C labeling experiments

Plants grown for longer periods in solid supports such as sand or soil repre-.sent the next level of complexity and, although other techniques are available, carbon flow is most frequently estimated using C labeling experiments. In the laboratory, COt can be supplied to shoots either as a short pulse or continuously, and the carbon flow can be monitored. In the field, due to technical limitations, only COi pulse labeling procedures are possible. A final approach, termed crop studies, involves the measurement of components of crop growth from which... 

The solvolysis of linalyl p-nitrobenzoate to (+)-a-terpineol has been shown, by deuterium and C labelling experiments, to be consistent with the anti process [e.g. (66)- (67)- (68)], rather than by the syn process proposed by Winstein (Vol. 3,... 

The results reveal that baker s yeast is a potent source for precursors of 2-acetyl-l-pyrroline. It appears likely that the flavor compound is formed in the yeast cells from proline and dihydroxyacetone phosphate via 1-pyrroline and pyruvaldehyde. This is corroborated by the results of c-labeling experiments which showed that the acetyl group in the Acp stems from a sugar degradation product and that the pyrroline ring was derived from proline. 

In laboratory microcosms, ira 5-permethrin was selectively degraded compared to the other diastereomer, cw-permethrin, by six bacterial strains [19]. These strains also preferentially biotransformed 15-cw-bifenthrin over their antipodal l/ -cw-enantiomers, which were more toxic to daphnids [19]. Enantioselectivity was more pronounced for cw-permethrin than for cw-bifenthrin, and was strain-dependent. The (—)-enantiomer of both pyrethroids was preferentially depleted in sediments adjacent to a plant nursery, suggesting that in situ microbial biotransformation was enantioselective [20]. Although all enantiomers of permethrin were hydrolyzed quickly in C-labeled experiments in soils and sediments, the degradates of both cis- and irara-permethrin s -enantiomers were mineralized more quickly than those of the 5-enantiomer, while degradation products of cA-permethrin were more persistent than those of the trans-isomex [185]. Enantioslective degradation of fenvalerate in soil slurries has also been reported [83]. These smdies underscore how enantiomer-specific biotransformation can affect pyrethroid environmental residues, the toxicity of which is also enantiomer-dependent [18-20]. 

Affrossman and Thomson used a C-labeling experiment to confirm that overreduction in the Rosenmund is a result of further reduction of the aldehyde product, and not a competing, direct reduction... 

However, the isomeric product distributions obtained upon photolysis of phenyl-methyl and various arylthiophenes did not conform to this simple mechanism. These results together with deuterium and C-labelling experiments led to the conclusion that the sequential order of the ring carbon atoms has changed during rearrangement. An attractive mechanism was proposed in which a valene thiophene is first formed, followed by collapse to a symmetrical Ladenburg structure, viz. 

The sodium salt of cyclopropanecarboxaldehyde tosylhydrazone (157) behaves similarly on heating to 125 135°C. Labelling experiments using the lithium cyclopropanecarboxaldehyde-p-tosylhydrazone salt has proved the occurrence of a cyclopropylcarbene-cyclobutene C3 - C4 ring expansion instead of a simple intramolecular carbene insertion in one of the four C-H bonds and bicyclobutane ring-opening . For other examples and discussion of such a rearrangement see Ref. 189. In an apparently related reaction, when cyclopropylmethyl tosylate (158) is treated with potassium t-butoxide in dimethyl sulphoxide at room temperature for 1 h, a quantitative mixture of cyclobutene and methylenecyclopropane is produced in equal amounts (equation 110) °. 

Any mechanistic proposal must comply with the following observations. (1) The Fischer-Tropsch hydrocarbon synthesis follows the formalism of polymerization kinetics with a Schulz-Flory distribution of the molecular weights. (2) a-Olefins and alcohols occur as the primary products. (3) The aliphatic final products are formed consecutively by hydrogenation of the olefins according to " C-labeling experiments [4 f, 30 b]. (4) Chain termination processes do not deactivate the catalyst centers because the chain-growth velocity stays constant for weeks. 

Apart from the theoretical significance, these carbene precursors constitute convenient sources of singlet carbenes which can be generated under non-basic conditions that are particularly suitable for small-scale preparations and C labeling experiments. Nevertheless, attempts at generating dialkylcarbenes from precursors of type 14 failed. ... 

The appearance of an intense M —1 peak in the mass. spectrum of benzocyclopropene was used as evidence for the formation of the benzocyclopropenyl cation upon electron impact. This view is, however, an oversimplification. It is now known from both and C labeling experiments that benzocyclopropene undergoes total hydrogen and carbon scrambling before fragmentation in the mass spectrometer. The nature of the rearranged species being unknown, the M —1 peak may not be attributed to the benzocyclopropenyl cation. 

The evidence presented so far does not exclude the possibility that the reaction proceeds by exchange of substituents while the triple bond remains intact. This, however, has been excluded by C-labelling experiments . ... 

Solvolysis of the complex (64) in aqueous acetone follows pseudo first-order kinetics with the formation of the ring-expanded compound (65 equation 198). A carbonium ion pathway is supported by C labeling experiments. 

Spectral Analysis. - The basic principles of metabolic modelling, applied to C-labeling experiments, have been reviewed. ... 

The C-labeling experiments of Roberts in 1953 put the existence of ortho-benzyne as an intermediate beyond doubt when it was found that treatment of 1-i C-chlorobenzene with potassium amide in liquid ammonia gave a 1 1 mixture of 1- and 2- C labeled aniline. The reaction had clearly proceeded through a symmetrical intermediate ortho-benzyne. The overall process whereby a nucleophile apparently enters ortho to the leaving group is referred to as cine substitution (Scheme 7.10). 

Hofmann, U., Maier, K., Niebel, A., Vacun, G. et al. (2008) Identification of metabolic fluxes in hepatic cells from transient C-labeling experiments part I. Experimental observations. Biotechnol. Bioeng., 100, 344-354. 

Fig. 4. Biosynthesis of mannosyiglycerate in Rhodothermus marinus. The reaction scheme was proposed based on measurements of enzyme activities in cmde cell extracts and C-labeling experiments. The numbers refer to the following enzymes 1, mannosyiglycerate synthase 2, mannosyl-3-phosphoglycerate synthase 3, mannosyl-3-phosphoglycerate phosphatase. Data from L. O. Martins, N. Empadinhas, J. D. Marugg, C. Miguel, C. Ferreira, M. S. da Costa, and H. Santos, J. Biol. Chem. 274,35407 (1999).

Wiechert, W., de Grairf, A. A. In Vivo Stationary Flux Analysis by C-Labeling Experiments. Vol. 54, p. 109... 

---