18< . isotopic labelling with

Isotopic labeling with CD3, substituents effects (56), and general trends in the thiazole series (271) allowed a complete assignement of the major infrared bands for a series of 2-methylthiothiazoles. 

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). 

Normal (without isotopic labeling) With - N labeling With N and C labeling... 

Retention of configuration is expected for conversion of 3 to 59a. It is curious, however, for the rearrangement of 59a to 60a, since the most obvious pathway—opening of the O—O bond followed by rotation and reclosing—should lead to inversion of configuration. Double isotopic labeling with, 80 was used to show that the rearrangement of 59a to 61a, as well as the initial oxidation of 3 to 59a, is fully intramolecular. 

The approach recruited to chemical proteomics in Reference [17] is called SILAC (stable isotope labeling with amino acids in cell culture) and is important in comparative proteomics (Figure 1). SILAC works well with cultured mammalian cells, but prokaryotes defeat it by metabolizing the label (usually supplied in lysine and arginine) into other amino acids. For applications beyond cultured eukaryotic cells, the reductive methylation route to differential labeling [18] is among the alternatives [15]-... 

Topper and Stetten (T2) fed rats on galactose isotopically labeled with Cu on carbon atom 1. They found that the glycogen formed in the liver was isotopically labeled and 70% of the C14 was on C-l of the glucose units. They concluded that galactose could not be split into 2 triose units which would then recombine, but that a Walden-type inversion must occur at C-4. 

Following the course of a reaction by NMR remains one of the most popular applications of this technique in homogeneous catalysis. The resulting kinetic information and/or the detection and identification of intermediates are important sources of mechanistic information. Often, isotopic labeling with [5-12] or [13-15] facilitates the acquisition and interpretation of the resulting NMR spectra. 

These initial investigations also included kinetic studies on catalyst initiation, polymer chain propagation and termination rates and the time dependence of the number-average molecular weight, using isotopic labelling with ( " CH3)2A1C1 [10]. 

Reetz and coworkers developed a highly efficient method for screening of enantioselectivity of asymmetrically catalyzed reactions of chiral or prochiral substrates using ESI-MS [60]. This method is based on the use of isotopically labeled substrates in the form of pseudo-enantiomers or pseudo-prochiral compounds. Pseudo-enantiomers are chiral compounds which are characterized by different absolute configurations and one of them is isotopically labeled. With these labeled compounds two different stereochemical processes are possible. The first is a kinetic separation of a racemic mixture, the second the asymmetric conversion of prochiral substrates with enantiotopic groups. The conversion can be monitored by measuring the relative amounts of substrates or products by electrospray mass spectrometry. Since only small amounts of sample are required for this method, reactions are easily carried out in microtiter plates. The combination of MS and the use of pseudo-enantiomers can be used for the investigation of different kinds of asymmetric conversion as shown in Fig. 3 [60]. 

In addition to the nature of the cation at the adsorption site, the superhyperfine tensor can also give information on neighboring atoms further away. For example, OJ adsorbed on MgO exhibits a superhyperfine tensor ascribed to the presence of a nearby proton, presumably as a hydroxyl group (68) and this has been confirmed by isotopic labeling with deuterium (see Section IV,A). Superhyperfine tensors indicating the presence of nearby protons have also been reported for O J adsorbed on ferrocene deposited on porous Vycor glass (PVG) (120) and for alkylperoxy radicals supported on Ti02 (90). 

Recently, Alia et al.m studied Yp site-specific isotopically labelled with 2H, 13C and 170 by EPR. The hyperfine couplings and the spin density distribution at all ring positions of the radical were obtained. A strong H-bond to the protein was detected and the bond length estimated to 1.5 A. Isotope labelling has also been used earlier by Babcock s group (reviewed in reference 385). 

Fitzgerald et al,54 studied the monitoring of calcium metabolism in patients in the final stages of renal disease using serum by AMS after isotopic labelling with a 41 Ca radiotracer. The authors hypothesized that bone resorption can be studied directly by serial measurements of the 41Ca/Ca,otal ratio in serum after in vivo labelling of the calcium pool with 41 Ca.54... 

Later experiments using isotopic labeling with 13C verified Knoop s proposals/but study of isolated enzymes was not possible until after the discovery of coenzyme A in 1950. Then, studies of fatty acid oxidation by extracts from isolated mitochondria established the details of the pathway/... 

A matrix reaction of trimethylsilane with oxygen atoms (from ozone) at 14-17 K gives a product that was tentatively identified as the silenol H2C=Si(OH)Me 592 by IR spectroscopy275. The finding was supported by isotopic labeling with lsO and D. 

To bypass this difficulty, BPA-PC isotopically labelled with 13 C at the carbonyl position has been synthesised and chemical shift anisotropy line shape has been studied from 100 °C down to - 255 °C [45]. 

Fig. 51 Experimental (solid line) and simulated (dotted line) 13C NMR spectrum at 25 MHz for BPA-PC isotopically labelled with 13C at the carboxyl position a T = - 177 °C and b T = 12 °C (from [45])...

For an octacoordinated iridium acetylacetonato derivative that is no longer aromatic, an interesting rearrangement of the diphenylpentadienic moiety was observed by means of isotopic labeling with 13C (asterisks) [294, 295],... 

Stable Isotope Labeling with Amino Acids... 

Which method is chosen depends on the requirements of the experiment. It is sometimes preferable to perform the initial stages of an experiment (e.g. isotopic labelling) with an unsynchronised exponentially growing culture and then to select cells at a given phase of the cycle by e.g. zone centrifugation for enzymic analysis. In such experiments the cells, following selection, are not generally replaced in culture and so they may be exposed to an otherwise deleterious environment. 

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