Tritium compounds, stability

For temperatures near ambient, most metal-hydrogen systems exhibit the so-called normal isotope effect, i.e., the stability order is protide > deuteride > tritide. There are, however, a few systems in which replacement of hydrogen by deuterium or tritium results in a more stable compound. A detailed review of the hydrogen isotope effect exhibited by metal-hydrogen systems is outside the scope of this discussion. It is pertinent, however, to note briefly those systems that are relevant to this work. Our discussion is limited to temperature at or near room temperature, and the inverse isotope effect is defined by the stability order tritide > deuteride > protide. 

When the acidities of hydrocarbons are compared in terms of the relative stabilities of neutral and anionic forms, the appropriate data are equilibrium acidity measurements, which relate directly to the relative stability of the neutral and anionic species. For compounds with pA > 35, it is difficult to obtain equilibrium data. In such cases, it may be possible to compare the rates of deprotonation, i.e., the kinetic acidity. These comparisons can be made between different protons in the same compound or between two different compounds by following an isotopic exchange. In the presence of a deuterated solvent, the rate of incorporation of deuterium is a measure of the rate of carbanion formation. Tritium ( H)-NMR spectroscopy is also a sensitive method for direct measurement of kinetic acidity. ... 

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. 

After covalent binding of the inactivator, the enzyme-analog compound is stabilized by reduction of its carbonyl to a carbinol group (Fig. 3). To 100 mg of inactivated enzyme in 0.15 M sodium pyrophosphate (pH 6.0) at 0° are added 10 mg of sodium borohydride. At this pH, formation of dihydro or tetrahydropyridine derivatives could not be observed. Excess sodium borohydride is removed by dialysis. With tritium-labeled sodium borohydride this method yields a tritiated protein complex. 

The organization of this book is as follows. The remainder of this chapter provides short accounts of purification, analysis and storage and stability of compounds labeled with carbon-14 and tritium, and descriptions of some common techniques and technologies unique to work with these isotopes. Chapter 2 discusses some strategies particularly appropriate for planning syntheses of compounds labeled with carbon-14 and tritium, an appropriate topic for inclusion because there are distinct differences vis-a-vis the ways nonisotopic synthetic problems are approached, and an appreciation of these differences is... 

Stability and Storage of Compounds Labeled with Tritium or Carbon-14... 

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