Studies with Deuterium

STUDIES WITH DEUTERIUM The Biosynthesis of Methionine Transmethylation 

Creatine was first isolated in 1835 by Chevreul 20 years later Dessaignes showed it to contain a methyl group. Choline was obtained from lecithin in bile by Strecker in 1849 and methionine isolated by Mueller in 1922. That methionine contained a methyl group linked to sulfur was demonstrated by Barger and Coyne in 1928. 

The first indication that methylation occurred in vivo was probably the observation that if pyridine was given to animals, methyl pyridine was excreted in their urine (His, 1887). From his studies on the fate of administered tellurides and their excretion as methyl telluride, Hofmeister (1894) proposed methylation might be a normal metabolic process in animals. This was conclusively established as a result of extensive work from du Vigneaud and his group between the late 1930s and 1955. Standard nutritional experiments with rats were combined with pioneering studies on the metabolism of 2H-labeled compounds (see du Vigneaud, 1952). 

The question therefore arose about the fate of the methyl group from methionine. When minimal amounts of methionine were used to supplement the diet of rats given homocysteine as their main source of sulfur, the rats did not usually thrive, and at death had fatty accumulations in their livers. Best and his co-workers had earlier reported the efficacy of choline as a lipotropic agent, facilitating the mobilization of fat from the liver. Du Vigneaud therefore tried supplementing homcys- 

The mechanism of transmethylation was then examined. A series of deuterium-labeled methylated compounds were synthesized by du Vigneaud s group, including arsenocholine, trimethylamine, dimethyl-glycine, and dimethylthetin. Of these only betaine and dimethylthetin served as methyl donors. In 1949 Dubnoff found that choline could only act as a donor under aerobic conditions, when it was oxidized to betaine. 

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As another example, studies with deuterium-labeled substrates have shown that the reaction of ethanol with the coenzyme NAD+ catalyzed by yeast alcohol dehydrogenase occurs with exclusive removal of the pro-R hydrogen from ethanol and with addition only to the Re face of NAD+. 

Early studies with deuterium-labeled57 58 and l4C-labeled compounds59 60 established that cleavage of the double bonds occurs. For example, the degenerate metathesis of 2-butene and perdeutero-2-butene57 results in the formation of [l,l,l,2-2H4]-2-butene as the only new labeled compound [Eq. (12.11)] which is consistent with alkylidene exchange [Eq. (12.10)] ... 

Katz, J. J. Chemical and Biological Studies with Deuterium/5 American Scientist, 48, 544 (1960). 

One reaction, mentioned in this section, combines some features of the meth-ylenecyclopropane rearrangement and the vinylcyclopropane rearrangement (Section III.C) it is the pyrolysis of 2-vinyl methylenecyclopropane. Here, too, studies with deuterium-labeled compounds have been valuable in helping to unravel the various mechanistic pathways involved. 

Most recently, the influence of the application of macroscopic tensile stress to the PPTA fiber on the microscopic dynamic structure has been studied with deuterium NMR [29]. 

Since no intermediates could be detected in the in vitro studies of the oxidation of acetylenic derivatives, it was assumed that very short-lived intermediates were probably formed. In order to further characterize this pathyway, studies with deuterium and labeled biphenylacetylene (acetylenic hydrogen and internal acetylenic carbon, respectively) were initiated. The mass and nuclear magnetic resonance spectra of the resulting biphenyl acetic acid derivative showed that ... 

A Stille-related paper reported different regioselectivities in a q clocarbo-palladation in which the multistep reaction described terminated in a Stille cross-coupling. Mechanistic studies with deuterium provided a background for a discussion of the different outcomes when alkynyl or vinyl stannanes were used. Microwave heating was found to increase the rate of the reaction with two vinyl stannanes and one heteroaromatic stannane as substrates [80]. 

The behavior of methine-labeled poIy(ethyl acrylate)-di (PEA-di), poly(wo-propyl acrylate)-di (PIPA-di), and poly(n-butyl aciylate)-di (PNBA-di) has been studied with deuterium NMR relaxation time measurements in concentrated solutions with chloroform. PEA-di and PNBA-di behaved similarly in terms of solution dynamics, but PIPA-di was found to reorient significantly faster at similar concentrations. The relaxation times were fitted to a log-normal distribution of correlation times and the resulting mean correlation times fit to Arrhenius behavior. The energies of activation were found to increase with increasing concentration from about 6 kJ/mol at lower concentrations to 10-20 kJ/mol from about 40 to 80 wt % polymer. 

Similarly, a tetrameric platina(IV)-cyclobutane intermediate 37 was postulated in the ring homologation reaction of tetracyclo[3.3.1.0 . 0 ]nonane (38), which on treatment with Zeise s dimer (C2H4PtCl2)2 led cleanly to the alkene 39 (equation 16). The mechanism of this rearrangement was established by studies with deuterium and labelling experiments. 

TBDMS derivatives have been applied to the analysis of organic acids in urine as oxime-TBDMS [91] and in tissues [92] as methoxime-TBDMS derivatives. The intense [M —57]" ions found in the spectra of the latter were shown to contain all the original hydrogen atoms. Thus, the derivatization should find application in quantification by isotope dilution and in metabolic studies with deuterium-labelled compounds. TMS and oxime-TMS derivatives, however, still remain the more popular derivatives for general screening, a consequence of the more structurally informative mass spectra, shorter analysis times and the ready availability of reference spectra. 

The equilibrium constant of this reaction depends on the temperature. The tantalacycle is exclusively trans-, / -disubstituted. This decomposes to give 2,3-dimethyl-1-butene. In the presence of an excess of propylene the dimer forms catalytically. Detailed studies with deuterium labelled olefins show that the dimer is not directly formed by reductive elimination from an alkenyl hydride intermediate. The most satisfactory explanation is that the tantalum hydride adds back to the alkenyl double bond to give a tantalacyclobutane which then rearranges to one of two possible olefins. This unexpected ring contraction supposes a rapid and irreversible decomposition of the less favored ring strained species. 

Stepanov AG, Alkaev MM, Shubin AA, Luzgin MV, Shegai TO, Jobic H. Dynamics of isobutane inside zeolite ZSM-5. A study with deuterium solid-state NMR. J Phys Chem B 2002 106 10114—20. 

The relative bioavailability and bioactivity of the different forms and food sources of vitamin K need more research. Preliminary studies with deuterium-labeled broccoli suggest that the bioavailability of endogenous vitamin K can be studied in humans by intrinsic stable isotope-labeling procedures. 

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