Evolution of intermetallic GaPd2_{2}/SiO2_{2} catalyst and optimization for methanol synthesis at ambient pressure

The CO$_{2}$ hydrogenation to methanol is efficiently catalyzed at ambient pressure by nanodispersed intermetallic GaPd$_{2}$/SiO$_{2}$ catalysts prepared by incipient wetness impregnation. Here we optimize the catalyst in terms of metal content and reduction temperature in relation to its catalytic activity. We find that the intrinsic activity is higher for the GaPd$_{2}$/SiO$_{2}$ catalyst with a metal loading of 13 wt.% compared to catalysts with 23 wt.% and 7 wt.%, indicating that there is an optimum particle size for the reaction of around 8 nm. The highest catalytic activity is measured on catalysts reduced at 550°C. To unravel the formation of the active phase, we studied calcined GaPd$_{2}$/SiO$_{2}$ catalysts with 23 wt.% and 13 wt.% using a combination of in situ techniques: X-ray diffraction (XRD), X-ray absorption near edge fine structure (XANES) and extended X-ray absorption fine structure (EXAFS). We find that the catalyst with higher metal content reduces to metallic Pd in a mixture of H$_{2}$/Ar at room temperature, while the catalyst with lower metal content retains a mixture of PdO and Pd up to 140°C. Both catalysts form the GaPd$_{2}$ phase above 300°C, albeit the fraction of crystalline intermediate Pd nanoparticles of the catalyst with higher metal loading reduces at higher temperature. In the final state, the catalyst with higher metal loading contains a fraction of unalloyed metallic Pd, while the catalyst with lower metal loading is phase pure. We discuss the alloying mechanism leading to the catalyst active phase formation selecting three temperatures: 25°C, 320°C and 550°C

A Novel OxyR Sensor and Regulator of Hydrogen Peroxide Stress with One Cysteine Residue in Deinococcus radiodurans

In bacteria, OxyR is a peroxide sensor and transcription regulator, which can sense the presence of reactive oxygen species and induce antioxidant system. When the cells are exposed to H2O2, OxyR protein is activated via the formation of a disulfide bond between the two conserved cysteine residues (C199 and C208). In Deinococcus radiodurans, a previously unreported special characteristic of DrOxyR (DR0615) is found with only one conserved cysteine. dr0615 gene mutant is hypersensitive to H2O2, but only a little to ionizing radiation. Site-directed mutagenesis and subsequent in vivo functional analyses revealed that the conserved cysteine (C210) is necessary for sensing H2O2, but its mutation did not alter the binding characteristics of OxyR on DNA. Under oxidant stress, DrOxyR is oxidized to sulfenic acid form, which can be reduced by reducing reagents. In addition, quantitative real-time PCR and global transcription profile results showed that OxyR is not only a transcriptional activator (e.g., katE, drb0125), but also a transcriptional repressor (e.g., dps, mntH). Because OxyR regulates Mn and Fe ion transporter genes, Mn/Fe ion ratio is changed in dr0615 mutant, suggesting that the genes involved in Mn/Fe ion homeostasis, and the genes involved in antioxidant mechanism are highly cooperative under extremely oxidant stress. In conclusion, these findings expand the OxyR family, which could be divided into two classes: typical 2-Cys OxyR and 1-Cys OxyR

Iron enzyme ribulose-5-phosphate 3-epimerase in Escherichia coli is rapidly damaged by hydrogen peroxide but can be protected by manganese

H2O2 is commonly generated in biological habitats by environmental chemistry and by cellular immune responses. H2O2 penetrates cells, disrupts metabolism, and blocks growth; it therefore is of interest to identify the major cellular molecules that H2O2 damages and the strategies by which cells protect themselves from it. We used a strain of Escherichia coli that lacks catalases and peroxidases to impose protracted low-grade H2O2 stress. Physiological analysis indicated that the pentose–phosphate pathway, in particular, was poisoned by submicromolar intracellular H2O2. Assays determined that ribulose-5-phosphate 3-epimerase (Rpe) was specifically inactivated. In vitro studies demonstrated that Rpe employs a ferrous iron atom as a solvent-exposed cofactor and that H2O2 rapidly oxidizes this metal in a Fenton reaction. The oxidized iron is released immediately, causing a loss of activity. Most Rpe proteins could be reactivated by remetallation; however, a small fraction of proteins were irreversibly damaged by each oxidation cycle, and so repeated cycles of oxidation and remetallation progressively led to permanent inactivation of the entire Rpe pool. Manganese import and iron sequestration are key elements of the H2O2 stress response, and we found that manganese can activate Rpe in vitro in place of iron, converting the enzyme to a form that is unaffected by H2O2. Indeed, the provision of manganese to H2O2–stressed cells protected Rpe and enabled the pentose–phosphate pathway to retain function. These data indicate that mononuclear iron enzymes can be primary targets of H2O2 stress and that cells adapt by shifting from iron- to manganese-centered metabolism

Anion Radicals of Bacteriochlorophylls c, d, and e. Likely Electron Acceptors in the Primary Photochemistry of Green and Brown Photosynthetic Bacteria

Representative redox, optical, ESR and ENDOR data and molecular orbital calculations are reported for the anion radicals of the homologous bacteriochlorophylls and bacteriopheophorbides c, d, and e. The chromophores (Chlorobium chlorophylls) have previously been identified or are suspected to exist in the reaction centers of green (c and d) and brown (e) photosynthetic bacteria which appear to straddle green plants and purple bacteria on an evolutionary scale. An early role in the light-driven electron-transport chain of these bacteria is proposed for the Chlorobium chlorophylls. Diagnostic optical and electron paramagnetic spectral signatures are presented for the anion radicals which distinguish them from subsequent acceptors and would identify them in vivo. © 1983, American Chemical Society. All rights reserved

The CorA Mg2+ Channel Is Required for the Virulence of Salmonella enterica Serovar Typhimurium▿ †

CorA is the primary Mg2+ channel in Salmonella enterica serovar Typhimurium. A corA mutant is attenuated in mice and defective for invasion of and replication within epithelial cells. Microarray studies show that several virulence effectors are repressed in a corA mutant strain, which ultimately manifests itself as a decrease in virulence

ZEUS HARDWARE CONTROL-SYSTEM

The ZEUS collaboration is building a system to monitor, control and document the hardware of the ZEUS detector. This system is based on a network of VAX computers and microprocessors connected via ethernet. The database for the hardware values will be ADAMO tables; the ethernet connection will be DECNET, TCP/IP, or RPC. Most of the documentation will also be kept in ADAMO tables for easy access by users