Crystal structures and cation ordering of Sr2AlSbO6 and Sr2CoSbO6

The two double perovskite oxides Sr₂AlSbO₆ and Sr₂CoSbO₆ were prepared and their structures studied with the X-ray powder diffraction method. At room temperature the crystal structure of Sr₂AlSbO₆ is cubic , with . It was found that depending on the preparation conditions, the Al³⁺ and Sb⁵⁺ cations can be either entirely or partially ordered. In the case of the partially ordered Sr₂AlSbO₆ sample, the extension of cation ordering was estimated from the -dependent broadening of the diffraction peaks and the results were interpreted as evidence of the formation of anti-phase domains in the material. Low-temperature Raman spectroscopic measurements demonstrated that the cubic phase of Sr₂AlSbO₆ is stable down to 79 K.The room-temperature crystal structure of Sr₂CoSbO₆ is trigonal (space group with and . At 470 K, however, the material undergoes a continuous phase transition and its structure is converted to cubic (space group . The studied Sr₂CoSbO₆ sample was partially ordered, but unlike Sr₂AlSbO₆, no indication of the formation of anti-phase domains was observed.     

Alkyl-CoA disulfides as inhibitors and mechanistic probes for FabH enzymes

The first step of the reaction catalyzed by the homodimeric FabH from a dissociated fatty acid synthase is acyl transfer from acyl-CoA to an active site cysteine. We report that C1 to C10 alkyl-CoA disulfides irreversibly inhibit Escherichia coli FabH (ecFabH) and Mycobacterium tuberculosis FabH with relative efficiencies that reflect these enzymes' differential acyl-group specificity. Crystallographic and kinetic studies with MeSSCoA show rapid inhibition of one monomer of ecFabH through formation of a methyl disulfide conjugate with this cysteine. Reaction of the second subunit with either MeSSCoA or acetyl-CoA is much slower. In the presence of malonyl-ACP, the acylation rate of the second subunit is restored to that of the native ecFabH. These observations suggest a catalytic model in which a structurally disordered apo-ecFabH dimer orders on binding either the first substrate, acetyl-CoA, or the inhibitor MeSSCoA, and is restored to a disordered state on binding of malonyl-ACP.     

Liquid crystalline mesophases of pluronics (L64, P65, and P123) and transition metal nitrate salts ([M(H2O)6](NO3)2)

The triblock poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) copolymers, Pluronics (L64, P65, and P123), form liquid crystalline (LC) mesophases with transition metal nitrate salts (TMS), [M(H(2)O)(n)](NO(3))(2), in the presence and absence of free water in the media. In this assembly process, M-OH(2) plays an important role as observed in a TMS:C(n)EO(m) (C(n)EO(m) is oligo(ethylene oxide) nonionic surfactants) system. The structure of the LC mesophases and interactions of the metal ion-nitrate ion and metal ion-Pluronic were investigated using microscopy (POM), diffraction (XRD), and spectroscopy (FTIR and micro-Raman) techniques. The TMS:L64 system requires a shear force for mesophase ordering to be observed using X-ray diffraction. However, TMS:P65 and TMS:P123 form well structured LC mesophases. Depending on the salt/Pluronic mole ratio, hexagonal LC mesophases are observed in the TMS:P65 systems and cubic and tetragonal LC mesophases in the TMS:P123 systems. The LC mesophase in the water/salt/Pluronic system is sensitive to the concentration of free (H(2)O) and coordinated water (M-OH(2)) molecules and demonstrates structural changes. As the free water is evaporated from the H(2)O:TMS:Pluronic LC mesophase (ternary mixture), the nitrate ion remains free in the media. However, complete evaporation of the free water molecules enforces the coordination of the nitrate ion to the metal ion in all TMS:Pluronic systems.     

ANALYSIS AND DESIGN OF MULTIPORT WAVEGUIDE JUNCTIONS WITH ARTIFICIAL INCLUSIONS FORMED OF CONDUCTING STRIPS

A rigorous electromagnetic theory is developed to analyze multiport waveguide junctions with artificial inclusions formed of the conducting strips and dielectric layers. The method is based on the Fourier transform technique combined with the mode matching method technique that takes into account the edge conditions in vicinity of the strip edges. The scattering characteristics of a 3-port junction are discussed with the numerical examples for the SWR maps, the power reflection and transmission coefficients, and the 2D plots of near fields distributions inside the junction area. A strategy of optimize matching properties of the 3-port waveguide junctions is also presented.     

Separate entrance and exit portals for ligand traffic in Mycobacterium tuberculosis FabH

Mycobacterium tuberculosis FabH initiates type II fatty acid synthase-catalyzed formation of the long chain (C(16)-C(22)) acyl-coenzyme A (CoA) precursors of mycolic acids, which are major constituents of the bacterial cell envelope. Crystal structures of M. tuberculosis FabH (mtFabH) show the substrate binding site to be a buried, extended L-shaped channel with only a single solvent access portal. Entrance of an acyl-CoA substrate through the solvent portal would require energetically unfavorable reptational threading of the substrate to its reactive position. Using a class of FabH inhibitors, we have tested an alternative hypothesis that FabH exists in an "open" form during substrate binding and product release, and a "closed" form in which catalysis and intermediate steps occur. This hypothesis is supported by mass spectrometric analysis of the product profile and crystal structures of complexes of mtFabH with these inhibitors.     

Seeded growth of titania colloids with refractive index tunability and fluorophore-free luminescence

Titania is an important material in modern materials science, chemistry, and physics because of its special catalytic, electric, and optical properties. Here, we describe a novel method to synthesize colloidal particles with a crystalline titania, anatase core and an amorphous titania-shell structure. We demonstrate seeded growth of titania onto titania particles with accurate particle size tunability. The monodispersity is improved to such an extent so that colloidal crystallization of the grown microspheres becomes feasible. Furthermore, seeded growth provides separate manipulation of the core and shell. We tuned the refractive index of the amorphous shell between 1.55 and 2.3. In addition, the particles show luminescence when trace amounts of aminopropyl-triethoxysilane are incorporated into the titania matrix and are calcined at 450 °C. Our novel colloids may be useful for optical materials and technologies such as photonic crystals and optical trapping.     

A Highly Stable Nonhysteretic {Cu2(tebpz) MOF+water} Molecular Spring

A molecular spring formed by a hydrophobic metal–organic framework Cu₂(tebpz) (tebpz=3,3′,5,5′‐tetraethyl‐4,4′‐bipyrazolate) and water is presented. This nanoporous heterogeneous lyophobic system (HLS) has exceptional properties compared to numerous reported systems of such type in terms of stability, efficiency, and operating pressure. Mechanical and thermal energetic characteristics as well as stability of the system are discussed and compared in detail with those of other previously reported HLS.     

Sono-thermal pre-treatment of waste activated sludge before anaerobic digestion

Sonication and thermalization can be applied successfully to disrupt the complex waste activated sludge (WAS) floc structure and to release extra and intra cellular polymeric substances into soluble phase along with solubilization of particulate organic matters, before sludge digestion. In this study, sonication has been combined with thermalization to improve its disintegration efficiency. It was aimed that rise in temperature occurring during the sonication of sludge was used to be as an advantage for the following thermalization in the combined pre-treatment. Thus, the effects of sonication, thermalization and sono-thermalization on physical and chemical properties of sludge were investigated separately under different pre-treatment conditions. The disintegration efficiencies of these methods were in the following descending order: sono-thermalization > sonication > thermalization. The optimum operating conditions for sono-thermalization were determined as the combination of 1-min sonication at 1.0 W/mL and thermalization at 80 °C for 1 h. The influences of sludge pre-treatment on biodegradability of WAS were experienced with biochemical methane potential assay in batch anaerobic reactors. Relative to the control reactor, total methane production in the sono-thermalized reactor increased by 13.6% and it was more than the sum of relative increases achieved in the sonicated and thermalized reactors. Besides, the volatile solids and total chemical oxygen demand reductions in the sono-thermalized reactor were enhanced as well. However, it was determined that sludge pre-treatment techniques applied in this study was not feasible due to their high energy requirements.