Isotope tracer experiments

Nuclear reaction analysis (NRA) is used to determine the concentration and depth distribution of light elements in the near sur ce (the first few lm) of solids. Because this method relies on nuclear reactions, it is insensitive to solid state matrix effects. Hence, it is easily made quantitative without reference to standard samples. NRA is isotope specific, making it ideal for isotopic tracer experiments. This characteristic also makes NRA less vulnerable than some other methods to interference effects that may overwhelm signals from low abundance elements. In addition, measurements are rapid and nondestructive. 

NRA is an effective technique for measuring depth profiles of light elements in solids. Its sensitivity and isotope-selective character make it ideal for isotopic tracer experiments. NRA is also capable of profiling hydrogen, which can be characterized by only a few other analytical techniques. Future prospects include further application of the technique in a wider range of fields, three-dimensional mapping with microbeams, and development of an easily accessible and comprehensive compilation of reaction cross sections. 

NO. The same parameters were then used to predict the responses for the isotopic tracer experiments reported in Figs. 9 and 10. 

To further test the model, calculations were performed to simulate the isotopic tracer experiments presented in Figs. 9 and 10. It should be noted that while the tracer experiments were performed at 438K, the rate coefficients used in the model were chosen to fit the experiments in which chemisorbed NO was reduced at 423 K. Figures 21 and 22 illustrate the nitrogen partial pressure and surface coverage responses predicted for an experiment in which 5 0 is substituted for l NO at the same time that H2 is added to the NO flow. Similar plots are shown in Figs. 23 and 24 for an experiment in which NO is substituted for during steady-state reduction. 

Early isotope tracer experiments by David Shemin permitted the elucidation of the formation of the immediate precursor of the porphyrin needed for the cytochromes and for hemoglobin. These studies indicated that the glycine methylene carbon and nitrogen were incorporated along with both carbons of acetate. Subsequent enzymatic studies in both bacteria and animals revealed a condensation reaction between succinyl-CoA and glycine to yield 5-amino-levulinate and C02 (presumably by way of an enzyme-bound /3-keto acid, a-amino-/3-ketoadipate) (fig. 22.13). 

Unlike the Rh-based hydrogenation of a-(acylamino)acrylates, the corresponding Ru chemistry has not been studied extensively. Ru complexes of (S)-BINAP and (S,S)-CHIRAPHOS catalyze the hydrogenation of (Z)-a-(acylamino)cinnamates to give the protected ( -phenylalanine with 92% ee [74] and 97% ee [75], respectively. It is interesting that the Rh and Ru complexes with the same chiral diphosphines exhibit an opposite sense of asymmetric induction (Scheme 1.6) [13,15,56,74,75]. This condition is due primarily to the difference in the mechanisms the Rh-catalyzed hydrogenation proceeds via Rh dihydride species [76], whereas the Ru-catalyzed reaction takes place via Ru monohydride intermediate [77]. The Rh-catalyzed reaction has been studied in more detail by kinetic measurement [78], isotope tracer experiments [79], NMR studies [80], and MO calculations [81]. The stereochemical outcome is understandable by considering the thermodynamic stability and reactivity of the catalyst-enamide complexes. 

Activation with 14 MeV neutrons has been used to determine the oxygen content of various metals such as beryllium 20>, Cl, F, O, Na, Si, and various rare earths in complex molten salt electrolytes 45>, the protein content of food products by means of the nitrogen content 46>, i60/180 and 14N/15N isotopic ratios in stable isotope tracer experiments 47,48), and in a wide variety of other applications. One application we... 

In 1933, Erdtman suggested [20] that the basic lignan structure involved the coupling of two phenylpropanoid monomer units. Isotope tracer experiments on lignans were later described [2,21,22]. Significant advances have however been made in the chemistry and biosynthesis of lignans by Lewis... 

Another reactor equation useful in reaction kinetics analyses represents the transient CSTR. This situation is encountered in temperature-programmed desorption and isotope tracer experiments. The material balance for the transient CSTR operating at constant total pressure for an ideal gas is... 

Evidence from isotopic tracer experiments suggests that acetoacetic acid is in fact formed in this way during fatty acid oxidation. 

MK biosynthesis by the OSB pathway has been elucidated on the basis of isotopic tracer experiments, isolation of mutants blocked in the various steps, isolation and identification of intermediates accumulated by the mutants, and by enzyme assays. Early isotopic tracer experiments with various bacteria established that methionine and prenyl PPi contribute to the methyl and prenyl substituents of the naphthoquinone. The early isotopic tracer studies and other work have been reviewed by Bentley and Meganathan. " In 1964, Cox and Gibson observed that [G- " C] shikimate was incorporated into both MK and ubiquinone by E. coli, thus providing the first evidence for the involvement of the shikimate pathway." Chemical degradation of the labeled isolated menaquinone (MK-8) showed that essentially all of the radioactivity was retained in the phthalic anhydride. It was concluded that the benzene ring of the naphthoquinone (sic) portion of vitamin K2 arises from shikimate in E. coli The authors further suggested that shikimate was first converted to chorismate before incorporation into MK. A more complete chemical degradation of the MK derived from... 

Since, as discussed above, chorismate (4) is the branch point of the pathway and SCO4506 (MqnA) is the enzyme responsible for the formation of futalosine (8), attempts were made to demonstrate the reaction in a cell-free system. Structural examination of futalosine indicated that the nucleoside moiety is likely derived from inosine (36). Isotopic tracer experiments established that the C-6 and C-7 positions of futalosine (8) between chorismate (4) and inosine (36) are likely derived from a C2 unit derived from either pyruvate or PEP. A recombinant enzyme from T. thermophilus (TTHAO803) was tested for the formation of futalosine (8). However, no futalosine was formed, but instead w-hydroxybenzoate and 3-(l-carboxyvinyloxy) benzoate were formed from chorismate, both in the presence and in the absence of flavin mononucleotide (FMN). 

A quite different use of isotopes in mechanistic studies involves their use as labels for ascertaining the location of a given atom involved in a reaction. As in kinetic experiments, the isotopic substitution will not qualitatively affect the course of the reaction. The nuclei most commonly used for isotopic tracer experiments in organic chemistry are deuterium, tritium, and the and isotopes of carbon. There are several means of locating isotopic labels. Deuterium can frequently be located by analysis of NMR spectra. In contrast to the normal isotope, deuterium... 

Scheme 10.23 Isotopic tracer experiments revealing exchange is faster in the presence of ethylene, formed in situ by productive RCM reactions.

Several postulated combinations of numbers of electrons and numbers of pathways have been proposed for activated states in which redox centers exchange electrons. Structures of such states are deduced from experimental evidence such as rate laws, product analyses, isotopic tracer experiments, and deuterium isotope effects. Let... 

The chemisorptive state of oxygen significantly exceeds the single-layer saturated adsorption quantity for many metals. This shows that the absorbed oxygen can penetrate into the bulk-phase of catalyst. With isotopic tracer experiments, it can be proved that the lattice oxygen in metal oxide is also involved in the reactions. 

In more recent times knowledge of the pathway of dissimilation of tyrosine has been advanced greatly by isotopic tracer experiments and by availability of enzyme systems that catalyze isolated reactions in the over-all oxidation of tyrosine. 

Evidence that tryptophan is the precursor of serotonin has been amply demonstrated by isotopic tracer experiments (S6ff, 366). 

The bacterial mutant technique failed in establishing noncyclic precursors of the aromatic amino acids. This gap, however, was bridged by isotopic tracer experiments. 

The system is designed for high volume throughput of samples 15 samples/hr/GC-MS) with immediate results for each sample printed out on the console before the next sample is injected. Estimates of relative abundances of up to 12 selected ions are provided for each GC peak. For stable isotope tracer experiments, the quantities of each of the isotopic variants of the compounds are estimated (6). The raw data are saved for possible more detailed graphic display or analysis at a later time. 

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