Hydrogen-deuterium-tritium exchange

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). 

J. W. Pyper and C. K. Briggs, Kinetics of the Radiation-induced Exchange Reactions of Hydrogen, Deuterium, and Tritium, Eawrence Eipermore Eaboratory Report... 

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... 

Exchange reactions between hydrogen, deuterium, or tritium located at C-8 have been examined in a wide variety of purines and their derivatives which include the parent member, adenine," guanosine, hypo-xanthine, ° xanthine, 6-chloropurine, 6-mercaptopurine, - and others. 

The evidence from experiments involving deuterium and tritium exchange for hydrogen, as well as that from investigations of its catalysis,... 

Dictamnine labeled on the furan hydrogens with tritium or deuterium was prepared by cyclization of aldehyde 247 with tritiated or deuterated PPA (J). Treatment of [2,3-2H2]dictamnine with aqueous acid did not result in exchange of deuterium atoms. In the formation of [2,3-2H2]dictamnine, exchange of deuterium between the 2- and 3-positions of the furan ring may occur by a 1,2-shift, (251) (252) (Scheme 19), although an alternative mechanism for exchange of the aldehyde proton involves addition of D+ to the enol form of aldehyde 250 assisted by the... 

Since 1952, most of the tritium measured in the atmosphere originates from thermonuclear explosions. Like hydrogen, deuterium and tritium also exhibit molecular isomerism. Because of the important differences between the relative atomic masses of the three isotopes, their physical properties (e.g., density, enthalpy of vaporization) differ greatly. This allows an easier isotopic separation than for any other element. Several separation processes are used for the enrichment and separation of hydrogen isotopes. Most of these processes use isotopic exchange reactions (e.g., H D-H O or NH3-HD) and to a lesser extent fractional distillation and water electrolysis (e.g., Norway, Canada). 

Isotope concentration While deuterium model reactions are often conducted with pure deuterium oxide as the isotope source, tritium oxide is rarely used at anything close to nuclidic purity (note that tritiated water at 50 Ci/mL, the highest specific activity normally available commercially, has a tritium/hydrogen ratio of only about 1.6/98.4). Therefore, the concentration of tritium in HHO is usually much lower than that of deuterium in HjO, and this difference will be important if the source concentration is a factor in the rate equation. Analogously, model exchange reactions with deuterium gas are often done at one atmosphere of pressure, whereas in most cases tritium gas is used at lower pressures. This can result in substantially slower tritium exchange rates. 

Several experimental techniques have have been used to study conformational flexibility in protein molecules. Spectroscopic methods such as NMR, ESR, Mossbauer, and fluorescence techniques provide direct and detailed studies of internal motions in proteins. Hydrogen-deuterium (or tritium) exchange methods are particularly convenient for kinetic studies. Immuno-chemistry may be also a very helpful method to study conformational fluctuations. These results are discussed in the second part of this volume. 

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