Positive labelling

Elements such as C, N, O, S, and Cl that are components of many organic compounds exist naturally as mixtures of stable isotopes. The ratios of these in a compound reflect the different rates of reaction at isotopically labeled positions, and therefore reflect the fractionation—biotic or abiotic—by which it was synthesized or to which the compound has been subjected. Techniques have been developed whereby the ratios C/ C (5 C), (5 N), (5 0),... 

A wooden or metal containment box surrounding the treated area is commonly used when a small quantity of test material is to be applied. The box, typically rectangular in shape and partially buried beneath the soil surface, serves to isolate the treated area from surrounding soil and protect against wind and water erosion. A one- to two-nozzle application boom that moves along guy wires or tracks is often used to ensure even application. Radiolabeled materials having two or more label positions often serve as replicates in these studies. 

Electrons pass through the external circuit into the copper electrode, where they are consumed in reducing Cu2+ ions in the solution to copper atoms. This is represented by the reduction half reaction (c). The copper electrode which pulls electrons from the external circuit is labeled positive, and is the cathode of the cell. 

Omnivorous reading made Carothers a brilliant and witty conversationalist in small groups of friends. Hill thought that, after a drink or two, Carothers became the funniest man alive. His humor was understated. When referring to a transmission box labeled positively infinitely variable, Carothers always added dryly, Within finite limits. At parties, he charmed guests by pulling from his pants pockets wooden blocks assembled into molecules. Stereochemical approaches to complicated compounds were becoming important in chemistry, he emphasized. 

The immunosuppressant compound170 FK-506, similar in effect to cyclosporin A, the leading drug for use in immune system suppression to prevent rejection of transplanted organs171, has been labelled at carbon atoms 10, 16, 18, 21a, 24 and 26 by fermentative biosynthesis using sodium [l-14C]propionate as a precursor172. The same 13C-labelled positions were derived from [l-13C]propionate. FK-506 producing culture Streptomyces tsukubaenis no 9993 has been utilized in this biosynthesis (120 h incubation at 29 °C). 

Fig. 7 Cytochrome P450-mimic oxidation of fenvalerate (9) by iron(ffl) porphyrin. The values in parentheses are percent of the applied 14C (Asterisk labeled position) after 50 min...

Kim et al. (67) P. placenta Polyclonal antiserum was produced to P. placenta extracellular metabolites red spruce and birch were degraded by P. placenta using the soil-block procedure degraded wood-block samples were prepared for TEM and the immunoelectron localization of wood-degrading enzymes Extracellular membrane structures (matrix) were observed surrounding hyphae, which degraded spruce and birch wood the matrix labeled positively with antisera produced to P. placenta extracellular metabolites... 

Figure 13.2 Illustration of the various types of injection methods for lab-on-a-chip applications (a) floating, (b) direct, (c) pinched, and (d) gated. The labeled positions are as follows for panels (a), (b), and (c) reservoir 1 is for buffer, reservoir 2 for sample, reservoir 3 for sample waste, and reservoir 4 is for buffer waste. For panel (d), reservoir 1 is for sample and reservoir 2 is for buffer.

Figure 7-17 The structure of insulin. (A) The amino acid sequence of the A and B chains linked by disulfide bridges. (B) Sketch showing the backbone structure of the insulin molecule as revealed by X-ray analysis. The A and B chains have been labeled. Positions and orientations of aromatic side chains are also shown. (C) View of the paired N-terminal ends of the B chains in the insulin dimer. View is approximately down the pseudo-twofold axis toward the center of the hexamer. (D) Schematic drawing showing packing of six insulin molecules in the zinc-stabilized hexamer.

One must be aware of the possible occurrence of certain problems in isotope dilution analysis. One of these is incomplete isotopic exchange, in which the active and inactive atoms do not mix. This lack of exchange can be due to differing physical and chemical states of tracer and inactive materials. Steps must be taken to ensure complete exchange. One must also be sure that the labeled position in any compound is relatively inert. If the atom in question is very labile, one can get a reduction in specific activity without any dilution having taken place. To compare specific activities, all samples must be counted under identical conditions with proper corrections for self-absorption in samples of varying mass. 

Most stretch and shrink labels are added to containers in a tubular form, generally relying on the stretch/shrink tightness of the material to retain label position for the life of the product. An additional feature is that the label may be extended over the closure to form a tamper-evident seal on suitable packs. 

This example demonstrates that high-quality 13C NMR spectra can be obtained in the continuous-flow mode. Thus, it is possible to monitor electrochemical reactions of high-concentration compounds (0.1 M solutions) [3], or even to use diluted samples with 13C-labelled positions [4-6],... 

Figure 1. Fungicidal substances applied and C-labeling positions ( ).

In parallel to this investigations the biostability of the labeling position in the radiolabeled test compound is examined. For this purpose, the volatile radioactivity in the cage air after administration is monitored over 24 hours. 

Compounds 9 and 10 with R = OC2H5 lose also a C3H402 neutral in the metastable time frame and the resulting fragment ion loses in turn a methyl radical. This has been explained8 by the initial transfer of a H from the ethyl chain as shown in Scheme 13, followed by the loss of ethylene and C02. The 5D atoms incorporated in the star-labelled positions are all retained in the fragment ion m/z (71 + 5). 

Eight methyl AT-(l/f-benzimidazol-2-yl)carbamates 78 with various 5-substituents have been labelled with 13C at carbon-2, and at the carbonyl and methoxy carbons according to the synthetic scheme of equation 25 using commercially available 13C-enriched (91-92 atom%) carbon tetrachloride, methanol and thiourea63. The three labelled positions indicated with asterisks are at or near the site involved in binding to tubulin (which performs a variety of vital functions in the cell). A correlation has been found between anthelmintic potency of the benzimidazole carbamate group of anthelmintic drugs widely used to control nematode parasites in sheep and their ability to bind tubulin. 

C-labelled propionates double-labelled with deuterium or fluorine have been needed to study the stereochemistry of the chain elongation steps in the biosynthesis of polyether antibodies75. NMR spectroscopy allows one to identify the stable isotope-labelled positions in these compounds, thus avoiding a chemical degradation76. 

Roberts seems to have been the first to propose the existence of a carbonium ion intermediate with a bridging methyl group (a structure that we would now call a corner-protonated cyclopropane), in part as a result of a study on the deamination of propylamine-1-in an aqueous medium, in which it was believed that a small amount of label migration to both C(2) and C(3) of the 1-propanol product was observed (Figure 50). In fact, the analysis of the label position was later shown to be in error but, by then, other experiments had suggested the existence of some kind of protonated cyclopropane. Among these were studies showing the formation of cyclopropanes from deamination of alkylamines in non-aqueous media. This process was subjected to further scrutiny... 

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