1.2- Dioxetanes firefly bioluminescence

As mentioned earlier (see p. 122) the previously postulated dioxetane intermediate in firefly bioluminescence has been challenged as no 180 is in-corporated in the carbon dioxide released during oxidation of firefly luciferin with 18C>2. In view of the crucial significance of the 180. experiments De Luca and Dempsey 202> rigorously examined the reliability of their tracer method. They conclude from their experiments that all available evidence is in favour of a linear, not a cyclic peroxide intermediate — in contrast to Cypridina bioluminescence where at least part of the reaction proceeds via a cyclic peroxide (dioxetane) as concluded from the incorporation of 180 into the carbon dioxide evolved 202,203). However, the dioxetane intermediate is not absolutely excluded as there is the possibility of a non-chemiluminescent hydrolytic cleavage of the four-membered ring 204>. 

A. Firefly Bioluminescence as a Prototype for a Dioxetane-based Triggered... 

Chemically Initiated Electron Exchange Luminescence (CIEEL) acridinium salts, 1256 alkaline phosphatase, 1193-8 1,2-dioxetanes, 1182-200 firefly bioluminescence, 1191-3 intermolecular, 1213-15, 1231-6 intramolecular, 1214-15, 1236-8 luminol, 1247-8... 

Chemically initiated electron-exchange luminescence (CIEEL) constitutes a general phenomenon, the important example of which is the firefly bioluminescence. A longstanding mechanistic dichotomy on the CIEEL process concerns concerted versus stepwise cleavage of the dioxetane ring. As it is shown in Scheme 1 (on the left), the... 

More than twenty years ago, Schaap et al discovered chemiluminescence from phenoxide substituted dioxetanes (PHOD) as a model for firefly bioluminescence. McCapra had suggested a charge or an electron transfer is involved for such process, but confirmative evidences had not been presented at that time. Recently successive synthetic efforts on phenoxide.substituted dioxetanes produced a series of efficient chemiluminescent molecules. ... 

The real breakthrough in exploiting dioxetanes as analytical tools stems from an initial idea by McCapra (M17, M19, M24) to explain firefly bioluminescence. 

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. 

The bioluminescence of the American firefly (Photinus pyralis) is certainly the best-known bioluminescent reaction, thanks to the work of Me Elroy and coworkers and E. H. White and his group (for references see P, p. 138, 6,168,169)) The substrate of this enzyme-catalyzed chemiluminescent oxidation is the benzothiazole derivative 107 (Photinus luciferin) which yields the ketone 109 in a decarboxylation reaction. The concept of a concerted cleavage of a dioxetane derivative has been applied to this reaction 170> (see Section II. C.). Recent experiments with 18C>2 have challenged this concept, as no 180-containing carbon dioxide was detected from the oxidation of 107 171>. 

Several other examples of 1,2-dioxetane derivatives containing easily oxidizable groups have been reported and the high singlet quantum yield observed in their decomposition was attributed to the occurrence of the intramolecular CIEEL sequence Based on this concept, Schaap and coworkers have introduced the concept of induced chemiluminescence, which is very relevant for investigations into firefly luciferin bioluminescence and has led to the development of chemiluminescent probes widely used in immunoassays (Section N. 

CIEEL is of particular interest for the development of modern chemiluminescent bioassays. The most popular clinical bioassays utilize thermally persistent spiro-adamantyl-substituted dioxetanes with a protected phenolate moiety. These designed 1,2-dioxetanes include an energy source, a fluorophore, and a trigger grouping, and are therefore structurally similar to bioluminescent substrates such as firefly luciferin. Three main commercial dioxetanes 75 are available as one-reagent assays for alkaline phosphatase and are sold under the name of AMPPD (R1 = R2 = H), CSPD (R1 = Cl, R2 = H), and CDP-Star (R1 = R2 = Cl) <2006S1781, 2003ANA279>. These substrates are sensitive to 10 21 mol of alkaline phosphatase in solution. 

The bioluminescence observed in glowworms and fireflies is due to the decomposition of 1,2-dioxetan-3-ones [12]. 

Chemiluminescence and Bioluminescence.—The function of thiazole and benzothiazole derivatives in the processes of bioluminescence and chemiluminescence continues to form the subject of significant investigations. Extensive contributions have been provided by White et al., ° who have reviewed this interesting and active field. They have shown that the luminescence of firefly luciferins involves intermediates having dioxetan structures. 

It might be thought that these four-membered cyclic peresters would most readily be compared with dioxetans. However there is a marked difference in their behaviour. The dioxetanones are considerably less stable than the dioxetans, and their reactions are much more susceptible to catalysis by electron donating compounds and substituents. They are key intermediates in firefly and coelenter-ate bioluminescence, and being a part of an electron-rich molecule, suffer decomposition as soon as they form. 

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