Correction of Pre-Steady-State KIEs for Isotopic Impurities and the Consequences of Kinetic Isotope Fractionation†

2008 ◽  
Vol 112 (50) ◽  
pp. 13109-13115 ◽  
Author(s):  
Sam Hay ◽  
Christopher R. Pudney ◽  
Parvinder Hothi ◽  
Nigel S. Scrutton
Keyword(s):  
Steady State ◽  
Isotope Fractionation ◽  
Kinetic Isotope

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.

Assessing the Blank Carbon Contribution, Isotope Mass Balance, and Kinetic Isotope Fractionation of the Ramped Pyrolysis/Oxidation Instrument at NOSAMS

Radiocarbon ◽  
2017 ◽  
Vol 59 (1) ◽  
pp. 179-193 ◽  
Author(s):  
Jordon D Hemingway ◽  
Valier V Galy ◽  
Alan R Gagnon ◽  
Katherine E Grant ◽  
Sarah Z Rosengard ◽  
...  
Keyword(s):  
Mass Balance ◽  
Isotope Fractionation ◽  
Standard Reference Materials ◽  
Weighted Mean ◽  
Kinetic Isotope ◽  
Carbon Mass ◽  
C Value ◽  
Ramped Pyrolysis ◽  
Minimal Mass ◽  
Mass Dependent

AbstractWe estimate the blank carbon mass over the course of a typical Ramped PyrOx (RPO) analysis (150–1000°C; 5°C×min–1) to be (3.7±0.6) μg C with an Fm value of 0.555±0.042 and a δ13C value of (–29.0±0.1) ‰ VPDB. Additionally, we provide equations for RPO Fm and δ13C blank corrections, including associated error propagation. By comparing RPO mass-weighted mean and independently measured bulk δ13C values for a compilation of environmental samples and standard reference materials (SRMs), we observe a small yet consistent 13C depletion within the RPO instrument (mean–bulk: μ=–0.8‰; ±1σ=0.9‰; n=66). In contrast, because they are fractionation-corrected by definition, mass-weighted mean Fm values accurately match bulk measurements (mean–bulk: μ=0.005; ±1σ=0.014; n=36). Lastly, we show there exists no significant intra-sample δ13C variability across carbonate SRM peaks, indicating minimal mass-dependent kinetic isotope fractionation during RPO analysis. These data are best explained by a difference in activation energy between 13C- and 12C-containing compounds (13–12∆E) of 0.3–1.8 J×mol–1, indicating that blank and mass-balance corrected RPO δ13C values accurately retain carbon source isotope signals to within 1–2‰.

scholarly journals Tyr-143 facilitates interdomain electron transfer in flavocytochrome b2

1992 ◽  
Vol 285 (1) ◽  
pp. 187-192 ◽  
Author(s):  
C S Miles ◽  
N Rouvière-Fourmy ◽  
F Lederer ◽  
F S Mathews ◽  
G A Reid ◽  
...  
Keyword(s):  
Electron Transfer ◽  
Steady State ◽  
Isotope Effects ◽  
Mutant Enzyme ◽  
Stopped Flow ◽  
Wild Type ◽  
Wild Type Enzyme ◽  
Flavocytochrome B2 ◽  
Rate Determining Step ◽  
Kinetic Isotope

The role of Tyr-143 in the catalytic cycle of flavocytochrome b2 (L-lactate:cytochrome c oxidoreductase) has been examined by replacement of this residue with phenylalanine. The electron-transfer steps in wild-type and mutant flavocytochromes b2 have been investigated by using steady-state and stopped-flow kinetic methods. The most significant effect of the Tyr-143----Phe mutation is a change in the rate-determining step in the reduction of the enzyme. For wild-type enzyme the main rate-determining step is proton abstraction at the C-2 position of lactate, as shown by the 2H kinetic-isotope effect. However, for the mutant enzyme it is clear that the slowest step is interdomain electron transfer between the FMN and haem prosthetic groups. In fact, the rate of haem reduction by lactate, as determined by the stopped-flow method, is decreased by more than 20-fold, from 445 +/- 50 s-1 (25 degrees C, pH 7.5) in the wild-type enzyme to 21 +/- 2 s-1 in the mutant enzyme. Decreases in kinetic-isotope effects seen with [2-2H]lactate for mutant enzyme compared with wild-type, both for flavin reduction (from 8.1 +/- 1.4 to 4.3 +/- 0.8) and for haem reduction (from 6.3 +/- 1.2 to 1.6 +/- 0.5) also provide support for a change in the nature of the rate-determining step. Other kinetic parameters determined by stopped-flow methods and with two external electron acceptors (cytochrome c and ferricyanide) under steady-state conditions are all consistent with this mutation having a dramatic effect on interdomain electron transfer. We conclude that Tyr-143, an active-site residue which lies between the flavodehydrogenase and cytochrome domains of flavocytochrome b2, plays a key role in facilitating electron transfer between FMN and haem groups.

Fundamental studies on kinetic isotope effect (KIE) of hydrogen isotope fractionation in natural gas systems

2011 ◽  
Vol 75 (10) ◽  
pp. 2696-2707 ◽  
Author(s):  
Yunyan Ni ◽  
Qisheng Ma ◽  
Geoffrey S. Ellis ◽  
Jinxing Dai ◽  
Barry Katz ◽  
...  
Keyword(s):  
Natural Gas ◽  
Isotope Effect ◽  
Hydrogen Isotope ◽  
Isotope Fractionation ◽  
Kinetic Isotope Effect ◽  
Kinetic Isotope

High-Temperature Processes: Is it Time for Lithium Isotopes?

Elements ◽  
2020 ◽  
Vol 16 (4) ◽  
pp. 247-252
Author(s):  
Horst R. Marschall ◽  
Ming Tang
Keyword(s):  
High Temperature ◽  
Isotope Fractionation ◽  
Isotope Geochemistry ◽  
Kinetic Isotope ◽  
High Temperature Processes ◽  
Li Isotope ◽  
Isotope Studies ◽  
Short Time ◽  
Diffusion Profiles ◽  
Geologic Processes

The field of high-temperature Li isotope geochemistry has been rattled by major paradigm changes. The idea that Li isotopes could be used to trace the sources of fluids, rocks, and magmas had to be largely abandoned, because Li diffusion causes its isotopes to fractionate at metamorphic and magmatic temperatures. However, diffusive fractionation of Li isotopes can be used to determine timescales of geologic processes using arrested diffusion profiles. High diffusivity and strong kinetic isotope fractionation favors Li isotopes as a tool to constrain the durations of fast processes in the crust and mantle, where other geochronometers fall short. Time may be the parameter that high-temperature Li isotope studies will be able to shed much light on.

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