Hydrogen embrittlement risk of high strength galvanized steel in contact with alkaline media

The critical conditions for hydrogenembrittlement (HE) risk of highstrengthgalvanizedsteel (HSGS) wires and tendons exposed to alkaline concrete pore solutions have been evaluated by means of electrochemical and mechanical testing. There is a relationship between the hydrogenembrittlementrisk in HSGS and the length of hydrogen evolution process in alkalinemedia. The galvanizedsteel suffers anodic dissolution simultaneously to the hydrogen evolution which does not stop until the passivation process is completed. HSGS wires exposed to a very highalkalinemedia have showed HE risk with loss in mechanical properties only if long periods with hydrogen evolution process take place with a simultaneous intensive galvanized coating reduction.

A link between protein structure and enzyme catalyzed hydrogen tunneling

We present evidence that the size of an active site side chain may modulate the degree of hydrogen tunneling in an enzyme-catalyzed reaction. Primary and secondary kH/kT and kD/kT kinetic isotope effects have been measured for the oxidation of benzyl alcohol catalyzed by horse liver alcohol dehydrogenase at 25°C. As reported in earlier studies, the relationship between secondary kH/kT and kD/kT isotope effects provides a sensitive probe for deviations from classical behavior. In the present work, catalytic efficiency and the extent of hydrogen tunneling have been correlated for the alcohol dehydrogenase-catalyzed hydride transfer among a group of site-directed mutants at position 203. Val-203 interacts with the opposite face of the cofactor NAD+ from the alcohol substrate. The reduction in size of this residue is correlated with diminished tunneling and a two orders of magnitude decrease in catalytic efficiency. Comparison of the x-ray crystal structures of a ternary complex of a high-tunneling (Phe-93 → Trp) and a low-tunneling (Val-203 → Ala) mutant provides a structural basis for the observed effects, demonstrating an increase in the hydrogen transfer distance for the low-tunneling mutant. The Val-203 → Ala ternary complex crystal structure also shows a hyperclosed interdomain geometry relative to the wild-type and the Phe-93 → Trp mutant ternary complex structures. This demonstrates a flexibility in interdomain movement that could potentially narrow the distance between the donor and acceptor carbons in the native enzyme and may enhance the role of tunneling in the hydride transfer reaction.

Regulation of the OxyR transcription factor by hydrogen peroxide and the cellular thiol—disulfide status

The Escherichia coli transcription factor OxyR is activated by the formation of an intramolecular disulfide bond and subsequently is deactivated by enzymatic reduction of the disulfide bond. Here we show that OxyR can be activated by two possible pathways. In mutants defective in the cellular disulfide-reducing systems, OxyR is constitutively activated by a change in the thiol—disulfide redox status in the absence of added oxidants. In wild-type cells, OxyR is activated by hydrogen peroxide. By monitoring the presence of the OxyR disulfide bond after exposure to hydrogen peroxide in vivo and in vitro, we also show that the kinetics of OxyR oxidation by low concentrations of hydrogen peroxide is significantly faster than the kinetics of OxyR reduction, allowing for transient activation in an overall reducing environment. We propose that the activity of OxyR in vivo is determined by the balance between hydrogen peroxide levels and the cellular redox environment.

A gain-of-function of an amyotrophic lateral sclerosis-associated Cu,Zn-superoxide dismutase mutant: An enhancement of free radical formation due to a decrease in Km for hydrogen peroxide.

Cu,Zn-superoxide dismutase (SOD) is known to be a locus of mutation in familial amyotrophic lateral sclerosis (FALS). Transgenic mice that express a mutant Cu,Zn-SOD, Gly-93--> Ala (G93A), have been shown to develop amyotrophic lateral sclerosis (ALS) symptoms. We cloned the FALS mutant, G93A, and wild-type cDNA of human Cu,Zn-SOD, overexpressed them in Sf9 insect cells, purified the proteins, and studied their enzymic activities for catalyzing the dismutation of superoxide anions and the generation of free radicals with H2O2 as substrate. Our results showed that both enzymes contain one copper ion per subunit and have identical dismutation activity. However, the free radical-generating function of the G93A mutant, as measured by the spin trapping method, is enhanced relative to that of the wild-type enzyme, particularly at lower H2O2 concentrations. This is due to a small, but reproducible, decrease in the value of Km for H2O2 for the G93A mutant, while the kcat is identical for both enzymes. Thus, the ALS symptoms observed in G93A transgenic mice are not caused by the reduction of Cu,Zn-SOD activity with the mutant enzyme; rather, it is induced by a gain-of-function, an enhancement of the free radical-generating function. This is consistent with the x-ray crystallographic studies showing the active channel of the FALS mutant is slightly larger than that of the wild-type enzyme; thus, it is more accessible to H2O2. This gain-of-function, in part, may provide an explanation for the association between ALS and Cu,Zn-SOD mutants.

Formate is the hydrogen donor for the anaerobic ribonucleotide reductase from Escherichia coli.

During anaerobic growth Escherichia coli uses a specific ribonucleoside-triphosphate reductase (class III enzyme) for the production of deoxyribonucleoside triphosphates. In its active form, the enzyme contains an iron-sulfur center and an oxygen-sensitive glycyl radical (Gly-681). The radical is generated in the inactive protein from S-adenosylmethionine by an auxiliary enzyme system present in E. coli. By modification of the previous purification procedure, we now prepared a glycyl radical-containing reductase, active in the absence of the auxiliary reducing enzyme system. This reductase uses formate as hydrogen donor in the reaction. During catalysis, formate is stoichiometrically oxidized to CO2, and isotope from [3H]formate appears in water. Thus E. coli uses completely different hydrogen donors for the reduction of ribonucleotides during anaerobic and aerobic growth. The aerobic class I reductase employs redox-active thiols from thioredoxin or glutaredoxin to this purpose. The present results strengthen speculations that class III enzymes arose early during the evolution of DNA.

Electrochemical synthesis at pre-pilot scale of 1-phenylethanol by cathodic reduction of acetophenone using a solid polymer electrolyte

The pre-pilot scale synthesis of 1-phenylethanol was carried out by the cathodic hydrogenation of acetophenone in a 100 cm2 (geometric area) Polymer Electrolyte Membrane Electrochemical Reactor. The cathode was a Pd/C electrode. Hydrogen oxidation on a gas diffusion electrode was chosen as anodic reaction in order to take advantage of the hydrogen evolved during the reduction. This hydrogen oxidation provides the protons needed for the synthesis. The synthesis performed with only a solid polymer electrolyte, spe, has lower fractional conversion although a higher selectivity than that carried out using a support–electrolyte–solvent together with a spe. However, the difference between these two cases is rather small and since the work-up and purification of the final product are much easier when only a spe is used, this last process was chosen for the pre-pilot electrochemical synthesis of 1-phenylethanol.

Environmentally friendly reduction of a platinum catalyst precursor supported on polypyrrole

Supported metals are traditionally prepared by impregnating a support material with the metal precursor solution, followed by reduction in hydrogen at elevated temperatures. In this study, a polymeric support has been considered. Polypyrrole (PPy) has been chemically synthesized using FeCl3 as a doping agent, and it has been impregnated with a H2PtCl6 solution to prepare a catalyst precursor. The restricted thermal stability of polypyrrole does not allow using the traditional reduction in hydrogen at elevated temperature, and chemical reduction under mild conditions using sodium borohydride implies environmental concerns. Therefore, cold RF plasma has been considered an environmentally friendly alternative. Ar plasma leads to a more effective reduction of platinum ions in the chloroplatinic complex anchored onto the polypyrrole chain after impregnation than reduction with sodium borohydride, as has been evidenced by XPS. The increase of RF power enhanced the effectiveness of the Ar plasma treatment. A homogeneous distribution of platinum nanoparticles has been observed by TEM after the reduction treatment with plasma. The Pt/polypyrrol catalyst reduced by Ar plasma at 200 watts effectively catalyzed the aqueous reduction of nitrates with H2 to yield N2, with a very low selectivity to undesired nitrites and ammonium by-products.

Functionalization of carbon nanotubes using aminobenzene acids and electrochemical methods. Electroactivity for the oxygen reduction reaction

Functionalized carbon nanotubes (CNTs) using three aminobenzene acids with different functional groups (carboxylic, sulphonic, phosphonic) in para position have been synthesized through potentiodynamic treatment in acid media under oxidative conditions. A noticeable increase in the capacitance for the functionalized carbon nanotubes mainly due to redox processes points out the formation of an electroactive polymer thin film on the CNTs surface along with covalently bonded functionalities. The CNTs functionalized using aminobenzoic acid rendered the highest capacitance values and surface nitrogen content, while the presence of sulfur and/or phosphorus groups in the aminobenzene structure yielded a lower functionalization degree. The oxygen reduction reaction (ORR) activity of the functionalized samples was similar to that of the parent CNTs, independently of the functional group present in the aminobenzene acid. Interestingly, a heat treatment in N2 atmosphere with a very low O2 concentration (3125 ppm) at 800 °C of the CNTs functionalized with aminobenzoic acid produced a material with high amounts of surface oxygen and nitrogen groups (12 and 4% at., respectively), that seem to modulate the electron-donor properties of the resulting material. The onset potential and limiting current for ORR was enhanced for this material. These are promising results that validates the use of electrochemistry for the synthesis of novel N-doped electrocatalysts for ORR in combination with adequate heat treatments.

Control of hypersonic turbulent skin friction by boundary-layer combustion of hydrogen

Shvab-Zeldovich coupling of flow variables has been used to extend Van Driest's theory of turbulent boundary-layer skin friction to include injection and combustion of hydrogen in the boundary layer. The resulting theory is used to make predictions of skin friction and heat transfer that are found to be consistent with experimental and numerical results. Using the theory to extrapolate to larger downstream distances at the same experimental conditions, it is found that the reduction in skin-friction drag with hydrogen mixing and combustion is three times that with mixing alone. In application to flow on a flat plate at mainstream velocities of 2, 4, and 6 knits, and Reynolds numbers from 3 X 10(6) to 1 x 10(8), injection and combustion of hydrogen yielded values of skin-friction drag that were less than one-half of the no-injection skin-friction drag, together with a net reduction in heat transfer when the combustion heat release in air was less than the stagnation enthalpy. The mass efficiency of hydrogen injection, as measured by effective specific impulse values, was approximately 2000 s.