Extent Of Reoxidation Research Articles

Failure Analysis of Ni-8YSZ Electrode under Reoxidation Based on the Real Microstructure.

During the operation of solid oxide fuel cells (SOFCs), the Ni-8YSZ anodes are subjected to thermal mismatch and reoxidation, accompanied by the risk of damage and failure. These damages and failures are generally induced by small defects at the microscopic level, leading to the degradation of the structural bearing capacity. Therefore, the distribution and quantification of the stresses in the real microstructure of Ni-8YSZ electrodes is essential. In this study, the real Ni-8YSZ microstructure was reconstructed based on nano-computed tomography, and the stress distribution of the real microstructure was analyzed based on the finite element method under reoxidation and different operating temperatures. The failure probability of 8YSZ at different degrees of reoxidation was evaluated according to the Weibull method, and the amount of damaged 8YSZ elements was statistically counted. The study results indicate a high level of stress in the thin necks and relatively sharp areas of the microstructure. The 8YSZ has a high failure probability at a reoxidation extent of 5-10%.

Investigation of reactive perovskite materials for solar fuel production via two-step redox cycles: Thermochemical activity, thermodynamic properties and reduction kinetics

The investigation and optimization of solar fuels production by H2O and CO2 splitting reactions using non-stoichiometric redox materials as oxygen carriers relies on materials-related studies. The thermochemical cycles performance strongly rely on the thermodynamics and kinetics of redox reactions, as well as the chemical composition and morphology of the reactive redox materials commonly based on ceria and perovskites. This study focusses on the evaluation and selection of suitable non-stoichiometric metal oxides for two-step thermochemical cycles with high fuel production yields, rapid reaction rates, and performance stability. The redox activities of different A- and B-site substituted perovskite materials (ABO3) were experimentally investigated (with A = La, Sr, Y, Ca, Ce, Pr, Sm and B = Mn, Co, Fe, Mg, Al, Ga, Cr). The reactive powders were synthesized via modified Pechini methods providing a porous microstructure especially suitable for thermochemical cycles, while their redox activity was evaluated by thermogravimetric analysis. This experimental screening highlighted the difficulty to combine high reduction extent (δ) achievable by the reactive material with complete re-oxidation extent and fast reaction rates. From the redox activity study of manganite perovskites, La0.5Sr0.5Mn0.9Mg0.1O3 (LSMMg) was pointed out as a good compromise between CO2 splitting activity and thermal stability, possibly competing with ceria as a promising material for two-step thermochemical cycles. Both thermodynamic and kinetic studies were also performed to provide a better understanding of the mechanisms involved in thermochemical cycles. Thermodynamic properties derived from experimental δ(T,pO2) diagrams were used to predict the upper bounds for both reduction extent and fuel production performance at equilibrium. Regarding kinetics, the activation energy during LSMMg reduction was shown to increase with the increase of the non-stoichiometry extent.

Two-step CO2 and H2O splitting using perovskite-coated ceria foam for enhanced green fuel production in a porous volumetric solar reactor

Solar thermochemical cycles offer a viable option for the production of green synthetic fuels from CO2 and H2O. Two-step cycles using redox materials consist of a high-temperature reduction creating oxygen vacancies, followed by a re-oxidation step with an oxidant gas (CO2 and/or H2O), resulting in CO and/or H2 production. This study focuses on the thermochemical performance in a solar reactor of a new kind of composite reactive material: reticulated ceria foam with uniform perovskite coating, forming a dual-phase layered heterostructure. La0.5Sr0.5Mn0.9Mg0.1O3 (LSMMg) was selected as perovskite coating due to its high fuel productivity and thermochemical stability upon cycling. The perovskite coating improves the reduction step by increasing the reduction extent reached by the reactive material. The results revealed significant enhancement of oxygen exchange in the dual-phase composite material during reduction compared to individual components. The enhanced reduction extent had a beneficial effect on the oxygen release rate and the total amount of fuel produced by CO2 and H2O splitting, whereas an adverse impact on the peak rate during H2/CO evolution was noticed. Decreasing the reduction pressure allowed enhancing the non-stoichiometric oxygen extent, while the re-oxidation extent increased with the inlet oxidant molar fraction. With suitable operating conditions (reduction at 0.100 bar and oxidation in 100 % CO2), the LSMMg-coated ceria foam produced a higher amount of fuel but with lower fuel production rate in comparison with pure ceria foam. This study points out the beneficial effects of composite redox materials consisting of LSMMg-coated ceria foam in enhancing the oxygen exchange capacity of ceria for solar fuel production.

Role of transition metal in perovskites for enhancing selectivity of methane to syngas

This work investigates the role of transition metal at the B-site of ABO3–type perovskite that affects the lattice oxygen transfer for the partial oxidation of methane. Two types, LaBO3 and La0.6Ca0.4BO3 (B = Fe, Mn, Co), were synthesized and investigated under repeated redox cycles. The Fe-based perovskite had a high tendency of partial oxidation, while the Co-based perovskite mainly led to the full oxidation of methane. The Co-based perovskite was enriched with the surface oxygen component due to the enhanced oxygen transfer from the lattice to the surface vacant site. On the other hand, the Fe-based perovskite showed a relatively low lattice oxygen transfer to the surface oxygen vacancy although it had the highest lattice oxygen ratio among the different B-site perovskites. The selectivity and the production amount of the syngas were improved when the amount of the surface oxygen was controlled by adjusting the re-oxidation extent of the carriers during the oxidation step. Through these experiments, La0.6Ca0.4FeO3, which showed the highest syngas productivity with utilizing earth-abundant metals, was selected as an optimal oxygen carrier for methane reforming.

Methane reforming and water splitting by zirconia-supported cerium-tungsten composite oxides for cyclic production of syngas and hydrogen

Redox cycles of methane reforming (MR) and water splitting (WS) of cerium-tungsten composite oxides were investigated at 1223 K for the cyclic production of syngas and hydrogen. Among various composite oxides tested having different Ce/W ratios, the sample with the Ce/W molar ratio of 1, CeW1/Zr, showed the best performance. When the sample was stabilized after several cycles, most of the Ce was present as Ce3+ and thus only W participated in the redox process. Production of CO2 during MR by the cerium–tungsten composite sample was much less than that by the WO3-only sample, yielding a H2/(CO + CO2) ratio much closer to the desired value of 2. The extent of reduction of W by MR remained nearly constant during repeated cycles, but re-oxidation by WS was not complete during the first several cycles. However, the re-oxidation extent increased to full re-oxidation during repeated cycles; the redox system became very stable after the sixth cycle. These are both beneficial effects owing to the presence of Ce.

快速光脉冲藻类光合作用测量方法的激发条件研究

The excitation intensity and excitation duration are the essential experimental conditions of the high-frequency flash method for the measurement of algae photosynthetic parameters.The fraction of reduced primary electron acceptors and the reoxidation extent of these reduced primary electron acceptors were analyzed under different excitation intensities and excitation durations.The analysis results indicate that 30 000μmol quanta·m-2·s-1 is the optimal excitation intensity and 70μs is the optimal excitation duration.Under this optimal excitation condition,96.08% of the primary electron acceptors are reduced and only 9.81% primary electron acceptors reoxidize.

Methods for handling redox-sensitive smectite dispersions

AbstractRedox activation (reduction of structural Fe) of smectites greatly alters their chemical reactivity and physical properties, which may be exploited for various environmental, agricultural or industrial purposes. Their re-oxidation during preparation, characterization, and use is, however, a significant risk to their utility. In this study, methods and apparatus were developed and described which mitigated reoxidation. Ferruginous smectite (sample SWa-1, Na saturated) was used as the model smectite. It was reduced with sodium dithionite in a citrate-bicarbonate buffer solution at 70°C for 4 h, which achieved a maximum Fe(II)/total Fe ratio of 0.9113 ± 0.0048. The first step in rendering reduced samples useful is to remove from them the reducing agents and other solutes present during reduction. This was accomplished in the present study by reducing the sample in an inert-atmosphere reaction tube (IRT) (a 50 mL centrifuge tube equipped with a removable septum cap), then removing solutes from the suspension by centrifuge washing. The washing steps were performed with the aid of a controlled-atmosphere liquid exchanger (CALE) which provided connections between the sample suspension and deoxygenated solutions. The reduced state was measured by 1, 10-phenanthroline or by Mössbauer spectroscopy at 77 K to give Fe(II)/total Fe ratios. Some samples were freeze dried after washing. Results revealed that if reduced smectites are washed without protection from atmospheric O2, the extent of reoxidation is on the order of 40 to 60%. If the sample is subsequently dried, reoxidation increases to more than 76%. If the sample is protected using the IRT and the CALE, however, reoxidation is decreased to less than 2%. Freeze drying in a glove box allowed reoxidaton to increase to slightly more than 10%. These results indicate that more reoxidation occurred during the drying stage than during the washing stage. These observations lead to the conclusions that (1) protection of reduced samples from atmospheric O2 is essential if extensive reoxidation is to be prevented, and (2) the methods and apparatus described herein are effective for accomplishing that purpose in abiotically reduced smectites. They may also be effective if applied to microbially reduced smectites.

Exploitation of thermochemical cycles based on solid oxide redox systems for thermochemical storage of solar heat. Part 1: Testing of cobalt oxide-based powders

Thermochemical storage of solar heat exploits the enthalpy effects of reversible chemical reactions for the storage of solar energy. Among the possible reversible gas–solid chemical reactions, utilization of a pair of reduction–oxidation (redox) reactions of solid oxides of multivalent metals can be directly coupled to Concentrated Solar Power (CSP) plants employing air as the heat transfer fluid avoiding thus the need for separate heat exchangers. The redox pair of cobalt oxides Co3O4/CoO in particular, is characterized by high reaction enthalpies and thus potential heat storage capacity.Parametric testing of cobalt oxide-based powder compositions via Thermo-Gravimetric Analysis/Differential Scanning Calorimetry was performed to determine the temperature range for cyclic reduction–oxidation and optimize the process parameters for maximum reduction and re-oxidation extent. The heating/cooling rate is an important means to control the extent of the oxidation reaction which is slower than reduction. Complete re-oxidation was achieved within reasonable times by performing the two reactions at close temperatures and by controlling the heating/cooling rate. Under proper operating conditions Co3O4 powders exhibited long-term (30 cycles), complete and reproducible cyclic reduction/oxidation performance within the temperature range 800–1000°C. No benefits occurred by using Ni, Mg and Cu cobaltates instead of “pure” Co3O4. The Co3O4 raw material’s specific surface area is an influential factor on redox performance to which observed differences among powders from various sources could be attributed. Presence of Na was also shown to affect significantly the evolution of the products’ microstructure, though not necessarily combined with improved redox performance.

Thermochemical Solar Energy Storage Via Redox Oxides: Materials and Reactor/Heat Exchanger Concepts

Thermochemical Storage of solar heat exploits the heat effects of reversible chemical reactions for the storage of solar energy. Among the possible reversible gas-solid chemical reactions, the utilization of a pair of redox reactions of multivalent solid oxides can be directly coupled to CSP plants employing air as the heat transfer fluid bypassing the need for a separate heat exchanger.The present work concerns the development of thermochemical storage systems based on such oxide-based redox materials and in particular on cobalt oxide; in the one hand by tailoring their heat storage/release capability and on the other hand via their incorporation in proper reactor/heat exchanger devices. In this respect the first stage of the work involved parametric testing of cobalt oxide compositions via Thermo-Gravimetric Analysis to comparatively investigate the temperature range for cyclic oxidation-reduction and optimize the cycle conditions for maximum reduction and re-oxidation extent. Subsequently, two reactor concepts for the coupling of solar energy to the redox reactions have been implemented and tested. These reactor concepts include in one hand structured ceramic reactors/heat exchangers based on redox-oxide-coated honeycombs and on the other hand powder-fed, solar-heated, rotary kiln reactors. The two reactor concepts were tested within non-solar-aided lab-scale and solar- aided campaigns, respectively. The feasibility of both concepts was shown and good chemical conversions were achieved. The experiments pointed out the challenging points related to the manufacture of pilot-scale reactors/heat exchangers with enhanced heat storage capacity. A numerical model using commercial CFD software is developed to define optimal geometrical characteristics and operating conditions and refine the pilot scale design in order to achieve efficient, long-term off-sun operation.

New Basic Insight into Reductive Functionalization Sequences of Single Walled Carbon Nanotubes (SWCNTs)

The reactivity of reduced single walled carbon nanotubes (SWCNTs) (carbon nanotubides), prepared under strict inert conditions in a glovebox with respect to the covalent functionalization with hexyl iodide and subsequent exposure to ambient conditions (air, moisture), was systematically investigated by Raman, absorption, fluorescence, and IR spectroscopy as well as by TG/MS measurements. We have discovered that the alkylation does not lead to a complete discharging of the tubes since follow-up reactions with moisture still take place leading to mixed functionalized carbon nanotube derivatives containing H- and OH-addends (but no carboxylates) next to the hexyl groups. This was confirmed by the exposure of carbon nanotubides to ambient conditions. The degree of hexylation determined both under strict inert (ic) and ambient (ac) conditions increases with an increasing K:C ratio of the reduced SWCNT starting material. The presence of OH-groups covalently attached to the nanotubes was also confirmed by postfunctionalization reactions with 2-thiophenecarbonyl chloride, leading to the corresponding esters. Control experiments with KO2 give rise to the formation of the same oxygen functionalities. These combined findings allowed for the suggestions of a plausible reaction mechanism, describing all the observed reactions on the SWCNTs side walls. The amount of subsequent side reactions after the treatment of reduced SWCNTs with electrophiles is strongly influenced by the reduction potential of the electrophile, which is responsible for the extent of reoxidation. Incomplete quenching of negative charges allows stronger oxidants/electrophile (e.g., O2) to perform follow-up reactions.

Dynamics of Tc Immobilization in Soils Under Flooded Conditions and Extent of Reoxidation Following Aeration

The dynamics and bioavailability of 99Tc, an important radioactive contaminant, depend largely on its redox state. Both biotic and abiotic reactions determine the rate, extent and reversibility of Tc immobilization. We monitored Tc solubility and chemical extractability in five contrasting soils. Tc remained water-soluble in aerated soils, but was immobilized under flooded conditions. Immobilization was only partly reversible. Immobilized Tc was associated with soil organic matter and was possibly chemically reduced in one soil. Water solubility was a sensitive probe of Tc dynamics. The extent of immobilization and reversibility were not simple functions of soil chemical or biological properties.

Influence of the enzyme dissimilatory sulfite reductase on stable isotope fractionation during sulfate reduction

The stable isotopes of sulfate are often used as a tool to assess bacterial sulfate reduction on the macro scale. However, the mechanisms of stable isotope fractionation of sulfur and oxygen at the enzymatic level are not yet fully understood. In batch experiments with water enriched in 18O we investigated the effect of different nitrite concentrations on sulfur isotope fractionation by Desulfovibrio desulfuricans. With increasing nitrite concentrations, we found sulfur isotope enrichment factors ranging from −11.2 ± 1.8‰ to −22.5 ± 3.2‰. Furthermore, the δ 18O values in the remaining sulfate increased from approximately 50–120‰ when 18O-enriched water was supplied. Since 18O-exchange with ambient water does not take place in sulfate, but rather in intermediates of the sulfate reduction pathway (e.g. SO 3 2 - ), we suggest that nitrite affects the steady-state concentration and the extent of reoxidation of the metabolic intermediate sulfite to sulfate during sulfate reduction. Given that nitrite is known to inhibit the production of the enzyme dissimilatory sulfite reductase, our results suggest that the activity of the dissimilatory sulfite reductase regulates the kinetic isotope fractionation of sulfur and oxygen during bacterial sulfate reduction. Our novel results also imply that isotope fractionation during bacterial sulfate reduction strongly depends on the cell internal enzymatic regulation rather than on the physico-chemical features of the individual enzymes.

Inhibiting the reoxidation of directly reduced iron by vapour phase inhibitors

The inhibition of the reoxidation of direct reduced iron was investigated by establishing different contents of vapour phase inhibitors in oxidising gas mixture He‐O2 at 150 and 200 °C. The raw materials were iron oxide powder and iron ore pellets which were first pre‐reduced by H2 at 500 °C and then reoxidised. Three stages of oxidation are identified, a rapid initial stage, an intermediate stage in which the rate of reoxidation decreases significantly, and a final stage, in which the rate of oxidation is very low. The length of each stage and the rate of reoxidation are related to temperature and to the status of raw materials.Four inhibitors were tested. Benzylamine, ammonium benzoate, and the commercial VCI cortec 101 exert a good inhibition on the reoxidation of iron. The inhibition by ammonia, however, is not significant. Presence of water vapour enhances the reoxidation. XPS analysis reveals amines on the surface when benzylamine is added in the oxidation gases.The ignition temperature of reduced iron decreases with the addition of inhibitors in the oxidation gas atmosphere. This unwanted effect probably is due to organic deposits on iron surface, initiating the burning of iron. The extent of reoxidation, however, is significantly diminished by the inhibitors, thus heating of the material is suppressed and the danger of ignition is decreased.

Correlation between protein flexibility and electron transfer from QA-* to QB in PSII membrane fragments from spinach.

To analyze a possible correlation between the extent of QA-* reoxidation and protein dynamics, fluorometric and Mössbauer spectroscopic measurements were performed in photosystem II membrane fragments from spinach. Numerical evaluation of the flash-induced change of the normalized fluorescence quantum yield revealed that the extent of reoxidation starts to decrease below 275 K and is almost completely suppressed at 230 K. Detailed analyses of Mössbauer spectra measured at different temperatures in 57Fe-enriched material indicate that the onset of fluctuations between conformational substates of the protein matrix occurs also at around 230 K. Based on this correspondence, protein flexibility is inferred to play a key role for QA-* reoxidation in photosystem II. Taking into account the striking similarities with purple bacteria and the latest structural information on these reaction centers [Stowell, M. H. B., McPhillips, T. M., Rees, D. C., Soltis, S. M., Abresch, E., and Feher, G. (1997) Science 276, 812-816], it appears most plausible that also the headgroup of plastoquinone-9 bound to the QB-site in PSII requires a structural reorientation for its reduction to the semiquinone.

Study of the deactivation mechanism of Al2O3-supported cobalt Fischer-Tropsch catalysts

The influence of water on alumina-supported cobalt catalysts has been studied. The deactivation of supported Co catalysts was studied in a fixed-bed reactor using synthesis gas feeds containing different concentrations of water vapour. Supporting model studies were carried out using H2O/H2 feeds in conjunction with XPS and gravimetry. Rapid deactivation occurs on Re-promoted CO/Al2O3 catalysts when H2/CO/H2O feeds are used, whereas unpromoted CO/Al2O3 shows more stable activity. The results from the gravimetric studies suggest that only a small fraction of the bulk cobalt metal initially present reoxidizes to cobalt oxide during reaction. However, the XPS results indicate significant reoxidation of surface cobalt atoms or highly dispersed cobalt phases, which is likely to be the cause of the observed deactivation. Rhenium is shown to have a marked effect on the extent of reoxidation of alumina-supported cobalt catalysts.

Electron transfer in sulfite oxidase: effects of pH and anions on transient kinetics.

Intramolecular electron transfer (ET) rates in sulfite oxidase (SO) were measured using flavin semiquinone reductants [5-deazariboflavin (dRFH.) and lumiflavin (LFH.)] generated by laser flash photolysis. Rapid bimolecular reduction of the heme by the dRF semiquinone occurred (k = 4 x 10(8) M-1 s-1 at pH 6; 1 x 10(8) M-1 s-1 at pH 9), followed by heme Fe(II) reoxidation due to intramolecular electron transfer to Mo(VI). Flash-induced difference spectra indicated that only spectral processes due to reduction and oxidation of the b-type heme prosthetic group were observed, with no detectable spectral contribution from the Mo cofactor. The extent of reoxidation decreased greatly from pH 6 to 9 (50% to 3%), as expected from the shifts in the redox potentials of the heme and Mo cofactor with pH, consistent with an electron transfer equilibrium between the two redox centers. The observed rate constant for the Fe(II) to Mo(VI) electron transfer decreased from 1650 s-1 at pH 6 to 60 s-1 at pH 9 and showed a maximum of 2400 s-1 at pH 7. Increases in salt concentration greatly decreased intramolecular ET rate constants (direct reduction by flavin semiquinone was unchanged), due to the binding of anions. Titration with the sodium salts of Cl-, SO4(2-), and H2PO4-/HPO4(2-) resulted in decreases in rate constants of intramolecular ET from 1500 s-1 to < 100 s-1 at pH 6 and 7. Similar dissociation constants were measured for the binding of these anions by flash photolysis and by steady-state enzyme kinetics using the inhibition of the sulfite/cytochrome c assay reaction for sulfite oxidase. A mechanism is proposed in which anion binding to the enzyme inhibits the rate of intramolecular electron transfer.

Aqueous humor hydrogen peroxide analysis with dichlorophenol-indophenol

Hydrogen peroxide is now reported to be a normal aqueous humor component present, in the low μ m concentration range, in the animal species which have been studied. This finding was established with the exclusive use of the dichlorophenol-indophenol method of analysis. In this procedure, aqueous humor is added to a blue, oxidized dichlorophenol-indophenol solution. The 605 nm absorbance of this solution immediately decreases in response to the reducing action of ascorbate present in the sample. The extent of reoxidation of the solution upon the addition of peroxidase, as measured by the increase in its 605 nm absorbance, can be quantitatively related to the concentration of H 2O 2 in the sample. A close examination of this method revealed that reduced dichlorophenol-indophenol spontaneously reoxidizes at a rate of 0·03 nmol min −1 μ m −1, with generation of H 2O 2. H 2O 2 generation was unequivocally established by analysis of the temporal dependency of the absorbance increase produced by peroxidase in the absence of added H 2O 2 and by the sensitivity of this phenomenon to catalase. This spontaneous production of H 2O 2, on the other hand, cannot be attributed to ascorbate auto-oxidation because added ascorbate quantitatively reacts with dichlorophenol-indophenol, provided that an excess of the latter is maintained. This method then has an enormous potential to overestimate H 2O 2 in any sample. On the other hand, the response of the assay system to a given level of H 2O 2 depends on the level of reduction previously produced by ascorbate. This results in an artifactual positive correlation between ascorbate and H 2O 2 levels in samples containing variable amounts of ascorbate. In spite of these serious limitations the method can still be useful to measure H 2O 2 if appropriate precautions are taken. When using it for the analysis of rabbit aqueous humor H 2O 2 without correcting for the H 2O 2 generated during the assay and ignoring differences in the level of ascorbate in the samples, we obtained an average value of 25·3 μ m H 2O 2, which coincides with that reported in the literature for the rabbit, but is obviously incorrect. When analysing aqueous humor there was the additional variable of the aqueous humor itself inhibiting the rate of dichlorophenol-indophenol auto-oxidation and so the final, corrected figure for H 2O 2 concentration in the aqueous humor became uncertain, since the auto-oxidation of the substrate could not be properly subtracted. Nevertheless, by inhibiting dichlorophenol-indophenol oxidation with argon, the apparent concentration of H 2O 2 measured in the aqueous humor became a function of the autooxidation rate of the phenol, H 2O 2 being no longer detectable for extrapolated zero auto-oxidation rates. We conclude that rabbit aqueous humor does not contain H 2O 2 that can be detected with the dichlorophenol-indophenol reagent. H 2O 2 could still be present in this ocular fluid, however, beyond the lower limit of detection of the method, in the n m range or below.

Protective effects of a PGI2 analogue OP-2507 on hemorrhagic shock in rats--with an evaluation of the metabolic recovery using near-infrared optical monitoring.

The protective effects of the PGI2 analogue, OP-2507 against hypoxic tissue injury were investigated in a 60-70 min acute hemorrhagic shock model in 29 rats. To assess the metabolic recovery of mitochondria after tissue injury induced by hemorrhagic hypotension with a mean arterial pressure of 30 mmHg, we have non-invasively monitored changes in the brain tissue parameters of cyt. aa3 redox state, blood oxygenation and relative blood volume by spectrophotometry through the closed skull and intact skin. Pretreatment with 0.1 mg/Kg s.c. of OP-2507 at 30-40 min before induction of shock was performed on 14 rats (OP-treated group). The remaining 15 rats were used as a control (control group). There was a consistent prolongation of survival time and a significant improvement in survival rate after reinfusion of the shed blood in the OP-treated group. In this group there was a rapid and complete reoxidation of cyt. aa3 with a mean overshoot of 9 +/- 5.5% above the baseline value after reinfusion. On the other hand, in the control group the extent of reoxidation was significantly lower, with a minimal 11 +/- 3.2% below the base line. In order to evaluate the mechanisms involved 10 mg of NaCN i.v. was administered to the living rats at 60-70 min after reinfusion of the blood in both groups. In the OP-treated group, brain Hb saturation increased up to 20% above the pre-cyanide infusion level. However in the control group there was a non-significant increase in the Hb oxygenation level. These observation indicate that in the OP-treated group oxygen consumption by mitochondria is significantly higher than that in the control group. Thus, enhanced oxygen utilization could lead to the active restoration of injured tissue by promoting oxidative phosphorylation. Under these experimental conditions the oxidative response of cyt. aa3 is concluded to correlate closely with the prognosis of shock animals in both groups. These results indicate the potential usefulness of OP-2507 in protecting the brain and other organs from oxygen insufficiency as a result of tissue ischemia and anoxia.

Direct measurement of intramolecular electron transfer between type I and type III copper centers in the multi-copper enzyme ascorbate oxidase and its type II copper-depleted and cyanide-inhibited forms

Transient kinetics of reduction of zucchini squash ascorbate oxidase (AO) by lumiflavin semiquinone have been studied by using laser flash photolysis. Second-order kinetics were obtained for reduction of the type I copper with a rate constant of 2.7 X 10(7) M-1 s-1, which is comparable to that obtained with other blue copper proteins such as plastocyanin. Following reduction, the type I copper was reoxidized in a protein concentration independent (i.e., intramolecular) reaction (kobs = 160 s-1). Comparison with literature values for limiting rate constants in transient single-turnover kinetic experiments suggests that intramolecular electron transfer probably is the rate-limiting step in enzyme catalysis. The extent of reoxidation of type I copper was approximately 55%, which is consistent with the approximately equal redox potentials of the type I and type III copper centers. Neither azide nor fluoride caused any significant changes in kinetics, although they are enzyme inhibitors and are thought to bind to the type II copper. In contrast, cyanide caused a concentration-dependent decrease in the extent of intramolecular electron transfer (with no change in rate constant), and decreased the rate constant for reduction of the type I copper by a factor of 2. The apparent dissociation constant for cyanide (0.2-0.4 mM) is similar to that reported for inhibition of enzyme activity. Removal of the type II copper from AO only marginally affected the kinetics of electron transfer to type I copper (k = 3.2 x 10(7) M-1 s-1) and slightly increased the extent but did not alter the rate constant of intramolecular electron transfer.(ABSTRACT TRUNCATED AT 250 WORDS)

The kinetics of reoxidation of yeast complex III. An evaluation of the Q-cycle.

The reoxidation of reduced yeast Complex III by oxidants believed to react with cytochrome c1 exhibited multiple phases for both cytochrome c1 and the cytochromes b; the reoxidation of cytochrome b, but not cytochrome c1, was markedly slowed by the presence of antimycin. The data are consistent with the Q-cycle or any other scheme which proposes a branched path for electron transport between the cytochrome b centers and the endogenous Q6, provided certain constraints are relaxed. The reoxidation of the endogenous quinone proceeded at a rate comparable to that of the rapidly reacting cytochrome b and appeared to be complete within 100 ms. Removal of the endogenous quinone did not change the rate or extent of reoxidation of any of the heme centers, demonstrating that quinone is not required for electron transport between cytochromes b and the iron-sulfur cluster. This result is inconsistent with the requirements of the Q-cycle. Funiculosin completely inhibited the reoxidation of cytochrome b whereas the reoxidation of cytochrome c1 exhibited simple first-order kinetics in the presence of this inhibitor, implying that the iron-sulfur cluster is on the direct path of electron transfer from cytochrome b to cytochrome c1. Potent inhibition of cytochrome b oxidation was also observed with myxothiazol and mucidin. The reaction of reduced Complex III with Q1 also exhibited multiple phases in the oxidation of the cytochrome b centers; these phases were unaffected by the presence of myxothiazol. Addition of antimycin, or removal of the endogenous quinone, eliminated the rapid phases; only one of the cytochrome b centers was oxidized under these conditions. Epr showed that it is the low-potential cytochrome b that is the species rapidly oxidized.