Isotope incorporation

In the early 1930s the American chemists Gilbert Newton Lewis and Harold Clayton Urey worked on the preparation of samples containing high proportions of deuterium. 

In 1935 Urey was successful in preparing by evaporation a sample of water considerably enriched in D2O, and he realized the importance of this substance for establishing the details of chemical reactions in living systems. Rudolf Schoenheimer took advantage of this development at once, and until his death in 1941 pioneered an extensive series of biological investigations. 

A rapid turnover was also found to occur with protein molecules. A number of amino acids were labeled with the stable isotope 7N, which had also been concentrated by Urey, and were found to undergo an interchange with the tissue proteins. 

The photosynthetic reaction is an enormously complicated one, and involves a number of enzymes and reaction intermediates. Some of the elementary processes which occur are extremely rapid. A great deal of research has been concerned with elucidating the mechanism of the process, the most outstanding contribution being made by the American chemist Melvin Calvin, who made use of radioactive carbon-14, 

Calvin and his coworkers used carbon dioxide enriched in carbon-14, and allowed the photosynthetic process to occur for very short intervals of time. By this means they isolated and identified a number of intermediates containing carbon-14, After much painstaking work they were able to deduce the main details of the mechanism of the photosynthetic process. 

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Isotope incorporation experiments have demonstrated the essential correctness of the scheme presented m this and preceding sections for terpene biosynthesis Considerable effort has been expended toward its detailed elaboration because of the common biosyn thetic origin of terpenes and another class of acetate derived natural products the steroids... 

Swart PK (1980) The enviromnental geochemistry of scleractinian corals. An experimental study of trace element and stable isotope incorporation. PhD Dissertation, University of Londort, Londort, United... 

The benefits of using ionic compounds in microwave-enhanced reactions led us to explore the possibility of using ionic solvents i.e. ionic liquids, as donors for both deuterium and tritium. Whilst D20 is now relatively inexpensive and available at high isotopic enrichment, tritiated water is usually employed, for safety reasons, at low isotopic incorporation (we typically use HTO at 5 or 50 Ci mLT1 specific activity corresponding to 0.2-2% isotopic incorporation). This is a serious limitation when there is a need to provide compounds at high specific activity. 

One of the most attractive features of borohydride reductions is that under micro-wave-enhanced conditions they can be performed in the solid state, and rapidly. We were attracted by the work of Loupy [57], and in particular Varma [58, 59] who has shown that irradiation of a number of aldehydes and ketones in a microwave oven in the presence of alumina doped NaBH4 for short periods of time led to rapid reduction (0.5-2 min) in good yields (62-93%). In our study [60] seven aldehydes and four ketones were reduced (Tab. 13.3). Again reduction was complete within 1 min, the products were of high purity (>95%), of high isotopic incorporation (95%, same as the NaBD4) and the reactions completely selective. 

The authors constructed an 80-well hydrogenator. Fifteen catalysts were screened and the isotopic incorporation assessed by LC/MS. The regiospecificity was determined by VAST Direct Injection NMR [82] in conjunction with SPADEZ [83], a multispectrum analysis tool - this is capable of displaying and quantifying up to 96 spectra. So far it has not been possible to vary the solvents as, under the experimental conditions, the more volatile distil over into the less volatile solvents. 

Radioisotopes are continuously produced in the atmosphere, the stratospheric production being roughly twice the tropospheric one. As shown in figure 4 it is useful to distinguish between isotopes of noble gases, isotopes incorporated in molecules of gases or vapors, and isotopes which get attached to aerosols. 

R. P. Rodgers, E. N. Blumer, C. L. Hendrickson, and A. G. Marshall. Stable Isotope Incorporation Triples the Upper Mass Limit for Determination of Elemental Composition by Accurate Mass Measurement. J. Am. Soc. Mass Spectrom., 11(2000) 835-840. 

Any geochronometric method for estimating the age of objects based upon the generation of radioactive isotopes by cosmic radiation, followed by isotopic incorporation into the biosphere/geosphere, and their subsequent first-order decay with release of radiation and/or accumulation of daughter isotopes. These methods take advantage of the lack of any dependence of the decay rate on temperature, pressure, pH, or other physical parameters. See Radiocarbon Dating... 

N]-, [ C]-, pHjleucine or p Sjmethionine in the case of proteins) for various periods, after which the cells are lysed and the protein of interest is purified (often by immunoprecipitation with specific antibodies). The time course of isotope incorporation gives information about the rate (slope of curve) and extent (amplitude of curve) of the proteins synthesis. To measure degradation, cells are first pulse-labeled (i.e., exposed to radiolabeled precursor for a fixed period, after which sufficient nonla-beled precursor is added to reduce the radiospecific activity of the precursor). Then, the cells are further incubated, and the radiospecific activity of a particular protein of interest is determined (again usually after immunoprecipitation or some other means for achieving its isolation from other cellular proteins). The key point is that the chase allows one to stop radiolabel uptake almost instantaneously, thereby permitting the kinetic... 

It is difficult to reconcile the unique chemical structure of tetrodotoxin with that of an animal product. Its structure is not related to that of other animal products by any readily recognized biosynthetic scheme. It is not a terpenoid, not obviously formed from amino acid or carbohydrate units, and apparently not constructed from acetate or propionate units. Nor does it resemble any of the various plant alkaloid patterns. It thus appears to be a very unlikely animal product to result from known biogenetic pathways. In this connection the metabolic incorporation of radioactive precursors using torosa and ]C. granulosa salamanders was studied by Shimizu et al. (47). They observed significant isotopic incorporation into amino acids and steroid metabolites, but they found no such incorporation associated with tetrodotoxin. 

As shown in Table III, there is no significant difference in glucose uptake by cotton leaf discs immediately after exposure, but after 24 hr the uptake has nearly doubled. The percentage distribution of radioactivity in various extraction fractions is approximately the same in control and ozone-treated tissues. CO2 evolution from the added glucose is virtually identical in control and treated tissues. Isotope incorporation into the water soluble fraction, which would include amino acids, organic acids, and soluble carbohydrate, is also quite similar. Some distributional differences in the insoluble fraction are noted between... 

The elucidation of the biosynthetic pathway of apiose was founded on extensive, isotope-incorporation experiments in L. minor,85,66-82,83... 

The hydrogenation of /3,7-unsaturated carboxylic acids proceeds in a similar manner by way of five-membered organoruthenium intermediates that explain the isotope incorporation pattern of Scheme 37. 

The results of deuterium labeling experiments shown in Scheme 37 clearly show the operation of a monohydride mechanism in the BINAP-RuOI) catalyzed hydrogenation of unsaturated carboxylic acids. However, with many olefinic substrates with a neutral, rather than anionic, secondary binding site, the products exhibit a similar degree of isotope incorporation at the two hydrogenated centers (Scheme 39). The out-... 

Isotope incorporation 489 Isotope exchange kinetics 47, 235, 236,240,563-569 Isozyme 461 -465... 

Synthesis of podophyllotoxin (3.86) in cell culture of Linum album results in yields comparable to those of the most efficient tissue cultures of Podophyllum hexandrum. In order to further improve L. album cultures, Seidel et al. (2002) investigated the biosynthesis of podophyllotoxin (3.86). They fed a number of labeled compounds that to L. album cell cultures to identify which of these compounds could be used as precursors to podophyllotoxin. They determined that the substitution pattern on the benzene ring is critical. The substitution has to be either 3-methoxy, 4-hydroxy, as in ferulic acid (3.33), or, alternatively, 3,4-methylenedioxycinnamic acid (3.90) can serve as precursor. The precursor of podophyllotoxin in L. album appears to be deoxypodophyllotoxin (3.83), based on the higher level of isotope incorporation in the latter compound. This means that 7-hydroxymatairesinol, the precursors of 5-methoxypodophyllotoxin in L. flavum (Xia et al., 2000), is not a precursor of podophyllotoxin in L. album. 

Drugs that have a radioactive isotope incorporated into their structures are invariably used in studies to establish safe withdrawal periods. The presence of the radioactive isotope is harmless to the animal, but offers a highly sensitive means of determining the biological half-life of the drug and the total residue concentrations in different tissues in different animals. 

Graziani, E. I., Andersen, R. J., Krug, P. J., and Faulkner, D. J., Stable isotope incorporation evidence for the de novo biosynthesis of terpenoic acid glycerides by dorid nudibranchs, Tetrahedron, 52, 6869, 1996. 

Interestingly, hydroboration-amination as an isotope incorporation methodology has led to the synthesis of isomerically pure 13N- and 15N-labeled primary amines from labeled ammonia [23-25] (Scheme 8). These labeled amines are valuable intermediates in pharmacology. 

In the isomerization of 2M3BN to 3PN, 7.46 and 7.47 are supposed to be involved that is, formation of free butadiene is proposed. Starting with perdeuterated butadiene and HCN, based on this mechanism what would be the expected pattern of isotopic incorporation ... 

Compound administered Time isolated (h) Isotope incorporated (%)... 

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