Isotope-labeled carbon dioxide

The Fick s law diffusion coefficient of a permeating molecule is a measure of the frequency with which the molecule moves and the size of each movement. Therefore, the magnitude of the diffusion coefficient is governed by the restraining forces of the medium on the diffusing species. Isotopically labeled carbon in a diamond lattice has a very small diffusion coefficient. The carbon atoms of diamond move infrequently, and each movement is very small—only 1 to 2 A. On the other hand, isotopically labeled carbon dioxide in a gas has an extremely large diffusion coefficient. The gas molecules are in constant motion and each jump is of the order of 1000 A or more. Table 2.1 lists some representative values of diffusion coefficients in different media. 

This mechanism was tested by use of C-labeled carbon dioxide (Barker, 1943 Buswell and Sollo, 1948 Stadtman and Barker, 1949, 1951 Pine and Barker, 1956 Baresi et al, 1978). Essentially none of the methane was found to be derived from carbon dioxide. Methane is derived entirely from the methyl carbon atoms and carbon dioxide is derived exclusively from carboxyl carbon atoms. Van Neil s mechanism is clearly not valid because the methyl carbon atom is not oxidized to carbon dioxide. Other work has been done to ascertain whether hydrogen atoms are removed during the fermentation of acetic acid, and whether the methyl group is incorporated intact into methane (Pine and Barker, 1954). Water and heavy water were used with deuterated and nondeuterated acetic acid. Acetic acid labeled in the methyl group, when used as the substrate, showed that the isotopic content of acetic acid and methane are the same. Unlabeled acetic acid fermented in the presence of heavy water indicated that about one atom of deuterium per molecule of methane formed is derived from heavy water. It was concluded that the methyl group is transferred from acetic acid to methane as a unit without the loss of attached hydrogen or deuterium atoms. 

Carboxyl-labeled dicarboxylic acid 68 has been prepared from l,5-di(bromomagnesio) pentane and labeled carbon dioxide [Eq. (27) 51]. However, when the di-Grignard reagent 67 was heated with isotopically labeled barium carbonate, labeled 4-f-butylcyclohexanone 69 was isolated [52]. 

Knowles and co-workers observed (V/K) at C-1 of glyoxalate by analyzing the isotopic composition of the malate produced in the reaction at early and late stages (23). In order to measure this ratio accurately they developed a procedure to convert C-2 of malate to carbon dioxide. Through a series of enzymic reactions, C-2 was converted to the carbonyl carbon of the acetyl group of acetyl-carnitine (through the reaction sequence malate — pyruvate — acetyl-CoA - acetylcarnitine). The labeled carbon then was carried flirough another cycle back to malate, but in this case the label was in the carboxylate carbon at C-4 (acetyl-carnitine acetyl-CoA 4-[ 3C]malate). Malic enzyme converts this to pyruvate and labeled carbon dioxide, whose isotopic ratio is determined by mass spectrometry. 

Starch synthesis is closely related to sucrose synthesis (see Chapters IX and XII). Experiments with isotopically labeled sugars have shown that leaves can form starch from externally supplied glucose, fructose, and sucrose, as well as from several other compounds. Starch is also formed from carbon dioxide during photosynthesis. It was found (78) that when labeled carbon dioxide was fed to leaves, the starch became labeled before the free sugars. Labeled glucose and glucose 1-phosphate did not contribute an... 

From metabolic studies, an isotopic caffeine breath test has been developed that detects impaired liver function using the quantitative formation of labeled carbon dioxide as an index. From the urinary excretion of an acetylated uracil metabolite, human acet-ylator phenotype can be easily identified and the analysis of the ratio of the urinary concentrations of other metabolites represents a sensitive test to determine the hepatic enzymatic activities of xanthine oxidase and microsomal 3-methyl demethylation, 7-methyl demethylation, and 8-hydroxylation. Quantitative analyses of paraxanthine urinary metabolites may be used as a biomarker of caffeine intake. Fecal excretion is a minor elimination route, with recovery of only 2-5% of the ingested dose. 

An alcohol of normal composition is equilibrated in one tube with carbon dioxide of known composition fi (usually tank carbon dioxide) and another with labeled carbon dioxide of known composition n . After equilibration, the measured atom fractions of in the COg are % and 2, respectively. If a sample of the same alcohol of unknown isotopic composition, d, is then analyzed under the same conditions, using tank carbon dioxide (of composition n,), it can be shown that... 

Let us first introduce some important definitions with the help of some simple mathematical concepts. Critical aspects of the evolution of a geological system, e.g., the mantle, the ocean, the Phanerozoic clastic sediments,..., can often be adequately described with a limited set of geochemical variables. These variables, which are typically concentrations, concentration ratios and isotope compositions, evolve in response to change in some parameters, such as the volume of continental crust or the release of carbon dioxide in the atmosphere. We assume that one such variable, which we label/ is a function of time and other geochemical parameters. The rate of change in / per unit time can be written... 

Isotope labeling studies furnished a convincing body of evidence to indicate that even in the presence of carbon monoxide, only carbon dioxide is converted to methanol [Eq. (3.2)].394-398 In a review article371 Chinchen considers this as the dominant route, with CO merely serving as a source of C02 via the water-gas shift reaction. 

Equation (115) is the same as (6) studied by James and co-workers (62) in the CO reduction of RhCl3.] The labeling experiment also revealed information on the stability of the hydroxycarbonyl intermediate in (115). If this species, Rh—COOH, was formed in an equilibrium concentration, then proton transfer and the reverse reaction would lead to incorporation of labeled oxygen in the carbonyl ligand and therefore to the observation of doubly labeled C02. However, comparison of the abundances of the three isotopic carbon dioxide molecules found (masses 44, 46 and 48) with distributions calculated assuming (i) equilibrium formation of the hydroxycarbonyl and (ii) immediate decomposition of the intermediate clearly showed that the hydroxycarbonyl intermediate reacts to form C02 immediately after it is formed, with no indication of a substantial equilibrium or incorporation of lsO in the carbonyl ligand. 

Incorporation studies with isotopes showed that when anthranijate was converted to tryptophan, the carboxyl group df anthranilate was lost as carbon dioxide, but the nitrogen was retained. Because the enzymes in the tryptophan biosynthetic pathway have only a limited specificity, it was possible to substitute 4-methyl-anthranilate in E. coli extracts that could convert anthranilate to indole. This nonisotope label was conserved during the conversion to yield 6-methyl indole. 

The use of labelled initiators to obtain accurate information concerning the relative reactivities of radical scavengers towards reference radicals, has been reviewed (4). Product analysis, by isotope dilution analysis, has been used in the examinations of the decompositions of certain initiators in the absence of monomers. It is possible to work with very dilute solutions and small extents of decomposition so that the conditions are similar to those prevailing when the substances are used as initiators of polymerizations. The decompositions of dibenzoyl (12) and di-anisoyl peroxides (13), for example, have been followed by determinations of the carbon dioxide evolved. 

The reaction of iV-(2,4-dinitrophenyl)amino acids with base in aqueous dioxane has been shown to give benzimidazole iV-oxides (7). The rate-determining step is likely to be formation of an iV-alkylidene-2-nitrosoaniline intermediate (6), which is followed by rapid cyclization and decarboxylation.19 The loss of carbon dioxide from perbenzoate anions has been investigated by mass spectrometry and electronic structure calculations. The results, including isotopic labelling experiments, support a mechanism involving initial intramolecular nucleophilic attack at either the ortho- or ipso-ring positions. They also indicate that epoxides may be intermediates en route to the phenoxide products.20 There has also been a theoretical study of the formation of trichlorinated dibenzo-/ -dioxins by reaction of 2,4,5-trichlorophenolate ions with 2,4-dichlorophenol.21... 

The sample (biological sample or collected fraction) is oxidized to produce carbon dioxide which is then converted to graphite (Getachew et ah, 2006). The sample is then ionized, and ions are accelerated to high levels of energy in the MS. The ratio of the rare isotope to the abundant isotope is determined ( C/ C, for example). Because of the extreme sensitivity of AMS (in the attomole range), only a small amount of radio-labeled carotenoid is necessary to study absorption and metabolism. 

Katakis and Taube examined the reactions of mixtures of ozone and carbon dioxide irradiated by ultraviolet light, using isotopically labeled reactants to ascertain the mechanism. They found an isotopic exchange to occur between O3 and CO2, and concluded that 0( D) was the intermediate which was responsible. They did not find CO as a product, and thus they felt that reaction (11) was not important in their system, but that exchange occurred via... 

HOOC CHj CH, C OOH + 2H 2CH,-C OOH These investigators have shown that suspensions of Aerobacier indologenes convert carboxyl-labelled acetate into isotopic succinate and vice versa. In both cases the isotope remained fixed in the carboxyl groups. Furthermore, doubly labelled acetate is converted to succinic acid which has the isotope equally distributed between the carboxyl and methylene groups. This is strong evidence that acetic acid can undergo condensation to form succinic acid without participation of carbon dioxide as an intermediate. 

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