Insights Into the Known 13C Depletion of Methane—Contribution of the Kinetic Isotope Effects on the Serine Hydroxymethyltransferase Reaction

We determined the kinetic isotope effect on the serine hydroxymethyltransferase reaction (SHMT), which provides important C1 metabolites that are essential for the biosynthesis of DNA bases, O-methyl groups of lignin and methane. An isotope effect on the SHMT reaction was suggested being responsible for the well-known isotopic depletion of methane. Using the cytosolic SHMT from pig liver, we measured the natural carbon isotope ratios of both atoms involved in the bond splitting by chemical degradation of the remaining serine before and after partial turnover. The kinetic isotope effect 13(VMax/Km) was 0.994 0.006 and 0.995 0.007 on position C-3 and C-2, respectively. The results indicated that the SHMT reaction does not contribute to the 13C depletion observed for methyl groups in natural products and methane. However, from the isotopic pattern of caffeine, isotope effects on the methionine synthetase reaction and on reactions forming Grignard compounds, the involved formation and fission of metal organic bonds are likely responsible for the observed general depletion of “activated” methyl groups. As metal organic bond formations in methyl transferases are also rate limiting in the formation of methane, they may likely be the origin of the known 13C depletion in methane.

The Diffusion Isotope Effect and Diffusion Mechanism in Liquid Cu-Ag and Cu-Ni Alloys

In this paper, the diffusion isotope effect and diffusion mechanism are investigated by means of molecular dynamics simulations in two liquid alloys, Ni-Ag and Ni-Cu. The values for the diffusion isotope effect parameter allow for the estimate of the number of atoms which are moving cooperatively in a basic diffusion event as experienced by a given atomic species. It is shown that the composition dependence of ND is typically very small. However, the temperature dependence of this parameter is much more pronounced. In addition, it is shown that, on average, in these alloys and temperatures considered, ND is limited to the range: 5<ND<17. This is consistent with results of molecular dynamics simulations on the average coordination number calculations. This would suggest that, together with a given atom, depending on temperature, the neighbouring atoms are all involved in the basic diffusion event.

Hydrogen Isotope Effect on Self-Organized Radial Electric Field Criticality During Electron Internal Transport Barrier Formation in Toroidal Plasmas

Self-organized structure formation in magnetically confined plasmas is one of the most attractive subjects in modern experimental physics. Nonequilibrium media are known to often exhibit phenomena that cannot be predicted by superposition of linear theories. One representative example of such phenomena is the hydrogen isotope effect in fusion plasmas, where the larger the mass of the hydrogen isotope fuel is the better the plasma confinement becomes, contrary to what simple scaling models anticipate. In this article, threshold condition of a plasma structure formation is shown to have a strong hydrogen isotope effect. To investigate the underlying mechanism of this isotope effect, the electrostatic potential is directly measured by a heavy ion beam probe. It is elucidated that the positive radial electric field structure can be driven by less input power normalized by plasma density in plasmas with larger isotope mass across the structure formation.

Joint meeting of 9th Asia Pacic-Transport Working Group (APTWG) & EU-US Transport Task Force (TTF) workshop

This conference report summarizes the contributions to, and discussions at the joint meeting of the 9th Asia Pacific-Transport Working Group (APTWG) & EU-US Transport Task Force (TTF) workshop held online, hosted by Kyushu University, Japan, during 6-9 July 2021. The topics of the meeting were organized under five main topics: 1)Isotope effect on transport and physics on isotope mixture plasma, 2)Turbulence spreading and coupling in core-edge-SOL, 3)Interplay between MHD topology/instability and turbulent transport, 4)Interaction between energetic particle driven instability and transport, 5)Model reduction and experiments for validation.

Quantitative Analysis of Nitrogen by Atom Probe Tomography Using Stoichiometric γ′-Fe4N Consisting of 15N Isotope

The nitrogen deficiency in steels measured by atom probe tomography (APT) is considered to arise from the obscurement of singly charged dimer nitrogen ions (N2+) by the iron-dominant peak (56Fe2+) at 28 Da. To verify this by quantifying the amount of N2+ ions, γ′-Fe4N consisting of the 15N isotope was prepared on iron substrates by plasma nitriding using a nitrogen isotopic gas (15N2). Although considerable amounts of 15N2+ were observed at 30 Da without overlap with any iron peak, the observed nitrogen concentrations of γ′-Fe4N were clearly lower than the stoichiometric composition (19–20 at%), using both pulsed voltage and pulsed laser atom probes. The origin of the missing nitrogen, excluding nitrogen obscured by other ion species, was predicted to be the occurrence of neutral nitrogen or nitrogen gas molecules in field evaporation. The generation rate of iron nitride ions (FeN2+) for 15N was significantly lower than that for 14N in γ′-Fe4N, which affected the amount of the missing nitrogen. The isotope effect suggests that the isotopic ratio cannot always be determined from only one ion species among the multiple species observed in the APT analysis. We discuss the mechanism of the isotope effect in FeN2+ formation by field evaporation.

Electronic Structure and Thermodynamic Properties of High Temperature Superconductors

<p>The generic doping dependence of the thermodynamic, electrodynamic and transport properties of high-temperature superconductors remains a puzzle despite many years of study. We are still awaiting a rigorous scientific theory that explains the resistance-free flow of electric current in these novel materials. In conventional superconductors, observations of the predicted dependence of the superconducting transition temperature on isotopic mass played a key role in identifying a phononic pairing mechanism. In order to elucidate the role of phonons in the high-Tc superconductors, the oxygen isotope effect in the separate components of the penetration depth tensor of the high-temperature superconductor YBa2Cu4O8 was determined from AC susceptibility measurements, performed on biaxially-aligned powders set in epoxy. The results, extracted after assuming values for the upper cut-off radii in the particle size distributions, show that the isotope effect in the bc-plane is negligible compared to those of the ab- and ac-planes. This suggests that the electrons prefer to couple to phonon modes in which the motion of the atoms is perpendicular to the plane of transport. The electronic entropy, superfluid density, Raman response, spin susceptibility and thermoelectric power were calculated from energy-momentum dispersions determined by angle-resolved photoemission spectroscopy (ARPES). An excellent match with experimental data was obtained. This is a highly significant result because it provides the first comprehensive link between these bulk properties and the ARPES measurements which are dominated by the outermost CuO2 layer. Thus, in most respects surface effects do not appear to seriously modify or obscure the band structure which governs bulk properties. The calculations reveal the presence of a van Hove singularity (vHs) at the Fermi level (EF ) in the heavily overdoped regime to be a universal feature of the cuprates. The evolution of these properties with temperature and doping can be fully explained by the retreat of EF from the vHs and the opening of a normal state pseudogap as doping is decreased. Consequently, the pairing potential amplitude is found to be a strongly decreasing function of hole concentration, similar to the doping dependence of the exchange interaction, J. The pairing interaction is possibly a universal function of the EF â EvHs with the maximum in the transition temperature (Tc) governed by the exact magnitude of the density of states on the flanks of the vHs. These are key new discoveries which may provide a route forward to solving the puzzle of high-temperature superconductivity.</p>