Isotopic labelling with tritium

Synthesis of tritium-labelled brefeldin-A by catalytic isotope exchange with tritium gas... 

Reduction of aldonolactones and their derivatives with isotopically modified reducing agents leads to sugars labeled at the anomeric center. Glycosides substituted with deuterium or labeled with tritium are widely employed for kinetic isotope-effect measurements, mechanistic studies, isotope-tracing experiments, and so on. 

More experimental data where the secondary KIE was larger than the EIE were subsequently published by Subramanian and Saunders (1984). The 2-arylethyl system was employed in these studies because other relevant data, such as the primary deuterium KIE, were available for this reaction. Special techniques were developed to determine the primary and the secondary tritium KIEs for this system. Three isotopically distinct elimination reactions (49-51) are possible for a 2-arylethyl derivative tracer labelled with tritium at the 2-position. 

When the hydrogen transferred as hydride to the cofactor is retransferred to the same carbon atom in the product, the movement is far more difficult to detect. The conversion of D-glucose 6-phosphate (58) into lL-mt/o-inositol 1-phosphate (61) occurs by cyclization of the carbon skeleton, with formation of a new bond between C-l and C-6. When each carbon atom in turn was specifically labeled with tritium, there was complete retention of tritium, even in the presence of added NADH, although there was an apparent, small isotope-effect with D-glucose-5-t 6-phosphate.19 The mechanism proposed for the cyclization19 was an initial oxidation at C-5 to give NADH and xylo-hexos-5-ulose 6-phosphate (59), followed by an aldol reaction causing cyclization to lL-myo-inosose-2 1-phosphate (60), which is then... 

Syntheses and uses of isotopically labelled dienes and polyenes 821 2. Synthesis of simple seven-membered ring compounds labelled with tritium... 

The validity of the above scheme was further explored with cadaverine samples chirally labelled with tritium at C-1. (The samples were obtained by decarboxylation of L-lysine mediated by L-lysinedecarboxylase from Bacillus cadaveris. The absolute configuration of the two materials is unknown and they were accordingly named [L4- H]- and [15- H]-cadaverine.) The labelling pattern of the derived N-methylpelletierine (22) was in accord with stereospecific oxidative deamination to A -piperideine (30) and in agreement with the proposed model. Both cadaverine samples afforded iV-methylpelletierine (22) with a label at C-6 but only [l/4- H]cadaverine labelled C-2. (The puzzling loss of 25% of the tritium from non-chirally labelled [l- C,l- H]cadaverine on incorporation into N-methylpelletierine is now explained in terms of this model, the tritium loss being exactly as predicted. It seems that subsequent elaboration of N-methylpelletierine (22) to pseudopelletierine (24) is accompanied by preferential tritium retention at C-6 by a primary isotope effect. )... 

In spite of the developments achieved in the preparation of samples labelled with tritium and carbon-14, however, no rapid combustion methods and suitable instruments have been available for the preparaticxi of multiple labelled organic substances containing other conbinatic is of soft beta omitting isotopes. On the other hand, the Improvenient of precision and automation as well as reduction of operational and maintenance costs seams also desirable. 

The adduct, which presumably is enzyme-bound, is thought to imdergo an intramolecular rearrangement to form thymidylate and dihydrofolate, which then leave the enzyme. Experiments with H4-folate, labeled with tritium exclusively in the 6-position hydrogen, have shown that the chick embryo synthetase also forms tritiated thymidylate (7) evidently an intramolecular transfer of a hydride ion is involved because no dilution of radioactivity occurs during introduction of the isotope into the methyl group. 

The mechanism of the reaction has been investigated with water labeled with tritium and with O. Neither isotope was found to enter the sugar. These results eliminate all mechanisms that require the... 

Labeling with tritium in position 15 could easily be accomplished by reduction of retinaldehyde with sodium borotritide (Mayer and Isler, 1971) or of retinoic acid esters with lithium aluminum tritide, and, again, oxidation of the tritiated retinol (Kaegi et aL, 1982c) to retinaldehyde-15- H. Because of the favorable isotope effect, most of the isotope should stay attached to the aldehyde group. However, Futterman et aL (1979) found that all-frany-retinaldehyde-15- H is inadequate as a tracer for 11-c/y-retinaldehyde formation in a biological system. 

Evans, E.A., in Isotopic Labeling with Carbon-14 and Tritium Essential Chemistry and Applications, Elvidge, J.A. and Jones, J.R., Eds. The Chemical Society BurUngton House, London, 1980, pp. 47 8. 

An application of this approach is the synthesis of chiral primary alcohols labeled with tritium at one of the enantiotopic a-hydrogens. Liver alcohol dehydrogenase, for example, reduces benzaldehyde to (7 )-[methylene- H]benzyl alcohol (27) in the presence of NAD/NAD H, which is regenerated by oxidation of (/ ,5)-[l- Hi]ethanol to acetaldehyde. Because the enzyme transfers exclusively the Re-positioned isotope of (R,5)-[ 1 - Hj Jethanol, the theoretical radiochemical yield is only 50% (the other 50% remains in acetaldehyde), and the specific activity of the product is only half that of this isotope source. One way to overcome this drawback is to use achiral, secondary alcohols such as [1- H]-cyclohexanol or [l- H]cyclopentanol (2S) as isotope sources. The corresponding (5)-antipode of 27 would be accessible by analogous reduction of benz[ H]aldehyde using nonisotopic cofactors. 

Decay products of the principal radionuclides used in tracer technology (see Table 1) are not themselves radioactive. Therefore, the primary decomposition events of isotopes in molecules labeled with only one radionuclide / molecule result in unlabeled impurities at a rate proportional to the half-life of the isotope. Eor and H, impurities arising from the decay process are in relatively small amounts. Eor the shorter half-life isotopes the relative amounts of these impurities caused by primary decomposition are larger, but usually not problematic because they are not radioactive and do not interfere with the application of the tracer compounds. Eor multilabeled tritiated compounds the rate of accumulation of labeled impurities owing to tritium decay can be significant. This increases with the number of radioactive atoms per molecule. 

The Brensted catalysis law can be applied to the problem of determination of acidity of very weak acids in the following way. First, a suitable base is chosen the base must be sufficiently strong to remove protons from the carbon acids in question at a measurable rate. The acids to be investigated are then prepared with deuterium or tritium substituted for hydrogen, and the rate of exchange of the isotopic label out of the carbon acid in the presence of the base is measured. 

In general, deuterium substitution can be used to decide which hydrogen atom in a molecule is the one most concerned in transfer processes. Introduction of deuterium at this position will reduce the transfer constant, and if transfer is accompanied by retardation, it will also reduce the extent of retardation. For work of this type, tritiumlabelling of the additive is not suitable substances so labelled usually contain only a very small proportion of molecules actually containing tritium and although there is a large isotope effect with these molecules, their number is so small relative to that of the unlabelled molecules that their influence cannot be detected. 

Because it is possible to label gibberellic acid with tritium, isotopic procedures are eminently suited for collecting the necessary data, circumventing some of the... 

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