Tritium tracer experiments

To address this issue directly. Moss and Frey performed tritium tracer experiments to establish whether hydrogen migration is an intramolecular process, or whether the hydrogen initially removed from the 3-pro-R position of the substrate equilibrates with a group or pool of hydrogen atoms located apart from the substrate. In the latter case, the number of constituents therein would govern on a statistical basis how often the particular hydrogen atom removed from the... 

Quay P., Broecker W. S., Hesslein R. H., and Schindler D. W. (1980) Vertical diffusion rates determined by tritium tracer experiments in the thermocline and hypoUmnion of two lakes. Limnol. Oceanogr. 25(2), 201—218. 

A quite different use of isotopes in mechanistic studies involves their use as labels for ascertaining the location of a given atom involved in a reaction. As in kinetic experiments, the isotopic substitution will not qualitatively affect the course of the reaction. The nuclei most commonly used for isotopic tracer experiments in organic chemistry are deuterium, tritium, and the and isotopes of carbon. There are several means of locating isotopic labels. Deuterium can frequently be located by analysis of NMR spectra. In contrast to the normal isotope, deuterium... 

A third and most significant study involves tracer experiments with Papaver rhoeas L. Both 135- and 13/ -tritium labeled scoulerines were incorporated into rhoeadine, but the rhoeadine isolated from plants fed with 135-labeled scoulerine had lost 19% of the tritium present in the precursor, whereas the 13/ -Iabeled scoulerine afforded rhoeadine which had retained 74% of the original tritium. A stereospecific loss of the pro-5 hydrogen must, therefore, have occurred from the C-13 of scoulerine at some specific stage of its biotransformation into rhoeadine. It seems likely that scoulerine is converted first into stylopine which is oxidized to 42. A-Methylation and rearrangement then provide the correct skeleton from which rhoeadine can arise... 

So far, the only published use of (cod)Ir(hfpd) in tritium labeling has been a tracer experiment with a model compound, but the method should work generally with tritium gas. It remains to be seen how broad the applicability will be to complex substrates. The utility of this catalyst class appears to be largely complementary to that of the iridium phosphine class, mainly because of its apparently general ability to label benzylamines and anilines and its activity in DMF and DMA solvents. Nitro groups tend to be reduced. But as with iridium phosphine complexes, (cod)Ir(hlipd) tends to reduce double bonds but... 

The incorporation of a chapter on deuteration in a steroid monograph is quite reasonable since development of a number of the most important deuteration reactions have actually had their impetus through steroid research. The field of steroid chemistry offers possibly the largest variety of deuteration reactions of any area of organic chemistry. Many of these deuteration techniques have also been used for tritium labeling, which is especially pertinent in view of the large demand for tritiated steroids as tracers in biological experiments. 

Tracer studies have been used in an attempt to determine the nature of the ends of the chain but these were as unsatisfactory as for propylene. Feldman and Perry (83) used triterated methanol to react the polyethylene from a titanium tetrachloridetrialkylaluminum catalyst. They found a continual increase in the number of polymeric chain ends which react with the tritium. This agrees with the results of Roha and Beears (84) who showed the very rapid exchange of alkyls which took place when ethylene was grown on a Ziegler catalyst in the presence of excess alkylaluminum chloride. In these experiments only an extremely small... 

Reaction (14) can be considered as irreversible because the tritium is present at tracer levels. For most aromatic systems the intermediate benzenonium ion is present in low concentrations compared with ArT under the conditions of the experiment and hence using the steady state approximation the rate of loss of tritium from the aromatic substrate is given by... 

Weston s experiment was carried out with tritium at the tracer level and, therefore, in the labeled molecule only 1 hydrogen atom out of 6 equivalent ones is replaced. The reaction studied thus has both an intermolecular and an intramolecular isotope effect. Up to the approximation of no secondary isotope effects, the high-pressure rate of isomerization of a protium atom in the labeled molecule will not differ significantly from that of the unlabeled molecule. With these assumptions Weston obtains (kHjkr)Ptst00 = 3.74 I.9at 775°K. The absolute error of the measurement is naturally magnified by the statistical correction for the intramolecular dilution. Because of this intramolecular dilution, a reliable temperature coefficient of the ratio of rate constants cannot be obtained by a least square treatment of the experimental data. As yet, the temperature dependence as a function of pressure has not been investigated. 

Radioactive isotopes are commonly used for competitive KIE measurements in a double-label experiment, yielding kn/kj or ko/kj ratios on kcat/ M- This technique typically utilizes tracer-level radioactivity in the position of interest (primary or secondary) to monitor the transfer of radioactivity from reactant to product, and requires a remote label (e.g. C) in order to measure the conversion of unlabelled substrate to product. As an example, [ring- C(U)]benzyl alcohol and [l- H]benzyl alcohol (Scheme 10.2) can be used to simultaneously measure the primary and a-secondary kn/kj effects in the reaction catalyzed by alcohol dehydrogenase (ADH), as the tracer tritium is incorporated randomly into primary and a-secondary positions [6, 10]. 

One reason Saunders proposed using the set of isotopomers presented in Eq. (11.17) was for synthetic considerations. For reactants with two hydrogenic sites attached to the same atom, it is typically much simpler to prepare the DD isotopomer than it is to synthesize a compound with high abundance D in an H D compound. The HT and DT isotopomers are generally easier to prepare because the tritium is at tracer levels. Figure 11.11 shows an example [89] of the labeled reactants needed for an experiment. 

Radioisotopic tracer techniques were applied to study the coadsorption of n-octadecane and stearic acid on a metal surface immersed in a n-octadecane solution of stearic acid. Dual labeling was employed for determining the surface concentrations of both n-octadecane and stearic acid. n-Octadecane was labeled with tritium and stearic acid with carbon-14. The results of half-hour adsorption experiments provide direct proof of coadsorption of polar and nonpolar materials on iron, copper, silver, and platinum surfaces. The films produced on silver and copper by 19-hour adsorption consisted of approximately one molecular layer of stearic acid and two molecular layers of octadecane. A new model is proposed to describe the structure of this thick coadsorbed film. 

It is obvious that tritium-labeled polar impurities were formed in the stored solvent. On the basis of these results it was decided to conduct all subsequent adsorption experiments with solutions prepared not more than 2 days before. The authors are aware that experimenters in this field appreciate the general requirement for solvent purity in adsorption experiments. However, it was thought that these data should be presented to emphasize the special importance of solvent purity when studying coadsorption of solvent and solute by tracer techniques and to point out the relatively short time required for impurities to develop. 

The methods employed for the miscible displacement studies were similar to those used in previous experiments (Hu and Brusseau, 1998). We connected a high-performance liquid chromatography (HPLC) pump (Model 301 from Alltech Associates Inc., Deerfield, IL) to the column, and placed a three-way valve in-line to facilitate switching between treatment solutions. Several iodine species (iodide, iodate, and 4-iodoaniline) were used to study transport behavior. We also examined the transport of tritium and bromide, commonly used conservative tracers, so that we could compare their transport behavior with iodine species. For transport experiments of 4-iodoaniline, which is used as a representative refractory organic iodine species, the solution was allowed contact only with glass or stainless steel, to avoid potential interaction of organoiodine with plastics in the column system. Column effluents were collected with an automated fraction collector (Retriever 500, ISCO Inc., Lincoln, NE) for chemical analysis, as described below. 

Radiochemical purity. The radiochemical purity is the fraction of the stated radioactive nuclide present in the stated chemical form. For tracers of elements stabilized in two or more oxidation states, it is necessary to check their oxidation state by their chemical behavior, ion exchange for example, preferably just before the experiment. In organic compounds labeled with a radioactive nuclide, it is desirable that the number and position of labeling of the radioactive nuclide are unique. However, when the number and position of the nuclide in a compound do not essentially affect its chemical behavior as is often the case in tritium-labeled ones, use of a mixture of a compound labeled with different number of the stated nuclide or labeled at different positions is acceptable. The purity of some labeled compounds decreases gradually due to oxidation, self- or radiolytic decomposition during long storage. Such a labeled compound should be assayed and purified, if necessary, before use. 

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