Chemical reactions isotopic labeling

The work on HIV protease demonstrates how chemical protein synthesis allowed isotope labeling of a 22-kDa protein with atomic precision and provided further insights into the chemical basis of the proteolytic cleavage reaction. Isotope labeling with atomic precision has since then been used to reveal structural features of other either chemically synthesized or semisynthetic proteins (27-29). 

The stereochemical relationship between the reactant and the product revealed by the isotopic labeling shows that oxygen becomes bonded to carbon on the same side from which H IS lost As you will see m this and the chapters to come determining the three dimensional aspects of a chemical or biochemical transformation can be a subtle yet powerful tool for increasing our understanding of how these reactions occur... 

Slightly removed from this in rigor is the use of a substituent to make a pure exchange into a net chemical reaction. No isotopic label is then needed. For example, the first reliable estimate of the rate constant for the exchange of ferrocenium ions and ferrocene was made on the basis of kinetic data for processes such as... 

The chemical reactivity of the organoruthenium and -osmium porphyrin complexes varies considerably, with some complexes (M(Por)R2, M(Por)R and Os(OEP)(NO)R) at least moderately air stable, while most are light sensitive and Stability is improved by handling them in the dark. Chemical transformations directly involving the methyl group have been observed for Ru(TTP) NO)Me, which inserts SO2 to form Ru(TTP)(N0) 0S(0)Me and Ru(OEP)Me which undergoes H- atom abstraction reactions with the radical trap TEMPO in benzene solution to yield Ru(OEP)(CO)(TEMPO). Isotope labeling studies indicate that the carbonyl carbon atom is derived from the methyl carbon atom. "" Reaction of... 

Isotopic-labeled tracers behave like the components in the fluid of interest. For example, tritium water behaves like water. If less similar chemicals are used as tracers, selective adsorption, chemical reaction, and liquid-liquid distribution must be considered. The tracer must be chosen so that the analytic method is sufficiently sensitive to detect the tracer in the desired amounts. 

In addition to the stable isotope labeling ( 0 versus 0) of proteins for quantifiable proteomic analyses as described above, chemical approaches to the protein-labeUng problem have developed in great variety. These so-called affinity tags can be used to label specific side chain groups such as sulfhydryl or amino groups, active sites for serine and cysteine hydrolases and many others. This active research area has been reviewed recently by A. Leitner and W. Lindner in a Proteomics article entitled Chemistry meets proteomics The use of chemical tagging reactions for MS-based proteomics. ... 

Solution phase chemists have developed a tremendous variety of tools to elucidate mechanisms. Spectroscopy, kinetics, isotopic labeling, and many more are all in the chemical mechanic s tool kit, for use in mapping out reaction pathways. In contrast, the tool kit for the gas phase reaction mechanic is far more limited. The low concentration and short lifetime of gas phase reaction intermediates and products severely limits the use of many of the conventional tools. Gas phase ion-molecule chemists have therefore both adapted solution phase tools to their unique needs and developed many new ones. 

The chemical reactions involving positron emitters have to be specially designed to take into account the short half life of the radionuclide, the limited number of labelled precursors and the sub-micromolar amounts of these precursors. Moreover, the reactions must be possible without any addition of the stable isotope (especially when ligands of receptors are synthesized). Several practical considerations that influence the design of positron-emitter radiotracers with a high specific radioactivity and their experimental handling have been reviewed [4,8,14-19]. 

Isotopic labels (and especially enriched materials) have proven crucial in the investigation of the mechanisms of homogeneously catalyzed reactions [130]. Further, isotope effects on the rate or the equilibrium constant of a reaction can be diagnostic, and structural information can be provided by isotope-induced changes in the chemical shifts of neighbouring nuclei, and/or alterations in the coupling pattern of the detected spectra. The isotope- and position-specific information inherent to NMR techniques are ideally suited for the analysis of isotope effects in catalysis [131]. 

Another approach combines the mass spectrometric derivatization approach with chemical amplification (Reiner et al., 1997, 1998). In this instrument, H02 and R02 are converted to OH through the reactions in the chemical amplifier approach discussed below, and the OH is then converted to H2S04 by reaction with S02 and measured by chemical ionization mass spectrometry using NO, (HN03) clusters as described earlier. In this case, the use of isotopically labeled S02 is not necessary, since the ambient H2S04 concentration is much smaller than that of the peroxy radicals. 

Frequently, the general nature or detailed structure of an intermediate is inferred from reaction products. Radical ions are invoked in reactions between electron donors and acceptors in polar solvents. Probing the fate of chirality, stereochemistry, or an isotopic label or substituent in the products of a chemical transformation exemphfies the classical approach to mechanism. Of course, this approach is not without shortcomings. 

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