Isotope labelling

All of the compounds can be deuterated by repeated crystalhzation from D2O. For SQA, labelhng can be accomplished by heating the dissolved compound in about 20% 0-labelled H2O, in a closed vessel [26]. Both the labelled water and squaric acid should be refrigerated until used. The isotope labelling should be confirmed by mass spectrometry or NMR spectroscopy. In general, a 10% label is quite satisfactory for good NMR signal 

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Studies to determine the nature of intermediate species have been made on a variety of transition metals, and especially on Pt, with emphasis on the Pt(lll) surface. Techniques such as TPD (temperature-programmed desorption), SIMS, NEXAFS (see Table VIII-1) and RAIRS (reflection absorption infrared spectroscopy) have been used, as well as all kinds of isotopic labeling (see Refs. 286 and 289). On Pt(III) the surface is covered with C2H3, ethylidyne, tightly bound to a three-fold hollow site, see Fig. XVIII-25, and Ref. 290. A current mechanism is that of the figure, in which ethylidyne acts as a kind of surface catalyst, allowing surface H atoms to add to a second, perhaps physically adsorbed layer of ethylene this is, in effect, a kind of Eley-Rideal mechanism. 

When a molecule adsorbs to a surface, it can remain intact or it may dissociate. Dissociative chemisorption is conmion for many types of molecules, particularly if all of the electrons in the molecule are tied up so that there are no electrons available for bonding to the surface without dissociation. Often, a molecule will dissociate upon adsorption, and then recombine and desorb intact when the sample is heated. In this case, dissociative chemisorption can be detected with TPD by employing isotopically labelled molecules. If mixing occurs during the adsorption/desorption sequence, it indicates that the mitial adsorption was dissociative. 

Dihydrogen (H2) is similarly protonated to by superacids, as was shown by studies using isotopic labeling. The structure of again involves 2e-3c bonding. 

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 study of the infrared spectrum of thiazole under various physical states (solid, liquid, vapor, in solution) by Sbrana et al. (202) and a similar study, extended to isotopically labeled molecules, by Davidovics et al. (203, 204), gave the symmetry properties of the main vibrations of the thiazole molecule. More recently, the calculation of the normal modes of vibration of the molecule defined a force field for it and confirmed quantitatively the preceeding assignments (205, 206). 

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). 

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... 

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... 

When benzoic acid is allowed to stand in water enriched in isotopic label becomes incorporated into the benzoic acid The reaction is cat alyzed by acids Suggest an explanation for this observation... 

In an extension of the work described m the preceding section Bender showed that basic ester hydrolysis was not concerted and like acid hydrolysis took place by way of a tetrahedral intermediate The nature of the experiment was the same and the results were similar to those observed m the acid catalyzed reaction Ethyl benzoate enriched m 0 at the carbonyl oxygen was subjected to hydrolysis m base and samples were isolated before saponification was complete The recovered ethyl benzoate was found to have lost a por tion of Its isotopic label consistent with the formation of a tetrahedral intermediate... 

A more recent experimental technique employs C as the isotopic label Instead of locating the position of a label by a laborious degradation procedure the NMR spectrum of the natural product is recorded The signals for the carbons that are enriched m are far more intense than those corresponding to carbons m which IS present only at the natural abundance level... 

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. 

Isotopically labeled compounds may be described by inserting the italic symbol of the isotope in brackets into the name of the compound for example, H C1 is hydrogen chloride[ CZ] or hydrogen chloride-36, and H C1 is hydrogen H chloride[ CZ] or hydrogen-2 chloride-38. 

Troost and Olavesen investigated the application of an internal standardization to the quantitative analysis of polynuclear aromatic hydrocarbons. The following results were obtained for the analysis of the analyte phenanthrene using isotopically labeled phenanthrene as an internal standard... 

Before sample preparation, surrogate compounds must be added to the matrix. These are used to evaluate the efficiency of recovery of sample for any analytical method. Surrogate standards are often brominated, fluorinated, or isotopically labeled compounds that are not expected to be present in environmental media. If the surrogates are detected by GC/MS within the specified range, it is... 

The proportionality between the concentration of chromophores and the measured absorbance [Eqs. (6.8) and (6.9)] requires calibration. With copolymers this is accomplished by chemical analysis for an element or functional group that characterizes the chromophore, or, better yet, by the use of isotopically labeled monomers. 

John Wiley Sons, Cornell University ms 125,000 CD-ROM, tape isotopic labeled compound QuaUty Index algorithm... 

Moleeular biology eloning expression isotope labeling of protein target... 

Alternatively, radiohalogen-labeled compounds may be prepared by way of isotopic labeling from the unlabeled bromo or iodo derivatives by various two-step reaction sequences. Examples include the use of trialkylsilyl synthons as described in References 10—13, and the use of boronic acid synthons as described in References 14 and 15. 

Benzoates. The selective debenzoylation of sucrose octabenzoate [2425-84-5] using isopropylamine in the absence of solvents caused deacylation in the furanose ring to give 2,3,4,6,1/3/6 -hepta- and 2,3,4,6,1/6 -hexa-O-benzoyl-sucroses in 24.1 and 25.4% after 21 and 80 hours, respectively (54). The unambiguous assignment of partially benzoylated sucrose derivatives was accompHshed by specific isotopic labeling techniques (54). Identification of any benzoylated sucrose derivative can thus be achieved by comparison of its C-nmr carbonyl carbon resonances with those of the assigned octabenzoate derivative after benzoylation with 10 atom % benzoyl—carbonyl chloride in pyridine. 

The study of the biosynthesis of vitamin B 2 is a saga whose resolution, due primarily to Battersby (80—83) and Scott (84,85), requited an effort on the same magnitude as the total synthesis. It was only when recent molecular biology tools became available to complement en2ymology, isotopic labeling, chemical synthesis, and spectroscopy that solution of this problem became possible. 

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