Tritium-hydrogen isotope exchange

Most of the chemical properties of tritium are common to those of the other hydrogen isotopes. However, notable deviations in chemical behavior result from isotope effects and from enhanced reaction kinetics induced by the ( -emission in tritium systems. Isotope exchange between tritium and other hydrogen isotopes is an interesting manifestation of the special chemical properties of tritium. 

A large part (65%) of the label located at the C(2)(H) position in 152 is probably due to base-catalysed hydrogen isotope exchange still operating even in the presence of prereduced Pt02. The specific activities of the tritium-labelled epoxides by the same procedure are lower than for the imidazoles. The specific activities of the tritiated imidazoles can be improved by using tritiated water of higher specific activity. 

Another case of the combination of old and new data involves hydrogen isotope exchange with chloroform (as CCI3T), again in basic water/DMSO mixtures(42). The rapidly increasing rate of loss of tritium limited the accessible solvent range to 0-24 % DMSO. The... 

Solvent-free methods of hydrogen isotope exchange labeling usually utilize tritium gas as the isotope source, and the substrate is used either neat, as a solid, or as a component of a solid phase. Therefore in contrast to the methods of Sections 3.1—3.5 this class of methods are, at least at a gross level, gas-solid two-phase systems. 

Isotopic Exchange Reactions. Exchange reactions between the isotopes of hydrogen are well known and well substantiated. The equihbrium constants for exchange between the various hydrogen molecular species have been documented (18). Kinetics of the radiation-induced exchange reactions of hydrogen, deuterium, and tritium have been critically and authoritatively reviewed (31). The reaction T2 + H2 — 2HT equiUbrates at room temperature even without a catalyst (30). 

Hydrogen-Tritium Exchange Studies. Isotope exchange experiments were conducted as described previously for the 7-phase hydrides (except TiMo2, that does not form the 7-phase). The results of the isotope separation determinations are shown in Table VI, where the parameter a is defined as ... 

Isotopic exchange in most cases does not greatly influence the physico-chemical properties of the total compound. In the case of tritium for hydrogen exchange however, the isotope effect of tritium can influence the pKA of a basic moiety in the vicinity of the tritium atom [1,2]. Although only a small shift in the pKA is observed (0.05-0.1), this can be of great importance for the biological activity of a radiopharmaceutical. 

The data given in Table 7 indicate that the deuterium-tritium kinetic isotope effect in hydrogen exchange with bases is in fact positive and lies within the limits of approximate theoretical estimates (Bigeleisen, 1949 Melander, 1960 Zollinger, 1958). The differences in the values... 

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