18O Isotope Exchange Research Articles

Oxidation activity and 18O-isotope exchange behavior of nickel oxide-stabilized cubic zirconia

A series of NiO–ZrO 2 samples with 2 to 40 mol% of NiO were prepared using a sol–gel synthesis technique and calcined at 873 K. XRD characterization of the samples revealed the stabilization of a cubic zirconia- (fluorite) phase containing nickel oxide up to 20 mol%. Bulk NiO characteristics were observed above 20 mol% loading of NiO. The linear decrease in lattice parameter up to 20 mol% of NiO indicates the probable incorporation of Ni 2+ into the lattice position of Zr 4+ ions. The NiO–ZrO 2 sample with 20% NiO retained its cubic phase even after prolonged heating at 1273 K, indicating 20 mol% as an optimum content of NiO for a thermally stable cubic zirconia phase. The BET surface areas of these samples were in the range of 40 to 70 m 2 g −1. XPS spectra along with XRD data indicated that at low (5 mol%) concentrations of NiO, Ni 2+ enters ZrO 2 lattice substitutionally creating oxygen vacancies. TPR of NiO–ZrO 2 showed the reduction of nickel oxide at 533 and 633 K, indicating nickel in two different environments, at a substitutional position and at a surface/interstitial position. 18O-isotope exchange studies of these samples showed a partial heterogeneous exchange and the T onset was found to be lowest for the sample containing 20 mol% NiO. The activity for CH 4 and CO oxidation was investigated by 18O-isotope exchange as well as catalytic studies in complete oxidation of CH 4 and CO. The activity for CH 4 oxidation was highest for the NiO–ZrO 2 sample with 20 mol% NiO. CH 4 and CO oxidation with 18O isotope over NiO–ZrO 2 catalysts showed the formation of CO 2 with 16O (amu 44), suggesting that the bulk oxygen is acting as an active species for methane oxidation. The structure of NiO–ZrO 2 samples with varying NiO content and its correlation with catalytic activity mechanism is discussed.

ATP Binding Modulates the Nucleic Acid Affinity of Hepatitis C Virus Helicase

The helicase of hepatitis C virus (HCV) unwinds nucleic acid using the energy of ATP hydrolysis. The ATPase cycle is believed to induce protein conformational changes to drive helicase translocation along the length of the nucleic acid. We have investigated the energetics of nucleic acid binding by HCV helicase to understand how the nucleotide ligation state of the helicase dictates the conformation of its nucleic acid binding site. Because most of the nucleotide ligation states of the helicase are transient due to rapid ATP hydrolysis, several compounds were analyzed to find an efficient unhydrolyzable ATP analog. We found that the beta-gamma methylene/amine analogs of ATP, ATPgammaS, or [AlF4]ADP were not effective in inhibiting the ATPase activity of HCV helicase. On the other hand, [BeF3]ADP was found to be a potent inhibitor of the ATPase activity, and it binds tightly to HCV helicase with a 1:1 stoichiometry. Equilibrium binding studies showed that HCV helicase binds single-stranded nucleic acid with a high affinity in the absence of ATP or in the presence of ADP. Upon binding to the ATP analog, a 100-fold reduction in affinity for ssDNA was observed. The reduction in affinity was also observed in duplex DNA with 3' single-stranded tail and in RNA but not in duplex DNA. The results of this study indicate that the nucleic acid binding site of HCV helicase is allosterically modulated by the ATPase reaction. The binding energy of ATP is used to bring HCV helicase out of a tightly bound state to facilitate translocation, whereas ATP hydrolysis and product release steps promote tight rebinding of the helicase to the nucleic acid. On the basis of these results we propose a Brownian motor model for unidirectional translocation of HCV helicase along the nucleic acid length.

Reactivity of oxygen of complex cobaltates La 1− xSr xCoO 3− δ and LaSrCoO 4

Complex oxides La 1− x Sr x CoO 3− δ ( x=0.0, 0.3, 0.5) with perovskite-like structure and LaSrCoO 4 with layered K 2NiF 4-type structure have been prepared using freeze-drying technique. The oxygen content of the single-phase samples was determined by iodometric titration. The oxygen mobility and catalytic activity of all cobaltate samples in the reaction of CH 4 oxidation were studied under dynamic-thermal 18O-isotope exchange conditions. Catalytic reactions of CH 4 and CO oxidation have also been performed under steady flow conditions. The correlations between oxygen mobility and structural features of the catalyst and its initial oxygen deficiency has been established. La 0.5Sr 0.5CoO 2.92 and La 0.7Sr 0.3CoO 2.96 with deficiency in oxygen sublattice were more active in isotope exchange and catalysis, while LaCoO 3 and LaSrCoO 4 with the stoichiometric oxygen content demonstrated lower activity. The mobility of oxygen in layered LaSrCoO 4 is significantly higher than in LaCoO 3. Analysis of the methane oxidation using 18O 18O allowed to clarify the steps of the catalytic process. It was found also that methane is oxidized by the lattice oxygen of the catalyst.

Comparison of 18O isotopic exchange after and during the interaction of N2O, NO, and NO2 with Fe ions in ferrierites

Nitrous oxide deposits its oxygens on Fe-ferrierites at 200–250 °C in contrast with nitric oxide and nitrogen dioxide. This oxygen is readily exchangeable for 18O2 at room temperature and the reaction proceeds via a single-step exchange mechanism. All three nitrogen oxides in a mixture with 18O labeled dioxygen undergo isotopic exchange (IE) at 200–250 °C, N2O via the same single-step mechanism, while NO and NO2 react via a multiple-step mechanism. Zeolitic oxygens participate in IE above 250 °C during temperature-programmed desorption of surface species formed in the reaction of nitrogen oxides with 18O2.

Isotopic Studies of Sn-Cr Binary Oxide Catalysts for Methane Total Oxidation

A series of Sn-Cr binary oxide catalysts were prepared by a co-current co-precipitation method and tested for methane total oxidation. The binary oxide catalysts have much higher surface areas and catalytic activities for methane oxidation than pure SnO2. CrO x /SnO2 with a Cr/Sn atomic ratio of 3:7 displays the highest activity. Selected samples were subjected to temperature-programmed 18O isotope-exchange measurements. Both complete and partial heteromolecular 18O isotope exchange, as well as oxygen release, was observed for all catalysts. Reaction between CH4 and 18O2 under static conditions was performed to investigate the reaction mechanism and it was found that the total oxidation of methane over Sn-Cr binary oxide catalysts occurs via a redox cycle with the chromium ion in a high oxidation state as the active center. Oxygen mobility of the catalyst plays an important role in the total oxidation of methane, but too high a mobility leads to very high oxygen release and a reduction of the surface reoxidability. This causes a decrease in the catalytic oxidation activity.

Kinetic characterization of wild-type and proton transfer-impaired variants of β-carbonic anhydrase from Arabidopsis thaliana

We have cloned and overexpressed a truncated, recombinant form of β-carbonic anhydrase from Arabidopsis thaliana. The wild-type enzyme and two site-directed variants, H216N and Y212F, have been kinetically characterized both at steady state by stopped-flow spectrophotometry and at chemical equilibrium by 18O isotope exchange methods. The wild-type enzyme has a maximal k cat for CO 2 hydration of 320 ms −1 and is rate limited by proton transfer involving two residues with apparent p K a values of 6.0 and 8.7. The mutant enzyme H216N has a maximal k cat at high pH that is 43% that of wild type, but is only 5% that of wild type at pH 7.0. 18O exchange studies reveal that the effect of the mutations H216N or Y212F is primarily on proton transfer steps in the catalytic mechanism and not in the rate of CO 2–HCO 3 − exchange. These results suggest that residues His-216 and Tyr-212 are both important for efficient proton transfer in A. thaliana carbonic anhydrase.

Stable Isotopes and Carbon Cycle Processes in Forests and Grasslands

Abstract: Scaling and partitioning are frequently two difficult challenges facing ecology today. With regard to ecosystem carbon balance studies, ecologists and atmospheric scientists are often interested in asking how fluxes of carbon dioxide scale across the landscape, region and continent. Yet at the same time, physiological ecologists and ecosystem ecologists are interested in dissecting the net ecosystem CO2 exchange between the biosphere and the atmosphere to achieve a better understanding of the balance between photosynthesis and respiration within a forest. In both of these multiple‐scale ecological questions, stable isotope analyses of carbon dioxide can play a central role in influencing our understanding of the extent to which terrestrial ecosystems are carbon sinks. In this synthesis, we review the theory and present field evidence to address isotopic scaling of CO2 fluxes. We first show that the 13C isotopic signal which ecosystems impart to the atmosphere does not remain constant over time at either temporal or spatial scales. The relative balances of different biological activities and plant responses to stress result in dynamic changes in the 13C isotopic exchange between the biosphere and atmosphere, with both seasonal and stand‐age factors playing major roles influencing the 13C biosphere‐atmosphere exchange. We then examine how stable isotopes are used to partition net ecosystem exchange fluxes in order to calculate shifts in the balance of photosynthesis and respiration. Lastly, we explore how fundamental differences in the 18O isotopic gas exchange of forest and grassland ecosystems can be used to further partition terrestrial fluxes.

18O-isotope exchange behavior and oxidation activity of La1-xSrxMnO3+δ

Different preparation routes derived both from the usual high-temperature solid state synthesis and the sol-gel process were applied for the synthesis of crystalline La1-xSrxMnO3+δ phases with 0≤x≤ 0.2. They were investigated by means of temperature programmed isotope exchange (TPIE) under various conditions. The 18O isotope exchange experiments yielded information on the dependence of the oxygen mobility on temperature and the Mn4+/Mn3+ratio. Both are related to the defect structure of the solid. TPIE was applied for studying the interaction between 18O2 and CH4 under static conditions and was compared with results obtained from catalytic investigations, i.e. the behavior of La1-xSrxMnO3+δ (0≤x≤0.2) in both CO and CH4 oxidation with normal oxygen 16O2 under steady flow conditions.

Decomposition of Nitrous Oxide Over Fe-Ferrierites. Effect of Deposited Oxygen on the Framework Oxygens Studied by 18O Isotopic Exchange

About 50-75% of oxygen captured during decomposition of N2O at 200 °C over Fe-FER (Fe/Al 0.03-0.6) was exchanged by 18O at room temperature. Complete desorption of captured oxygen containing mostly 16O isotope was reached at higher temperature. The 18O deficiency was rationalized by assuming labilization of the framework oxygen in Fe-FER.

2-Hydroxy-6-keto-nona-2,4-diene 1,9-dioic acid 5,6-hydrolase: Evidence from 18O isotope exchange for gem-diol intermediate

The mechanism-based inactivation and subsequent identification of the nucleophilic residue using mass spectrometry have been successfully applied and used to identify the active-site nucleophile in numerous β-glycosidases, as illustrated using C. fimi exoglycanase. Evidence for a covalent glycosyl-enzyme intermediate has come from X-ray crystallographic analysis of trapped complexes, the first being that of the trapped fluoroglycosyl-enzyme intermediate of Cex. The crystal structure of the trapped fluorocellobiosyl-enzyme complex for Cex has provided useful insights into catalysis and the roles of specific residues at the active site. In addition, information about the conformation of the natural sugar in the covalently bound state and the interactions at the active site was obtained using a mutant form of Cex.

On the accurate measurement of oxygen self-diffusivities and surface exchange coefficients in oxides via SIMS depth profiling

Oxygen diffusivity in oxides is usually determined via gas/solid exchange experiments in an atmosphere enriched as to the concentration of the rare stable isotope 18O. The kinetics of the 16O and 18O isotope exchange at the surface is usually described by the surface exchange coefficient, K. The analytical solution of the diffusion problem suggests the definition of a characteristic time constant, τ≡ D/ K 2. This time constant determines the duration that is necessary to reach equilibrium between the 18O gas concentration and the 18O concentration at the surface of the solid. The simultaneous determination of D and K by secondary ion mass spectrometry (SIMS) depth profiling works well in a large parameter range of D and K. However, at low temperatures, the simultaneous determination of D and K may be subject to severe errors. To overcome this problem, the possibility to determine D and K separately will be discussed.

Oxidation Activity and 18O-Isotope Exchange Behavior of Cu-Stabilized Cubic Zirconia

A series of Cu–ZrO2 samples with varying concentrations of Cu from 1 to 33 mol% have been prepared by sol–gel technique and calcined at 873 K. XRD characterization of the samples with a copper content of 2 to 20 mol% reveals the stabilization of zirconia into cubic (fluorite) phase. The Cu–ZrO2 samples with higher Cu content (>20 mol%) revealed the presence of bulk CuO. A linear decrease in lattice parameter with increase in Cu content up to 20 mol% indicates the possible incorporation of Cu2+ in the lattice position of Zr4+ ions. XPS data of these samples provide further evidence for the incorporation of Cu2+ ions in the ZrO2 lattice up to 5 mol%. At higher concentrations of Cu, about half of the input of Cu goes into the lattice and the remaining stays as extra lattice Cu, possibly on the surface or subsurface layer of ZrO2. The BET surface area of these samples was in the range of 2 to 8 m2 g−1. The activity of these samples in CH4 and CO oxidation was investigated by 18O-isotope exchange as well as by catalytic reaction studies in complete oxidation of CH4 and CO. The Cu–ZrO2 with 20 mol% Cu was found to be the most active sample in the series, which has the maximum amount of copper in the substitutional position. For comparison, yttrium-stabilized zirconia samples with and without Cu was also prepared following the same procedure. Yttrium-stabilized zirconia without Cu was almost inactive in complete methane oxidation, whereas Cu-containing sample was more active. This confirms that the presence of Cu species in substitutional positions along with oxygen vacancies in zirconia lattice are substantially responsible for the catalytic activity in CH4 and CO oxidation as well as in complete heterogeneous 18O exchange processes. The light-off temperature for 50% conversion of CH4 (T50) decreases with an increase in Cu content up to 20 mol% and matches well with the results of 18O exchange measurements. The shapes of the curves of T50 and Texchange follow a similar trend indicating that both CH4 oxidation and 18O exchange processes occur via a completely heterogeneous mechanism.

Evidence for the binding of two substrate water molecules in the S2 state of photosystem II.

We present the first evidence to show that both substrate water molecules are bound to the catalytic site in the S2 state of photosystem II (PSII). Earlier 18O isotope exchange measurements between added H218O and the photogenerated O2 products (W. Hillier & T. Wydrzynski, 2000, Biochemistry 39, 4399-4405) revealed that at least one substrate water molecule is bound in the S2 state. However, due to experimental limitations, the results left open the possibility that the second substrate water molecule only enters the catalytic site after the S2 to S3 state transition. Here we show for PSII samples depleted of the 16 and 23 kDa extrinsic proteins that both substrate water molecules are bound in the S2 state. Rapid 18O exchange measurements of the protein-depleted samples at m/e = 34 (which measures the mixed labelled 16O18O product) reveal two resolvable exchange phases in the S2 state, representing the independent exchange processes by the two bound water molecules. The exchange rates measured were 34k1 = 117 s-1 and 34k2 = 2.9 s-1 at 10 oC. The addition of 15 mM CaCl2 or MgCl2, resulted in a slowing in both phases of exchange, with rates of 34k1 = 90 s-1 and 34k2 = 1.8 s-1 or 34k1 = 74 s-1 and 34k2 = 1.6 s-1, respectively. The addition of CaCl2 increased the steady state yield of O2 evolution to 90% of the untreated control whereas MgCl2 had little effect on the residual activity, which was 20% of the control. This result indicates that under these conditions, substrate water interactions at the catalytic site are influenced by the local ionic environment. Implication of these results will be discussed.

Tracer diffusion and chemical diffusion of oxygen in acceptor doped SrTiO3

Oxygen tracer and oxygen chemical diffusion coefficients have been determined for single crystalline SrTiO3 under defined temperature (f), oxygen partial pressure (p(O2)), and acceptor (Fe) dopant concentration (mFe) conditions. Oxygen tracer diffusion results were obtained (873K≤T≤1173K,p(O2)=105Pa) by means of 18O isotope exchange with subsequent analysis of the 18O in-diffusion profiles by secondary ion mass spectrometry (SIMS). In the case of chemical diffusion an in situ and spatially resolved, optical relaxation technique was applied (673K≤T≤973K,10Pa≤p(O2)≤105Pa). The dopant concentration in both experiments was varied between 4.3×1018cm−3≤mFe≤4.9×1019cm−3. The evaluation of (ex situ) tracer and (in situ) concentration profiles are shown to be in excellent agreement with defect chemical calculations. In contrast to the tracer diffusion coefficients (D∗) the chemical diffusion coefficients (Dδ) are sensitive to ionic–electronic (short-range) defect interactions (internal buffer effects influence the thermodynamic factor) caused by valence changes of the redox-active Fe-doping.

The Reactivity of Manganites in 18O Isotope Exchange Reactions

The effect of the structural properties and the oxidation state of Mn on the 18O isotope exchange behaviour of ternary manganites (La1−xSrxMnO3, La0.5Sr1.5MnO4 and SrMnO3) has been studied. All types of 18O isotope exchange homomolecular, partially and completely heteromolecular) take place on the very active manganites with perovskite (LaMnO3 and La0.7Sr0.3MnO3) and perovskite-like (SrMnO3) structure, but not on the less active K2NiF4-structure (La0.5Sr1.5MnO4). The highest 18O exchange activity is observed for La0.7Sr0.3MnO3, for which the completely heteromolecular 18O exchange starts to occur at 520 K, already, a Ton which is typical for excellent redox catalysts. The influence of the structural properties on the 18O exchange and oxygen diffusion behaviour of the manganites is much more pronounced than that of the Mn3+/Mn4+ ratio. The different reduction behaviour of the manganites with perovskite and K2NiF4-structure can be explained by means of the bond-valence model.

The Cl2O2+ Cation: Preparation and Structural Investigation of Cl2O2+SbF6- and Cl2O2+Sb2F11-

Cl2 reacts with O2+SbF6- in anhydrous HF, forming subsequently a violet, a blue, and a yellow solution. Crystals from the violet solution are identified as Cl2O2+SbF6- or Cl2O2+Sb2F11- by means of single-crystal structural determinations, Raman spectroscopy, magnetic and EPR measurements, and ab initio calculations. The cation has a planar trapezoid structure with Cl−Cl distances of 190.9(1)−191.6(5) pm, O−O distances of 118.5(11)−120.7(5) pm, and Cl−O distances of 240.8(4)−242.8(4) pm. The bonding between the two parts of the cation may be described as a π*−π* interaction, resulting in a charge transfer of about 0.6 electrons with respect to isolated Cl2+ and O2 fragments. The bonding of O2 to Cl2+ is reversible, as is shown by 18O isotope exchange. The nature of the blue solution remains unknown. Both deeply colored compounds turn into yellow Cl3+ if warmed to room temperature, especially in the presence of excess Cl2. The Cl3+ cation is symmetrical and bent with Cl−Cl = 197.23−199.44 pm and Cl−Cl−Cl = 104.51(10)−105.62(7)°.

Effects induced by O-16-O-18 oxygen isotope exchange in manganite ceramics and films

Unique sensitivity of electrical resistivity to the 16O→18O isotope exchange was observed for (La1−yPry)0.7Ca0.3MnO3 ceramics and epitaxial thin films. At y=0.7, substitution of 16O by 18O in ceramic samples results in the reversible transition from a metal-like to insulating state. In thin films, this metal–insulator transition occurs at y=0.5 due to the strains induced by substrate. The applied magnetic field (H≥2 T) transforms the sample with 18O back to the metallic state. In such magnetic fields the temperature dependence of resistivity exhibits a peak corresponding to the onset of metallic state. The shift of the peak toward low temperatures after the 16O→18O exchange exceeds 50 K at 3 T for both ceramic and thin film samples. The results are interpreted in terms of the isotope dependence of the effective electron bandwidth which shifts the balance between the charge ordered insulating and the ferromagnetic metal-like states.

Investigation of Lattice Effects in Rare Earth Manganites by 18O-isotope Exchange

The strong coupling between the electron spin and lattice arising from the Jahn-Teller effect of manganese ions plays an important role in the mechanism of colossal magnetoresistance and related properties of the rare earth manganites. The lattice effects in this class of oxides have been extensively studied through the application of hydrostatic as well as chemical pressures and magnetic fields. The recently observed giant 18O isotope effect provides direct evidence for the formation of lattice polarons in manganites. Here we report the preliminary results of our investigations on a variety of normal as well as 18O isotope-exchanged perovskite manganites exhibiting colossal magnetoresistance and charge ordering. The observed isotope shift of Tc as well as that of Tco is correlated with the key parameters controlling the lattice such as the Mn 3+ content, the average ionic radius of the A-site cation ⟨rA⟩ , and the A-site ionic size disorder σ2 .

Oxygen ion diffusivity, surface exchange and ionic conductivity in single crystal Gadolinia doped Ceria

In this paper we present the first data on a single crystal of Gadolinia doped Ceria (CGO). The crystal was prepared using inductive melting in a cold crucible. The cell parameter ( a=5.426 Å) was estimated using XRD on a powdered sample. The conductivity was measured by impedance spectroscopy in N 2, O 2 and air between 190 and 550°C. The activation energy was found to be 1.1 eV. The diffusion and surface exchange coefficients were determined using 18O isotope exchange and SIMS. The activation energy for the diffusion process was found to be 1.16 eV. A transition in the activation energy of the surface exchange coefficient from low to high was seen at around 650°C.

Kinetic determination of the fast exchanging substrate water molecule in the S3 state of photosystem II.

In a previous communication we showed from rapid isotopic exchange measurements that the exchangeability of the substrate water at the water oxidation catalytic site in the S3 state undergoes biphasic kinetics although the fast phase could not be fully resolved at that time [Messinger, J., Badger, M., and Wydrzynski, T. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 3209-3213]. We have since improved the time resolution for these measurements by a further factor of 3 and report here the first detailed kinetics for the fast phase of exchange. First-order exchange kinetics were determined from mass spectrometric measurements of photogenerated O2 as a function of time after injection of H218O into spinach thylakoid samples preset in the S3 state at 10 degreesC. For measurements made at m/e = 34 (i. e., for the mixed labeled 16,18O2 product), the two kinetic components are observed: a slow component with k1 = 2.2 +/- 0.1 s-1 (t1/2 approximately 315 ms) and a fast component with k2 = 38 +/- 4 s-1 (t1/2 approximately 18 ms). When the isotopic exchange is measured at m/e = 36 (i.e., for the double labeled 18,18O2 product), only the slow component (k1) is observed, clearly indicating that the substrate water undergoing slow isotopic exchange provides the rate-limiting step in the formation of the double labeled 18,18O2 product. When the isotopic exchange is measured as a function of temperature, the two kinetic components reveal different temperature dependencies in which k1 increases by a factor of 10 over the range 0-20 degreesC while k2 increases by only a factor of 3. Assuming simple Arrhenius behavior, the activation energies are estimated to be 78 +/- 10 kJ mol-1 for the slow component and 39 +/- 5 kJ mol-1 for the fast component. The different kinetic components in the 18O isotopic exchange provide firm evidence that the two substrate water molecules undergo separate exchange processes at two different chemical sites in the S3 state, prior to the O2 release step (t1/2 approximately 1 ms at 20 degreesC). The results are discussed in terms of how the substrate water may be bound at two separate metal sites.