Isotope labeling, deuterium

The acidity probes discussed above are the most commonly used. However, the use of many different probes has been reported in the literature. This list includes nitriles, alkanes, amines, water, di-hydrogen, deuterium, isotopically labeled molecules, benzene, etc. Probe molecules can also be used to measure basicity on zeohtes. In this case, weakly acidic molecules such as CO2, pyrrole, acetic acid and halogenated light paraffins have been used. Space does not permit discussion of these in any detail, but information about these probes and their applications can be found in the following references [87, 127-130]. 

This reduction almost certainly proceeds via the activation of the aldehyde carbonyl both by bond polarization in the ground-state and by stabilization of bonding interactions in the transition state. Although deuterium isotope labeling experiments show that hydrogen transfer is direct, comparison of kinetic isotope effects with isotope discrimination studies (23) suggest that the transition state of this reaction may not be a simple transfer of hydride ion (see Section IV—2). 

Only a year later, Gagosz and coworkers [32] published a synthesis of 10-membered medium-sized cycloalkynes 28 from 1,10-diynes 27. Like in the earlier investigation of Houk and Toste, the ligand was a bulky phosphane ligand. Two important mechanistic proposals were included in this publication the one shown in Scheme 4.10a which is based on the proposal of Houk and Toste [31], and also the alternative pathway shown in Scheme 4.10b. Both pathways are in accordance with the deuterium isotope labeling experiments conducted. The Houk/Toste catalyst transfer step was not considered by Gagosz. On the other hand, Gagosz already addressed a possible role of the counterion as a proton acceptor, a motive later discussed by Corma [33] and Widenhoefer [34] for the formation of species related to I and Q. 

These three key findings, along with a series of deuterium isotope labeling experiments and the important additional computational results of Zhang s group, led to the following new mechanistic cycle for the formation of 35 (Scheme 4.15). 

In order to obtain some detail about this transformation, a series of deuterium isotope-labeling experiments were conducted. By the treatment of diphenylacetylene with D2O instead of H2O, the deuterated indanone was... 

The first mass spectrometric investigation of the thiazole ring was done by Clarke et al. (271). Shortly after, Cooks et al., in a study devoted to bicydic aromatic systems, demonstrated the influence of the benzo ring in benzothiazole (272). Since this time, many studies have been devoted to the influence of various types of substitution upon fragmentation schemes and rearrangements, in the case of alkylthiazoles by Buttery (273) arylthiazoles by Aune et al. (276), Rix et al. (277), Khnulnitskii et al. (278) functional derivatives by Salmona el al. (279) and Entenmann (280) and thiazoles isotopically labeled with deuterium and C by Bojesen et al. (113). More recently, Witzhum et al. have detected the presence of simple derivatives of thiazole in food aromas by mass spectrometry (281). 

Substitution of deuterium for hydrogen at the a carbon atom of an aldehyde or a ketone is a convenient way to introduce an isotopic label into a molecule and is readily carried out by treating the carbonyl compound with deuterium oxide (D2O) and base... 

Isotopically Labeled Compounds. The hydrogen isotopes are given special names H (protium), H or D (deuterium), and H or T (tritium). The superscript designation is preferred because D and T disturb the alphabetical ordering in formulas. 

Below — 140°C, the EPR spectrum observed was that of the cyclopropylmethyl radical. If the photolysis was done above — 140°C, however, the spectmm of a second species was seen, and above — 100°C, this was the only spectmm observed. This second spectmm could be shown to be that of the 3-butenyl radical. This study also established that the 3-butenyl radical did not revert to the cyclopropylmethyl radical on being cooled back to — 140°C. The conclusion is that the ring opening of the cyclopropyl radical is a very facile process and that the lifetime of the cyclopropyl radical above — 100°C is very short. Even though the equilibrium favors the 3-butenyl radical, the reversible ring closure can be detected by isotopic labeling experiments, which reveal the occurrence of deuterium migration ... 

A disadvantage of this technique is that isotopic labeling can cause unwanted perturbations to the competition between pathways through kinetic isotope effects. Whereas the Born-Oppenheimer potential energy surfaces are not affected by isotopic substitution, rotational and vibrational levels become more closely spaced with substitution of heavier isotopes. Consequently, the rate of reaction in competing pathways will be modified somewhat compared to the unlabeled reaction. This effect scales approximately as the square root of the ratio of the isotopic masses, and will be most pronounced for deuterium or... 

Although the radioactive isotope H has been extensively used for studies on the uptake of xenobiotics into whole cells, the intrusion of exchange reactions and the large isotope effect renders this isotope rather less straightforward for metabolic studies. Both deuterium H-labeled substrates, and oxygen and OH2 have, however, been extensively used in metabolic studies, since essentially pure labeled compounds are readily available and mass spectrometer facilities have become an essential part of structural determination. 

Further evidence for different reaction pathways is obtained in isotope labelling experiments, illustrated in fig.6. Here we present a comparison of the effects of preadsorbing various amounts of deuterium with methyl groups cm the two metal surfaces. 

Figure 16.2 The original design of the ICAT compound. The iodoacetyl group provides reactivity with thiol groups. The isotopically labeled spacer arm typically is substituted with eight deuterium atoms.

Figure 16.4 A more advanced ICAT design uses an acid-cleavable spacer arm to facilitate elution of labeled peptides from a (strept)avidin affinity column. The use of 14C isotopes instead of deuterium labels permits precise reverse phase separations prior to mass spec that show no elution peak time differences between isotope-labeled and normal atom-labeled peptides.

The use of a deuterium-labeled organosilicon hydride and location of the deuterium isotope in the reduced product shows that 1,2-hydride shifts also occur. Thus, reduction of 1-bromohexane with triethylsilane-A yields hexane with all of the deuterium at C2 (Eq. 51) similar treatment of cyclohexylmethyl bromide produces melhyIcyclohexane-1 -di (Eq. 52).186... 

Sections I-V of this chapter deal with the syntheses of unsaturated organic compounds playing an essential role in biochemical processes of life. Numerous polyunsaturated compounds have been synthesized in order to elucidate their physiological role, for instance in brain. However, the main impact on permanent searches for new improved methods of synthesis of isotopically labelled dienes and polyenes comes from nuclear medicine and nuclear pharmacy. The deuterium and carbon-13 labelled polyunsaturated compounds are needed as internal standards in mass spectral determinations of very low concentrations of biologically active substances in biological fluids. 

The authors have also synthesized134 fatty acids labelled with deuterium and carbon-11 in order to investigate if kinetic isotope effects related to fatty acid metabolism can be observed in vivo by pet133,135-137. In vitro, the large kinetic deuterium isotope effects are observed in the oxidation of deuteriated aliphatic carboxylic acids with alkaline permanganate and manganate135-139. 

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. 

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