General Acid and Base Catalysis by Phosphate in Peroxyoxalate Chemiluminescence

Bright and long-lasting aqueous peroxyoxalate chemiluminescence in cellulose microspheres

Water decreases the brightness of the peroxyoxalate chemiluminescence partially due to the hydrolysis of the oxalate reagent. Here, we show that encapsulation of an oxalate ester and the fluorescent activator in microspheres of cellulose esters increases the emission intensity 30 times compared to the same reaction in water without encapsulation, whereas the emission intensity decay rate constants are considerably lower. Emission intensities, rate constants and chemiluminescence quantum yields increase with increasing hydrogen peroxide concentrations. These results expand the potential of application of chemiluminescence, contributing for the development of ultrasensitive analytical methods.

An Update on General Chemiexcitation Mechanisms in Cyclic Organic Peroxide Decomposition and the Chemiluminescent Peroxyoxalate Reaction in Aqueous Media

Four-membered ring peroxides are intimately linked to chemiluminescence and bioluminescence transformations, as high-energy intermediates responsible for electronically excited state formation. The synthesis of 1,2-dioxetanes and 1,2-dioxetanones enabled mechanistic studies on their decomposition occurring with the formation of electronically excited carbonyl products in the singlet or triplet state. The third member of this family, 1,2-dioxetanedione, has been postulated as the intermediate in the peroxyoxalate reaction, recently confirmed by kinetic studies on peroxalic acid derivatives. Several general chemiexcitation mechanisms have been proposed as model systems for the chemiexcitation step in efficient bioluminescence and chemiluminescence transformations. In this review article, we discuss the validity and efficiency of the most important chemiexcitation mechanisms, extended to aqueous media, where the efficiency is known to be drastically reduced, specifically in the peroxyoxalate reaction, highly efficient in anhydrous environment, but much less efficient in aqueous media. Mechanistic studies of this reaction will be discussed in diverse aqueous environments, with special attention to the catalysis involved in the thermal reaction leading to the formation of the high-energy intermediate and to the chemiexcitation mechanism, as well as emission quantum yields. Finally, several recent analytical and bioanalytical applications of the peroxyoxalate reaction in aqueous media will be given.

Water Effect on Peroxyoxalate Kinetics and Mechanism for Oxalic Esters with Distinct Reactivities

The peroxyoxalate reaction is being widely used for various analytical and bioanalytical applications, and however, few mechanistic studies are performed in aqueous media, important mainly for bioanalytical applications, where low chemiluminescence emission quantum yields are obtained. In this sense, we report here kinetic studies on the peroxyoxalate reaction, using two commercially available and widely utilized esters, bis(2,4-dinitrophenyl) oxalate (DNPO) and bis(2,4,6-trichlorophenyl) oxalate (TCPO), in 1,2-dimethoxyethane:water mixtures. The reaction of the much more reactive DNPO, in anhydrous and aqueous media, occurs by a direct nucleophilic attack of H₂ O₂ to the oxalic ester, not involving nucleophilic catalysis by imidazole. Contrary, in the reaction of the less reactive TCPO with H₂ O₂ , imidazole acts mainly as nucleophilic catalyst. For both esters, experimental conditions are established where precise kinetic data and emission quantum yields can be obtained. Interestingly, the quantum yields in 1,2-dimethoxyethane water mixtures increase up to a water concentration of 0.7 mol L^-1 and decrease significantly with higher concentrations.

Two Pyrene-Based Metal-Organic Frameworks for Chemiluminescence

Fluorescent agents play an important role in the peroxyoxalate chemiluminescence system. However, the effect of different frameworks on chemiluminescence (CL) has not been explored. Herein two pyrene-based metal-organic frameworks (MOFs), [Pb₂L]_n·2nDMA·2nH₂O (1) and [(Ca₂L)·(DMF)₃]_n·2.5nDMF·6nH₂O (2) (H₄L = 5,5'-(-pyrene-1,6-diyl)-diisophthalic acid; DMA = N,N'-dimethylacetamide; DMF = N,N'-dimethylformamide), have been successfully synthesized and are applied to CL. They both exhibit strong and lasting emission that is visible to the naked eye and is significantly stronger than that of the ligand. More importantly, compared with 2, 1 has notably better CL performance. We infer that the reason may be that 1 has higher stability and larger open channels, which can avoid the aggregation of organic ligands as well as provide an effective pathway for the active substance to diffuse into the channels.

Mechanistic Study of the Peroxyoxalate System in Completely Aqueous Carbonate Buffer

The peroxyoxalate reaction is one of the most efficient chemiluminescence transformations, with emission quantum yields of up to 50%; additionally, it is widely utilized in analytical and bioanalytical assays. Although the real reason for its extremely high efficiency is still not yet understood, the mechanism of this transformation has been well elucidated in anhydrous medium. Contrarily, only few mechanistic studies have been performed in aqueous media, which would be of great importance for its application in biological systems. We report here our experimental results of the peroxyoxalate reaction in completely aqueous carbonate buffer, using fluorescein as chemiluminescence activator. The kinetics are very fast in the used basic conditions (pH > 9); despite this, reproducible kinetic results were obtained. The reaction proceeds by specific base catalysis, with rate-limiting attack of hydrogen peroxide anion to the oxalic ester, in competition with ester hydrolysis by hydroxide ion. Emission quantum yields increase with the hydrogen peroxide concentration up to an optimal concentration of 10 mmol L^-1 . The infinite singlet quantum yield of (5.8 ± 0.2) × 10^-7 is much lower than in anhydrous medium; however, it is similar to quantum yields measured before in partially aqueous media.

Revisiting the Mechanism of Hydrolysis of Betanin

Betanin (betanidin 5-O-β-D-glucoside) is a water-soluble plant pigment used as a color additive in food, drugs and cosmetic products. Despite its sensitivity to light and heat, betanin maintains appreciable tinctorial strength in low acidic and neutral conditions, where the color of other plant pigments, such as anthocyanins, quickly fades. However, betanin is an iminium natural product that experiences acid- and base-catalyzed hydrolysis to form the fairly stable betalamic acid and cyclo-DOPA-5-O-β-D-glucoside. Here, we show that the decomposition of betanin in aqueous phosphate solution pH 2-11 is subject to general base catalysis by hydrogen phosphate ion and intramolecular general acid and base catalysis, providing new insights on the mechanism of betanin hydrolysis. UV/Vis absorption spectrophotometry, ¹ H NMR spectroscopy and mass spectrometry were used to investigate product formation. Furthermore, theoretical calculations support the hypothesis that the nitrogen atom of the tetrahydropyridine ring of betanin is doubly protonated, as observed for structurally simpler amino dicarboxylic acids. Our results contribute to the study of betanin and other pigments belonging to the class of betalains and to deepen the knowledge on the chemical properties of imino acids as well as on iminium-catalyzed modifications of carbonyl compounds in water.