New Evidence for Symmetry Dependent Isotope Effects: O+CO Reaction

1989 ◽  
Vol 44 (5) ◽  
pp. 435-444 ◽  
Author(s):  
S. K. Bhattacharya ◽  
M. H. Thiemens
Keyword(s):  
Isotopic Exchange ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Large Mass ◽  
Reaction Rates ◽  
Kinetic Evaluation ◽  
Mass Independent Fractionation ◽  
New Evidence ◽  
Activated Complex ◽  
Fractionation Mechanism

The isotopic fractionation associated with the O + CO reaction has been studied using oxygen atoms produced by room temperature O2 photolysis at two different wavelengths, 185 and 130 nm. A large mass-independent isotopic fractionation is observed in the product CO2, extending the range of this type of reaction beyond O + O2 and SF5 + SF5. Kinetic evaluation of the data restricts the source of the mass-independent fractionation mechanism to the O + CO recombination step rather than O2 photolysis, secondary ozone formation, or O2 photodissociation. At least one, and most likely two other fractionation processes appear to occur in the experiments, and interpretation of the isotopic results is tentative at present. Based on the relevant reaction rates and the value for the reduced partition function for isotopic exchange between O and CO, it is suggested that this process may occur prior to the δ17O≅δ18O recombination process. Secondary CO2 photolysis may superimpose an additional fractionation. The experimental data are also examined in the context of a model based upon energy randomization rates versus the lifetime of the activated complex.

Effect of Isotopic Exchange upon Symmetry Dependent Fractionation in the O + CO → CO2 Reaction

1989 ◽  
Vol 44 (9) ◽  
pp. 811-813 ◽  
Author(s):  
S. K. Bhattacharya ◽  
M. H. Thiemens
Keyword(s):  
Experimental Data ◽  
Isotopic Exchange ◽  
Isotopic Fractionation ◽  
Large Mass ◽  
Kinetic Evaluation ◽  
Mass Independent Fractionation ◽  
Kinetic Treatment

Abstract In a recent study, it was demonstrated that the mechanism associated with the O + CO reaction produces a large, mass independent isotopic fractionation in the product CO2. A kinetic treatment of the data demonstrated that isotopic exchange between the O atom, produced by O2 photolysis and CO, occurred prior to the O + CO recombination. It was concluded that the likely source of the mass independent fractionation was the O + CO recombination. The present paper includes a kinetic evaluation of the added role of O + O2, along with O + CO, isotopic exchange. The new determinations provide a better fit of the experimental data.

scholarly journals Viewpoint: Isotopic fractionation by plant nitrate reductase, twenty years later

2006 ◽  
Vol 33 (6) ◽  
pp. 531 ◽  
Author(s):  
Guillaume Tcherkez ◽  
Graham D. Farquhar
Keyword(s):  
Nitrate Reductase ◽  
Isotope Effect ◽  
Nitrate Reduction ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Nitrogen Isotope ◽  
Reaction Rates ◽  
Rate Limiting Step ◽  
18O Isotope ◽  
N Isotopes

Plant nitrate reductase, the enzyme that reduces nitrate (NO3–) to nitrite (NO2–), is known to fractionate N isotopes, depleting nitrite in 15N compared with substrate nitrate. Nearly 20 years ago, the nitrogen isotope effect associated with this reaction was found to be around 1.015. However, the relationships between the isotope effect and the mechanism of the reaction have not yet been examined in the light of recent advances regarding the catalytic cycle and enzyme structure. We thus give here the mathematical bases of the 14N / 15N and also the 16O / 18O isotope effects as a function of reaction rates. Enzymatic nitrate reduction involves steps other than NO3– reduction itself, in which the oxidation number of N changes from +V (nitrate) to +III (nitrite). Using some approximations, we give numerical estimates of the intrinsic N and O isotope effects and this leads us to challenge the assumptions of nitrate reduction itself as being a rate-limiting step within the nitrate reductase reaction, and of the formation of a bridging oxygen as a reaction intermediate.

scholarly journals Quantifying the nitrogen isotope effects during photochemical equilibrium between NO and NO<sub>2</sub>: implications for <i>δ</i><sup>15</sup>N in tropospheric reactive nitrogen

2020 ◽  
Vol 20 (16) ◽  
pp. 9805-9819 ◽  
Author(s):  
Jianghanyang Li ◽  
Xuan Zhang ◽  
John Orlando ◽  
Geoffrey Tyndall ◽  
Greg Michalski
Keyword(s):  
Atmospheric Chemistry ◽  
Isotopic Exchange ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Field Measurements ◽  
Nitrogen Isotope ◽  
Dominant Factor ◽  
Reactive Nitrogen ◽  
Fractionation Factor ◽  
The Impact

Abstract. Nitrogen isotope fractionations between nitrogen oxides (NO and NO2) play a significant role in determining the nitrogen isotopic compositions (δ15N) of atmospheric reactive nitrogen. Both the equilibrium isotopic exchange between NO and NO2 molecules and the isotope effects occurring during the NOx photochemical cycle are important, but both are not well constrained. The nighttime and daytime isotopic fractionations between NO and NO2 in an atmospheric simulation chamber at atmospherically relevant NOx levels were measured. Then, the impact of NOx level and NO2 photolysis rate on the combined isotopic fractionation (equilibrium isotopic exchange and photochemical cycle) between NO and NO2 was calculated. It was found that the isotope effects occurring during the NOx photochemical cycle can be described using a single fractionation factor, designated the Leighton cycle isotope effect (LCIE). The results showed that at room temperature, the fractionation factor of nitrogen isotopic exchange is 1.0289±0.0019, and the fractionation factor of LCIE (when O3 solely controls the oxidation from NO to NO2) is 0.990±0.005. The measured LCIE factor showed good agreement with previous field measurements, suggesting that it could be applied in an ambient environment, although future work is needed to assess the isotopic fractionation factors of NO+RO2/HO2→NO2. The results were used to model the NO–NO2 isotopic fractionations under several NOx conditions. The model suggested that isotopic exchange was the dominant factor when NOx>20 nmol mol−1, while LCIE was more important at low NOx concentrations (<1 nmol mol−1) and high rates of NO2 photolysis. These findings provided a useful tool to quantify the isotopic fractionations between tropospheric NO and NO2, which can be applied in future field observations and atmospheric chemistry models.

scholarly journals Oxygen and magnesium mass-independent isotopic fractionation induced by chemical reactions in plasma

2021 ◽  
Vol 118 (52) ◽  
pp. e2114221118
Author(s):  
François Robert ◽  
Marc Chaussidon ◽  
Adriana Gonzalez-Cano ◽  
Smail Mostefaoui
Keyword(s):  
Chemical Reactions ◽  
Isotope Composition ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Chem Phys ◽  
Mass Independent Fractionation ◽  
Protosolar Nebula ◽  
Micrometer Size ◽  
Linear Correlations ◽  
Isotopic Effects

Enrichment or depletion ranging from −40 to +100% in the major isotopes 16O and 24Mg were observed experimentally in solids condensed from carbonaceous plasma composed of CO2/MgCl2/Pentanol or N2O/Pentanol for O and MgCl2/Pentanol for Mg. In NanoSims imaging, isotope effects appear as micrometer-size hotspots embedded in a carbonaceous matrix showing no isotope fractionation. For Mg, these hotspots are localized in carbonaceous grains, which show positive and negative isotopic effects so that the whole grain has a standard isotope composition. For O, no specific structure was observed at hotspot locations. These results suggest that MIF (mass-independent fractionation) effects can be induced by chemical reactions taking place in plasma. The close agreement between the slopes of the linear correlations observed between δ25Mg versus δ26Mg and between δ17O versus δ18O and the slopes calculated using the empirical MIF factor η discovered in ozone [M. H. Thiemens, J. E. Heidenreich, III. Science 219, 1073–1075; C. Janssen, J. Guenther, K. Mauersberger, D. Krankowsky. Phys. Chem. Chem. Phys. 3, 4718–4721] attests to the ubiquity of this process. Although the chemical reactants used in the present experiments cannot be directly transposed to the protosolar nebula, a similar MIF mechanism is proposed for oxygen isotopes: at high temperature, at the surface of grains, a mass-independent isotope exchange could have taken place between condensing oxides and oxygen atoms originated form the dissociation of CO or H2O gas.

scholarly journals 2H/1H variation in microbial lipids is controlled by NADPH metabolism

2019 ◽  
Vol 116 (25) ◽  
pp. 12173-12182 ◽  
Author(s):  
Reto S. Wijker ◽  
Alex L. Sessions ◽  
Tobias Fuhrer ◽  
Michelle Phan
Keyword(s):  
Metabolic Flux ◽  
Isotope Effects ◽  
Full Range ◽  
Isotopic Fractionation ◽  
Flux Analysis ◽  
Organic Substrates ◽  
Nadph Metabolism ◽  
Highly Correlated ◽  
Geologic Record ◽  
Ancient Ecosystems

The hydrogen-isotopic compositions (2H/1H ratios) of lipids in microbial heterotrophs are known to vary enormously, by at least 40% (400‰) relative. This is particularly surprising, given that most C-bound H in their lipids appear to derive from the growth medium water, rather than from organic substrates, implying that the isotopic fractionation between lipids and water is itself highly variable. Changes in the lipid/water fractionation are also strongly correlated with the type of energy metabolism operating in the host. Because lipids are well preserved in the geologic record, there is thus significant potential for using lipid 2H/1H ratios to decipher the metabolism of uncultured microorganisms in both modern and ancient ecosystems. But despite over a decade of research, the precise mechanisms underlying this isotopic variability remain unclear. Differences in the kinetic isotope effects (KIEs) accompanying NADP+ reduction by dehydrogenases and transhydrogenases have been hypothesized as a plausible mechanism. However, this relationship has been difficult to prove because multiple oxidoreductases affect the NADPH pool simultaneously. Here, we cultured five diverse aerobic heterotrophs, plus five Escherichia coli mutants, and used metabolic flux analysis to show that 2H/1H fractionations are highly correlated with fluxes through NADP+-reducing and NADPH-balancing reactions. Mass-balance calculations indicate that the full range of 2H/1H variability in the investigated organisms can be quantitatively explained by varying fluxes, i.e., with constant KIEs for each involved oxidoreductase across all species. This proves that lipid 2H/1H ratios of heterotrophic microbes are quantitatively related to central metabolism and provides a foundation for interpreting 2H/1H ratios of environmental lipids and sedimentary hydrocarbons.

Why magnesium isotope fractionation is absent from basaltic melts under thermal gradients in natural settings

2019 ◽  
Vol 157 (7) ◽  
pp. 1144-1148
Author(s):  
Yingkui Xu ◽  
Dan Zhu ◽  
Xiongyao Li ◽  
Jianzhong Liu
Keyword(s):  
Isotope Fractionation ◽  
Isotope Effects ◽  
Isotopic Fractionation ◽  
Chemical Diffusion ◽  
Kinetic Isotope Effects ◽  
Thermal Gradients ◽  
Magnesium Isotopes ◽  
Magnesium Isotope ◽  
Natural Systems ◽  
Kinetic Isotope

AbstractLaboratory experiments have shown that thermal gradients in silicate melts can lead to isotopic fractionation; this is known as the Richter effect. However, it is perplexing that the Richter effect has not been documented in natural samples as thermal gradients commonly exist within natural igneous systems. To resolve this discrepancy, theoretical analysis and calculations were undertaken. We found that the Richter effect, commonly seen in experiments with wholly molten silicates, cannot be applied to natural systems because natural igneous samples are more likely to be formed out of partially molten magma and the presence of minerals adds complexity to the behaviour of the isotope. In this study, we consider two related diffusion-rate kinetic isotope effects that originate from chemical diffusion, which are absent from experiments with wholly molten samples. We performed detailed calculations for magnesium isotopes, and the results indicated that the Richter effect for magnesium isotopes is buffered by kinetic isotope effects and the total value of magnesium isotope fractionation can be zero or even undetectable. Our study provides a new understanding of isotopic behaviour during the processes of cooling and solidification in natural magmatic systems.

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