluloseazetatmembranen, die zur Wasserentsalzung nach dem Prinzip der umgekehrten Osmose geeignet sind, wird untersucht. Die wichtigsten Herstellungsparameter wie Zusammensetzung der Membranlösung, Fällungstemperatur, Anlaßzeit- und Anlaßtemperatur werden diskutiert und interpretiert. Es wird davon ausgegangen, daß die eigentliche Bildung der Membran auf einer Fällungsreaktion aus einem Mehrkomponentensystem mit Mischungslücke beruht und die asymmetrische Struktur auf die Kinetik der Fällung aus einer stark übersättigten Lösung zurückzuführen ist. Hierzu wird das Mischungsdiagramm des Systems Zelluloseazetat-Aceton-Formamid-Wasser aufgenommen. Die Membranen werden nach Literaturvorschrift hergestellt, ihre Transporteigenschaften in Filtrationsversuchen mit einer l-gew.%igen NaCl-Lösung bei 100 atm hydrostatischem Druck bestimmt und ihre Struktur röntgenologisch und elektronenmikroskopisch untersucht.

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Summary The formation mechanism of asymmetric cellulose acetate membranes, which are suited for reverse osmosis desalination, is studied. The main parameters in the membrane preparation technique, i.e., the casting solution composition, the wash bath temperature, and the annealing time and temperature, are discussed and rationalized. The actual membrane formation is interpreted as precipitation of the polymer from a multicomponent system which is only partially miscible. The observed asymmetric membrane structure is related to the kinetics of precipitation from supersaturated solutions. The phase diagram of the system cellulose acetate-acetone-formamide-water is presented. Membranes are made according to a recipe given in the literature. Their transport properties are determined in filtration tests with a 1% sodium chloride solution at an applied hydrostatic pressure of 100 atm. X-ray diffraction and electron microscopy are used to study their structures.

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Die vorliegende Arbeit wurde am Institut für Physikalische Chemie der T. H. Aachen unter der Leitung von Prof.U. Franck angefertigt. Wir danken Herrn Prof.Franck für seine Anregungen und die Förderung dieser Arbeit. Herrn Dr.Spei und Herrn Dr.Burchard danken wir für die Anfertigung der Röntgen-bzw. der Elektronenmikroskopaufnahmen.