18O Exchange Reactions Research Articles

A method for isotope exchange kinetics in mineral-water system-an in-situ study on oxygen isotope exchange in Na2CO3-H2O system.

The experiments on the exchange reaction rate of the oxygen isotopes in the mineral-water system with initial conditions of 100°C and 240MPa, 100°C and 924MPa, 118°C and 170MPa are taken. The oxygen isotope 18O exchange reactions between aqueous C16O32- and H218O are mainly traced by measuring the Raman peak intensity of oxygen-containing elements (CO32-) in sodium carbonate solution. The inconsistent trends between the molar fraction and the concentration of C18O216O2- indicate oxygen isotope exchange between sodium carbonate and heavy water accompanied by dissolution and recrystallization between supersaturated sodium carbonate solution and solid sodium carbonate. In the heterogeneous experimental systems the order of oxygen isotope exchange reactions are more than 1 and the dynamics of oxygen isotope exchange conform to JMAK kinetics model with the nucleation and growth processes of sodium carbonate crystal.

Quantum mechanical study of the 16O + 18O18O → 16O18O + 18O exchange reaction: Integral cross sections and rate constants.

The isotopic exchange reaction, 16O + 18O18O → 16O18O + 18O, involving excited ozone, , as intermediate complex, was investigated by means of a time independent quantum mechanical approach using the TKTHS potential energy surface (PES) [V. G. Tyuterev et al., J. Chem. Phys. 139, 134307 (2013)] of ozone. State-to-state integral cross sections were calculated for collision energies lower than 0.4 eV. Then specific and thermal rate constants were computed between 10 K and 350 K using these cross sections. The full quantum thermal rate constant is found to be in better agreement with the most recent experimental data than with previous studies where other O3 PESs were employed, confirming therefore the higher accuracy of the TKTHS PES. However, the present theoretical thermal rate constant still remains below the measured rate, maybe due to the neglect of non-adiabtic couplings.

Absence of oxygen isotope fractionation/exchange of (hemi-) cellulose derived sugars during litter decomposition

Aiming at developing a novel tool for palaeoclimate research, we recently proposed a new method for determining the oxygen isotope composition of monosaccharides (Zech, M., Glaser, B., 2009. Compound-specific δ 18O analyses of neutral sugars in soils using GC-Py-IRMS: problems, possible solutions and a first application. Rapid Communications in Mass Spectrometry 23, 3522–3532). In order to answer the question whether isotope fractionation and oxygen exchange reactions during litter decomposition affect the climatically controlled δ 18O values of plant derived sugars, such as for instance xylose and arabinose from hemicelluloses, we studied the compound specific δ 18O values of five different litter species having been decomposed in a field litterbag experiment for 27 months. While δ 18O values of bulk organic matter yielded a systematic 18O depletion of 3.3‰ (pine) to 4.6‰ (spruce) accompanying total cellulose decomposition of 51% (beech) to 86% (ash), δ 18O values of individual sugars show no significant trend with time. In addition to the observed absence of isotope fractionation during decomposition, we also rule out 18O exchange reactions affecting the δ 18O signature of sugar molecules during diagenetic processes as well as during analytical procedure based on experimental findings and on theoretical biochemical mechanistic considerations. We conclude that our new method may become an analytical tool that elegantly overcomes extraction, purity and hygroscopicity problems of previous 18O cellulose methods. It furthermore has the potential to be applied to a wide range of climate archives such as tree rings, lacustrine sediments and loess palaeosol sequences.

Characterization of Titanium Species Incorporated into Dealuminated Mordenites by Means of IR Spectroscopy and 18O-Exchange Technique

Titanium mordenite, Ti-M, was prepared by the solid−gas reaction (atom-planting method) between highly dealuminated mordenites and TiCl4 vapor at elevated temperatures (473−873 K), and the coordination state of the incorporated Ti was studied comparing with that in titanosilicate with MFI structure (TS-1). The amount of Ti incorporated leveled off at 1.7 kPa of TiCl4 vapor for a process time of 1 h, and only trace amount of Al and Si was released during the treatment. IR spectra confirmed that TiCl4 reacted with internal SiOH groups in hydroxyl nests. The concentration of oxygen atoms bound to the framework Ti atoms in both TS-1 and Ti-M zeolites was measured by the 18O-exchange reaction between C18O2 and the zeolite. The number of oxygen atoms bound to a Ti atom was ca. 4 for TS-1 zeolites and varied from 5 to 4 for Ti-M zeolites with increasing temperature of the TiCl4 treatment. From these results, a reaction mechanism was proposed for the reaction of TiCl4 with hydroxyl nests in highly siliceous mordenites. Ti-M exhibited an IR band at 963 cm-1, while it was observed at 960 cm-1 for TS-1. The relationship between the relative intensity of the 963 cm-1 band and the Ti content also depended on the treatment temperature. The 18O exchange with C18O2 shifted the 963 cm-1 band for Ti-M to 928 cm-1. On the basis of the 18O exchange and IR measurements, we assign the characteristic IR band around 960 cm-1 for titanium zeolites to the stretching vibration of Si−O−Ti-bonds.

Raman Spectrometric Determination of the Oxygen Self‐Diffusion Coefficients in Oxides

New methods of determining the oxygen self‐diffusion coefficients (D*o) in oxides have been developed using Raman spectroscopy combined with the 16O–18O exchange technique. From the depth‐profiles of the 18O concentration in the 16O–18O exchanged oxides, which was measured by Raman microscope with a spatial resolution of 5 μm, D*o was determined for 2.8 mol% Y2O3‐containing tetragonal zirconia polycrystall (the depth‐profile method). Thus‐obtained results are expressed as D*O,D‐P= 1.82(+0.41−0.40) × 10−1·exp{−(139.3 ± 0.2) [kJ/mol]/RT} [cm2/s] in the temperature range of 700–950°C. We also determined D*o for the same sample from the Raman spectrometric monitoring of the ambient gas during the 16O–18O exchange reaction (the gas‐monitoring method). Thus‐obtained results are expressed as D*O,G‐M= 1.14(+0.05−0.04) × 10−2 exp{−(117.5 ± 0.4) [kJ/mol]/RT} [cm2/s] in the temperature range of 700–1165°C. The results obtained from the above two different methods virtually agree with each other, indicating that reliable D*o can be obtained by either of these two methods. We demonstrate that Raman spectroscopy is a useful tool for determining D*o in oxides.

Evidence from 18O Exchange Studies for an Exocyclic Methylene Intermediate in the Reaction Catalyzed by T4 Deoxycytidylate Hydroxymethylase

18O exchange experiments were designed to identify the final intermediate in the catalytic mechanism of bacteriophage T4 deoxycytidylate (dCMP) hydroxymethylase (CH). CH catalyzes the formation of 5-(hydroxymethyl)-dCMP (HmdCMP) from dCMP and methylenetetrahydrofolate (CH2-THF). CH resembles thymidylate synthase (TS), an enzyme of known three-dimensional structure, in both amino acid sequence and the reaction catalyzed. The final intermediate in the reaction catalyzed by TS or CH has been proposed to be the nucleotide with an exocyclic 5-methylene group covalently linked to the enzyme. This intermediate is then hydrated to HmdCMP (by CH) or reduced to deoxythymidylate (by TS). We report here that CH catalyzes the incorporation of 18O from solvent water into the product, HmdCMP, in the presence of tetrahydrofolate (THF). The cause of this exchange is a reverse reaction followed by a resynthesis. CH also catalyzes the exchange of 18O from solvent water into HmdCMP in the absence of exogenous THF and in the presence of THF analogues that lack N-5. N-5 is the nitrogen that is likely to be bound to the methylene as it is transferred to dCMP. A CH variant that lacks the nucleophilic Cys 148 is incapable of promoting these 18O exchange reactions. The THF analogues lacking N-5 do not promote a CH-catalyzed reverse reaction. Rather, we propose that the CH-catalyzed 18O exchange reaction promoted by these THF analogues occurs via 5-methylene-dCMP linked to the enzyme through Cys 148. We conclude here that enzyme-bound 5-methylene-dCMP is the final intermediate during catalysis by CH, as has also been proposed for TS and dUMP.

Apparently irreversible GTP hydrolysis attends tubulin self‐assembly

The pathway of GTP hydrolysis associated with microtubule polymerization was investigated using an assay of intermediate 18O-exchange reactions. Under a variety of conditions influencing tubulin self-assembly, GTP was hydrolyzed without any evidence of multiple reversals characteristic of reversible phosphoanhydride-bond cleavage. These results also accord with published findings that ATP hydrolysis during actin polymerization fails to display intermediate exchange reactions [Carlier, M. F., Pantaloni, D., Evans, J. A., Lambooy, P. K., Korn, E. D. and Webb, M. R. (1988) FEBS Lett. 235, 211-214].

Surface Diffusion and Surface Layer in Ceramics

Methods of measuring surface diffusion coefficients and related phenomena have been reviewed. These include grain boundary grooving, sinusoidal profile decay, isolated scratch decay, tracer, 18O-exchange reaction, sintering, spheroidization of cylindrical voids, work function, etc. Most of the surface diffusion coefficients of oxides measured by these methods involve Al2O3, MgO and NiO. The activation energies of surface diffusion measured at low temperature were generally smaller than those measured at high temperature. In MgO and NiO, the surface diffusion coefficient of oxygen is slower than that of cations.Methods of measuring surface layer thickness at elevated temperature are also reviewed and the results for oxides are summarized. In general, the surface layer thicknesses measured by solid state reaction are larger than those measured by 18O-exchange reaction.

Studies on zinc enzymes and models

Carboxypeptidase A catalyzes the hydrolysis of peptides and related esters. For one special type of ester an anhydride intermediate has been suggested [1], but our work on 18O exchange reactions [2] indicated that normal peptide substrates do not use a mechanism involving an anhydride intermediate but instead proceed by direct hydrolysis. A loophole in this proof involves the ‘trapped water’ problem: 18O exchange with solvent might not be observed if the water molecule produced in one step is enzyme bound, and not equilibrated with solvent. Kinetic 18O isotope effect studies have now been performed to close this loophole and establish the true enzymatic mechanism. In kinetic isotope effect studies a reaction partitions between heavy and light water. Another partitioning of interest is between water and methanol. We find that even the unusual substrate which reportedly [1] forms an anhydride with carboxypeptidase A will not incorporate methanol. By contrast, model systems [3] for the zinc-catalyzed cleavage of an anhydride show a preference for methanolysis over hydrolysis. This contrasting behavior suggests that the enzymatic reaction involves two proton transfers. Carbonic anhydrase is a zinc enzyme catalyzing a deceptively simple hydration reaction. A variety of ligands have been prepared [4] which form zinc complexes related to the active site of the enzyme, but few of the complexes show striking catalytic ability. The special requirements for catalysis will be discussed.

Isotopic and chemical composition of parbati valley geothermal discharges, North-West Himalaya, India

The isotopic compositions of the waters discharged from Parbati Valley geothermal areas indicate a higher altitude meteoric origin, with discharge temperatures reflecting variations in the depth of penetration of the waters to levels heated by the existence of a ‘normal’ geothermal gradient. On the basis of mixing models involving silica, tritium, discharge temperatures and chloride contents, deep equilibration temperatures of 120–140°C were obtained for Manikaran, possibly reaching 160°C at even greater depth. Geothermometers based on sulfate-water 18O exchange and gas reactions point to similar temperatures. Exceptionally high helium contents of the discharges correspond to apparent crustal residence times of the waters in the order of 10–100 Ma; relative nitrogen-argon contents support a largely meteoric origin of the waters with a possible fossil brine, but no detectable magmatic component.

A study of the 18O-exchange reactions catalyzed by cardiac myosin and its subfragment 1.

Isotope-exchange reactions (H2(18)O in equilibrium with KH2PO4) during ATP hydrolysis catalyzed by myosin and its subfragment 1 from rabbit, dog, and human cardiac muscle were studied. All preparations of myosin and subfragment 1 in the presence of Mg2+ catalyzed two types of 18O-exchange reactions similar to those of skeletal muscle: intermediate and direct 18O exchange. The dependences of both reactions on divalent metals and nucleotides were studied. Data on 18O exchange of subfragment 1 from rabbit, dog, and human cardiac muscle were obtained for the first time. They indicate the similarity of molecular mechanisms of ATP energy use in cardiac and skeletal muscle contraction.

The interaction of myosine S1 with phosphorothioates of ADP: An 18O exchange study by 31P NMR

The 18O exchange reaction which labeled P i undergoes in the presence of complexes of myosin subfragment 1, MgCl 2, and the different phosphorothioates of ADP has been observed by 31P NMR. From these experiments it can be concluded that ADP and ADP (α-S) (A) on the one hand and ADP (β-S) and ADP (α-S) (B) on the other hand form similar complexes as far as the number of reversals of the nucleoside triphosphate formation step from the nucleoside diphosphate and P i, is concerned. In addition, the same seems to hold for the rate constant k −2, which describes the binding step of free P i, to the subfragment 1 nucleoside diphosphate complex. These observations support former kinetic experiments which yielded the same similarities for the rate parameters describing association and dissociation of the subfragment 1 nucleotide complexes.

Oxygen exchange between oxo-anions and water in basic media: molybdate(2–) and tungstate(2–)

The 18O-exchange reaction between [MoO4]2– and [WO4]2– respectively and water has been investigated in alkaline solutions at constant total concentration of ionic equivalents J= 1.0 mol dm–3(Na[ClO4]). Over the range 3 × 10– 3⩽[OH–]⩽ 1.5 × 10–l mol dm–3 at 5 °C ([MoO4]2–) and 2 × 10–3⩽[OH–]⩽7.5 × 10–3 mol dm–3 at 1 °C ([WO4]2–) the exchange occurs by spontaneous and hydroxide-ion catalyzed paths (X = MO or W) according to the rate equation (i). The rate constants extrapolated at 25 °C are k0= 0.33 s–l and k1= 2.22 dm3R/[XO42–]T=k0+k1[OH–](i) mol–1 s–1 for [MoO4]2– and k0= 0.44 s–l and k1= 273 dm3 mol–l s–l for [WO4]2–. The following activation parameters were obtained: ΔHo:‡= 62.8 ± 2.5 kJ mol–1ΔS0‡=–43.5 ± 9.0 J K–l mol–1, ΔH1‡= 70.3 ± 5.0 kJ mol–l, and ΔS1‡=–2.5 ± 13.0 J K–l mol–l([MoO4]2–); and ΔH0‡= 29.25 ± 3.22 kJ mol–l, ΔSo†=–153.55 ± 11.30 J K–l mol–1, ΔH1‡= 71.1 ± 8.0 kJ mol–l. ΔS1‡= 40.2 ± 28.9 J K–l mol–l([WO4]2–). The values for the kinetic solvent deuterium-isotope effects k0H/k0D= 3.6 and k1H/k1D= 2.3 (5 °C, [MoO4]2–) and k0H/k0d= 3.0 and k1H/k1D= 1.7 (1 °C, [WO4]2–), indicate proton transfer in the rate-determining step. The mechanisms of the spontaneous paths are described as solvent-assisted oxygen dissociations with association between H2O and [XO4]2– playing an important role, whereas the [OH]–- dependent paths are tentatively viewed associative reaction between [OH]– and [XO4]2–.

Oxygen exchange between iodate and water: catalysis by hydrogen and hydroxide lons

The 18O-exchange reaction between iodate ions and water has been investigated using a rapid chemical-quenching technique. Over the range 2.1 < pH < 12.5 the exchange occurs by hydrogen- and hydroxide-ion catalysed paths according to the rate law (i). At 5 ° C and I= 1.0 mol dm–3(Li[ClO4]) the rate constants are k0 < 0.01 s–1, R/[IO3–]=k0+k1[H+]+k2[H+]2+k3[OH–](i)k1=(3.20 ± 0.11)× 103 dm3 mol–1 s–1, k2=(1.38 ± 0.24)× 105 dm6 mol–2 s–1 and k3=(2.49 ± 0.16)× 102 dm3 mol–1 s–1 respectively. Activation parameters from data at 5–25 °C are ΔH1‡= 61.5 ± 3.8 kJ mol–1,ΔS1‡= 43.1 ± 13.0 J K–1 mol–1, ΔH3‡= 47.4 ± 1.7 kJ mol–1, and ΔS3‡=–28.3 ± 5.9 J K–1 mol–1. The following values for the kinetic solvent deuterium-isotope effect were obtained (5 °C) : klH/klD= 2.1, k2H/k2D= 0.49, and k3H/k3D= 1.7. The normal isotope effect suggests proton transfer as the rate-determining steps corresponding to k1 and k3, whereas the inverse isotope effect on k2 can be explained by a rapid acid–base pre-equilibrium. These results provide further evidence for associative substitution mechanisms with iodine(V).

Occurrence and Characteristics of 18O Exchange Reactions Catalyzed by Sodium- and Potassium-dependent Adenosine Triphosphatases

Occurrence and Characteristics of 18O Exchange Reactions Catalyzed by Sodium- and Potassium-dependent Adenosine Triphosphatases

Occurrence and Possible Mechanism of 32P and 18O Exchange Reactions of Photophosphorylation

Abstract The incorporation of water oxygen into ATP during photophosphorylation by chloroplasts varies from l0.5 to g1.0 atom of oxygen per mole of ATP formed. The ATP ⇄ HOH exchange, particularly in the presence of ascorbate and phenazine methosulfate or ferricyanide, is accompanied by small but significant Pi ⇄ HOH and Pi ⇄ ATP exchanges. The results are compatible with all exchanges occurring as a result of dynamic reversal of ATP formation at the catalytic site. Such interpretation is supported by ATP acceleration of a light-dependent Pi ⇄ HOH exchange, and can be rationalized with Pi and ADP inhibitory effects on the ATP ⇄ HOH exchange. Mg++ in excess of the ATP concentration is required for maximal ATP ⇄ HOH exchange. A small Pi ⇄ HOH exchange as well as a more prominent ATP ⇄ HOH exchange accompanies ATP synthesis in the dark resulting from an acid-base transition of chloroplasts.

Oxygen-18 exchange reactions between gaseous oxygen and certain oxygen-containing inorganic salts

Rate measurements have been carried out on the 18O exchange reaction between oxygen gas and certain alkali metal chromates, molybdates, tungstates, phosphates and sulphates in the 600–850°C temperature range. It was observed that the rate of exchange was influenced by the nature of the cation and the nature of the central atom. In the alkali metal chromate series, the potassium salt exhibited the least tendency to exchange, but the activation energy for the exchange process was observed to decrease with an increase in the cation radius. In contrast, both the rate of exchange and the activation energy decreased as the effective nuclear charge of the central atom increased, as the central atom-oxygen bond distance decreased, and as the oxidation state of the central atom increased. A mechanism is proposed for the exchange reaction and a correlation between the rate of exchange and various thermal and thermodynamic properties is presented.

32P and 18O Exchange Reactions in Soluble Fractions Prepared from Rat Liver Mitochondria

32P and 18O Exchange Reactions in Soluble Fractions Prepared from Rat Liver Mitochondria