Isotope effect, fixing

Figure 5A shows experimentally derived profiles of pH vs rate for reactions in H2O and D2O [30, 50, 71]. The magnitude of the apparent isotope effect (ratio of rate constants in H2O and D2O) is 4.4 and the profiles appear to support the possibility that a proton is transferred from (Mg -bound) water molecules. However, careful analysis led us to conclude that a metal ion binds directly to the 5 -oxygen. Since the concentration of the deproto-nated 2 -oxygen in H2O should be higher than that in D2O at a fixed pH, we must take into account this difference in pKa, namely ApKa (=pKa °-pKa ), when we analyze the solvent isotope effect of D2O [30, 50, 68, 71]. We can estimate the pKa in D2O from the pKa in H2O using the linear relationship shown in Fig. 5B [30, 68, 73-75]. If the pKa for a Mg -bound water molecule in H2O is 11.4, the ApKa is calculated to be 0.65 (solid line in Fig. 5B). Then, the pKa in D2O should be 12.0. Demonstrating the absence of an intrinsic isotope effect (kH2o/kD20=l)> the resultant theoretical curves closely fit the experimental data, with an approximate 4-fold difference in...

In principle, any property of a reacting system which changes as the reaction proceeds may be monitored in order to accumulate the experimental data which lead to determination of the various kinetics parameters (rate law, rate constants, kinetic isotope effects, etc.). In practice, some methods are much more widely used than others, and UV-vis spectropho-tometric techniques are amongst these. Often, it is sufficient simply to record continuously the absorbance at a fixed wavelength of a reaction mixture in a thermostatted cuvette the required instrumentation is inexpensive and only a basic level of experimental skill is required. In contrast, instrumentation required to study very fast reactions spectrophotometrically is demanding both of resources and experimental skill, and likely to remain the preserve of relatively few dedicated expert users. 

How do kinetic isotope effects come about Even in its lowest energy state a covalent bond never stops vibrating. If it did it would violate a fundamental physical principle, Heisenberg s uncertainty principle, which states that position and momentum cannot be known exactly at the same time a nonvibrating pair of atoms have precisely zero momentum and precisely fixed locations. The minimum vibrational energy a bond can have is called the zero point energy (Eo) - given by the expression Eq = j/iv. 

If substrate-binding were essentially irreversible the product ratio achieved with 16 as the substrate should not reflect any kinetic isotope effect, since regardless of which half of the molecule was fixed in the reactive position the substrate would be converted into product. If, on the other hand, substrate-binding were reversible and faster than hydrogen abstraction, then the product ratio should reflect the maximum isotope effect. The observed isotope effects indicate that neither of these extreme situations is realized, i.e., the substrate binding and H-transfer must have comparable activation energies as illustrated in Fig. 16. 

The concentration of D2O is fixed at the same valne for all the solntions in order to hold constant any possible isotope effect on the rates. Caution Since pymvic acid is irritating to the skin and concentrated HCl can canse severe bnms, solntion handling should be carried out wearing gloves. 

As already pointed out in chapter 6.2.2.1, isotope abundances, especially of primary products, are primarily fixed by those of their original elemental source. Within a closed system , e.g. a plant, all secondary compounds must be isotopically correlated to the primary assimilation products (carbohydrates), and their isotope abundances or patterns can be characterised by moderate shifts from here in the course of their biosynthesis by isotope effects of the reactions involved. These will only become manifest in the case of metabolic branching and would, depending on material fluxes in different directions, lead to isotopically depleted compounds in one... 

This deuterium isotope effect cannot be explained by a purely electronic process but could be explained by a proton-coupled electron transfer. The population decay rate of the excited state at a fixed energy is successfully decomposed into two components an isotope independent solvation term and a proton-coupled electron transfer term with a marked deuterium effect. The latter terms for the CH3OH overlayer are found to be about twice those for the CH3OD overlayer. Thus, with time-resolved 2PPE, the ultrafast dielectric response of a protic/solvent metal-oxide interface has been revealed. 

When very accurate dipole moments are deduced, it is proper to query the significance of a breakdown of the Bom-Oppenheimer approximation. This approximation justifies the assignment of molecular property tensors, such as dipole moments and polarizabilities, to specific directions in a molecule-fixed frame and supports the use of a property function or surface representing the variation of the property with nuclear position. The dipole moment of HD (5.85 X 1(T4 D)26 arises solely from the breakdown of the approximation and may have the sense H D-.27-29 In HC1 and DC1, there is an isotope effect on the dipole moment that has been attributed to a violation of the Born-Oppen-heimer approximation30 there is an apparent difference of 0.0010 0.0002 D between the dipole functions of HC1 and DC1, with HC1 having the bigger moment. This result is in accord with a recent theoretical analysis by Bunker.31... 

Two important aspects of the carbon isotopic fractionation imposed by the C4 pathway are summarized graphically in Figures 9 and 10. The first schematically indicates the carbon-isotopic relationships between dissolved CO2, bicarbonate, and the carbon that is added to phosphoenolpyruvate in the reaction catalyzed by phosphoenolpyruvate carboxylase. It shows that, because the kinetic isotope effect associated with PEP carboxylase is smaller than the equilibrium isotope effect between dissolved CO2 and bicarbonate, the fixed carbon is enriched in relative to that in the dissolved CO2. As resi lt, the CO2 subsequently made available to rubisco in cam and C4 plants is enriched in relative to atmospheric CO2. If that p arbon were fixed with perfect efficiency, the biomass of the plant would be enriched in relative to CO2 from the atmosphere—Sp4 would be negative, indicating an inverse fractionation. 

Individual lipids have been isotopically analyzed in only three Archaea, all of them methanogens (see entries for Methanosarcina barkeri, Methanococcoides burtonii, and Methanobacterium thermoautotrophicum in Table 5). All reflect strong depletion of in lipids relative to biomass. Methanogens are commonly described as fixing carbon by use of the acetyl-CoA pathway. This can provide the feedstock required for synthesis of lipids, but C3 and C4 carbon skeletons are required for the synthesis of amino acids (which, in the form of proteins, account for most of the biomass). In methanogens, these are produced by additional C02-fixing steps (White 1995, p. 261). If the isotope effects associated with those reactions are much smaller than those associated with the production of acetyl-CoA, the isotopic contrast between the lipids and the biomass can be accounted for. 

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