Stable Dioxetane Precursors as Selective Trap-and-Trigger Chemiluminescent Probes for Singlet Oxygen

Light/X-ray/ultrasound activated delayed photon emission of organic molecular probes for optical imaging: mechanisms, design strategies, and biomedical applications

Conventional optical imaging, particularly fluorescence imaging, often encounters significant background noise due to tissue autofluorescence under real-time light excitation. To address this issue, a novel optical imaging strategy that captures optical signals after light excitation has been developed. This approach relies on molecular probes designed to store photoenergy and release it gradually as photons, resulting in delayed photon emission that minimizes background noise during signal acquisition. These molecular probes undergo various photophysical processes to facilitate delayed photon emission, including (1) charge separation and recombination, (2) generation, stabilization, and conversion of the triplet excitons, and (3) generation and decomposition of chemical traps. Another challenge in optical imaging is the limited tissue penetration depth of light, which severely restricts the efficiency of energy delivery, leading to a reduced penetration depth for delayed photon emission. In contrast, X-ray and ultrasound serve as deep-tissue energy sources that facilitate the conversion of high-energy photons or mechanical waves into the potential energy of excitons or the chemical energy of intermediates. This review highlights recent advancements in organic molecular probes designed for delayed photon emission using various energy sources. We discuss distinct mechanisms, and molecular design strategies, and offer insights into the future development of organic molecular probes for enhanced delayed photon emission.

An Aggregation-Induced Emission Nanosensor for Real-Time Chemiluminescent Sensing of Light-Independent Intracellular Singlet Oxygen

Characterizing the transient ultratrace light-independent intracellular singlet oxygen (¹O₂), which plays a vital role in multiple biological processes in living organisms, brings about tremendous help for understanding the nature of ¹O₂-mediated or related bioevents. Nevertheless, an approach to detect the light-independent intracellular ¹O₂ is hard to find. Herein, we developed a chemiluminescent nanosensor by compacting a great number of TPE-N(Ph)-DBT-PH molecules in one nanostructure via autoaggregation. Taking advantage of the aggregation-induced emission property, this TPE-N(Ph)-DBT-PH nanosensor is highly fluorescent and promises a bright red-light CL and the convenience of mapping in vivo sensor distribution. Experiments demonstrate the nanosensor's unprecedented selectivity toward ¹O₂ against other reactive oxygen species. The 3.7 nmol L^-1 limit of detection renders this nanosensor with the best-known sensitivity of ¹O₂ chemical sensors. Meanwhile, fluorescence confocal microscope imaging results suggest that our nanosensor simultaneously targets mitochondria and lysosomes in RAW 264.7 cells via the energy-dependent endocytosis pathway, thereby implying an attractive potential for the detection of intracellular ¹O₂. Such a potential is demonstrated by detecting ¹O₂ in RAW 264.7 cells during a lipopolysaccharide and phorbol myristate acetate stimulated respiration burst. This study represents the first approach to detect light-independent intracellular ¹O₂ during cell bioregulation. Thus, our nanosensor provides an effective tool for investigating the ¹O₂-related bioprocesses and pathological processes.

Chemiluminescent Probes Based on 1,2-dioxetane Structures For Bioimaging

Chemiluminescent probes based on 1,2-dioxetane scaffold are one of the most sensitive imaging modalities for detecting disease-related biomarkers and can obtain more accurate biological information in cells and in vivo . Due to the elimination of external light excitation, the background autofluorescence problem in fluorescence technology can be effectively avoided, providing ultra-high sensitivity and signal-to-noise ratio for various applications. In this minireview, we highlight a comprehensive but concise overview of activatable 1,2-dioetxane-based chemiluminescent probes by reporting significant advances in accurate detection and bioimaging. The design principles and applications for reactive species, enzymes, and other disease-related biomarkers are systematically discussed and summarized. The challenges and potential prospects of chemiluminescent probes are also discussed to further promote the development of new chemiluminescence methods for biological analysis and diagnosis.

Near-Infrared Chemiluminescent Probe for Real-Time Monitoring Singlet Oxygen in Cells and Mice Model

Singlet oxygen (¹O₂) plays a vital role in metabolism. However, because of its extremely high reactivity and short-lived state, the in vivo detection of ¹O₂ is challenging. To address this issue, for the first time, we herein constructed a near-infrared (NIR) chemiluminescent probe (CL-SO) by caging the precursor of phenoxy-dioxetane scaffolds and a dicyanomethylchromone acceptor for selective ¹O₂ detection. This probe can detect ¹O₂ in vitro with a tremendous turn-on chemiluminescence signal in the NIR region (700 nm) and image intracellular ¹O₂ produced by the photosensitizer during the simulated action of photodynamic therapy (PDT). Notably, ¹O₂ level changes in the abdominal cavity and tumor of the various mice model under different stimulations and PDT action were effectively monitored by CL-SO, providing a novel chemiluminescence imaging platform to explore ¹O₂ generation in PDT-associated applications.

Emissive Enhancement of the Singlet Oxygen Chemiluminescence Probe after Binding to Bovine Serum Albumin

A chemiluminescence probe for singlet oxygen ¹O₂ (SOCL) was investigated in phosphate buffer saline (PBS), either in the absence of proteins or containing bovine serum albumin (BSA). In the protein-free PBS, the reactivity of SOCL for methylene blue (MB)-photosensitized ¹O₂ was found to be moderate or low. The reaction yield increased with temperature and/or concentration of dissolved molecular oxygen. Unexpectedly, the presence of BSA boosted both the emissive nature and the thermal stability of the phenoxy-dioxetane intermediate formed in the chemiexcitation pathway. Isothermal titration calorimetry showed that SOCL has a moderate binding affinity for BSA and that entropy forces drive the formation of the SOCL-BSA complex. A model with two identical and independent binding sites was used to fit the binding isotherm data. Co-operative binding was observed when MB was present. Local viscosity factors and/or conformational restrictions of the BSA-bound SOCL phenoxy-dioxetane were proposed to contribute to the formation of the highly emissive benzoate ester during the chemically initiated electron exchange luminescence (CIEEL) process. These results led us to conclude that hydrophobic interactions of the SOCL with proteins can modify the emissive nature of its phenoxy-dioxetane, which should be taken into account when using SOCL or its cell-penetrating peptide derivative in living cells.

Real-Time Mapping of Ultratrace Singlet Oxygen in Rat during Acute and Chronic Inflammations via a Chemiluminescent Nanosensor

Sensing nonradiation-induced singlet oxygen (¹ O₂ ) in whole-animal is deemed as one of the most challenging tasks in noninvasive techniques due to the µs level lifetime of ¹ O₂ and quenching by numerous reductants in tissues. Here a distinct chemiluminescent (CL) nanosensor (NTPE-PH) that boasts ultrahigh concentrated CL units in one nanoparticle is reported. Taking advantage of the intramolecular energy transfer mechanism that promises high energy transfer efficiency and the aggregation-induced emission behavior that guarantees high CL amplification, the NTPE-PH sensor is sensitive to a nm level ¹ O₂ . Experiments demonstrate that the NTPE-PH yields a highly selective CL response toward ¹ O₂ among common reactive oxygen species. With proved low cytotoxicity and good animal compatibility, real-time mapping of ultratrace ¹ O₂ in whole-animal during acute and chronic inflammations is first achieved. It is anticipated that the NTPE-PH sensor can be a useful tool for monitoring ¹ O₂ variation during immune response and pathological processes corresponding to different stimuli, even with drug treatment included.

Structure-activity study of furyl aryloxazole fluorescent probes for the detection of singlet oxygen

In this study, we report the synthesis and the photochemical behavior of a series of new "click-on" fluorescent probes designed to detect singlet oxygen. They include a highly fluorescent chemical structure, an aryloxazole ring, linked to a furan moiety operating as singlet oxygen trap. Their activity depends on both the structure of the aryloxazole fluorophore and the electron-donating and electron-accepting properties of the substituents attached to the C-5 of the furan ring. All probes are selectively oxidized by singlet oxygen to give a single fluorescent product in methanol and produce negligible amounts of singlet oxygen themselves by self-sensitization. The most promising dyad, (E)-2-(2-(5-methylfuran-2-yl)vinyl)naphtho[1,2-d]oxazole, FN-6, shows outstanding reactivity and sensitivity: it traps singlet oxygen with a rate constant (5,8 ± 0.1) x 1(07) M-1 s-1 and its fluorescence increases by a factor of 500 upon reaction. Analysis of the dyads reactivity in terms of linear free energy relationships using the modified Swain and Lupton parameter F and the Fukui condensed function for the electrophilic attack, suggests that cycloaddition of singlet oxygen to the furan ring is partially concerted and possibly involves an exciplex with a "more open" structure than could be expected for a concerted cycloaddition.

Ergothioneine stands out from hercynine in the reaction with singlet oxygen: Resistance to glutathione and TRIS in the generation of specific products indicates high reactivity

The candidate vitamin ergothioneine (ET), an imidazole-2-thione derivative of histidine betaine, is generally considered an antioxidant. However, the precise physiological role of ET is still unresolved. Here, we investigated in vitro the hypothesis that ET serves specifically to eradicate noxious singlet oxygen (¹O₂). Pure ¹O₂ was generated by thermolysis at 37°C of N,N'-di(2,3-dihydroxypropyl)-1,4-naphthalenedipropanamide 1,4-endoperoxide (DHPNO₂). Assays of DHPNO₂ with ET or hercynine (= ET minus sulfur) at pH 7.4 were analyzed by LC-MS in full scan mode to detect products. Based on accurate mass and product ion scan data, several products were identified and then quantitated as a function of time by selected reaction monitoring. All products of hercynine contained, after a [4+2] cycloaddition of ¹O₂, a carbonyl at position 2 of the imidazole ring. By contrast, because of the doubly bonded sulfur, we infer from the products of ET as the initial intermediates a 4,5-dioxetane (after [2+2] cycloaddition) and hydroperoxides at position 4 and 5 (after Schenck ene reactions). The generation of single products from ET, but not from hercynine, was fully resistant to a large excess of tris(hydroxymethyl)aminomethane (TRIS) or glutathione (GSH). This suggests that ¹O₂ markedly favors ET over GSH (at least 50-fold) and TRIS (at least 250-fold) for the initial reaction. Loss of ET was almost abolished in 5mM GSH, but not in 25mM TRIS. Regeneration of ET seems feasible, since some ET products - by contrast to hercynine products - decomposed easily in the MS collision cell to become aromatic again.

Amplified Chemiluminescence Signal for Sensing Fluoride Ions

Bringing together the concepts of self-immolative linkers and chemiluminogen dioxetane modules, a chemiluminescence-based sensor for fluoride with signal amplification is presented. Signal amplification is obtained by triggering two chemiluminescence events for each reacting fluoride ion that in turn releases two fluoride ions for each ion. As expected, the chemiluminescence signal starts to rise following an induction period. In addition to the analytical potential, this chemical system is also of interest as a demonstration of positive feedback loop character.

The Role of Singlet Oxygen in Surface Water Photochemistry

Singlet oxygen, (1O2, 1Δg), is a selective oxidant produced in sunlit surface waters. It is an electrophile produced from the quenching of excited state triplet natural organic matter (3NOM) by dissolved oxygen and it reacts with electron-rich alkenes, sulfides, and phenols. The concentration of 1O2 is high near the NOM molecules that sensitize its production and significantly decreases moving away from the NOM source. This chapter discusses the formation, quenching, reactivity, and detection of 1O2 and includes examples of surface water contaminants that react with 1O2.

Photochemical production of singlet oxygen from particulate organic matter

Dissolved organic matter is established as one of the most relevant photosensitizers in aquatic environments, producing singlet oxygen (1O2) alongside other photochemically produced reactive intermediates. While the production of 1O2 from DOM has been well studied, the relative importance of particulate organic matter (POM) to the overall 1O2 production is less well understood. POM is known to play an important role in pollutant fate through the sorption and transport of hydrophobic pollutants. If POM is directly involved in 1O2 production, sorbed molecules would be expected to undergo enhanced photodegradation. In this work, synthetic POM was prepared by coating silica particles with commercial humic acid. The photochemical behavior of these POM samples was compared to dissolved commercial humic acids (DOM). Suspended natural sediment was also studied to test the environmental relevance of the synthetic POM model. Synthetic POM particles appear to simulate well the 1O2-production of suspended sediment. The 1O2 concentrations experienced by POM-sorbed probe molecules was up to 30% higher than experienced by DOM-sorbed ones, even though the aqueous concentration of 1O2 in irradiated POM suspensions was much lower than the analogous DOM solutions. These results were interpreted with a reaction-diffusion model, which suggested that the production rate of 1O2 by POM is lower than DOM, but the loss of 1O2 from the POM-phase is also lower than DOM. Based on the experimental results of this study, calculations were conducted to estimate the impact of removing POM on 1O2-mediated processes. These calculations indicate that compounds with a log Koc value near 4 will be most affected by removal of POM and that the magnitude of the effect is proportional to the fraction of the total organic matter represented by POM. This study demonstrates that particles can play an important role in the degradation of organic compounds via aquatic photochemistry.

A novel chemiluminescence from the reaction of singlet oxygen with β-diketonates of europium(iii), neodymium(iii) and ytterbium(iii)

Decomposition of 1,4-dimethylnaphthalene endoperoxide, which is the source of singlet oxygen, in the presence of β-diketonates of europium(iii), neodymium(iii) and ytterbium(iii) is accompanied by bright chemiluminescence in visible and near infra-red spectral region due to characteristic emission from the lanthanides.

Autocatalytic-assisted photorelease of a sensitizer drug bound to a silica support

The photorelease of a sensitizer from a fluorinated silica surface occurs by a reaction of singlet oxygen with the vinyl ether bond linker with scission of a dioxetane intermediate. Irradiation of the released sensitizer generates singlet oxygen, which accelerates the release of more sensitizer via an autocatalytic reaction. Sigmoidal behavior of sensitizer release in n-butanol and n-octanol occurs at an optimal temperature of 20 °C. The photorelease efficiency was reduced at low temperatures, where the sensitizer was retained on the surface due to a long-lived dioxetane with inefficient scission, and also reduced at high temperatures, due to a slower reaction of (1)O2 with the vinyl ether bond. Immediate acceleration is a result of released sensitizer being used as a dopant to eliminate the induction step, further implicating an autocatalytic mechanism. However, the sigmoidal sensitizer release was not correlated to solvent viscosity, heat, or light from the dioxetane decomposition or to minor O2 solubility enhancements caused by the fluorinated silica. The mechanistic information collected here can be used to help control the pace of drug release; however, it remains to be seen whether an autocatalytic-based drug delivery system has an advantage to those with non-sigmoidal kinetics.

Singlet oxygen formation from wastewater organic matter

Singlet oxygen ((1)O2) plays an important role in the inactivation of pathogens and the degradation of organic contaminants. The present study looks at the surface steady-state concentration of (1)O2 and quantum yields (ΦSO) for organic matter present in or derived from wastewater (WWOM), including those that are partially treated and after undergoing oxidation. The surface steady state concentrations of (1)O2 ranged from 1.23 to 1.43 × 10(-13) M for bulk wastewaters under simulated sunlight. The ΦSO values for these samples varied from 2.8% to 4.7% which was higher than the values observed for the natural organic matter isolates evaluated (1.6-2.1%). Size fractionation of WWOM resulted in ΦSO increases, with a value of up to 8.6% for one of the <1 kDa fractions. Furthermore, oxidation of WWOM by hypochlorous acid (HOCl) and molecular ozone also resulted in an increase in ΦSO, with the highest measured value being 9.3%. This research further explores the correlations between the photosensitizing properties of WWOM and optical characteristics (e.g., absorbance, E2:E3 ratio). Making use of easily measurable absorbance values, a model for the prediction of (1)O2 steady-state concentrations is proposed.

Microheterogeneous concentrations of singlet oxygen in natural organic matter isolate solutions

The binding affinity of a hydrophobic singlet oxygen probe toward natural organic matter isolates was investigated. A linear phase-partitioning model was used to calculate partition coefficients and intramicellar concentrations of singlet oxygen several orders of magnitude larger than those reported by traditional singlet oxygen probes. From the obtained data, a kinetic model was developed to describe the microscopic environment experienced by hydrophobic compounds in natural water systems. Micellar radii and molecular weights were derived from the experimental data and evaluated. The data obtained provides additional support of a microheterogeneous environment within bulk natural solutions. The enhanced concentrations of photogenerated reactive intermediates within these microenvironments may improve understanding of hydrophobic pollutant degradation in the environment.

A phosphinate-based red fluorescent probe for imaging the superoxide radical anion generated by RAW264.7 macrophages

4',9'-Bis(diphenylphosphinyl)naphthofluorescein (PNF-1) has been designed and synthesized as a highly selective, sensitive, cell-permeable, red fluorescent probe for detecting O(2) (.-) in biological systems. The design strategy for the probe is based on the nucleophilic mechanism of O(2) (.-) to mediate deprotection of the probe to naphthofluorescein, the emission spectrum of which is just in the spectral region of low background fluorescence interference in biological systems. Upon treatment with O(2) (.-), the probe exhibits a strong fluorescence response and high selectivity for O(2) (.-), rather than other reactive oxygen species or biological compounds. A linear calibration curve for PNF-1 showed a detection limit of 0.1 nM O(2) (.-). This new type of fluorescent probe allows nanomolar changes in O(2) (.-) concentrations in living cells to be detected by confocal microscopy.

Online phototransformation-flow injection chemiluminescence determination of triclosan

A highly selective and sensitive chemiluminescence method for the determination of triclosan is proposed. The method is based on the phototransformation of triclosan to a light-emitting precursor in the presence of fluorescein in alkaline medium and the chemiluminescence reaction is then triggered by strong base or oxidants such as N-bromosuccinimide. Based on this reaction an online phototransformation-flow injection manifold was developed, in which the photoreactor comprises a 150-cm-long x 0.8-mm-i.d. piece of PTFE tubing coiled around a 25-W fluorescent lamp, and the phototransformed products were then injected into a carrier stream of borate buffer. After mixing with the oxidant stream the produced light was detected by a photomultiplier. A wide calibration range from 8.0 x 10(-8) to 1.0 x 10(-4) mol L(-1) was obtained under the optimized conditions, and the detection limit was as low as 5.0 x 10(-8) mol L(-1). The whole process of analysis, including the online phototransformation and subsequent chemiluminescence detection, could be completed in 6 min. Most of the foreign substances tested showed high tolerance levels, and the proposed method was directly applied to the determination of triclosan in toothpaste samples without any pre-separation procedure. Figure Schematic representation of the phototransformation of triclosan and subsequent chemiluminescence reaction.

A Europium(III) Complex as an Efficient Singlet Oxygen Luminescence Probe

A new europium(III) complex, [4'-(10-methyl-9-anthryl)-2,2':6',2"-terpyridine-6,6"-diyl]bis(methylenenitrilo) tetrakis(acetate)-Eu(3+), was designed and synthesized as a highly sensitive and selective time-gated luminescence probe for singlet oxygen ((1)O2). The new probe is highly water soluble with a large stability constant of approximately 10(21) and a wide pH available range (pH 3-10), and can specifically react with (1)O2 to form its endoperoxide (EP-MTTA-Eu(3+)) with a high reaction rate constant at 10(10) M(-1) s(-1), accompanied by the remarkable increases of luminescence quantum yield from 0.90% to 13.8% and lifetime from 0.80 to 1.29 ms, respectively. The wide applicability of the probe was demonstrated by detection of (1)O2 generated from a MoO(4)(2-)/H(2)O2 system, a photosensitization system of 5,10,15,20-tetrakis(1-methyl-4-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP), and a horseradish peroxidase catalyzed aerobic oxidation system of indole-3-acetic acid (IAA). In addition, it was found that the new probe could be easily transferred into living HeLa cells by incubation with TMPyP. A time-gated luminescence imaging technique that can fully eliminate the short-lived background fluorescence from TMPyP and cell components has been successfully developed for monitoring the time-dependent generation of (1)O2 in living cells.

A new chemiluminescence probe for singlet oxygen based on tetrathiafulvalene-anthracene dyad capable of performing detection in water/alcohol solution

A new tetrathiafulvalene-anthracene dyad 1 with two "tetraethylene glycol" units was synthesized and characterized. Strong chemiluminescence was observed upon reaction of dyad 1 with singlet oxygen (1O2), and this reaction shows fairly good selectivity toward 1O2 over other reactive oxygen species. Due to the introduction of two hydrophilic "tetraethylene glycol" units, the detection of 1O2 with dyad 1 can be performed in alcohol/water solution, which is relatively a mild medium when compared with water/tetrahydrofuran solution required by other tetrathiafulvalene-anthracene dyads. Dyad 1 may have a wider use for detection of 1O2 in biological systems.

A new terbium(III) chelate as an efficient singlet oxygen fluorescence probe

A terbium(III) chelate fluorescence probe for detection of singlet oxygen (1O2) in aqueous media, N,N,N1,N1-[2,6-bis(3'-aminomethyl-1'-pyrazolyl)-4-(9''-anthryl)pyridine] tetrakis (acetate)-Tb3+ (PATA-Tb3+), was designed and synthesized. The new chelate is highly water soluble, is almost nonfluorescent, and can specifically react with 1O2 to yield a strongly fluorescent chelate, the endoperoxide of PATA-Tb3+, accompanied by a remarkable increase in the fluorescence quantum yield from 0.46 to 10.5%. The long fluorescence lifetime of the endoperoxide (2.76 ms) allows the probe to be used favorably for time-resolved fluorescence detection of 1O2. The studies of fluorescence property and reaction specificity indicate that the new probe is highly sensitive and selective for 1O2. The probe was used for quantitative detection of 1O2 generated from a MoO4(2-) -H2O2 system to give a detection limit of 10.8 nM. In addition, the good applicability of the probe was demonstrated by the real-time monitoring of the kinetic process of 1O2 generation in a horseradish peroxidase-catalyzed oxidation system of indole-3-acetic acid in a weakly acidic buffer.

Microheterogeneity of singlet oxygen distributions in irradiated humic acid solutions

Singlet oxygen (1O2) is a highly reactive species formed through solar irradiation of organic matter in environmental waters. Implicated in a range of reactions, it has proven difficult to quantify its spatial distribution in natural waters. We assessed the microheterogeneous distribution of 1O2 in irradiated solutions containing chromophoric dissolved organic matter (CDOM) by using molecular probes of varying hydrophobicity. The apparent 1O2 concentrations ([1O2]app), measured by recently developed hydrophobic trap-and-trigger chemiluminescent probe molecules, were orders of magnitude higher than those measured by the conventional hydrophilic probe molecule furfuryl alcohol. The differential [1O2]app values measured by these probes reflect a steep concentration gradient between the CDOM macromolecules and the aqueous phase. A detailed kinetic model based on the data predicts probabilistic 1O2 distributions under different solvent conditions.

Determination of H2O2-dependent generation of singlet oxygen from human saliva with a novel chemiluminescence probe

Singlet oxygen (1O2) has been shown to play an important role in salivary defense system, but its generation process and level from human saliva remain uncertain due to the lack of a reliable detection method. We have previously reported 4,4'(5')-bis[2-(9-anthryloxy)ethylthio]tetrathiafulvalene (BAET) as a novel chemiluminescence probe for 1O2. In this work, the probe is successfully used to characterize H2O2-dependent generation of 1O2 from saliva in real time. However, the yield of 1O2 is found to be very low, for example, being about 0.13 nmol from 200 microL saliva in the presence of 1 mM of hydrogen peroxide over a 5-s reaction period. The result is also compared with that obtained with another 1O2 probe 2-methyl-6-phenyl-3,7-dihydroimidazo[1,2-a]pyrazin-3-one (CLA), demonstrating that, besides 1O2, the other reactive oxygen species such as hydroxyl radical may also be involved in the reaction of saliva with H2O2. Furthermore, the present study shows that the selectivity of BAET for 1O2 is much higher than that of CLA and thus BAET is highly suited for the detection of 1O2 in the presence of other reactive oxygen species in biological systems.

Singlet Oxygen Production in the Reaction of Superoxide with Organic Peroxides

[reaction: see text] A selective chemiluminescent probe for singlet oxygen has been employed to detect and quantify singlet oxygen in the reactions of superoxide with organic peroxides. The production of singlet oxygen has been quantified in the reaction of superoxide with benzoyl peroxide (BP). No singlet oxygen was detected in the reactions of superoxide with cumyl peroxide, tert-butyl peroxide, or tert-butyl hydroperoxide. On the basis of these results and on the temperature dependence of the reaction, we proposed a mechanism for singlet oxygen formation in the reaction of superoxide with BP.

Quantification of Singlet Oxygen Production in the Reaction of Superoxide with Hydrogen Peroxide Using a Selective Chemiluminescent Probe

A sensitive chemiluminescent probe that selectively reacts with singlet oxygen in the presence of superoxide and hydrogen peroxide has been used to quantify the production of singlet oxygen in the reaction of superoxide with hydrogen peroxide. The yield of singlet oxygen from this reaction was found to be low (0.2% relative to the initial superoxide concentration). No evidence for the formation of hydroxyl radical was observed in this reaction, ruling out the Haber-Weiss mechanism as a major singlet oxygen formation pathway. No singlet oxygen production was observed in the reaction of superoxide with 2-nitrobenzoic acid, which has a pKa similar to that of hydrogen peroxide, rendering the protonation of superoxide, followed by its disproportionation, an unlikely explanation for the formation of singlet oxygen in this system. The low yields of singlet oxygen and hydroxyl radical suggest that their formation in this reaction should be relatively unimportant in biological systems.