Reaction pathways molecular labeling

One is the concerted decomposition of a dioxetanone structure that is proposed for the chemiluminescence and bioluminescence of both firefly luciferin (Hopkins et al., 1967 McCapra et al., 1968 Shimomura et al., 1977) and Cypridina luciferin (McCapra and Chang, 1967 Shimomura and Johnson, 1971). The other is the linear decomposition mechanism that has been proposed for the bioluminescence reaction of fireflies by DeLuca and Dempsey (1970), but not substantiated. In the case of the Oplopborus bioluminescence, investigation of the reaction pathway by 180-labeling experiments has shown that one O atom of the product CO2 derives from molecular oxygen, indicating that the dioxetanone pathway takes place in this bioluminescence system as well (Shimomura et al., 1978). It appears that the involvement of a dioxetane intermediate is quite widespread in bioluminescence. 

Crossover experiments often use methyl or other alkyl groups as labels. There is always the possibility, however, that a label may alter the course of a reaction. Therefore, the minimum label that will allow us to distinguish the reaction pathway is best, and an isotopic replacement is the smallest perturbation to the molecular structure we can envision. In the case of the Claisen reaction, labeling the allyl functionality of 34 with an isotope of carbon as shown in Figure 6.9 led to 35, but 36 was not detected. This result was further evidence that the mechanism does not involve molecular fragmentation, since a dissociative process would be expected to give both products. ... 

However, the linear bond cleavage hypothesis of the firefly bioluminescence was made invalid in 1977. It was clearly shown by Shimomura et al. (1977) that one O atom of the CO2 produced is derived from molecular oxygen, not from the solvent water, using the same 180-labeling technique as used by DeLuca and Dempsey. The result was verified by Wannlund et al. (1978). Thus it was confirmed that the firefly bioluminescence reaction involves the dioxetanone pathway. Incidentally, there is currently no known bioluminescence system that involves a splitting of CO2 by the linear bond cleavage mechanism. 

Attempts to use molecular oxygen as the oxidant failed except in solvents that undergo efficient autoxidation to the corresponding hydroperoxide (e.g., THF). Mechanistic studies, including isotopic labeling studies, indicate that fBuOOH is the source of the oxygen atom incorporated into the product, and the reaction proceeds via a hydride-shift pathway that avoids formation of an enol intermediate (Scheme 12). 

A number of isotopically different forms of water can be prepared, which greatly facilitates experimental studies. Replacing both of the usual hydrogen atoms with deuterium (2H) results in heavy water, or deuterium oxide, with a molecular weight of 20. The role of water in chemical reactions can then be studied by analyzing the deuterium content of substances involved as reactants or products. Tritium (3H), a radioactive isotope with a half-life of 12.4 years, can also be incorporated into water. Tritiated water has been used to measure water diffusion in plant tissues. Another alternative for tracing the pathway of water is to replace the usual 160 isotope with lsO. This labeling of water with lsO helped determine that the O2 evolved in photosynthesis comes from H20 and not from CO2 (Chapter 5, Section 5.5A). 

In the same study66 further pathways to generate 68 were elucidated by using labeled precursors. For example, 68 is generated from the molecular ion 72 via two entirely different mechanisms. The first one involves, again, interaction of an ether oxygen atom with a silicenium ion (reaction 40 72 73 68 + 74). In the second one, in the initial step... 

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