Tritium Subject

Tritium is the subject of various reviews (6—8), and a book (9) provides a comprehensive survey of the preparation, properties, and uses of tritium compounds. Selected physical properties for molecular tritium, are given in Table 1. 

A large scale preparation of E. coli 045 was subjected to enzyme purification using the assay for 3,5-epimerase. Protamin sulfate precipitation, ammonium sulfate fractionation was followed by DEAE-chroma-tography. The fraction containing enzymatic activity, as measured by tritium exchange, was eluted from the DEAE column early. This fraction was incapable of producing any net synthesis of TDP-6-deoxy-L-... 

TLC-Radio-label Procedure — The volunteer subjects (described in Clinical Protocol) all received 100 yCi of tritium labeled A9-THC, along with the standard 4.0-5.0 mg intravenous dose. Two to three ml aliquots of plasma were analyzed by the procedure described by Wall (4). ... 

The postexchange mixture in the tritium-labelling experiment has not been subjected to oxidation, and if the tritiated (saturated) B-ring product had been formed, it probably would not have undergone reoxidation and its bromoacetylated derivative has been separated from 148 in the course of its HPLC purification. 

Tritium is a very sensitive subject for public acceptance of fusion and will play a central role in the operation of a next-step experimental fusion facility, which will routinely use large amounts of tritium as fuel (e.g., 100 times more in ITER than in present experiments) in a mixture with deuterium. Tritium retention is a regulatory issue since the amount that can potentially be released in an accident sets the limits on plasma operation without removal. Fuel economy has never been an issue in deuterium-fuelled experiments and only recently have the limitations associated with the use of tritium, and its incomplete recovery in experiments in TFTR and in JET, brought the issue of fuel retention under closer scrutiny [56,57]. Table 12.3 provides a list of key quantities related to tritium in existing tokamaks and a next-step device [18,57-59]. 

The mechanism of the Wilzbach method applied to the labeling of aromatic compounds was also the subject of a number of investigations, usually involving the measurement of the intramolecular tritium distribution within the labeled aromatic substance. Whilst toluene was the most frequently employed aromatic substrate (Aliprandi et al., 1960 Cacace andPossagno, 1960 Cacace, 1961a Ache etal., 1961, 1962 Ache and Herr, 1962 Ache, 1967), chlorobenzene, nitrobenzene and anisole (Cacace et al., 1960) and anthranilic acid (Crawford and Garnett, 1965 Garnett et al., 1965) have also been studied. 

The rates of reaction of hypophosphorous acid with iodine bromine ", chlorine ", iodine chlorides , iodate , selenious and tel-lurous acids, silver nitrate , cupric chloride and mercuric chloride" (all forming phosphorous acid or phosphites) have been measured, and the results of the earlier work summarized clearly" . All the data are consistent with the hypothesis that there is prior transformation to some reactive form (I). This form (I) does not discriminate very effectively between different oxidants and thus the oxidation steps are presumed to have rates close to the diffusion-controlled limit. The rates of formation of I deduced in these studies are close enough to the rates of deuterium and tritium exchange for the residual difference to represent an isotope effect. Mitchell wrote the formula H5PO3 for I. Others have supposed it to be a tautomer e.g. HPO(OH)2. Both the isotopic exchange results and the oxidation studies require that its formation and decomposition be subject to acid catalysis. For the general mechanism... 

Since all of the tritium in the crops was in the form of tritiated water, it seems reasonable to assume that the uptake into the plants derives from the aqueous fraction in the soil. The bulk of the tritium in the rooting zone in the soil was however still in the form of OBT. The uptake into the crop would therefore appear to come from a mobile pool of tritiated water. This pool would be subject to losses via transfer into the plant, downward leaching out of the rooting zone and transpiration into the atmosphere as water vapour. 

The answer is found in a series of studies begun by Lars Melander (of the Nobel Institute of Chemistry, Stockholm) and extended by many other workers. A variety of aromatic compounds labeled w ith deuterium or tritium were subjected to nitration, bromination, and Friedel-Crafts alkylation. It was found that in these reactions deuterium or tritium is replaced at the same rate as protium there is no significant isotope effect. 

The use of incorporation efficiencies as a measure of the relative importance of various precursors within a biosynthetic pathway must often be subject to considerable uncertainty since it is difficult to achieve identical conditions in successive feeding experiments. This problem is overcome " if the precursors are fed together to the same plant (or culture), distinction between them being made by use of different isotopic labels e.g. and H. A problem with this approach is that, particularly, tritium (or deuterium) may be lost from one of the precursors, leading to a false result. An ingenious solution to this difficulty is to run a second experiment in which only the substrate which bears the tritium label in the first experiment is fed, but labelled now with C and H. The ratio observed in the metabolite then... 

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