Isotope experimental determination

By experimentally determining the ratio of abundances of C and isotope peaks for CO2 dissolved in sea water at various temperatures, a graph can be drawn relating the solubility of CO2 compared with that of CO2 (the ratio described above). On extracting the CO2 from sediment containing the shells (calcium carbonate) of dead sea creatures by addition of acid, a ratio (R) of abundances of CO2 to CO2 can be measured. If this value is read from the graph, a temperature T is extrapolated, indicating the temperature of the sea at the time the sediment was laid down. Such experiments have shown that 10,000 years ago the temperature of the Mediterranean was much as it is now. 

The relative abundances of the various isotopes of the light elements Li, Be and B therefore depend to some extent on which detailed model of the big bang is adopted, and experimentally determined abundances may in time permit conclusions to be drawn as to the relative importance of these processes as compared to x-process spallation reactions. 

Mariotti, A., Germon, J.C., Hubert, P., Kaiser, R, Letolle, R., Tardieux, A. and Tardieux, P. 1981 Experimental determination of nitrogen kinetic isotope fractionation some principles illustration for the denitrification and nitrification processes. Plant and Soil 62 413-430. 

The density functional theory calculations of primary 14C KIE and secondary deuterium kinetic isotope effects (SKIE)220 did not reproduce satisfactorily all the experimentally determined 14C KIE and deuterium (4,4-2H2)- and 6,6-2H2-SKIE, though the non-local DFT methods provide transition state energies on a par with correlated molecular orbital theory221. 

It can be concluded that the [3+2] pathway seems to be the only feasible reaction pathway for the dihydroxylation by permanganate. The study on the free activation energies for the oxidation of a. P unsaturated carboxylic acids by permanganate shows that the [3+2] mechanism is in better agreement with experimental data than the [2+2] pathway. Experimentally determined kinetic isotope effects for cinnamic acid are in good agreement with calculated isotope effects for the [3+2] pathway, therefore it can be concluded that a pathway via an oxetane intermediate is not feasible. 

Equation 11.57 signifies that when the competitive method is used (i.e., both iso-topomers are present simultaneously in the reaction mixture) the experimentally determined kinetic isotope effect corresponds to the isotope effect on V/K regardless of the actual concentration of the substrate. In other words, one cannot measure the isotope effect on Vmax using this method even when concentration is much larger than the Michaelis constant Km-... 

The practical usefulness of Equations 11.46 through 11.53 has been demonstrated for the malic enzyme catalyzed conversion of L-malate to pyruvate (Equation 11.72). Table 11.1 lists experimentally determined isotope effects for this reaction. Comparison of carbon kinetic isotope effects for protio and deutero-malate substituted at position 2 (the carbon that undergoes sp3 to sp2 transition) rules out the possibility that the hydride transfer and the decarboxylation events are concerted. This conclusion follows from Equation 11.48 which, for a concerted reaction, predicts that 13(V/K) should be smaller than 13(V/K)D, which is opposite to the order observed experimentally. 

The haloalkane dehalogenase DhlA mechanism takes place in two consecutive Sn2 steps. In the first, the carboxylate moiety of the aspartate Aspl24, acting as a nucleophile on the carbon atom of DCE, displaces chloride anion which leads to formation of the enzyme-substrate intermediate (Equation 11.86). That intermediate is hydrolyzed by water in the subsequent step. The experimentally determined chlorine kinetic isotope effect for 1-chlorobutane, the slow substrate, is k(35Cl)/k(37Cl) = 1.0066 0.0004 and should correspond to the intrinsic isotope effect for the dehalogenation step. While the reported experimental value for DCE hydrolysis is smaller, it becomes practically the same when corrected for the intramolecular chlorine kinetic isotope effect (a consequence of the two identical chlorine labels in DCE). 

In addition to providing the means for calculating the isotopic compositions of ancient fluids based on analysis of minerals, mineral-fluid isotope fractionation factors provide an opportunity to combine fractionation factors when there is a common substance such as water. A fundamental strategy for compiling databases for isotopic fractionation factors is to reference such factors to a common substance (e.g., Friedman and O Neil 1977). For example, the quartz-water fractionation factor may be combined with the calcite-water fractionation factor to obtain the quartz-calcite fractionation factor at some temperature. It is now recognized, however, that the isotopic activity ratio of water in a number of experimental determinations of mineral-fluid fractionation factors has been variable, in part due to dissolution of... 

Figure 10. Summary of experimentally determined fractionation factors for Ca isotopes in the formation of foraminifera and coccolith shell carbonate, and for rapid inorganic precipitation of aragonite from an Mg-Ca-Cl solution. Data for the foraminifer G. ornatissima and the coccolith E. huxleyi are from De La Rocha and DePaolo (2000). Data on G. sacculifer are from Nagler et al. (2000). Data for O. universa and aragonite are from Gussone et al. (2003). Two of the forams and the coccolith E. huxleyi have similar fractionation behavior, with an overall fractionation factor of-1 to -1.5%o, and a small temperature dependence of about 0.02 per °C. The foram G. sacculifer appears to have a strongly temperature dependent fractionation factor.

Wiesli RA, Beard BL, Johnson CM (2003b) Experimental determination of Ee isotope fractionation between aq. Ee(ll), green rust , and siderite. Geochim Cosmochim Acta 67 A533... 

Just as in the preceding examples, early indications of tunneling in enzyme-catalyzed reactions depended on the failure of experiments to conform to the traditional expectations for kinetic isotope effects (Chart 3). Table 1 describes experimental determinations of -secondary isotope effects for redox reactions of the cofactors NADH and NAD. The two hydrogenic positions at C4 of NADH are stereochemically distinct and can be labeled individually by synthetic use of enzyme-catalyzed reactions. In reactions where the deuterium label is not transferred (see below), an... 

Fig. 5A The dependence on pH of the deuterium isotope effect in the hammerhead ri-bozyme-catalyzed reaction. Black circles show rate constants in H2O gray circles show rate constants in D2O. Solid curves are experimentally determined curves. The apparent plateau of cleavage rates above pH 8 is due to disruptive effects on the deprotonation of uridine and guanosine residues. Dotted lines are theoretical lines calculated from pKa values of hydrated Mg ions of 11.4 in H2O and 12.0 in D2O and on the assmnption that there is no intrinsic isotope effect (a=kH2o/kD2o=l is the coefficient of the intrinsic isotope effect). The following equation was used to plot the graph of pL vs log(rate) log kobs=log(kmax)-log(l+10

Table 1.3 illustrates the closeness of the approximation. Considering experimental uncertainties in isotope ratio determinations (typically > 0.1 %o), these approximations are excellent for differences in 8-values less than abont 10 and for 5-values that are relatively small in magnitude. 

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