Reoxidation Kinetics Research Articles

Fast re-oxidation kinetics and conduction pathway in Spark Plasma Sintered ferroelectric ceramics

The re-oxidation kinetics of BaTiO3 ceramics sintered by Spark Plasma Sintering (SPS) was investigated using in-situ impedance spectroscopy. Thanks to the flexibility of the SPS process, the grain size of the dense ceramics was tuned from 0.5μm to 10μm. The re-oxidation kinetics are found to be very fast regardless of the grain size and a full re-oxidation of the ceramics are achieved after 20h of exposure to an ambient environment at only 600°C. The residual density of charge carriers is reduced when using finer starting powders. SPS ceramics made with micrometer size grains demonstrate a residual charge-carrier density that is one tenth that of ceramics made from 10μm particles. Grain-boundary conduction is dominant through fine-grain SPS ceramics. This latter feature is similar to BaTiO3 sintered using the conventional route with 10μm size grain. Finally, the critical grain size for optimal dielectric permittivity is found to shift from 0.7μm in standard ceramics to 1.5μm in SPS ceramics.

TEMPORARY REMOVAL: Effects of oxathiapiprolin on photosynthetic activity of Chlorella pyrenoidosa probed by chlorophyll fluorescence and thermoluminescence assays

Oxathiapiprolin is the first member of the piperidinyl thiazole isoxazoline fungicides developed globally as DuPont™ Zorvec™ for the control of plant diseases caused by Oomycete pathogens. Although they have great application potential, ecotoxicological data, especially their non-target effects on photosynthesis of algae and higher plants are completely lacking. Effects of a novel oxathiapiprolin biocide on the photosynthesis of Chlorella pyrenoidosa were therefore investigated using chlorophyll a fluorescence and thermoluminescence techniques. Effects of the bioicide on photosynthetic electron transport and energy absorption and dissipation were concentration dependent. Inhibition of the electron donor side of PSII significantly increased as oxathiapiprolin concentration increased from 0 to 100 μM. The oxathiapiprolin treatment increased the effective antenna size per reaction center and the dissipated energy flux per reaction center. It damaged PSII reaction centers and blocked the electron transport flux, thus inhibiting photochemical activity in Chlorella pyrenoidosa. Exposure to oxathiapiprolin at concentrations up to 50 μM had little effect on QA− reoxidation kinetics on the electron acceptor side. Only a higher concentration (100 μM) of the oxathiapiprolin biocide exerted obvious inhibitory effects on QA− reoxidation. At 100 μM, electron transport beyond QA was drastically blocked, and more QA was reoxidized through S2(QAQB)− charge combination. As the concentration increased from 0 to 100 μM, the thermoluminescence C band decreased with increasing oxathiapiprolin concentration, whereas the intensity of the Q band increased which was associated with the decrease of B1 and B2 band intensity. This confirmed that the bioicide hindered electron transfer after QA and caused an increase of S2/S3QB− and TyrD+ QA− recombination. This study implies that when oxathiapiprolin biocides are applied to farmlands, they may pose a potential risk to algae and higher plants.

Effects of 28-homobrassinoloid on key physiological attributes of Solanum lycopersicum seedlings under cadmium stress: Photosynthesis and nitrogen metabolism

Heavy metal accumulation due to environmental pollution, especially in agricultural ecosystem can cause serious deterioration of crop yield and quality. In present study we assessed the effect of exogenous 28-homobrassinoloid (HBL; 10−8 M) on growth, photosynthesis, indices of chlorophyll a fluorescence and nitrogen metabolism in Solanum lycopersicum seedlings grown under two doses (Cd1: 3 mg kg−1 sand and Cd2: 9 mg kg−1 sand) of cadmium. Accumulation of Cd in root tissues was considerably higher than shoot hence, Cd declined the growth, pigment contents, and photosynthetic O2 yield in its concentration dependent manner. Chlorophyll a fluorescence due to Cd stress was negatively affected as shown by decreased QA − reoxidation kinetics: φP0, ψ0, φE0 and PI_ABS and increased energy flux parameters: ABS/RC, TR0/RC, ET0/RC and DI0/RC. HBL application under Cd stress improved the photochemistry of photosystem II (PS II) by affecting these parameters positively. Treatment of Cd in test seedlings resulted into significant decrease in nitrate reductase, nitrite reductase, glutamine synthetase and glutamate synthase activities, and induced enhancing effect on ammonium content and glutamate dehydrogenase activity. Exogenous HBL treatment alleviated the negative effect of Cd on growth, photosynthesis, contents of protein, carbohydrate and inorganic nitrogen and nitrogen assimilating enzymes. The data indicate that exogenous HBL protects the test seedlings during the early growth phase against Cd phytotoxicity by regulating Cd accumulation in tissues and two key metabolic processes; photosynthesis and nitrogen metabolism.

Self-assembly of a 1-eicosanethiolate layer on InSb(100)

1-Eicosanethiolate molecules form relatively weak bonds with the surface of InSb(100) limiting the order of the self-assembled monolayer despite the long length of the alkyl chain. Heating to only 225°C in vacuum completely desorbed the eicosanethiolate layer from the surface based on X-ray photoelectron spectroscopy. Even after deposition times as long as 20h in ethanol, the asymmetric methylene stretch was at 2925cm−1 in the attenuated total reflection Fourier transform infrared spectrum, which is indicative of alkane chains that are incompletely ordered. Atomic force microscopy images combined with ellipsometry showed that the eicosanethiolate layer conformed to the rough InSb(100) starting surface (2.3±0.2nm RMS). The reoxidation kinetics in air of InSb(100) and InSb(111)B covered with eicosanethiolate layers was the same despite the lower surface roughness of the latter (0.64±0.14nm). The bond that the S head group makes with the substrate is the primary factor that determines the cohesiveness of the molecules on the surface. Although interactions between the alkane chains in the layer are sufficient to form a self-assembled layer, the fluidity of the molecules in the layer compromised the chemical passivation of the surface resulting in reoxidation in air after 20min.

The stability of microbially reduced U(IV); impact of residual electron donor and sediment ageing

The stimulation of microbial U(VI) reduction to precipitate insoluble U(IV) has been proposed as a means of remediating mobile uranium groundwater contamination. Crucial to the success of such a remediation strategy is determining the longevity of U(IV) biominerals in the subsurface, particularly if the groundwater becomes oxidising. Here we describe experiments to assess the susceptibility of microbially-reduced U(IV) to oxidative remobilisation both via aeration and by the addition of nitrate at environmentally-relevant conditions. Additional factors examined include the possibility of biogenic U(IV) becoming more crystalline (and potentially more recalcitrant) during a period of ageing, and the role played by residual electron donor in controlling the long-term fate of the uranium. Biogenic U(IV) was precipitated as a non-crystalline U(IV) or “monomeric” phase, with a small but increasing contribution to the EXAFS spectra from nanocrystalline uraninite occurring during 15months of ageing. Despite this, no evidence was observed for an increase in recalcitrance to oxidative remobilisation. However, the presence of residual electron donor post-biostimulation was shown to exert a strong control on U(IV) reoxidation kinetics, highlighting the importance of maintaining the presence of electron donor in the subsurface, in order to protect biogenic U(IV) from oxidative remobilisation.

Kinetic studies of a ferredoxin-dependent cyanobacterial nitrate reductase.

A flash photolysis study of electron transfer (ET) kinetics from reduced ferredoxin (photoreduced by Photosystem I) to the ferredoxin-dependent nitrate reductase from the cyanobacterium Synechococcus sp. PCC 7942 has been carried out. In the presence of nitrate, under conditions where only a single electron is transferred to nitrate reductase, the rate of enzyme reduction shows a biphasic concentration dependence: At low enzyme concentrations the dependence is approximately linear, with an estimated second-order rate constant of 7.4 ± 0.8 × 10(7) M(-1) s(-1); at concentrations above 2 μM, the rate increases nonlinearly to an asymptotic value of approximately 300 s(-1), indicating the presence of a rate-limiting step in the process. The spectrum of the one-electron reduced enzyme suggests that Mo centers are largely reduced with a minor contribution of iron-sulfur cluster reduction. Under conditions favoring two-electron reduction of the enzyme, the redox difference spectrum can be accounted for by the oxidation of two reduced ferredoxins, suggesting that the enzyme has completed one full catalytic cycle. The spectral changes observed in the absence of nitrate are significantly different from those seen in the presence of nitrate. Experiments in the absence of nitrate revealed that the singly reduced enzyme exhibits different absorption characteristics and reoxidation kinetics, compared to those observed with nitrate present, and exhibits a much faster binding by reduced ferredoxin than the oxidized enzyme. The implications of these observations for understanding the enzyme mechanism are discussed.

Oxidative Remobilization of Technetium Sequestered by Sulfide-Transformed Nano Zerovalent Iron

Our previous study showed that formation of TcS2-like phases is favored over TcO2 under sulfidic conditions stimulated by nano zerovalent iron. This study further investigates the stability of Tc(IV) sulfide upon reoxidation by solution chemistry, solid phase characterization, and X-ray absorption spectroscopy. Tc dissolution data showed that Tc(VII) reduced by sulfide-transformed nZVI has substantially slower reoxidation kinetics than Tc(VII) reduced by nZVI only. The initial inhibition of Tc(IV) dissolution at S/Fe = 0.112 is due to the redox buffer capacity of FeS, which is evidenced by the parallel trends in oxidation-reduction potentials (ORP) and Tc dissolution kinetics. The role of FeS in inhibiting Tc oxidation is further supported by the Mössbauer spectroscopy and micro X-ray diffraction data at S/Fe = 0.112, showing persistence of FeS after 24-h oxidation but complete oxidation after 120-h oxidation. X-ray absorption spectroscopy data for S/Fe = 0.011 showed significantly increasing percentages of TcS2 in the solid phase after 24-h oxidation, indicating stronger resistance of TcS2 to oxidation. At S/Fe = 0.112, the XAS results revealed significant transformation of Tc speciation from TcS2 to TcO2 after 120-h oxidation. Given that no apparent Tc dissolution occurred during this period, the speciation transformation might play a secondary role in hindering Tc oxidation. Collectively, the results indicate that sequestrating Tc as TcS2 under stimulated sulfate reduction is a promising strategy to improve the long-term stability of reduced Tc in subsurface remediation.

Ceria-based electrospun fibers for renewable fuel production via two-step thermal redox cycles for carbon dioxide splitting.

Zirconium-doped ceria (Ce(1-x)Zr(x)O2) was synthesized through a controlled electrospinning process as a promising approach to cost-effective, sinter-resistant material structures for high-temperature, solar-driven thermochemical redox cycles. To approximate a two-step redox cycle for solar fuel production, fibrous Ce(1-x)Zr(x)O2 with relatively low levels of Zr-doping (0 < x < 0.1) were cycled in an infrared-imaging furnace with high-temperature (up to 1500 °C) partial reduction and lower-temperature (∼800 °C) reoxidation via CO2 splitting to produce CO. Increases in Zr content improve reducibility and sintering resistance, and, for x≤ 0.05, do not significantly slow reoxidation kinetics for CO production. Cycle stability of the fibrous Ce(1-x)Zr(x)O2 (with x = 0.025) was assessed for a range of conditions by measuring rates of O2 release during reduction and CO production during reoxidation and by assessing post-cycling fiber crystallite sizes and surface areas. Sintering increases with reduction temperature but occurs primarily along the fiber axes. Even after 108 redox cycles with reduction at 1400 °C and oxidation with CO2 at 800 °C, the fibers maintain their structure with surface areas of ∼0.3 m(2) g(-1), higher than those observed in the literature for other ceria-based structures operating at similarly high temperature conditions. Total CO production and peak production rate stabilize above 3.0 mL g(-1) and 13.0 mL min(-1) g(-1), respectively. The results show the potential for electrospun oxides as sinter-resistant material structures with adequate surface area to support rapid CO2 splitting in solar thermochemical redox cycles.

Re-Oxidation Kinetics of Flash Reduced Iron Particles in O2–N2 Gas Mixtures Relevant to a Novel Flash Ironmaking Process

Re-Oxidation Kinetics of Flash Reduced Iron Particles in O2–N2 Gas Mixtures Relevant to a Novel Flash Ironmaking Process

Re-oxidation Kinetics of Flash-Reduced Iron Particles in H2-H2O(g) Atmosphere Relevant to a Novel Flash Ironmaking Process

A novel flash ironmaking process based on hydrogen-containing reduction gases is under development at the University of Utah. The goal of this work was to study the possibility of the re-oxidation of iron particles in a H2-H2O gas mixture in the lower part of the flash reactor from the kinetic point of view. The last stage of hydrogen reduction of iron oxide, i.e., the reduction of wustite, is limited by equilibrium. As the reaction mixture cools down, the re-oxidation of iron could take place because of the decreasing equilibrium constant and the high reactivity of the freshly reduced fine iron particles. The effects of temperature and H2O partial pressure on the re-oxidation rate were examined in the temperature range of 823 K to 973 K (550 °C to 700 °C) and H2O contents of 40 to 100 pct. The nucleation and growth kinetics model was shown to best describe the re-oxidation kinetics. The partial pressure dependence with respect to water vapor was determined to be of first order, and the activation energy of re-oxidation reaction was 146 kJ/mol. A complete rate equation that adequately represents the experimental data was developed.

Effect of polypyrrole conformational rearrangement on Al alloys corrosion protection

Polypyrrole (PPy) films were galvanostatically synthesized on 6082-T6 and 2024-T3 Al alloys. Experiments based on Otero’s Electrochemically Stimulated Conformational Relaxation (ESCR) model were used to investigate PPy re-oxidation kinetics as a function of polymer compaction. Chronoamperograms after pre-polarization at different cathodic potentials for various times were recorded in LiClO4/propylene carbonate solution. Physico-chemical parameters such as compaction coefficient, closing potential and diffusion parameter were used to characterize the relaxation and diffusion controlled processes of PPy films. All indicate that conformational rearrangements depend mainly on polymer structure, governed in turn by substrate nature. However, experiments in aggressive NaCl solution have confirmed the importance of the redox process dynamics on the protection performance of PPy against Al alloy degradation. Galvanic coupling has been confirmed to be the main factor for effective corrosion protection of Al 2024 by Ppy.

Effects of arsenic on growth and photosystem II (PSII) activity of Microcystis aeruginosa

Effects of arsenic on growth, pigments content, oxygen evolution and photosystem II (PSII) activity of Microcystis aeruginosa were investigated in the present study. Various concentrations of As(III) did not show significant effects on growth and total carotenoids content within 24h of treatment. After 48h of treatment, 10mgL−1 As(III) significantly inhibited the growth and synthesis of carotenoids of M. aeruginosa, while As(III) at concentrations ranging from 0 to 1mgL−1 showed no significant inhibition. Chlorophyll a synthesis, oxygen evolution and chlorophyll fluorescence were more sensitive to As(III) exposure than carotenoid synthesis and growth. Chlorophyll a content, fast fluorescence rise transients and fluorescence decay kinetics appeared to be affected after the cells were exposed to 1 and 10mgL−1 As(III) for more than 24h. Treatment with 10mgL−1 As(III) for 24h or longer led to flattening of the fluorescence transient and drastic decrease of amplitude of fast phase of QA− reoxidation kinetics. Exposure to As(III) mainly inhibited the quantum yield for primary photochemistry, density of reaction centers and photosynthesis performance index, and increased the dissipated energy. The decrease in amplitude of the fast and middle phases further revealed that once electron transfer from QA− to QB was inhibited by As(III), more QA− was reoxidized via S2(QAQB)− charge recombination. As(III) stress may result in an increased stability of the S2QB− and S2QA− recombination.

Tightly-bound ubiquinone in the Escherichia coli respiratory Complex I

NADH:ubiquinone oxidoreductase (Complex I), the electron input enzyme in the respiratory chain of mitochondria and many bacteria, couples electron transport to proton translocation across the membrane. Complex I is a primary proton pump; although its proton translocation mechanism is yet to be known, it is considered radically different from any other mechanism known for redox-driven proton pumps: no redox centers have been found in its membrane domain where the proton translocation takes place. Here we studied the properties and the catalytic role of the enzyme-bound ubiquinone in the solubilized, purified Complex I from Escherichia coli. The ubiquinone content in the enzyme preparations was 1.3±0.1 per bound FMN residue. Rapid mixing of Complex I with NADH, traced optically, demonstrated that both reduction and re-oxidation kinetics of ubiquinone coincide with the respective kinetics of the majority of Fe–S clusters, indicating kinetic competence of the detected ubiquinone. Optical spectroelectrochemical redox titration of Complex I followed at 270–280nm, where the redox changes of ubiquinone contribute, did not reveal any transition within the redox potential range typical for the membrane pool, or loosely bound ubiquinone (ca. +50–+100mV vs. NHE, pH 6.8). The transition is likely to take place at much lower potentials (Em ≤−200mV). Such perturbed redox properties of ubiquinone indicate that it is tightly bound to the enzyme's hydrophobic core. The possibility of two ubiquinone-binding sites in Complex I is discussed.

Effects of redox cycling of iron in nontronite on reduction of technetium

In situ technetium-99 ( 99Tc) immobilization by Fe(II) associated with clay minerals is a potential cost-effective method for Tc remediation at contaminated sites. The oxidation state of Fe is constantly cycled in sedimentary environments, however the effect of this cycle on Tc reduction and immobilization has not yet been investigated. The objective of this project was therefore to study how multiple cycles of reduction–reoxidation of Fe-rich clay mineral, nontronite, affected its reactivity toward Tc (VII) reduction. Iron-rich nontronite NAu-2 was used as a model clay mineral. NAu-2 suspension was first bioreduced by Shewanella putrefaciens CN32, which was subsequently re-oxidized by air. Three cycles of reduction–oxidation were conducted and bioreduced NAu-2 samples from all three cycles were collected and used for Tc(VII) reduction experiments. Each redox cycle resulted in a small fraction of dissolution of small size and/or poorly crystalline NAu-2. The released Fe(II) from the dissolution was likely adsorbed onto NAu-2 surface/edge sites with a high reactivity. Upon exposure to O 2, this reactive Fe(II) fraction was oxidized more rapidly than structural Fe(II) and may have accounted for a two-step reoxidation kinetics of NAu-2 associated Fe(II): rapid oxidation over first few hours followed by slow oxidation. Progressive increase of this reactive fraction of Fe(II), from increased dissolution, accounted for the successively higher rate of bioreduction and reoxidation with increased redox cycles. The same Fe redistribution accounted for two-step Tc(VII) reduction kinetics as well. Rapid Tc(VII) reduction in the first few hours may be attributed to a small fraction of highly reactive Fe(II) at the NAu-2 surface/edge sites, and more steady Tc(VII) reduction over longer time may be carried out by structural Fe(II). Similar to the increased rates of Fe(III) reduction and Fe(II) oxidation, the Tc(VII) reduction rate also increased with redox cycles and could be explained by progressive increase of the reactive Fe(II) on NAu-2 surface/edges. Iron-rich clay minerals undergo important changes after redox cycles, but eventually reach a steady state with continued reactivity toward heavy metals.

Binding Stoichiometry and Affinity of the Manganese-Stabilizing Protein Affects Redox Reactions on the Oxidizing Side of Photosystem II

It has been reported previously that the two subunits of PsbO, the photosystem II (PSII) manganese stabilizing protein, have unique functions in relation to the Mn, Ca(2+), and Cl(-) cofactors in eukaryotic PSII [Popelkova; (2008) Biochemistry 47, 12593]. The experiments reported here utilize a set of N-terminal truncation mutants of PsbO, which exhibit altered subunit binding to PSII, to further characterize its role in establishing efficient O(2) evolution activity. The effects of PsbO binding stoichiometry, affinity, and specificity on Q(A)(-) reoxidation kinetics after a single turnover flash, S-state transitions, and O(2) release time have been examined. The data presented here show that weak rebinding of a single PsbO subunit to PsbO-depleted PSII repairs many of the defects in PSII resulting from the removal of the protein, but many of these are not sustainable, as indicated by low steady-state activities of the reconstituted samples [Popelkova; (2003) Biochemistry 42 , 6193]. High affinity binding of PsbO to PSII is required to produce more stable and efficient cycling of the water oxidation reaction. Reconstitution of the second PsbO subunit is needed to further optimize redox reactions on the PSII oxidizing side. Native PsbO and recombinant wild-type PsbO from spinach facilitate PSII redox reactions in a very similar manner, and nonspecific binding of PsbO to PSII has no significance in these reactions.

The structure of maize polyamine oxidase K300M mutant in complex with the natural substrates provides a snapshot of the catalytic mechanism of polyamine oxidation

Polyamine oxidases are FAD-dependent enzymes catalyzing the oxidation of polyamines at the secondary amino groups. Zea mays PAO (ZmPAO) oxidizes the carbon on the endo-side of the N5-nitrogen of spermidine (Spd) and spermine (Spm). The structure of ZmPAO revealed that the active site is formed by a catalytic tunnel in which the N5 atom of FAD lies in close proximity to the K300 side chain, the only active-site residue conserved in all PAOs. A water molecule, (HOH309), is hydrogen-bound to the amino group of K300 and mutation of this residue results in a 1400-fold decrease in the rate of flavin reduction. The structural studies on the catalytically impaired ZmPAO-K300M mutant described here show that substrates are bound in an 'out-of-register' mode and the HOH309 water molecule is absent in the enzyme-substrate complexes. Moreover, K300 mutation brings about a 60 mV decrease in the FAD redox potential and a 30-fold decrease in the FAD reoxidation rate, within a virtually unaltered geometry of the catalytic pocket. Taken together, these results indicate that the HOH309-K300 couple plays a major role in multiple steps of ZmPAO catalytic mechanism, such as correct substrate binding geometry as well as FAD reduction and reoxidation kinetics.

The Sll0606 Protein Is Required for Photosystem II Assembly/Stability in the Cyanobacterium Synechocystis sp. PCC 6803

An insertional transposon mutation in the sll0606 gene was found to lead to a loss of photoautotrophy but not photoheterotrophy in the cyanobacterium Synechocystis sp. PCC 6803. Complementation analysis of this mutant (Tsll0606) indicated that an intact sll0606 gene could fully restore photoautotrophic growth. Gene organization in the vicinity of sll0606 indicates that it is not contained in an operon. No electron transport activity was detected in Tsll0606 using water as an electron donor and 2,6-dichlorobenzoquinone as an electron acceptor, indicating that Photosystem II (PS II) was defective. Electron transport activity using dichlorophenol indolephenol plus ascorbate as an electron donor to methyl viologen, however, was the same as observed in the control strain. This indicated that electron flow through Photosystem I was normal. Fluorescence induction and decay parameters verified that Photosystem II was highly compromised. The quantum yield for energy trapping by Photosystem II (F(V)/F(M)) in the mutant was less than 10% of that observed in the control strain. The small variable fluorescence yield observed after a single saturating flash exhibited aberrant Q(A)(-) reoxidation kinetics that were insensitive to dichloromethylurea. Immunological analysis indicated that whereas the D2 and CP47 proteins were modestly affected, the D1 and CP43 components were dramatically reduced. Analysis of two-dimensional blue native/lithium dodecyl sulfate-polyacrylamide gels indicated that no intact PS II monomer or dimers were observed in the mutant. The CP43-less PS II monomer did accumulate to detectable levels. Our results indicate that the Sll0606 protein is required for the assembly/stability of a functionally competent Photosystem II.

Solid-state kinetics and catalytic behavior of selective oxidation catalysts from time-resolved XAFS investigations

Time-resolved measurements are required to elucidate time-dependencies of the electronic and geometric structure of a catalyst under changing reaction conditions. Monitoring the evolution of the bulk structure of a catalyst under changing conditions reveals the solid-state kinetics of the corresponding reaction. X-ray absorption spectroscopy (XAS) permits to reveal quantitative phase composition and average valence together with the evolution of the local structure. Hence, combining time-resolved XAS with simultaneous catalysis measurements may elucidate correlations between catalytic performance, the catalyst state under reaction conditions, and its solid-state kinetics. Here, results from time-resolved in situ XAS investigations of various molybdenum-based selective oxidation catalysts are compared and discussed. Model systems (i.e. α-MoO 3, hexagonal MoO 3 supported on SBA-15, and H 4[PVMo 11O 40]) suitable to distinguish structural effects and promotion by additional metal centers have been studied under changing reaction conditions. Correlations between reduction and oxidation solid-state kinetics and catalytic performance reveal the dependence of the selectivity of the catalyst on its electronic structure. In particular the re-oxidation kinetics and the average valence under reaction conditions appear to be determined by the defect structure of the underlying catalyst bulk.

Modelling of surface exchange reactions and diffusion in composites and polycrystalline materials

Abstract Surface exchange reactions and chemical diffusion in composites, consisting of a dilute distribution of inclusions in a matrix, and polycrystalline materials have been modelled by application of both a square grain and a spherical grain model. The diffusion equations have been solved numerically by employing a finite element approach in the case of the square grain model and the Laplace transform method involving numerical Laplace inversion with respect to the spherical grain model. The boundary conditions refer to oxygen exchange reactions between a gas phase and a mixed ionically–electronically conducting ceramic sample within the linear response regime, i.e. small variations of the oxygen partial pressure. Diffusion profiles as well as the time dependence of the total amount of exchanged oxygen (relaxation curves) have been calculated. A necessary requirement for effective medium diffusion is proposed, and appropriate relations for the effective chemical surface exchange coefficient and the effective chemical diffusion coefficient are derived. On the contrary, when the time constant for diffusion from the matrix into the inclusions of a composite exceeds considerably the relaxation time for effective medium diffusion, relaxation curves with two separate time constants are observed. Analogously, in the case of polycrystalline materials the overall transport process is determined by slow (rate-limiting) bulk diffusion from the grain boundaries into the grains. Adequate formulae for the relaxation times are given based on analytical approximations of the solution functions to the diffusion equations. In addition, the spherical grain model is applied to interpret the re-oxidation kinetics of the positive temperature coefficient of resistivity (PTC) ceramics based on conductivity relaxation experiments.

The effects of simultaneous RNAi suppression of PsbO and PsbP protein expression in photosystem II of Arabidopsis

Interfering RNA was used to suppress simultaneously the expression of the four genes which encode the PsbO and PsbP proteins of Photosystem II in Arabidopsis (PsbO: At5g66570, At3g50820 and PsbP: At1g06680, At2g30790). A phenotypic series of transgenic plants was obtained that expressed variable amounts of the PsbO proteins and undetectable amounts of the PsbP proteins. Immunological studies indicated that the loss of PsbP expression was correlated with the loss of expression of the PsbQ, D2, and CP47 proteins, while the loss of PsbO expression was correlated with the loss of expression of the D1 and CP43 proteins. Q(A)(-) reoxidation kinetics in the absence of DCMU indicated that the slowing of electron transfer from Q(A)(-) to Q(B) was correlated with the loss of the PsbP protein. Q(A)(-) reoxidation kinetics in the presence of DCMU indicated that charge recombination between Q(A)(-) and donor side components of the photosystem was retarded in all of the mutants. Decreasing amounts of the PsbO protein in the absence of the PsbP component also led to a progressive loss of variable fluorescence yield (F(V)/F(M)). During fluorescence induction, the loss of PsbP was correlated with a more rapid O to J transition and a loss of the J to I transition. These results indicate that the losses of the PsbO and PsbP proteins differentially affect separate protein components and different PS II functions and can do so, apparently, in the same plant.