Diazaquinone, luminol oxidation

In the first proposal of a mechanism for chemiluminescent luminol oxidation, Albrecht postulates a bicyclic endoperoxide as the high-energy intermediate. The endoperoxide is presumably formed by nucleophilic attack of hydrogen peroxide monoanion on one of the diazaquinone 27 carbonylic groups to form 28, followed, after deprotonation to 29, by ring closure to 30 (Scheme 21) . 

Qualitatively at least luminol diazaquinone (40) can be trapped [52] in a luminol oxidation mixture (with H2O2/K3 Fe(CN)6) as the Diels-Alder adduct with cyclopentadiene (42). 

That a bridged peroxide 68 (for a more detailed discussion, see 1>, p. 84) arises from the intermediate open-chain peroxide formed either by recombination of hydrazide radical ion with. 02( > radical ion, such as 66, or by nucleophilic attack of ion on a carbonyl group of a diazaquinone, such as 67, appears to be very plausible. One of the most important experimental reasons for this assumption has been put forward by E. H. White and M. M. Bursey 106> who, on oxidation of luminol with... 

The CL reaction of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) (1) is one of the more commonly used nonenzymatic CL reactions and has been extensively studied [49, 57-59], It is well known that the luminol CL reaction is catalyzed by many kinds of substances, e.g., ozone, halogen-Fe complex, hemin, hemoglobin, persulfate, and oxidized transition metals. The most acceptable scheme is shown in Figure 10. Luminol forms a six-membered ring of peroxide (3) from a diazaquinone intermediate (2) and then, by the decomposition of 3, N2 gas and the Si-excited state of the phthalate dianion are produced, yielding... 

The source of chemiluminescence in the oxidation of luminol was explored by Merenyi and co-workers in detail (153). The oxidation of luminol yields aminophthalate as a final product and the reaction proceeds via a series of electron transfer steps. The primary oxidation product is the luminol radical which is transformed into either diazaquinone or the a-hydroxide-hydroperoxide intermediate (a-HHP). The latter oxidation step occurs between the deprotonated form of the luminol radical and O -. The chemiluminescence is due to the decomposition of the mono-anionic form of a-HHP into the final products ... 

In aprotic media, only molecular oxygen and a strong base are needed to produce chemiluminescence from luminol. In such media, an important intermediate in the reaction is the dianion of luminol, which can be oxidized by oxygen, resulting in the formation of the diazaquinone 27 and deprotonated hydrogen peroxide. The subsequent nucleophilic attack by hydrogen peroxide dianion to one of the diazaquinone carbonyls gives rise to the formation of a metastable peroxidic intermediate that, by several steps, results in chemiexcited 3-aminophthalate (Scheme 16)123,174, iso ... 

Several other chromophores have been used in the development of sensors based upon ECL. For example, the luminol reaction is a conventional chemi-luminence reaction that has been studied in detail and it is believed that the mechanism of the ECL reaction is similar, if not identical, to that of the chemiluminescence. As shown in Fig. 2, the luminol ion undergoes a one-electron oxidation to yield a diazaquinone, which then reacts with peroxide or superoxide ( OOH) to give the excited 3-aminophthalate which has an emission maximum of 425 nm. This reaction is particularly versatile and has been utilized in a variety of ECL assays, many of which have been previously summarized by Knight [1], The luminol ECL reaction can be used for the determination of any species labeled with luminol derivatives, hydrogen peroxide, and other peroxides or enzymatic reactions that produce peroxides. A couple of examples are described later. 

Sekura and Terao reported a luminol-based ECL sensor for lipid hydroperoxides, which are of great importance since they are involved in the regulation of prostaglandin biosynthesis, which in turn can lead to cancer development, aging, and other pathological conditions [70]. In this method, there are two approaches for generation of ECL. By the application of potentials between 0.5 and 1.0 V, luminol is oxidized to a diazaquinone which reacts with free lipid hydroperoxide to yield... 

The ECL reaction of luminol (5-amino-2,3-dihydro-l,4-phthalazinedione) with hydrogen peroxide in alkaline medium is similar to the CL of luminol triggered by chemical oxidation (34, 147). The general accepted mechanism is shown in Schane 13.14. In alkaline solution luminol deprotonates to form an anion, which then undergoes electrochonical oxidation to produce a diazaquinone. This species is further oxidized by peroxide or superoxide to give 3-aminophthalate in an excited state that emits light at 425 nm. 

The kinetic experiments on luminol chemiluminescence in the system water/ alkali/potassium persulfate/hydrogen peroxide [45] were interpreted as showing that a two-electron oxidation product of luminol (i.e. the diazaquinone) was a key intermediate. The kinetics of the chemiluminescent oxidation of 7-dimethyl-amino naphthalene-1,2-dicarboxylic hydrazide led to the same conclusion [51]. 

Since the preparation of the first simple diazaquinones [53, 54] (all of them being non-chemiluminescent ) chemiluminescent diazaquinones have been described. It should be pointed out that luminol diazaquinone has not been isolated as yet, nor has it been isolated from luminescent oxidation mixtures, except as the Diels-Alder adduct mentioned above. 

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