High-energy intermediate peroxyoxalates

Peroxyoxalate-based CL reactions are related to the hydrogen peroxide oxidation of an aryl oxalate ester, producing a high-energy intermediate. This intermediate (l,2-dioxetane-3,4-dione) forms, in the presence of a fluorophore, a charge transfer complex that dissociates to yield an excited-state fluorophore, which then emits. This type of CL reaction can be used to determine hydrogen peroxide or fluorophores including polycyclic aromatic hydrocarbons, dansyl- or fluores-camine-labeled analytes, or, indirectly, nonfluorescers that are easily oxidized (e.g., sulfite, nitrite) and quench the emission. The most widely used oxalate... 

The second system to be described is the CL obtained in the transformation of lucigenin and related derivatives here, too, the mechanisms which lead to chemiexcitation are still discussed in the literature. Finally, we will concentrate our discussion on one of the most efficient CL systems known, the peroxyoxalate reaction. After a brief discussion of kinetic results obtained with the different peroxyoxalate substrates, we will focus mainly on studies which attempt to elucidate the structure of the high-energy intermediate in these reactions and describe the experimental evidence obtained with respect to the mechanism of the excitation step. 

In this part of the chapter, we will focus essentially on mechanistic aspects of the peroxyoxalate reaction. For the discussion of the most important advances in mechanistic aspects of this chemiluminescent system, covering mainly literature reports published in the last two decades, we will divide the sequence operationally into three main parts (i) the kinetics of chemical reactions that take place before chemiexcitation, which ultimately produce the high-energy intermediate (HEI) (ii) the efforts to elucidate the structure of the proposed HEIs, either attempting to trap and synthesize them, or by indirect spectroscopic studies and lastly, (iii) the mechanism involved in chemiexcitation, whereby the interaction of the HEI with the activator leads to the formation of the electronically excited state of the latter, followed by fluorescence emission and decay to the ground state. 

Hexanal, hpid oxidation assessment, 669 1-Hexene, primary ozonide, 720 High-density lipoprotein (HDL) oxidation, 612 TEARS assay, 667 High-energy intermediate (HEI) chemiluminescence, 1215 activators, 1220, 1222 peroxyoxalates, 1188-9, 1257, 1261-6, 1267, 1269 stmcture, 1262, 1263... 

Peroxyoxalate chemiluminescence, continued) high-energy intermediate, 1188-9, 1257, 1261-6, 1267, 1269 mechanism, 1257-61... 

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). 

Lee JH, Rock JC, Park SB, Schlautman MA, Carraway ER. Study of the characteristics of three high-energy intermediates generated in peroxyoxalate chemiluminescence (PO-CL) reactions. J Chem Soc Perkin Trans 2 2002 802-9. 

Peroxyoxalate chemiluminescence is the most efficient nonenzymatic chemiluminescent reaction known. Quantum efficiencies as high as 22—27% have been reported for oxalate esters prepared from 2,4,6-trichlorophenol, 2,4-dinitrophenol, and 3-trif1uoromethy1-4-nitropheno1 (6,76,77) with the duorescers mbrene [517-51-1] (78,79) or 5,12-bis(phenylethynyl)naphthacene [18826-29-4] (79). For most reactions, however, a quantum efficiency of 4% or less is more common with many in the range of lO " to 10 ein/mol (80). The inefficiency in the chemiexcitation process undoubtedly arises from the transfer of energy of the activated peroxyoxalate to the duorescer. The inefficiency in the CIEEL sequence derives from multiple side reactions available to the reactive intermediates in competition with the excited state producing back-electron transfer process. 

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