1.2- Dioxetanones oxygen

Fig. 1.12 Mechanism of the bioluminescence reaction of firefly luciferin catalyzed by firefly luciferase. Luciferin is probably in the dianion form when bound to luciferase. Luciferase-bound luciferin is converted into an adenylate in the presence of ATP and Mg2+, splitting off pyrophosphate (PP). The adenylate is oxygenated in the presence of oxygen (air) forming a peroxide intermediate A, which forms a dioxetanone intermediate B by splitting off AMP. The decomposition of intermediate B produces the excited state of oxyluciferin monoanion (Cl) or dianion (C2). When the energy levels of the excited states fall to the ground states, Cl and C2 emit red light (Amax 615 nm) and yellow-green light (Amax 560 nm), respectively.

In the luminescence reaction of firefly luciferin (Fig. 1.12), one oxygen atom of the product CO2 is derived from the molecular oxygen while the other originates from the carboxyl group of luciferin. In the chemiluminescence reaction of an analogue of firefly luciferin in DMSO in the presence of a base, the analysis of the product CO2 has supported the dioxetanone pathway (White et al., 1975). 

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. 

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 imidazopyrazine substructure occurs in luciferins from a wide range of organisms living in the sea and in ostracod crustaceans, e.g. Cypridina. The chemistry of the luminescent effect in these systems involves reaction with oxygen to form a stable cyclic dioxetanone, similar to the hrefly luciferin as indicated in (3.108). 

Of the many types of bioluminescence in nature, that of the firefly represents the most thoroughly studied and best understood biological luminescent process. The molecular mechanism of light emission by the firefly was elucidated in the 1960s in which a dioxetanone (a-peroxy lactone) was proposed as an intermediate, formed by the luciferase-catalyzed enzymatic oxidation of the firefly luciferin with molecular oxygen (Scheme 15). This biological reaction constitutes one of the most efficient luminescent processes known to date . Hence, it is not surprising that the luciferin-luciferase system finds wide use... 

The following four-membered ring structures containing two oxygen atoms are known. Of these, the 1,2-dioxetanes 1 are most common and will form the majority of this chapter. 1,2-Dioxetanones 2 (a-peroxy lactones) and dioxetanimines 3 are rare but have been prepared and will be mentioned where relevant. 1,2-Dioxetenes 4, methylene-1,2-dioxetanes 5, and l,2-dioxetane-3,4-dione 6 have not been prepared in stable form and will only be discussed from a theoretical viewpoint. The isomeric 1,3-dioxetanes 7, l,3-dioxetane-2,4-dione 8, and methylene-1,3-dioxetanes 9 have also received attention from the theoretical viewpoint however, creditable chemical evidence for their existence is still lacking. 

In principle, the cyclic route is distinguishable from the alternative linear route by labeling experiments. The dioxetanone route predicts that one oxygen in the C02 is derived from the molecular oxygen the linear route predicts that one oxygen in the C02 is derived from the aqueous medium. In practice, experimental attempts to distinguish between the two possibilities have been the source of considerable controversy, as will be illustrated for the case of the firefly. The controversy has now been settled in favor of the dioxetanone route, however. 

Under basic condition (pH 7.8), this chromophore reacts with oxygen to produce a dioxetanone intermediate. The dioxetanone collapses to alpha-thio-amide with producing blue light (470 nm) and CO2. After the oxidized chromophore decomposes to a coelenteramine, the free active site cysteine becomes available again. 

Usami K, Isobe M. Two luminescent intermediates of coelenterazine analog, peroxide and dioxetanone, prepared by direct photo-oxygenation at low temperature. Tetrahedron Lett 1995 36 8613-6. 

SOC values between So and Tt states near a S0/Ti crossing seam. The 6,4 model was used for dioxetanones, and the 10,6 model (= 6,4 + two additional oxygen nonbonding orbitals) for dioxetanes. aSOC values between Si and Ti states. 

Very recently, Isobe et al.4,5 have shown that the charge-transfer (CT) configuration in Fig. 2C play an important role of the intersystem crossing from triplet (T) molecular oxygen (302) to the singlet one via the spin-orbit (SO) interaction in the course of dioxetane (DO) (dioxetanone (DON)) formation reactions. 

Oxygen-18 studies of the chemiluminescence of a firefly luciferin analogue have provided evidence which contradicts other earlier reports in that it suggests that the dioxetanone is an important intermediate in light production.304 Spin-statistical contributions in redox chemiluminescence quantum efficiency have been analysed,305 and a striking deuterium isotope effect in phosphorus chemiluminescence has been discovered.800... 

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