Isotopic labels chemical stability

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... 

Isotope labeling experiments and EPR spectroscopic studies have shown that the cyclopropylmethyl radical is a discrete chemical species with a finite lifetime = 7 x 10 s at 25°C in methylcyclopropane solution). Unlike the corresponding cyclopropylmethyl cation, 1 has no nonclassical or fluxional characteristics, and it does not rearrange to cyclobutane derivatives. Its rate of formation from diazenes, peroxides, and other precursors is slightly greater than that of model primary acyclic radicals, which indicates that it has a small thermodynamic stabilization. EPR spectroscopic studies have shown that rotation of the methylene group carrying the unpaired electron is not free and that the preferred conformation is bisected rather than perpendicular. 

The chemical stability of an analyte in a given solution that is stored under specific conditions for given time intervals is an important validation parameter that was discussed in general terms in Section 9.4.4f. An extreme form of instability is the propensity of the analyte to explode or be set afire, and such properties should be available in the MSDS information or, if the analyte is a new compound, the chemists associated with its synthesis or isolation will have discovered such properties before the analyst A crucial special case is the stability of the analyte (and thus of the analytical standard) in solution, as in stock, sub-stock and spiking solutions. The stabihty of analyte or of an analog internal standard in solution should be determined by comparing stored stock solution(s) to freshly made-up stock solution(s). For a stable isotope-labeled internal standard, stability data for the corresponding analyte are often used to estabhsh stahUity for such an SIS. 

General.—Since the electron-impact fragmentation patterns of many classes of natural products are now known, mass spectrometry can be used in conjunction with heavy isotopes in biosynthetic studies. A study of the fragmentation pattern of a metabolite with and without isotopic labelling yields direct information on the position of the labelled atom in the molecule without recourse to wet chemical degradation. Essential requirements for the biosynthetic application of mass spectrometry are (0 thermal stability of the metabolite at the probe inlet temperatures and (ii) a reasonably simple fragmentation pattern. 

Since the experimental parameters determine not only the quality of the process (i.e. amount of waste, capacity, costs) but also the chemical and physical characteristics (i.e. position of label, chemical and biological stability, isotopic abundance) of the labeled target molecule, they need to be carefully optimized in pilot studies. 

---