Showing papers on "Reaction rate constant published in 1994"
TL;DR: In this paper, the authors present a full-dimensional (6D) dynamics study for the H2+OH reaction using the time-dependent wavepacket approach and employ discrete variable representations for three radial coordinates and coupled angular momentum basis functions for three angular coordinates.
Abstract: Extending our previous studies for the H2+OH reaction in five mathematical dimensions (5D) [J. Chem. Phys. 99, 5615 (1993); 100, 2697 (1994)], we present in this paper a full‐dimensional (6D) dynamics study for the title reaction. The 6D treatment uses the time‐dependent wave‐packet approach and employs discrete variable representations for three radial coordinates and coupled angular momentum basis functions for three angular coordinates. The present 6D study employs an energy projection method to extract reaction probabilities for a whole range of energies from a single wave‐packet propagation, while previous studies produced only energy‐averaged reaction probability from a single wave‐packet propagation. The application of the energy‐projection method allows us to efficiently map out the energy dependence of the reaction probability on a fine grid which revealed surprisingly sharp resonancelike features at low collision energies on the Schatz–Elgersma potential surface. Our calculation shows that the potential‐averaged 5D treatment can produce reaction probabilities essentially indistinguishable from the full‐dimensional result. We also report initial state‐selected reaction cross sections and rate constants which are in good agreement with our previous calculations. The effect of OH vibration on H2+OH reaction is examined in the present study and our calculation shows that the OH vibration can enhance the rate constant by about a factor of 1.7 in good agreement with the experimental estimate of about 1.5.
418 citations
330 citations
TL;DR: Novel approach was developed for studying the kinetics of this reaction, which permitted simultaneous and continuous measurements of the concentrations of NO and the principal product, nitrite (NO2-), and confirmed that NO and NO2- were the only NOx species present in significant amounts and supported the validity of pseudo-steady-state assumptions.
Abstract: An understanding of the rate of reaction of nitric oxide (NO) with oxygen in aqueous solutions is needed in assessing the various actions of NO in the body. A novel approach was developed for studying the kinetics of this reaction, which permitted simultaneous and continuous measurements of the concentrations of NO and the principal product, nitrite (NO2-). Nitric oxide was measured using a chemiluminescence detector, with continuous sampling achieved by diffusion of NO through a membrane fitted into the base of a small, stirred reactor. The results with various initial NO and O2 concentrations confirmed that the rate of reaction is second-order in NO and first-order in O2. The rate of reaction of NO was described by the expression 4k1 [NO]2[O2], where k1 was (2.1 +/- 0.4) x 10(6) M-2 s-1 at 23 degrees C and (2.4 +/- 0.3) x 10(6) M-2 s-1 at 37 degrees C. The value of k1 was the same at pH 4.9 and 7.4. The rate of formation of NO2- equaled the rate of reaction of NO (within experimental uncertainty of a few percent), and there was no detectable formation of nitrate (NO3-). This confirmed that NO and NO2- were the only NOx species present in significant amounts and supported the validity of pseudo-steady-state assumptions for NO2 and N2O3, which are intermediates in the conversion of NO to NO2-.(ABSTRACT TRUNCATED AT 250 WORDS)
261 citations
TL;DR: The results indicate that the better an electron donor the amino acid residue is the more pronounced is the charge transfer contribution in the exciplex formed with 1O2 and the more likely it is to lead to charge separation and hence to a chemical reaction.
Abstract: Quenching of singlet oxygen (1O2) in D2O-ethanol by the amino acids tryptophan, tyrosine, histidine, methionine, cysteine and their derivatives was measured by exciting the sensitizers rose bengal or meso-tetra (N-methyl-4-pyridyl)porphyrin tetratosylate in the presence of oxygen and the above quenchers in solution. In our polar solvent, containing 75% D2O on a molar basis it was found that (1) substitution of the aromatic ring in indole, phenol and imidazole by the electron-donating methyl group increases the total (i.e. nonreactive and reactive) quenching rate constant by a factor of five to eight. Free or blocked amino and carboxyl groups removed by two methylene groups from the ring counteract the above increase in the rate constant. The reactive quenching of singlet oxygen, which leads to oxidative destruction of the aromatic ring, correlates with the above substitution effects. It has been proposed that the quenching process takes place by formation of an exciplex between 1O2 and the quencher. Thus our results indicate that the better an electron donor the amino acid residue is the more pronounced is the charge transfer contribution in the exciplex formed with 1O2 and the more likely it is to lead to charge separation and hence to a chemical reaction. (2) Oligopeptides in solution or peptide bonds linked to the amino acid residue have only a minor effect on singlet oxygen. It can therefore be expected that the polypeptide chains per se in the protein network will not interact significantly with the single oxygen molecules present.(ABSTRACT TRUNCATED AT 250 WORDS)
258 citations
TL;DR: In this article, the polymerization behavior of a series of multifunctional methacrylate and acrylate monomers was studied using differential scanning calorimetry to characterize the reaction rate profiles.
256 citations
TL;DR: In this article, it was shown that the thickness of the degraded layer is of the order of magnitude of D/k, where D is the diffusion coefficient and k the pseudo-first-order rate constant for reactant consumption.
Abstract: For an ageing process involving the consumption of a small molecule (typically O2 or H2O) by reaction with the polymer, there are critical conditions of reaction rate and/or thickness above which the process becomes kinetically controlled by the diffusion of the small molecule in the polymer. Suitable lifetime prediction models must then involve the thickness distribution of reaction products. This latter can be predicted from Fick's law, modified by a term relative to the rate of consumption of the diffusing species by the chemical reaction. Some problems related to the use of this approach are examined here. It appears that, in the most frequent case, the thickness of the degraded layer is of the order of magnitude of D/k, where D is the diffusion coefficient and k the pseudo-first-order rate constant for reactant consumption. Some examples of application related to photochemical, radiochemical and thermochemical ageing are examined. It can, for instance, be shown that in photochemical or radiochemical ageing, the thickness of the oxidized layer (TOL) is proportional to the reciprocal of lβ, where l is the radiation intensity and Β an exponent depending essentially on the radical chain mechanism. It is generally expected that in the case of thermal ageing, the TOL is a decreasing function of the temperature. Some consequences of diffusion control on accelerated and natural ageing methods are briefly examined. The consequences of the ageing-induced “skin-core” structure due to the diffusion control are examined. The main features of the observed polymer embrittlement can be explained in terms of fracture mechanics.
253 citations
TL;DR: In this paper, a simple global equation with an Arrhenius rate constant formulation satisfactorily represented experimental data spanning temperatures from 25 to 625°C obtained from crushed quartz crystals, quartz sand, and quartz slab samples.
251 citations
TL;DR: Model calculations for the reaction at estimated physiological concentrations of NO and O2 indicate that the simple autoxidation of NO will not limit its diffusion from the site of production in endothelial cells to a spatially removed target molecule such as guanylate cyclase in myocytes and platelets.
229 citations
TL;DR: In this article, the steady state dissolution rates of a K-rich feldspar (K081Na015Ba003Al105Si296O8) were measured as a function of chemical affinity and aqueous Si and Al concentration in solutions containing 5 × 10−3 m total K using a titanium mixed flow reactor at a temperature of 150°C and pH of 90.
224 citations
TL;DR: In this paper, a theory for electrode kinetics of surface-immobilized monolayers in cyclic voltammetry is developed based on the Marcus free energy-rate relation.
Abstract: : Theory for electrode kinetics of surface-immobilized monolayers in cyclic voltammetry is developed based on the Marcus free energy-rate relation. Numerical calculations show that when the applied over-potential exceeds ca. 30% of the reorganizational energy of the electrode reaction, voltammetry predicted from Marcus theory differs from that based on classical Butler-Volmer kinetics with regard to waveshape, peak currents and their dependence on potential sweep rate, and variation of peak potential with potential sweep rate. Estimates of the standard rate constant, K deg, can be made from E peak data without exact knowledge of reorganizational energies. Examples are given of evaluating K deg for monolayers of ferrocene alkanethiols chemisorbed on Au(111) electrodes, when the monolayers are highly ordered, and kinetically monodisperse, and when they are somewhat disordered, and kinetically disperse on bulk gold electrodes at room and 150K temperatures. Electrode kinetics, Monolayers, Cyclic voltammetry, Marcus equation.
220 citations
TL;DR: In this article, the presence of oxygen (gaseous O2 or O atoms via N2O decomposition) greatly enhanced the NOx conversion to N2, but its conversion decreased with increasing temperature at T > 500°C with a formation of small amount of O2.
TL;DR: In this article, the second-order rate constants for a wide range of inorganic and organic compounds, including a comprehensive series of phenols, have been determined using conventional batch-type and stopped-flow methods.
TL;DR: In this article, an experimental method is described which enables the rate constants of neutral-neutral gas phase reactions to be measured at ultralow temperatures by applying the pulsed laser photolysis (PLP), laser-induced fluorescence (LIF) technique of studying the kinetics of free radical reactions.
Abstract: An entirely new experimental method is described which enables the rate constants of neutral–neutral gas‐phase reactions to be measured at ultralow temperatures. The measurements are made by applying the pulsed laser photolysis (PLP), laser‐induced fluorescence (LIF) technique of studying the kinetics of free radical reactions in the ultracold environment provided by the gas flow in a Cinetique de Reaction en Ecoulement Supersonique Uniforme (CRESU) apparatus. The experimental method is described in some detail and its application and limitations are discussed. Results are reported for the reactions of CN radicals with O2 and NH3. For reaction (1) between CN and O2 data are reported for the temperature range T=13–295 K and the rate constants are well‐matched by the expression k1(T)=(2.49±0.17)×10−11 (T/298)(−0.63±0.04) cm3 molecule−1 s−1. For reaction (2) between CN and NH3, rate constants in the temperature range T=25–295 K fit the expression k2(T)=(2.77±0.67)×10−11 (T/298)(−1.14±0.15) cm3 molecule−1 s−1...
TL;DR: The rate constants of the forward reaction for the reversible hydrolysis of Cl 2 (aq) at μ=0.50 M increase from 1.9 s -1 at 0.0 o C to 30.5 s −1 at 30.0 O C as discussed by the authors.
Abstract: Rate constants of the forward reaction for the reversible hydrolysis of Cl 2 (aq) at μ=0.50 M increase from 1.9 s -1 at 0.0 o C to 30.5 s -1 at 30.0 o C. The activation parameters at 15.0 o C are ΔH = 63±3 kJ mol -1 and ΔS = -8∼4 J mol -1 K -1 for the forward reaction and ΔH = 27±1 kJ mol -1 and ΔS=-71±9 J mol -1 K -1 for the reverse reaction
TL;DR: In this article, the formation of nickel hydrides in the potential region of the her was previously analyzed by cyclic voltammetry and the electrode surface was pre-conditioned to minimize the effect of the hydride in the experimental response of the system.
TL;DR: Toward an Understanding of the High Enantioselectivity in the Osmium-Catalyzed Asymmetric Dihydroxylation (AD), Kinetics.
Abstract: Toward an Understanding of the High Enantioselectivity in the Osmium-Catalyzed Asymmetric Dihydroxylation (AD). 1. Kinetics
TL;DR: In this article, the authors employed nickel stripe patten electrodes prepared on the surface of 8m/o Y2O3-doped ZrO2 (YSZ), which have well defined length and morphology of the gas/nickel/YSZ triplet phase boundary (TPB).
TL;DR: A zero-order dependence on the CCl 4 concentration supports a surface-controlled reaction mechanism wherein the rate of reaction depends on the absorbed CCl 2 concentration.
Abstract: The reactivity of CCl 4 with pyrite was investigated by measuring the CCl 4 transformation rates and products under aerobic and anaerobic conditions. Under all reaction conditions, >90% of the CCl 4 was transformed within 12-36 days in the presence of 1.2-1.4 m 2 /L pyrite at 25 o C. A zero-order dependence on the CCl 4 concentration supports a surface-controlled reaction mechanism wherein the rate of reaction depends on the absorbed CCl 4 concentration
TL;DR: In this article, the second-order rate constants of a-CH deprotonation and a-desilylation reactions with bases and silophiles were determined for a series of tertiary amine cation radicals.
Abstract: Time-resolved laser spectroscopy has been used to generate and characterize a series of tertiary amine cation radicals and to determine the rates of their a-CH deprotonation and a-desilylation reactions with bases and silophiles. Laser excitation (308 nm) of a 60:40 Me0H:MeCN solution of PhNMe2 (DMA) and 1,4-dicyanobenzene (DCB) promotes SET-induced formation of the DMA cation radical (460 nm) and DCB anion radical (340 nm), which undergo decay by back electron transfer at nearly equal rates and with respective second-order rate constants of 1.1 X 1Olo and 1.3 X 1010 M-1 s-l (25 "C). The decay rate is lowered (ca. 4-fold) by the inclusion of salts (ca. 0.1 M) such as nBu4NC104, LiC104, nBu4NC1, nBu4NBF4, and nBu4N03SCF3 in MeOH-MeCN and by changing the solvent from MeCN to MeOH and to EtOH. The cation radical of PhNMeCHz(TMS) (480 nm) and the simultaneously generated DCB anion radical undergo second order decay in MeCN with respective rate constants of 1.2 X 10'0 and 9.9 X lo9 M-1 s-1 (25 "C). The silylamine cation radical decay rate was found to be governed by the concentration of silophiles (MeOH, H20, and nBu4NF) in MeCN solutions. The observations are consistent with a silophile-induced desilylation The rate of DMA cation radical decay is a function of base concentration. Both nBu4NOAc and nBu4N02CCF3 react with the DMA cation radical (in 60:40 Me0H:MeCN containing 0.1 M nBu4NC104) with second-order rate constants for a-CH deprotonation of 3.1 X 105 and 8 X lo4 M-l s-l (25 "C), respectively. Measurements with PhN(CD& and nBu4NOAc gave a kH/kD for a-CH deprotonation of 3.6 (60:40 MeOH:MeCN, 25 "C). Para-substituents have a pronounced effect on the rate of a-CH deprotonation by nBu4NOAc; second-order rate constants of 2.3 X lo4, 1.1 X lo5, and 2.5 X lo6 M-1 s-l were determined for thep-OMeC6H4NMez, p-MeC&NMez and p-CF3C6H4NMe2 cation radicals. Studies with Ph2NMe demonstrated that its cation radical (645 nm) can be generated by SET to DCB and that its decay through a-CH deprotonation by nBu4NOAc has a second-order rate constant of 9.5 X lo$ M-1 s-1 and a kH/kD value of 2.8 (25:75 MeOH:MeCN, 25 "C). Finally, the effects of a-substituents on the rates of nBu4NOAc-induced a-CH deprotonation of tertiary amine cation radicals were evaluated by use of the amines Ph2NCHRlR2. The second-order rate constants (25 "C, 25:75 Me0H:MeCN) are 2.3 X lo$ (RI = Me, RZ = H), 1.7 X 105 (RI = process with second-order rate constants of 8.9 X lo5 (MeOH), 1.27 X lo6 (HzO), and 3.1 X lo9 M-* s-l ( n Bu~NF) . R2 = Me), 3.2 X 106 (R1 = Ph, R2 = H), 2.6 X lo6 (R1 = CH=CH2, R2 = H), and 7.0 X lo7 M-1 s-l (RI = m C H , Rz = H).
TL;DR: Experiments designed to characterize several partial reactions of the Na,K-ATPase and to demonstrate that a model can be defined that reproduces most of the transport features of the pump with a single set of kientic parameters indicate further that the conformational transition E1P-->P-E2 is the rate limiting process of theNa+ translocation.
Abstract: Experiments were designed to characterize several partial reactions of the Na,K-ATPase and to demonstrate that a model can be defined that reproduces most of the transport features of the pump with a single set of kientic parameters. We used the fluorescence label 5-iodoacetamidofluorescein, which is thought to be sensitive to conformational changes, and the styryl dye RH 421, which can be applied to detect ion-binding and -release reactions. In addition transient electric currents were measured, which are associated mainly with the E1-->E2 conformational transition. Numerical simulations were performed on the basis of a reaction model, that has been developed from the Post-Albers cycle. Analysis of the experimental data allows the determination of several rate constants of the pump cycle. Our conclusions may be summarized as follows: (a) binding of one Na+ ion at the cytoplasmic face is electrogenic. This Na+ ion is specifically bound to a neutral binding site with an affinity of 8 mM in the presence of 10 mM Mg2+. In the absence of divalent cations, the intrinsic binding affinity was found to be 0.7 mM. (b) The analysis of fluorescence experiments with the cardiotonic steroid strophanthidin indicates that the 5-iodoacetamidofluorescein label monitors the conformational transition (Na3)E1-P-->P-E2(Na2), which is accompanied by the release of one Na+ ion. 5-IAF does not respond to the release of the subsequent two Na+ ions, which can be monitored by the RH 421 dye. These experiments indicate further that the conformational transition E1P-->P-E2 is the rate limiting process of the Na+ translocation. The corresponding rate constant was determined to be 22 s-1 at 20 degrees C. From competition experiments with cardiotonic steroids, we estimated that the remaining 2 Na+ ions are released subsequently with a rate constant of at least 5,000 s-1 from their negatively charged binding sites. (c) Comparing the fluorescence experiments with electric current transients, which were performed at various Na concentrations in the absence and presence of strophanthidin, we found that the transition (Na3).E1-P-->P-E2.(Na2) is the major charge translocating step in the reaction sequence Na3.E1-->(Na3).E1-P-->P-E2.(Na2)-->P-E2. The subsequent release of 2 Na+ ions contributed less than 25% to the total electric current transient. (d) The well known antagonism between cardiotonic steroids and K+ binding can be explained by a kinetic model. A quantitative description has been obtained under the assumption that these inhibitors bind only to the states P-E2(Na2) and P-E2(K2).(ABSTRACT TRUNCATED AT 400 WORDS)
TL;DR: In this article, the cumulative reaction probability for the H2+OH→H+H2O reaction in its full (six) dimensionality for total angular momentum J=0 was calculated.
Abstract: The cumulative reaction probability is calculated for the H2+OH→H+H2O reaction in its full (six) dimensionality for total angular momentum J=0. The calculation, which should give the (numerically) exact result for the assumed potential energy surface, yields the cumulative reaction probability directly, without having to solve the complete state‐to‐state reactive scattering problem. Higher angular momenta (J≳0) were taken into account approximately to obtain the thermal rate constant k(T) over the range 300°
TL;DR: The reaction of H atoms with solid CO molecules was studied in this article, where the solid CO film deposited on the cold head of the cryocooler was reacted with H atoms by spraying the plasma-activated hydrogen gas over the surface of the sample.
TL;DR: In this paper, carboxylic acid-terminated polystyrene (PS-COOH) and epoxy-tolerant poly(methyl methacrylate) (PMMA-E) were synthesized by anionic polymerization.
Abstract: Monodispese carboxylic acid-terminated polystyrene (PS-COOH) and epoxy-terminated poly(methyl methacrylate) (PMMA-E) heve been synthesized by anionic polymerization. To compare with coupling in homogeneous blends, PS-E was also synthesized. The amount of block copolymer formed by coupling of PS-COOH with PMMA-E to form the ester was measured by SEC. PMMA-E was melt mixed with PS-COOH with a maximum shear rate of 20 s -1 . PMMA-E particle diameter was 0.6 μm and stable after 5 min of mixing at 180°C. The block copolymer content increased to 2% at 20 min. The reaction is slow enough that the chain ends can sample the interface many times before reacting
TL;DR: The rate constants for refolding and unfolding of the P22 Arc repressor dimer have been determined by stop-flow fluorescence experiments and imply that a free Arc molecule must unfold and refold hundreds of times per generation in the cell.
Abstract: The rate constants for refolding and unfolding of the P22 Arc repressor dimer have been determined by stop-flow fluorescence experiments. Under most conditions, refolding is described well as a two-state reaction with a bimolecular rate-limiting step (kf approximately 10(7) M-1 s-1). A unimolecular step appears to become co-rate limiting at high protein concentrations. The urea dependence of the refolding reaction suggests that about 75% of the total burial of hydrophobic surface occurs between the unfolded state and the transition state for folding. Hydrophobic interactions are also evidenced by the temperature dependence of the refolding reaction; the rate increases with temperature and Arrhenius plots are curved, as expected for a reaction that proceeds with a significant heat capacity change. The refolding of Arc also proceeds more rapidly as the salt concentration is raised, presumably because repulsive interactions between monomers are screened. At a protein concentration of 10 microM, the apparent rate constant for refolding of the Arc dimer is approximately 100 s-1, as fast as the refolding of many monomeric proteins. The rate constant for unfolding is approximately 0.1 s-1, corresponding to a half-life of less than 10 s for the folded Arc dimer. This rate of unfolding is very fast in comparison to that of other characterized proteins and implies that a free Arc molecule must unfold and refold hundreds of times per generation in the cell.
TL;DR: In this paper, the electrocatalytic properties of pure [beta]-lead dioxide electrodes and bismuth-doped lead dioxide electrodes were compared for oxidation of dimethylsulfoxide (DMSO), phenol, and benzene in acidic media.
Abstract: The electrocatalytic activities are compared for pure [beta]-lead dioxide electrodes and bismuth-doped lead dioxide electrodes for oxidation of dimethylsulfoxide (DMSO), phenol, and benzene in acidic media. A correlation is observed between the bismuth content of Bi-PbO[sub 2] films on Au disk electrodes and the apparent heterogeneous rate constant for conversion of DMSO to DMSO[sub 2]. The oxidation of phenol occurs at a transport-limited rate for both the PbO[sub 2] and Bi-PbO[sub 2] film electrodes; however, the Bi-PbO[sub 2] electrodes exhibit significantly greater stability against surface fouling by polymeric films. This is a direct benefit of the greater electrocatalytic activity of Bi-PbO[sub 2] electrodes for oxidation of phenol beyond the phenolic radical. A ring-disk experiment for benzene oxidation at a Bi-PbO[sub 2] disk demonstrates that o-benzoquinone and p-benzoquinone are produced at the disk in the ratio ca. 0.25. However, the total ring current for reduction of these products from the disk reaction is significantly below that predicted from the disk current, and it must be concluded that a significant amount of electroinactive species is produced at the disk. This production probably corresponds to a ring-opening reaction with production of one or more organic acids. The product solution from exhaustive electrolysis ofmore » benzene contains p-benzoquinone as the only electroactive product; hence, the o-benzoquinone is concluded to undergo further oxidation at the Bi-PbO[sub 2] electrode to one or more electroinactive products. This is part of a larger program motivated by the search for new catalytic anode materials for achieving electrolytic production of useful compounds by oxidation of toxic chemical wastes. Also of interest is the total oxidative degradation of toxic organic wastes to their corresponding elemental oxides (CO[sub 2], etc.), a technology referred to as electrochemical incineration.« less
TL;DR: In this article, the authors introduce a model whereby the rate of an electron-transfer reaction can be modulated by an intervening proton-transmission reaction, and they find that the observed rate constant may have a significant isotope effect upon replacing the protons by deuterons and may also exhibit a temperature dependence above that expected for a conventional electron transfer reaction.
Abstract: We introduce a model whereby the rate of an electron-transfer reaction can be modulated by an intervening proton-transfer reaction. The mechanism for this modulation is the assumed dependence of the electronic matrix element, which enables the electron to transfer from donor to acceptor, on the configuration of the proton(s) that are potentially undergoing a proton-transfer reaction. As the proton configuration changes, so does the electronic matrix element and, consequently, the electron-transfer rate depends on the proton configuration and dynamics, for this proton-coupled electron-transfer reaction, we find that the observed rate constant may have a significant isotope effect upon replacing the protons by deuterons and may also exhibit a temperature dependence above that expected for a conventional electron-transfer reaction
TL;DR: In this article, the activated complex for the Diels-alder reactions of 5-substituted 1,4-naphthoquinones with cyclopentadiene is more polar in water than in other solvents.
Abstract: In order to check whether the activated complex for the Diels-Alder reactions of 5-substituted 1,4-naphthoquinones with cyclopentadiene is more polar in water than in other solvents, we have determined the substituent effects in seven different solvents. The substituent effects gradually decrease with increasing rate of the reaction in a specific solvent, indicating that the charge separation in the activated complex in water is not much different from that in the other solvents. We also compared solvent effects on the Diels-Alder reaction of methyl vinyl ketone with cyclopentadiene with effects on the corresponding reaction of methyl vinyl sulfone. The medium effects were separated into effects on initial state and activated complex. The destabilization of the initial state by water and the stabilization of the activated complex by 2,2,2-trifluoroethanol were less pronounced for the sulfone than for the ketone. These results further underline the importance of enforced hydrophobic interactions and changes in hydrogen bonding during the activation process in explaining the acceleration of Diels-Alder reactions in water.
TL;DR: P pH dependence studies suggest that the formation of compounds I and II is controlled by a residue in the enzyme with a pKa of about 4.0, but this does not consider this a dominant pathway because it fails to account for the fast spontaneous reduction of compound I to compound II.
Abstract: The conversion of myeloperoxidase to compounds I and II in the presence of H2O2 has been reinvestigated in order to explain the abnormal stoichiometry of compound I formation and the fast spontaneous decay of compound I to compound II. Rapid-scan studies show that at least a 20-fold excess of H2O2 is required to obtain a good spectrum of relatively pure compound I; a further increase in H2O2 concentration causes compound I to be reduced to compound II, which is a very stable intermediate. Compound I formation is reversible, with an apparent second-order forward rate constant of (1.8 +/- 0.1) x 10(7) M-1 s-1 and a reverse rate constant of 58 +/- 4 s-1, giving a constant of 3.2 microM for the dissociation of compound I to native enzyme and H2O. This reversibility is one factor that can explain the large excess of H2O2 required to form compound I. The apparent second-order rate constant for compound II formation from compound I and H2O2 is (8.2 +/- 0.2) x 10(4) M-1 s-1. We confirm pH dependence studies, which suggest that the formation of compounds I and II is controlled by a residue in the enzyme with a pKa of about 4.0. Excess H2O2 is also converted to O2 via catalase activity of the enzyme. However, we do not consider this a dominant pathway because it fails to account for the fast spontaneous reduction of compound I to compound II. The time courses for both the decay of compound I and the formation of compound II are biphasic.(ABSTRACT TRUNCATED AT 250 WORDS)
TL;DR: In this paper, chemical relaxation experiments were conducted on sintered samples of calcium-doped lanthanum chromites by abruptly changing the oxygen partial pressure in the atmosphere and following the time change of conductivity.
Abstract: Chemical relaxation experiments were conducted on sintered samples of calcium‐doped lanthanum chromites by abruptly changing the oxygen partial pressure in the atmosphere and following the time change of conductivity. The re‐equilibration kinetics was analyzed by fitting the relaxation data to the solutions of Fick's second law for appropriate boundary conditions. The diffusion equation ignoring the effect of surface reaction failed to describe the transient behavior especially for the initial stage, while that taking the surface effect into account gave a satisfactory interpretation of the overall relaxation process and allowed a precise determination of the two kinetic parameters: oxygen chemical diffusion coefficient and surface reaction rate constant. The chemical diffusion coefficients increased with a decrease of the oxygen partial pressure due to the corresponding change in the concentration of the moving species. The activation energy was similar to that of oxygen vacancy diffusion coefficients in other monocrystalline perovskites, suggesting that the measured diffusion coefficients were attributable to lattice diffusion. The surface reaction rate constant increased with a decrease of the oxygen partial pressure similarly to the reported oxygen nonstoichiometry, which implies that the presence of oxygen vacancies plays an important role in the surface reaction kinetics.