Reversed isotopic compositions

The same figure holds qualitatively true in the case of a depletion layer, but with reversed isotopic compositions. 

Here the effects of any one fractionating step can be expressed in a change in isotopic composition in a wider range of body tissue components, including the product as well as the precursor of a (reversible) reaction. The details depend on the explicit model, for example how rates depend on metabolite concentrations. Therefore, where a metabolic pathway is, or becomes, reversible, the effect on isofractionation on measured body components can be more widespread. 

Elgiire 11.3. A flow-model scheme intended to represent relevant nitrogen flows, especially with regard to which flows are reversihle. The labeled reactions 1, 11, 111 IV are all potentially iso-topically fractionating. Because reaction 11 is not reversible, subsequent fractionations in the excretory pathway should not influence the isotopic composition of the body protein pool. 

The protocol described in Section 7.1.2 involves isotopic competition, but with the different isotopomers held in separate containers. Equations 7.10 to 7.13 apply equally well to a type of competition experiment known in biochemistry as the perturbation method for determining KIE s of reversible enzyme catalyzed reactions. The perturbation method differs from simultaneous non-competitive measurements in several important ways. One begins by mixing equilibrium concentrations of substrate and product but with one component (substrate or product) at a different isotopic composition than the other. Thus, the mixture is in chemical, but not isotopic equilibrium. At this stage no enzyme is present and the interconversion is... 

Possible inter relationships of natural substances are important. Similarities of the low molecular weight alkane isomers from crude oil and Fischer-Tropsch synthesis product have been reported. A similar composition for high temperature coal carbonization has been found. The C4 to C7 alkane isomers from these sources can be calculated quantitatively with equations developed for Fischer-Tropsch products. A reversal of the concentrations of the monomethyl isomers from CG (2 Me > 3 Me) to C7 (3 Me > 2 Me) occurs in all three products comparisons at higher carbon numbers indicate some dissimilarities. Naphthene isomers for crude oil and high temperature coal carbonization also have similar compositions. Aliphatic hydrocarbons from low temperature coal processes are considerably different. The C1 isotopic composition of pure compounds from the various sources are being compared in order to provide information on their origin. 

Though unconventional reversible 0—0 is indicated by experiments which demonstrated that the isotope composition of the unreacted H202 was dramatically altered in 180-enriched water. The proposed mechanism has implications for the interpretation of the kinetic parameters for the enzymatic reaction,79 suggesting that kcat as well as /ccat/KM(H202) is determined by an irreversible step after 0—0 heterolysis. One possibility is the reduction of the iron(IV) oxo porphyrin + by the cosubstrate 2-methoxyphenol, as shown in Figure 9.13. 

Isotopic fractionation during evaporation causes fractionation during cloud formation the vapor in the clouds has a lighter isotopic composition than the ocean that supplied the water. Upon condensation from the cloud, during rainformation, the reverse is true the heavy water molecules condense more efficiently, leaving the cloud residual vapor depleted of deuterium and lsO. 

Recognition of the fact that elements always displayed the same chemical behavior - regardless of their isotopic composition - led to a reformulation of the periodic law. The idea that each element was characterized by a unique number had already been demonstrated experimentally by Hemy Moseley (1887-1915). By studying the X-ray diffraction patterns produced by a variety of elements, he discovered that the frequencies of the K lines differed from element to element in a predictable and consistent fashion. He went on to show that the frequency of any line in the X-ray spectrum is approximately proportional to A(N-b), where A and b are constants and N is an integer that he termed the atomic number of the element. Moseley was able to identify the number N with the number of protons in the atomic nucleus. Plots of the square root of the frequency for the K and L lines in the X-ray spectra of the elements versus their atomic number, reproduced in Figure 5, show almost straight lines. From this work, it became clear why the order in which certain element pairs appeared in the periodic table needed to be reversed. The pairs in question are argon (39.95) and potassium (39.10) cobalt (58.93) and nickel (58.69) and tellurium (127.60) and iodine (126.91), the... 

The selectivity values confirm the potential of reverse osmosis composite membranes in the treatment of effluents that contain radioactive isotopes. 

In principle, such consideration apply of course also to the reverse reaction. Indeed, labile deuterium atoms must be washed out of a labeled compound by water because use of such a compound requires that it can be purified to constant isotopic composition by distillation,... 

To obtain credible distinction, great experimental precision is required in measurements of these small differences in rates. One technique for detection of heavy-atom isotope effects has been to measure isotopic composition of products early in the reaction and again at 100% completion with high-precision ratio mass spectrometers (O Leary Kleutz, 1972). Another approach is a sensitive equilibrium-perturbation method for enzymatic experiments, where freely reversible reactions are studied (Schimerlik et a/, 1975 Cleland, 1980). 

A stock solution was prepared by dissolving the enriched isotope. Diluted solutions were prepared from this stock solution on a weight basis. The isotopic composition of this solution was determined experimentally by GC-MS analysis of the chelate. The internal standard solution was calibrated by reverse isotope dilution GC-MS using the atomic absorption primary standard. Weighed amounts of the primary standard solution were mixed with weighed amounts of the enriched isotope solution. Chelates were prepared from the spiked samples (as described below) and were used for mass spectrometric determination of isotope ratios. Concentration of the trace metal in the spike solution was calculated using the experimentally determined isotope ratios, the weights of the standard and... 

A protocol for accurate determination of lead concentrations in water samples using ICP-IDMS was established by the National Institute of Standards and Technology (NIST), Gaithersburg, MD, USA [17]. A comparable scheme is presented in Figure 8.3, which summarizes schematically the different steps of isotope dilution analysis via ICP-MS. In the first line of this schematic diagram the initial solutions are listed, in the second the necessary isotope dilution processes, and in the third all isotope ratio measurements required. Reverse isotope dilution refers to exactly the same process, but in which the isotopic spike is considered as a sample and a standard solution of natural isotopic composition as a spike. 

In this case, the spike added first is used as a tracer, for example, in metabolic studies, and the second for quantification of the elemental species. Meija et al. [99] investigated the redox interconversion reaction between Cr(III) and Cr(VI) in yeast using °Cr-emiched Cr(III) and Cr-enriched Cr(VI) by least-squares ion intensity-based IPD. They found that the results were in exact agreement with conventional isotope dilution calculations. An example in which IPD was applied for metabolism studies is the determination of the metabolism of butyltin compounds in rats [100]. Sn-labeled MET, Sn-labeled DBT, and Sn-labeled TBT dissolved in tap water were administered orally to the animals. Degradation of the butyltin compounds was studied in different organs by least-squares IPD and their concentrations were determined via reverse isotope dilution analysis using the corresponding compounds of natural isotopic composition as spikes. 

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