Luminol mechanism

These results involving the luminol radical anion are compatible with other proposals for the luminol mechanism. Steps after the peroxide intermediate are not specifically detected, and must occur with a rate constant of 10 s ... 

Cormier M.J.,Prichard P.M., An investigation of the mechanism of the luminescent peroxidation of luminol by stopped flow techniques, J. Biol. Chem. 1968 243 4706-4714. 

DPA) in dimethylphthalate at about 70°, yields a relatively strong blue Umax =435 nm) chemiluminescence the quantum yield is about 7% that of luminol 64>. The emission spectrum matches that of DPA fluorescence so that the available excitation energy is more than 70 kcal/mole. Energy transfer was observed on other fluorescers, e.g. rubrene and fluorescein. The mechansim of the phthaloyl peroxide/fluorescer chemiluminescence reaction very probably involves radicals. Luminol also chemiluminesces when heated with phthaloyl peroxide but only in the presence of base, which suggests another mechanism. The products of phthaloyl peroxide thermolysis are carbon dioxide, benzoic acid, phthalic anhydride, o-phenyl benzoic acid and some other compounds 65>66>. It is not yet known which of them is the key intermediate which transfers its excitation energy to the fluorescer. 

Hydrazide chemiluminescence has been investigated very intensively during recent years (for reviews, see 1>, p. 63, 2>, 90>). Main topics in this field are synthesis of highly chemiluminescent cyclic diacyl hydrazides derived from aromatic hydrocarbons, relations between chemiluminescence quantum yield and fluorescence efficiency of the dicarboxylates produced in the reaction, studies concerning the mechanism of luminol type chemiluminescence, and energy-transfer problems. 

In DMS0/tert.Bu0K/02 quantum yields found were 3 x I0 4 for 50 and 5 X 104 for 49. These values are not regarded as maximum values, as a strong influence of the reaction conditions was noted 104). Whereas, in luminol-type compounds, any substitution in the cyclic hydrazide ring system renders the compound non-chemiluminescent 105>, this is not the case with these open-chain hydrazides the methylated compounds 49, 50 (R=N(CH3)NH2) and 50 (R=N(CH3)NH(CH3)) were also found to be chemiluminescent, though less so than the unsubstituted hydrazide. E. H. White and coworkers 104> therefore suggest a mechanism via acyl anions for the non-cyclic hydrazides (see Section VI. C.). 

E. H. White and coworkers 2> have proposed a mechanism of luminol (Lum) chemiluminescence in aprotic media in which the luminol dianion (Lum2<-)) is a key intermediate ... 

The luminol dianion Lum2< > does not exist in appreciable quantities in aqueous solvents hydrogen peroxide and a catalyst such as hemin are required. Thus another mechanism seems to be at work here. Perhaps a hydrogen atom is abstracted from the luminol monoanion Lum( > to yield a luminol radical anion 55 which then reacts with oxygen or a radical ion derived from hydrogen peroxide according to 3,4,109)... 

Finally, we mention another experimental result which points to a radical mechanism in luminol chemiluminescence. 

Aqueous alkaline luminol solutions can be excited to chemiluminescence by pulse radiolysis, the only additional requirement being oxygen 119h The suggested mechanism is that hydroxyl radicals attacking luminol monoanions, followed by reaction of the luminol radical anion thus formed with oxygen ... 

Concerning the mechanism of the chemiluminescent reaction between luminol and molecular oxygen in DMSO M. T. Beck and F. Joo 209> performed kinetic experiments leading them to the conclusion that the formation of an oxygen containing intermediate (see p. 109) is a reversible step, in contrast to the opinion of E. H. White and M. M. Rauhut, D. M. Hercules and their coworkers, and others 2 3 109). 

Burdo and Seitz reported in 1975 the mechanism of the formation of a cobalt peroxide complex as the important intermediate leading to luminescence in the cobalt catalysis of the luminol CL reaction [116]. Delumyea and Hartkopf reported metal catalysis of the luminol reaction in chromatographic solvent systems in 1976 [117], while Yurow and Sass [118] reported on the structure-CL correlation for various organic compounds in the luminol-peroxide reaction. 

For analysis in solutions, the most frequently used CL reaction is alkaline oxidation of luminol and lucigenin in the presence of hydrogen peroxide as oxidant, although sodium hypochlorite, sodium perborate, or potassium ferricyanide may also be used. CL reactions involving alkaline oxidation have been used to indicate acid-base, precipitation, redox, or complexometric titration endpoints either by the appearance or the quenching of CL when an excess of titrant is present [114, 134], An example of these mechanisms is shown in Figure 14. 

Figure 5 Proposed mechanism for the ECL reaction of luminol. (Reprinted from Ref. 15, with permission from, and copyright, Elsevier Science.)...

Other cationic surfactants such as TTAB, DTAB, DODAB, STAC, CEDAB, and DDDAB have been used in CL reactions with less frequency. Thus, tetradecyltrimethylammonium bromide [TTAB] has been used to increase the sensitivity of the method to determine Fe(II) and total Fe based on the catalytic action of Fe(II) in the oxidation of luminol with hydrogen peroxide in an alkaline medium [47], While other surfactants such as HTAB, hexadecylpiridinium bromide (HPB), Brij-35, and SDS do not enhance the CL intensity, TTAB shows a maximum enhancement at a concentration of 2.7 X 10 2 M (Fig. 11). At the same time it was found that the catalytic effect of Fe(II) is extremely efficient in the presence of citric acid. With regard to the mechanism of the reaction, it is thought that Fe(II) forms an anionic complex with citric acid, being later concentrated on the surface of the TTAB cationic micelle. The complex reacts with the hydrogen peroxide to form hydroxy radical or superoxide ion on the... 

Independent of the precise mechanism of the chemiluminescence-producing reaction, the process of luminol-dependent PCL can be subdivided into two stages ... 

MF effects on FA relatives and healthy donors. (Fanconi anemia is an autosomal recessive disease associated with the overproduction of free radicals, Chapter 31.) It has been shown earlier [215] that FA leukocytes produce the enhanced amount of hydroxyl or hydroxyl-like free radicals, which are probably formed by the Fenton reaction. It was suggested that MF would be able to accelerate hydroxyl radical production by FA leukocytes. Indeed, we found that MF significantly enhanced luminol-amplified CL produced by non-stimulated and PMA-stimulated FA leukocytes but did not affect at all oxygen radical production by leukocytes from FA relatives and healthy donors (Table 21.3). It is interesting that MF did not also affect the calcium ionophore A23187-stimulated CL by FA leukocytes, indicating the absence of the calcium-mediated mechanism of MF activity, at least for FA leukocytes. 

As mentioned above, the formation of peroxynitrite might be measured by luminol-amplified CL [67]. CL response was greatly augmented by bicarbonate and inhibited by SOD. Radi et al. [67] proposed the following mechanism of carbonate-enhanced luminol CL stimulated by peroxynitrite ... 

The oxidative behaviour of glycolaldehyde towards hexacyanoferrate(III) in alkaline media has been investigated and a mechanism proposed, which involves an intermediate alkoxide ion. Reactions of tetranitromethane with the luminol and luminol-peroxide radical anions have been shown to contribute substantially to the tetranitromethane reduction in luminol oxidation with hexacyanoferrate(III) in aerated aqueous alkali solutions. The retarding effect of crown ethers on the oxidation of triethylamine by hexacyanoferrate(III) ion has been noted. The influence of ionic strength on the rate constant of oxidation of ascorbic acid by hexacyanofer-rate(III) in acidic media has been investigated. The oxidations of CH2=CHX (where X = CN, CONH2, and C02 ) by alkaline hexacyanoferrate(III) to diols have been studied. ... 

In this part of the chapter we will give a more detailed description of some highly efficient organic chemiluminescence systems, which occur with the involvement of peroxide intermediates. We have chosen to begin the subject with the well-known and widely applied luminol oxidation and will show that, even though this reaction has been exhaustively studied, several critical points in its mechanism remain unclear and are still the subject of... 

The chemiluminescence emission resulting from the oxidation of luminol (5-amino-2,3-dihydro-l,4-phthalazinedione) has been extensively studied since its discovery by Albrecht in 1928. Although luminol oxidation is one of the most commonly applied chemiluminescent reactions, to date no definitive mechanism is known . Efficient chemiluminescence emission is only observed when luminol (25) is oxidized under alkaline conditions. Depending on the medium, co-oxidants are required in addition to molecular oxygen for the observation of light emission, but under any condition, 3-aminophthalate (3-AP) and molecular nitrogen are the main reaction products (equation 10). 

Apparently, the mechanisms in protic and aprotic conditions only differ in their early steps, in which the protic system requires additional oxidants to convert luminol to... 

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) . 

Bacillus subtilis defense mechanism, 610 bovine semm albumin y-radiation, 614 generation inhibition, 612 hydroperoxide synthesis, 315, 320 ludgenin oxidation, 645, 1250-1 luminol oxidation, 643, 644, 1242-4 organic sulfur compounds, 1032-9 ozone water disinfection, 606 peroxynitrite generation, 10, 611-12 Superoxide dismutase (SOD)... 

The effect of the hydroxyl radical (HO ) on luminol chemiluminescence has also been intensively studied178 187 198 199. Although detailed mechanisms for the reaction of hydroxyl radicals with hydrazides remain unknown, two different processes are assumed to be involved oxidation of the hydrazide group and addition of hydroxyl radical to the aromatic ring (Scheme 19)180 1". 

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