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.     

N.m.r. studies in the heterocyclic series: XVII—carbon-13 spectra of indazole derivatives

The ¹³C chemical shifts of 17 indazole derivatives are discussed as a function of substituent effects and the N-1—H or N-2—H structure of indazole. The presence in solution of the N-1—H tautomer is confirmed.     

Reduction Des Groupes Fonctionnels Nitroaromatiques Par L'Hydrogene en Presence de Palladium Sur Charbon

Abstract

En utilisant la méthode de réduction habituelle ?hydrogénation catalysée par le palladium sur charbon sur des composés nitroaromatiques, nous nous sommes aperçus que la vitesse ?absorption ?hydrogène décroissait en fonction du temps et que ces réactions n'étaient qu'exceptionnelement complètes. Certains auteurs¹ signalent que cette réaction est plus rapide et plus sélective en milieu neutre qu'en milieu acide ou basique.     

Langmuir and Langmuir-Blodgett films of poly(ethylene oxide)-b-poly(epsilon-caprolactone) star-shaped block copolymers

Self-assembly of poly(ethylene oxide)-block-poly(epsilon-caprolactone) five-arm stars (PEO-b-PCL) was studied at the air/water (A/W) interface. The block copolymers consist of a hydrophilic PEO core with hydrophobic PCL chains at the star periphery. All the polymers have the same number of ethylene oxide repeat units (9 per arm), and the number of epsilon-caprolactone repeat units ranges from 0 to 18 per arm. The Langmuir monolayers were analyzed by surface pressure/mean molecular area isotherms, compression-expansion hysteresis experiments, and isobaric relaxation measurements, and the Langmuir-Blodgett (LB) films' morphologies were investigated by atomic force microscopy (AFM). PCL homopolymers crystallize directly at the A/W interface in a narrow surface pressure range (11-15 mN/m). In the same pressure region, the star-shaped block copolymers undergo a phase transition corresponding to the collapse and the crystallization of the PCL chains as shown by the presence of a pseudoplateau in the isotherms. The LB films were prepared by transferring the Langmuir monolayers onto mica substrates at various surface pressures. AFM imaging confirmed the formation of PCL crystals in the LB monolayers of the PCL homopolymers and of the copolymers, but also showed that the PCL segments can undergo additional crystallization after monolayer transfer during water evaporation. The PCL crystal morphologies were also strongly influenced by the surface pressure and by the PEO segments.     

Polystyrene-b-poly(tert-butyl acrylate) and polystyrene-b-poly(acrylic acid) dendrimer-like copolymers: two-dimensional self-assembly at the air-water interface

The two-dimensional self-assembly at the air/water (A/W) interface of two dendrimer-like copolymers based on polystyrene and poly(tert-butyl acrylate) (PS-b-PtBA) or poly(acrylic acid) (PS-b-PAA) was investigated through surface pressure measurements (isotherms, isochores, and compression-expansion hysteresis experiments) and atomic force microscopy (AFM) imaging. The two dendrimer-like block copolymers have an 8-arm PS core (Mn = 10 000 g/mol, approximately 12 styrene repeat units per arm) with a 16-arm PtBA (Mn = 230 000 g/mol, approximately 112 tert-butyl acrylate repeat units per arm) or PAA (Mn = 129 000 g/mol, approximately 112 acrylic acid repeat units per arm) corona. The PS-b-PtBA sample forms stable Langmuir monolayers and aggregates into circular surface micelles up to a plateau observed in the corresponding isotherm around 24 mN/m. Beyond this threshold, the monolayers collapse above the interface, resulting in the formation of large and irregular desorbed aggregates. The PS-b-PAA sample has ionizable carboxylic acid groups, and its A/W interfacial self-assembly was therefore investigated for various subphase pH values. Under basic conditions (pH = 11), the carboxylic acid groups are deprotonated, and the PS-b-PAA sample is therefore highly water-soluble and does not form stable monolayers, instead irreversibly dissolving in the aqueous subphase. Under acidic conditions (pH = 2.5), the PS-b-PAA sample is less water-soluble and becomes surface-active. The pseudoplateau observed in the isotherm around 5 mN/m corresponds to a pancake-to-brush transition with the PAA chains dissolving in the water subphase and stretching underneath the anchoring PS cores. AFM imaging revealed the presence of circular surface micelles for low surface pressures, whereas the biphasic nature of the pseudoplateau region was confirmed with the gradual aggregation of the micellar PS cores above the PAA chains. The aggregation numbers for both samples were estimated around 3-5 dendrimer-like copolymers per circular surface micelle. These rather low values confirmed the tremendous influence of molecular architecture on the two-dimensional self-assembly of block copolymers.     

Electrochemical and spectroscopic characterization of organic compound uptake in silica core-shell nanocapsules

Oil-filled silica nanocapsules consisting of a hydrophobic liquid core and a silicate shell have been shown to efficiently extract hydrophobic compounds from aqueous media. With a view toward quantifying the selectivity of these systems, a series of electrochemical and spectroscopic measurements was performed. Uptake and kinetics experiments were carried out through electrochemical measurements by using solutions of lipophilic electroactive molecules of different sizes and with different affinities for silica. Other solutions with fluorescent probes were used for spectrophotometry measurements. In this work we report the environment where the lipophilic compounds studied end up after absorption and the kinetics of their uptake by the oil-filled silica nanocapsules with different shell thicknesses.     

Chiral ethylhexyl substituents for optically active aggregates of pi-conjugated polymers

We report an efficient synthesis of chiral (2S)-ethylhexanol for functionalizing and solubilizing conjugated polymers. The alpha-substituted chiral ethylhexyl side chains were obtained through a powerful and flexible asymmetric synthesis using pseudoephedrine as a chiral auxiliary. The dependence of the properties of conjugated polymers on molecular structure is investigated by circular dichroism, fluorescence, and absorption spectroscopy on two new chiral conjugated polymers, poly(3,3-bis((S)-2-ethylhexyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine) (PProDOT((2S)-ethylhexyl)(2)) and poly(3,3-bis((S)-2-methylbutyl)-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine) (PProDOT((2S)-methylbutyl)(2)). The properties of PProDOT((2S)-ethylhexyl)(2)) differ significantly from those of its methylbutyl analog as investigated by chiral aggregation providing insight into the role of interchain interactions in these subsecond switching electrochromic polymers.