Carbon enriched substrates

Boutton, T.W. 1991 Tracer studies with C-enriched substrates humans and large animals. In Coleman, D.W. and Fry, B., eds.. Carbon Isotope Techniques. Academic Press, San Diego 219-242. 

The asymmetric synthesis of allenes by stereoselective manipulations of enantio-merically pure or enriched substrates relies on the availability of such optically active substrates. In contrast, a direct synthesis of allenes by the reaction of prochiral substrates in the presence of an external asymmetric catalyst is an almost ideal process [102]. Most of the catalytic asymmetric syntheses in organic chemistry involve the creation of chiral tetrahedral carbon centers [103], whereas the asymmetric synthesis of allenes requires the construction of an axis of chirality. 

High resolution l3C n.m.r. spectroscopy is a non-invasive technique that can be used to study metabolic pathways in intact organisms, organs or cells. Metabolism of l3C-enriched substrates may be followed spectroscopically for a number of hours. The rates of formation of metabolites can be monitored and end-products identified. This procedure provides the biochemist with a complete overview of the metabolism of a given substrate without destruction of the biological material and it has the additional advantage of enabling the observer to follow the metabolism of specific carbon nuclei and their metabolites. 

The glow discharge optical spectroscopy (GDOS) of the A 301L sample indicated the coating composition (Figs.7,8). The results of the depth profile analysis proved carbon enrichment in the interface of the steel substrate and the tantalum layer. 

In the asymmetric hydrogenation of the methyl ester of a-acetamido cinnamic acid, 3.24 cannot be observed by conventional NMR techniques. In situ PHIP-NMR studies in contrast do show two hydride signals at -19 and -2 ppm. Using a C-enriched substrate, the coupling between the asterisk-labeled carbon and the hydride at -2 ppm can also be observed. Based on the NMR data, the actual structure is concluded to be more like 3.25 than 3.24. 

Azirines can be prepared in optically enriched form by the asymmetric Neber reaction mediated by Cinchona alkaloids. Thus, ketoxime tosylates 173, derived from 3-oxocarhoxylic esters, are converted to the azirine carboxylic esters 174 in the presence of a large excess of potassium carbonate and a catalytic amount of quinidine. The asymmetric bias is believed to be conferred on the substrate by strong hydrogen bonding via the catalyst hydroxyl group <96JA8491>. 

Carbon isotope fractionation was examined during the aerobic degradation of TCE by Burkholderia cepacia strain G4 that possesses toluene monooxygenase activity (Barth et al. 2002). There were substantial differences in values of isotope shifts during degradation, from 57 to 17 ppm, and when the data were corrected to correspond to the same amount of substrate reduction the Releigh enrichment factor was 18.2. 

Labelling studies with 13C-enriched acetic acid, where 13C NMR data are consistent with carbon monoxide addition to the carbonyl carbon of the acid substrate. 

In the presence of organic substrates may be caused by simple carbon dioxide enrichment from Increased bacterial respiration (37, 38)>... 

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