Kinetic studies on the peroxyoxalate chemiluminescence reaction: determination of the cyclization rate constant

Hydrogen-bond-driven self-assembly of chemiluminophore affording long-lasting in vivo imaging

Developing chemiluminescence probe with a slow kinetic profile, even a constant emission within analytical time, would improve the analytical sensitivity, but still remains challenging. This work reports a novel strategy to afford long-lasting in vivo imaging by developing a self-assembled chemiluminophore HPQCL-Cl via the introduction of the hydrogen-bond-driven self-assembled dye HPQ to Schaap's dioxetane. Compared with classical chemiluminophore HCL, self-assembled HPQCL-Cl was isolated from the physiological environment, thereby lowering its deprotonation and prolonging its half-life. Based on HPQCL-Cl, the long-lasting in vivo imaging of 9L-lacz tumor was achieved by developing a β-gal-responsive probe. Its signals remained constant (<5% change) for about 20 min, which may provide a wide time window for the determination of β-gal. This probe also showed high tumor-to-normal tissue ratio throughout tumor resection, highlighting its potential in image-guided clinical surgery.

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.

Direct and Indirect Chemiluminescence: Reactions, Mechanisms and Challenges

Emission of light by matter can occur through a variety of mechanisms. When it results from an electronically excited state of a species produced by a chemical reaction, it is called chemiluminescence (CL). The phenomenon can take place both in natural and artificial chemical systems and it has been utilized in a variety of applications. In this review, we aim to revisit some of the latest CL applications based on direct and indirect production modes. The characteristics of the chemical reactions and the underpinning CL mechanisms are thoroughly discussed in view of studies from the very recent bibliography. Different methodologies aiming at higher CL efficiencies are summarized and presented in detail, including CL type and scaffolds used in each study. The CL role in the development of efficient therapeutic platforms is also discussed in relation to the Reactive Oxygen Species (ROS) and singlet oxygen (1O2) produced, as final products. Moreover, recent research results from our team are included regarding the behavior of commonly used photosensitizers upon chemical activation under CL conditions. The CL prospects in imaging, biomimetic organic and radical chemistry, and therapeutics are critically presented in respect to the persisting challenges and limitations of the existing strategies to date.

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.

Evidence for the Formation of 1,2-Dioxetane as a High-Energy Intermediate and Possible Chemiexcitation Pathways in the Chemiluminescence of Lophine Peroxides

A kinetic study of the chemiluminescent (CL) reaction mechanism of lophine-derived hydroperoxides and silylperoxides induced by a base and fluoride, respectively, provided evidence for the formation of a 1,2-dioxetane as a high-energy intermediate (HEI) of this CL transformation. This was postulated using a linear Hammett relationship, consistent with the formation of negative charge on the transition state of HEI generation (ρ > 1). The decomposition of this HEI leads to chemiexcitation with overall low singlet excited state formation quantum yield (Φ_S from 1.1 to 14.5 × 10^-5 E mol^-1); nonetheless, Φ_S = 1.20 × 10^-3 E mol^-1 was observed with both peroxides substituted with bromine. The use of electron-donating substituents increases chemiexcitation efficiency, while it also reduces the rate for both formation and decomposition of the HEI. Different possible pathways for HEI decomposition and chemiexcitation are discussed in light of literature data from the perspective of the substituent effect. This system could be explored in the future for analytical and labeling purposes or for biological oxidation through chemiexcitation.

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.

Opening a Gateway for Chemiluminescence Cell Imaging: Distinctive Methodology for Design of Bright Chemiluminescent Dioxetane Probes

Chemiluminescence probes are considered to be among the most sensitive diagnostic tools that provide high signal-to-noise ratio for various applications such as DNA detection and immunoassays. We have developed a new molecular methodology to design and foresee light-emission properties of turn-ON chemiluminescence dioxetane probes suitable for use under physiological conditions. The methodology is based on incorporation of a substituent on the benzoate species obtained during the chemiexcitation pathway of Schaap's adamantylidene-dioxetane probe. The substituent effect was initially evaluated on the fluorescence emission generated by the benzoate species and then on the chemiluminescence of the dioxetane luminophores. A striking substituent effect on the chemiluminescence efficiency of the probes was obtained when acrylate and acrylonitrile electron-withdrawing groups were installed. The chemiluminescence quantum yield of the best probe was more than 3 orders of magnitude higher than that of a standard, commercially available adamantylidene-dioxetane probe. These are the most powerful chemiluminescence dioxetane probes synthesized to date that are suitable for use under aqueous conditions. One of our probes was capable of providing high-quality chemiluminescence cell images based on endogenous activity of β-galactosidase. This is the first demonstration of cell imaging achieved by a non-luciferin small-molecule probe with direct chemiluminescence mode of emission. We anticipate that the strategy presented here will lead to development of efficient chemiluminescence probes for various applications in the field of sensing and imaging.

Lophine derivatives as activators in peroxyoxalate chemiluminescence

Lophine and four of its derivatives were applied for the first time as activators of the chemiluminescent peroxyoxalate reaction.

Simple and fast PO-CL method for the evaluation of antioxidant capacity of hydrophilic and hydrophobic antioxidants

A simple and fast procedure is described for evaluating the antioxidant activity of hydrophilic and hydrophobic compounds by using the peroxyoxalate-chemiluminescence (PO-CL) reaction of Bis(2,4,6-trichlorophenyl) oxalate (TCPO) with hydrogen peroxide in the presence of di(tert-butyl)2-(tert-butylamino)-5-[(E)-2-phenyl-1-ethenyl]3,4-furandicarboxylate as a highly fluorescent fluorophore. The IC50 values of the well-known antioxidants were calculated and the results were expressed as gallic equivalent antioxidant capacity (GEAC). It was found that the proposed method is free of physical quenching and oxidant interference, for this reason, proposed method is able to determine the accurate scavenging activity of the antioxidants to the free radicals. Finally, the proposed method was applied to the evaluation of antioxidant activity of complex real samples such as soybean oil and sunflower oil (as hydrophobic samples) and honey (as hydrophilic sample). To the best of our knowledge, this is the first time that total antioxidant activity can be determined directly in soybean oil, sunflower oil and honey (not in their extracts) using PO-CL reactions.

A study of peroxyoxalate-chemiluminescence of 4,4'-bis{[4,6-bis (2-hydroxyethyl)amino-1,3,5-triazin-2-yl]amino}stilbene-2,2'-disulfonic acid-disodium salt as a novel blue fluorescer

The chemiluminescence arising from the reaction of bis(2,4,6-trichlorophenyl)oxalate (TCPO) with hydrogen peroxide in the presence of brightener 4,4'-bis{[4,6-bis(2-hydroxyethyl)amino-1,3,5-triazin-2-yl]amino}stilbene-2,2'-disulfonic acid-disodium salt (Triazinyl) has been studied. The influence of concentration of TCPO, hydrogen peroxide, Triazinyl, base catalysts and temperature on the resulting chemiluminescence was investigated. The kinetic parameters for the peroxyoxalate-chemiluminescence (PO-CL) of Triazinyl were evaluated from computer fitting of the resulting intensity-time plots. The activation energies, E(a), were evaluated from temperature dependence of the corresponding rise and fall rate constants.

Chemiluminescence as a Probe of Polymer Oxidation

Many oxidation reactions of organic materials, including polymers, are accompanied by the emission of weak chemiluminescence (CL). From a study of the mechanism of this weak CL, it is shown that the time development of the CL intensity may provide the kinetics of the oxidation reaction and is thus a sensitive probe of the degradation of the material. The intensity of emission reflects the concentration of peroxidic species in the material. Whereas the kinetics of the oxidation may be described by a series of elementary, homogeneous free radical reactions, the use of imaging techniques has shown that the oxidation of polymers such as polypropylene is highly heterogeneous. A model that describes the oxidation as spreading through the material as an infection from a number of initiating sites is able to rationalize these observations and provide a new approach to the prediction of the useful lifetime of a polymeric material.

Unprecedented chemiluminescence behaviour during peroxyoxalate chemiluminescence of oxalates with fluorescent or electron-donating aryloxy groups

A series of diaryl and bis(4-styrylphenyl) oxalates with electron-donating substituents or fluorescent moieties were subjected to the peroxyoxalate chemiluminescence (PO-CL) reaction, some of which were found to behave in a unprecedented manner. The reaction of bis(p-methyoxyphenyl) oxalate, as a representative example, emits light due not only to the emission from the externally added excited fluorophore, but also from the presumable excimer of p-methoxyphenol. Also, during the reaction of the bis(4-styrylphenyl) oxalates, the emission based on the fluorescence as well as the excimer of the eliminating group were observed. These experimental results suggest that such emitting species would be formed by an intra- and intermolecular electronic interaction with a high-energy intermediate, such as a dioxetanone.