Tritium in Catalysis

Hydrogen isotope exchange (A) is therefore a very versatile reaction and as it can be acid-, base- or metal-catalysed it has been extensively used.l l It is also ideal for studying solvent effects in reaction kinetics. Recent attention has focused on energy-enhanced reactions, particularly those involving 

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The reduction of the pyrimidine to dihydropyrimidine is the reverse of the oxidation reaction carried out by DHODs. The structure of the FMN/pyrimidine-binding site is very similar to the structure of L. lactis DHODs. Three Asn residues form hydrogen bonds with the nitrogens and carbonyls of the pyrimidine analogous to DHODs. DPD has an active site cysteine proposed to act in acid/base chemistry similar to Class 1 DHODs. When mutated to alanine, only 1% of the wild-type activity was retained, indicating the importance of this residue in catalysis. Secondary tritium isotope effects using 5- H-uracil were determined in both H2O and D2O an inverse isotope effect was observed in H2O and the value became more inverse in D20. " This was taken as evidence of a stepwise mechanism in which hydride transfer to C6 is followed by protonation at C5. 

Figure 8.47 Stereochemistry in Catalysis. Biocatalyst stereochemistry may be followed with chiral substrates to reveal stereochemical changes during catalysis. Substrates include, chiral acetylCoA and phosphate (that undergo inversion or retention of configuration with reaction), and chiral NADH(D) designed to demonstrate whether prochiral (proR or proS) are employed in reduction. mMDH uses the proR hydrogen as illustrated by deuterium (D) transfer to the product (bottom). D, deuterium T, tritium.

A number of studies of the acid-catalyzed mechanism of enolization have been done. The case of cyclohexanone is illustrative. The reaction is catalyzed by various carboxylic acids and substituted ammonium ions. The effectiveness of these proton donors as catalysts correlates with their pK values. When plotted according to the Bronsted catalysis law (Section 4.8), the value of the slope a is 0.74. When deuterium or tritium is introduced in the a position, there is a marked decrease in the rate of acid-catalyzed enolization h/ d 5. This kinetic isotope effect indicates that the C—H bond cleavage is part of the rate-determining step. The generally accepted mechanism for acid-catalyzed enolization pictures the rate-determining step as deprotonation of the protonated ketone ... 

Electronically excited states of organic molecules, acid-base properties of, 12,131 Energetic tritium and carbon atoms, reactions of, with organic compounds, 2, 201 Enolisation of simple carbonyl compounds and related reactions, 18,1 Entropies of activation and mechanisms of reactions in solution, 1,1 Enzymatic catalysis, physical organic model systems and the problem of, 11, 1 Enzyme action, catalysis of micelles, membranes and other aqueous aggregates as models of, 17. 435... 

Isotope effects have also been applied extensively to studies of NAD+/NADP+-linked dehydrogenases. We typically treat these enzymes as systems whose catalytic rates are limited by product release. Nonetheless, Palm clearly demonstrated a primary tritium kinetic isotope effect on lactate dehydrogenase catalysis, a finding that indicated that the hydride transfer step is rate-contributing. Plapp s laboratory later demonstrated that liver alcohol dehydrogenase has an intrinsic /ch//cd isotope effect of 5.2 with ethanol and an intrinsic /ch//cd isotope effect of 3-6-4.3 with benzyl alcohol. Moreover, Klin-man reported the following intrinsic isotope effects in the reduction of p-substituted benzaldehydes by yeast alcohol dehydrogenase kn/ko for p-Br-benzaldehyde = 3.5 kulki) for p-Cl-benzaldehyde = 3.3 kulk for p-H-benzaldehyde = 3.0 kulk for p-CHs-benzaldehyde = 5.4 and kn/ko for p-CHsO-benzaldehyde = 3.4. 

Pfizer has also prepared both and " C-labelled ziprasidone (Schemes 18 and 19) to detennine its metabolism and tissue distnbution. It was envisioned that tritium could be introduced in the last step of e synthesis utilizing the selective replacement of a bromine atom on the benzisothiazole nng. Therefore, the synthesis began with the bromination of 39 using bromine in acetic acid with FeCls catalysis. The dibrominated regioisomer 60 was isolated by liquid chromatography in 18% yield and reacted with... 

A similar result was obtained by Schowen and Behn280, for the methanolysis of p-nitrophenyl acetate in the presence of tritium-labelled acetate ion. In this case the intermediate anhydride is effectively symmetrical, and methanol will attack equally at the labelled and unlabelled acetyl group. As expected for nucleophilic catalysis, 50% (within experimental error) of the methyl acetate produced was labelled T3CCOOCH3. 

A considerable amount of work, both with thymidylate synthetase itself and model systems, indicates that in the mechanism of thymidylate synthetase a key step is the attack of an enzymatic nucleophile (believed to be cysteine) on C-6 of dUMP to give a 5,6-dihydro-dUMP intermediate (B-77MI11003). A number of observations are in accord with such a nucleophilic catalysis by the enzyme. Among these are the demonstration that thymidylate synthetase, in the absence of the cofactor, catalyzes the exchange of tritium from [5-3H]dUMP for protons of water (79B2794), and the recent isolation of a covalent adduct formed between 5-nitro-dUMP and thymidylate synthetase (80JBC(255)5538 see also 80MI11003). 

When we speak of the papers of the 1950s, we must not fail to mention articles published in 1954-1960 on muon catalysis (for a review with complete bibliography, see [43]). Various aspects of cold muon catalysis in nuclear reactions of hydrogen, deuterium, and tritium were considered. In recent years the ideas which lay at the foundation of these papers have been further developed and verified. In particular, it was Ya.B. [40] who noted the decisive role of muon adhesion to the helium nucleus which is formed in the reaction this process restricts the number of reaction steps, and the future for applying this process in energy engineering depends on it. 

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. 

A single muon stopped in a target of deuterium-tritium mixture can catalyze more than 100 fusions, but this number is limited by two major bottle-necks. One is the rate at which a muon can go through the catalysis cycle before its decay (cycling rate), and another is a poisoning process called p-a sticking in which, with a probability u)s < 0.01, the muon gets captured after the fusion reaction to atomic bound states of the fusion product 4He, and hence lost from the cycle (see Section 5). 

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