Carbon labeled

Differences in reactivity of the double bond among the four isomers are controlled by substitution pattern and geometry. Inductive effects imply that the carbons labeled B in Table 3 should have less electron density than the A carbons. nmr shift data, a measure of electron density, confirm this. 

When these labeled oxaloacetates enter a second turn of the cycle, both of the carboxyl carbons are lost as CO2, but the methylene and carbonyl carbons survive through the second turn. Thus, the methyl carbon of a labeled acetyl-CoA survives two full turns of the cycle. In the third turn of the cycle, one-half of the carbon from the original methyl group of acetyl-CoA has become one of the carboxyl carbons of oxaloacetate and is thus lost as CO2. In the fourth turn of the cycle, further scrambling results in loss of half of the remaining labeled carbon (one-fourth of the original methyl carbon label of acetyl-CoA), and so on. 

Direct Methods. The classical approach has been to prepare tritiated or carbon labeled analogs of the parent hydrocarbons which may then be used in animal or in vitro experiments. Tritiated compounds are generally easier to prepare, using exchange reactions on the parent hydrocarbon, than their 14C analogs and have higher specific activities. However, during the metabolism of such compounds, some of the tritium is released as tritiated water, either directly or... 

The labeling trick also works backward. You could have to decide which carbons of glucose become labeled when you use pyruvate labeled on a given carbon. Labeling pyruvate on C-l will result in a glucose molecule that is labeled on both C-3 and C-4, again because of the TIM reaction. 

B14. Blomstrand, R., A study on the intestinal absorption of fat in normal adults and in non-tropieal sprue with carbon-labelled oleic acid and palmitic acid. Acta Med. Scand. 152, 129-138 (1955). 

Figure 14. Principle for measuring bidirectional fluxes by 13C metabolic flux analysis. In a carbon labeling experiment, 1 13C glucose is provided in the medium, and the culture is grown until a steady state is reached. Glucose can either go directly via the hexose phosphate pool (Glu 6P and Fru 6P) into starch, resulting in labeling hexose units of starch only at the Cj position, or it can be cleaved to triose phosphates (DHAP and GAP), from which hexose phosphates can be resynthesized, which will result in 50% labeling at both the Ci and the C6 position (assuming equilibration of label by scrambling at the level of triose phosphates). From the label in the hexose units of starch, the steady state fluxes at the hexose phosphate branchpoint can be calculated for example, if we observe 75% label at the Ci and 25% at the C6 position, the ratio of vs to V7 must have been 1 to 1. All other fluxes can be derived if two of the fluxes of Vi, V6, and V7 are known (e.g., V2 vi V3 V5 + v6).

In contrast, exposure of 14-VE (diene)MCp Cl complexes (M = Zr, Hf) to CO (1 atm) results in the formation of cyclopentadienes70. The mechanism proposed for this transformation was elucidated with a carbon labeled CO ( CO) as requiring an initial coordination of CO to generate a (diene)MCp (CO)Cl complex 153 (Scheme 37). For the hafnium complex, the intermediate 153 (M = Hf) was observed by infrared spectroscopy. Insertion of CO into the a2, jt diene generates a metallacyclohexenone, which undergoes reductive elimination to generate the dimeric metallaoxirane species 154. -Hydride elimination from 154 (M = Zr, Hf) followed by 1,2-elimination produces substituted cyclopentadienes and the polymeric metal-oxide 155. Treatment of (diene)TiCp Cl with CO leads to isolation of the metallaoxirane complex 154 (M = Ti). 

A carbon labelling study has elucidated the rearrangement mechanism for formation of chalcone (97) which accompanies formation of (91) by the expected vicinyl elimination of trimethylsilyl and benzotriazolyl groups from 2-benzotriazolyl-2-aryl-3-ketopropylsilanes, on reaction with fluoride ion in DMF. ° Thus, it has been possible to distinguish between the two alternative mechanisms depicted in Scheme 11 (via intermediates (93) or (95), respectively, by determining the fate of the labelled quaternary carbon of substrate (89). The results are consistent with the formation of a cyclopropane intermediate (95) which subsequently ring opens, with relief of strain, to form delocalized carbanion (96), from which the chalcone (97) is obtained (labelled... 

Figure 1 shows the proton noise-decoupled C-NMR spectrum of a polytetrahydrofurein (polytetramethylene ether glycol, PTMEG) dissolved in THF. In this spectrum the carbons numbered 1, 2 and 3 which cure a to the oxygen appear at lower field them the 6-carbons labeled as 4, 5 and 6. The carbon atoms in the polymer are clearly resolved from the corresponding carbons of the THF monomer. The fact that carbons 3 and 4 near the hydroxyl end-groups can be easily identified shows the excellent resolution of this technique. 

Rao KS, Recknagel RO. 1969. Early incorporation of carbon-labeled carbon tetrachloride into rat liver particulate lipids and proteins. Exp Mol Pathol 10 219-228. 

Exercise 30-14 Show the position(s) of an isotopic carbon label such as 14C in geranyl pyrophosphate biosynthesized from carboxyl-labeled CH3C 02H by way of 8 and 9. 

Glucose can be purchased with essentially any specific carbon labeled with 14C. If your objective is to assess the relative importance of glycolysis versus the pentose phosphate pathway in a particular tissue, how might you use radioactive glucose to answer this question ... 

Figure 1. The Cs geometry of the pentamethylbenzenium cation with carbons labeled as referred to in the text. Selected bond lengths in A are also shown. (Reproduced from reference 13. Copyright 1995 American Chemical Society.)...

Ghoos, Y.F., et al. 1993. Measurement of gastric emptying rate of solid by means of a carbon-labeled octanoic acid breath test. Gastroenterology 104 1640. 

Does the hydrogen come from the pro-R or the pro-S CH2C02H branch of the citric acid [This problem is, in itself, interesting since it is known from carbon labeling experiments discussed already in Section 2 (cf. Fig. 6) that the aeonitase-active branch is also the one derived from the oxaloacetic add, cf. the asterisks — for labeled carbons — in Fig. 61.]... 

Gertz, E. W., Wisneski, J. A., Stanley, W. C., and Neese, R. A. 1988. Myocardial substrate utilization during exercise in humans. Dual carbon-labeled carbohydrate isotope experiments. J Clin Invest 82 2017-2025. 

After a new (and unusual) mechanism, such as the benzyne mechanism for nucleophilic aromatic substitution, is proposed, experiments are usually designed to test that mechanism. A classic experiment supporting the benzyne mechanism used a radioactive carbon label. Examination of the mechanism shown in Figure 17.6 shows that the carbon bonded to the leaving chlorine and the carbon ortho to it become equivalent in the benzyne intermediate. Consider what would happen if the carbon bonded to the chlorine were a radioactive isotope of carbon (l4C) rather than the normal isotope of carbon (I2C). If we follow the position of the radioactive carbon label through the mechanism of Figure 17.6, we find that the label should be equally distributed between the carbon attached to the amino group in the product and the carbon ortho to it. 

Now we do have a reactive, primary electrophilic site, which undergoes an S 2 reaction with the f-BuS- under the conditions of the rearrangement. Notice how the black OH, which started on the carbon labelled 1, has ended up on carbon 2. 

On pyrolysis of isoxazoles, decarbonylation has also been observed. Carbon labeling experiments have indicated that the formation of 2,3-diphenyl-2/f-azirine (86) and 2-phenylindole from 3,5-diphenylisoxazole takes place as shown in Scheme 22 thus, 3,5-13C-labeled starting material gave 2-[2-13C]phenylindole containing 51% of the original 13C.205... 

---