The role of the oxidic support on the deactivation of Pt catalysts during the CO2 reforming of methane

Pt supported on ¿-Al2O3, TiO2 and ZrO2 are active catalysts for the CO2 reforming of methane to synthesis gas. The stability of the catalysts increased in the order Pt/¿-A12O3 < Pt/TiO2 < Pt/ZrO2. For all catalysts, the decrease in activity with time on stream is caused by carbon formation, which blocks the active metal sites for reaction. With Pt/TiO2 and Pt/ZrO2, deactivation started immediately after the start of the reaction, while the Pt/¿-A12O3 catalyst showed an induction period during which carbon was accumulated without affecting the catalytic activity

Geophysical Research

Contains research objectives and reports on two research projects.Joint Services Electronics Programs (U. S. Army, U.S. Navy, and U.S. Air Force) under Contract DA 36-039-AMC-03200(E)National Aeronautics and Space Administration (Grant NGR-22-009-131)National Aeronautics and Space Administration (Grant NGR-22-009-(114)

Leydig cells express neural cell adhesion molecules in vivo and in vitro

The neural cell adhesion molecule (NCAM) polypeptides are expressed by numerous tissues during embryonic development, where they are involved in cell-cell interactions. In the adult, NCAM expression is confined to a few cell types, including neurons and peptide-hormone-producing cells. Here we demonstrate that the Leydig cells of the adult rat, mouse, and hamster testes express NCAM as well. Western blotting showed that an NCAM of approximately 120 kDa was present in the adult testes of all three species investigated. This form was also found in freshly isolated mouse Leydig cells and in Leydig cells after 2 days in culture. After 4 days in culture, mouse Leydig cells expressed additional NCAM isoforms of approximately 140 and 180 kDa, indicating changes in alternative splicing of NCAM primary transcripts. Also, NCAM mRNA of all isoforms, as detected by S1-nuclease protection assays, increased with time in culture. The expression of the cell adhesion molecule NCAM by adult Leydig cells may explain the aggregation of Leydig cells in clusters in rodent testes, which could be a prerequisite for functional coordination of groups of Leydig cells. Furthermore, the presence of this neural and endocrine marker may indicate a closer relationship between Leydig cells and neural and peptide-hormone-producing cells than is considered to exist at the present time

Structure and Function of the Mycobacterial Type VII Secretion Systems

Bacteria have evolved intricate secretion machineries for the successful delivery of large molecules across their cell envelopes. Such specialized secretion systems allow a variety of bacteria to thrive in specific host environments. In mycobacteria, type VII secretion systems (T7SSs) are dedicated protein transport machineries that fulfill diverse and crucial roles, ranging from metabolite uptake to immune evasion and subversion to conjugation. Since the discovery of mycobacterial T7SSs about 15 y ago, genetic, structural, and functional studies have provided insight into the roles and functioning of these secretion machineries. Here, we focus on recent advances in the elucidation of the structure and mechanism of mycobacterial T7SSs in protein secretion. As many of these systems are essential for mycobacterial growth or virulence, they provide opportunities for the development of novel therapies to combat a number of relevant mycobacterial diseases

Geophysical Research

Contains reports on two research projects.Joint Services Electronics Programs (U. S. Army, U. S. Navy, and U. S. Air Force) under Contract DA 28-043-AMC-02536(E)National Aeronautics and Space Administration (Grant NGR-22-009-131)National Aeronautics and Space Administration (Grant NGR-22-009-114)National Aeronautics and Space Administration (Contract NAS 12-436

Iron impregnation on the amorphous shell of vapor grown carbon fibers and the catalytic growth of secondary nanofibers

Vapor grown carbon fibers (VGCFs) with diameters of several microns were synthesized and investigated by high resolution transmission electron microscopy. It was found that the shell of the VGCFs consisted of densely-packed domains embedded in loosely-packed matrix, and both were highly amorphous. Regular edge planes as observed on the surface of fishbone nanofibers do not exist on VGCFs. Hence, surface treatment is more important for the deposition of catalysts. Ammonium ferric citrate (AFC) was employed for the impregnation of iron, where the high viscosity of the aqueous solution of AFC is beneficial. Calcination was found to be a key step to improve the dispersion of the iron particles, which can be attributed to enhanced interactions between iron and carbon due to the gasification of carbon occurring at the iron-carbon interface. Quantitative analysis by X-ray photoelectron spectroscopy showed that the calcination of the supported AFC led to a higher atomic concentration of iron on the surface, indicating smaller particle size and higher dispersion. Secondary carbon nanofibers were grown subsequently on the VGCFs from cyclohexane. The specific surface area was enhanced considerably, from less than 1 m2 g-1 to 106 m2 g-1 after the growth of the secondary nanofibers. The obtained composites are promising materials as structured support in heterogeneous catalysis

Coherent Ro-vibrational Revivals in a Thermal Molecular Ensemble

We report an experimental and theoretical study of the evolution of vibrational coherence in a thermal ensemble of nitrogen molecules. Rotational dephasing and rephasing of the vibrational coherence is detected by coherent anti-Stokes Raman scattering. The existence of ro-vibrational coupling and the discrete energy spectrum of the rotational bath lead to a whole new class of full and fractional ro-vibrational revivals. Following the rich ro-vibrational dynamics on a nanosecond time scale with sub-picosecond time resolution enables us to determine the second-order ro-vibrational constant $gamma_e$ and assess new possibilities of controlling decoherence.Comment: submitted at Physical Review

Nuclear Magnetic Resonance and Hyperfine Structure

Contains reports on six research projects

Nuclear Magnetic Resonance and Hyperfine Structure

Contains reports on six research projects

Activation of Pseudomonas aeruginosa elastase in Pseudomonas putida by triggering dissociation of the propeptide-enzyme complex

The propeptide of Pseudomonas aeruginosa elastase functions both as an intramolecular chaperone required for the folding of the enzyme and as an inhibitor that prevents activity of the enzyme before its secretion into the extracellular medium. Since expression of the lasB gene, which encodes elastase, in Pseudomonas putida did not result in extracellular elastase activity, it has been suggested that the enzyme is not recognized by the Xcp secretion machinery of the heterologous host. Here, it is demonstrated that the proenzyme is normally processed in P. putida and that it is indeed not actively secreted by the Xcp machinery. Nevertheless, substantial amounts of the enzyme were detected in the extracellular medium. Co-immunoprecipitations revealed that the extracellular enzyme was associated with the propeptide, which explains the lack of enzymic activity. Since the propeptide-enzyme complex in P. putida apparently does not dissociate spontaneously, it is concluded that a host-specific factor is required to induce this event. Mutants were selected which showed extracellular elastase activity. Two mutations, located within the lasB gene, were further characterized. These mutations, resulting in the substitution of Ala and Thr at positions -15 and -153, respectively, of the propeptide (where position +1 is defined as the first residue of the mature enzyme) destabilized the propeptide-enzyme complex. It is concluded that Ala-15 and Thr-153 are required for the inhibitor function, but not for the chaperone function of the propeptide