Complex chemiluminescent systems

COMPLEX CHEMILUMINESCENT SYSTEMS 3.5.1 Excitation of additives in active nitrogen... 

In this part of the chapter we briefly outline the main classes of mechanisms occurring in chemiluminescent transformations of organic peroxides. The transformations involving isolated 1,2-dioxetanes and related species will not be extensively discussed, since a specific chapter is dedicated to these compounds. We therefore limit ourselves to describing the general decomposition mechanisms of these peroxides, as these are important in the context of the more complex CL systems that we will describe in the last part of this chapter. 

An indirect method has been used to determine relative rate constants for the excitation step in peroxyoxalate CL from the imidazole (IM-H)-catalyzed reaction of bis(2,4,6-trichlorophenyl) oxalate (TCPO) with hydrogen peroxide in the presence of various ACTs18. In this case, the HEI is formed in slow reaction steps and its interaction with the ACT is not observed kinetically. However, application of the steady-state approximation to the reduced kinetic scheme for this transformation (Scheme 6) leads to a linear relationship of 1/S vs. 1/[ACT] (equation 5) and to the ratio of the chemiluminescence parameters /ic vrAi), which is a direct measure of the rate constant of the excitation step. Therefore, this method allows for the determination of relative rate constants for the excitation step in a complex reaction system, where this step cannot be observed directly by kinetic measurements18. The singlet quantum yield at infinite activator concentrations ( °), where all high-energy intermediates formed interact with the activator, is also obtained from this relationship (equation 5). 

Directly labeled chemiluminescent systems produce <1 photon/label and require complex chemical synthesis to produce each new labeled molecule (1,2). In contrast, chemiluminescent detection of enzyme labels combines the advantages of a high specific activity label with the convenience of relatively simple coupling chemistries, which use commercial reagents. A number of enzyme labels can be detected via chemiluminescent or bioluminescent reac-... 

In addition to their implication as reactive intermediates in bioluminescence, dioxetanones have been proposed as key intermediates in several chemiluminescent systems. Most notable are the chemiluminescent oxidation reaction of acridan esters [19] and the chemiluminescent reaction of the related acridinium salts [20] (Rauhut et al., 1965a McCapra et al., 1977). Both reactions are quite efficient at generating singlet excited states (pCE = 10% and 2% respectively) and, owing to the elegant work of McCapra and others, are among the best understood complex chemiluminescent reaction mechanisms. 

The potential diagram for NO is shown in Figure 1.8. Baulch et al. [112] have recently reviewed the rate data and recommend a value for the third-order rate constant for recombination at 298°K with N2 as the third body of 1.03 x 10 32 cm6/molecule2-sec. The /8 (2f2II-X2n), y (A 2 +-X 2I I), d (C ari—A 2TI), and Ogawa (b 4S -a 4II) bands have all been identified in the complex chemiluminescence that accompanies the recombination. Young and Sharpless [113, 114] determined the total intensities of the first three of these systems at room temperature, and the temperature dependences of these processes have since been measured by Gross and Cohen [115]. 

Attention has been focused on four different chemiluminescent systems, two as substrates for enzymes and two as labels incorporated in the probes (activated after hybridization by H2O2/alkali (Table 7.6)), The latter are usually intended for diagnostic purposes and prepared by commercial suppliers, whereas those serving as a substrate for enzymes attached to the probe-hybrid complex can be readily adapted for many hybridization assays. 

As a close related compound to the chemiluminescence systems, the anthracene-9,10-endoperoxide has been used to describe the complex topography which characterises the region of crossing between the ground and excited state PESs in the 0-0 photolysis. Four eleetronic states involving different electron occupations of the aoo> < oo ... 

Chemiluminescence has been observed from chelate complexes of Eu ". For the case of the complex Eu(dbm)3(pyridine), the Do excited state can be populated by energy transfer from the electrogenerated singlet state exciplex BF /TPTA (where BP is benzophenone and TPTA is tri-p-tolylamine). A similar chemiluminescent system has been observed in the thermal decomposition of trimethyl-1,2-dioxetane to a triplet excited state in the presence of Eu(Ha)3(phen). The major fraction of the emission is observed from the Do state of Eu ", which is populated by intermolecular energy transfer. " ... 

It is a happy coincidence that the first luciferins whose structures were elucidated have a chemistry which leads to a satisfactory explanation of mechanism. However there are many other organisms with complex biochemical systems, and even where the structures of some of the luciferins are known, no chemiluminescence mechanism has yet been derived. Examples of some of the structures which await further investigation are shown. 

Seitz, Suydam, and Hercules 186> recently developed on the basis of luminol chemiluminescence a method for chromium-III ion determination which has a detection limit of about 0.025 ppb. The method is specific for free chromium-III ions as chromium-VI compounds have no catalytic effect and other metal ions can be converted to a non-catalytic form by complexing with EDTA, since the chromium-III complex of EDTA, which is in any case not catalytically active, is formed kinetically slowly 186>. To detect extremely small light emissions, and hence very small metal concentrations, a flow system was used which allows the reactants to be mixed directly in front of a multiplier. (For a detailed description of the apparatus, see 186>). 

The enhanced chemiluminescence associated with the autoxidation of luminol (5-amino-2,3-dihydro-1,4-phthalazinedione) in the presence of trace amounts of iron(II) is being used extensively for selective determination of Fe(II) under natural conditions (149-152). The specificity of the reaction is that iron(II) induces chemiluminescence with 02, but not with H202, which was utilized as an oxidizing agent in the determination of other trace metals. The oxidation of luminol by 02 is often referred to as an iron(II)-catalyzed process but it is not a catalytic reaction in reality because iron(II) is not involved in a redox cycle, rather it is oxidized to iron(III). In other words, the lower oxidation state metal ion should be regarded as a co-substrate in this system. Nevertheless, the reaction deserves attention because it is one of the few cases where a metal ion significantly affects the autoxidation kinetics of a substrate without actually forming a complex with it. 

Tris(2,2 -bipyridine)ruthenium(UI) complex ions (44) produce a chemiluminescence in the presence of amino acids in a FTA system. Amino acids containing secondary amino groups have the strongest response LOD 20 pmol for proline to 50 nmol for... 

Other detection methods have been used in optical MIP sensing systems. An MIP-based chemiluminescent flow-through sensor was developed for the detection of 1,10-phenanthroline (Lin and Yamada 2001). A metal complex was used to catalyze the decomposition of hydrogen peroxide and form the superoxide radical ion that can... 

Rauhut and coworkers proposed the occurrence of a charge transfer complex between the HEI and the ACT in order to explain the electronically excited-state generation in the peroxyoxalate system. Chemiluminescence quantum yield (4>cl) measurements with different activators have shown that the lower the ACT half-wave oxidation potential (Ei/2° ) or singlet energy (Es), the higher the electronically excited-state formation rate and 4>cl- According to the mechanistic proposal of Schuster and coworkers for the CIEEL... 

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