Vitamin B2-sensitised photooxidation of the ophthalmic drugs Timolol and Pindolol: kinetics and mechanism

The kinetics and mechanistic aspects of the riboflavin-photosensitised oxidation of the topically administrable ophthalmic drugs Timolol (Tim) and Pindolol (Pin) were investigated in water-MeOH (9:1, v/v) solution employing light of wavelength > 400 nm. riboflavin, belonging to the vitamin B(2) complex, is a known human endogenous photosensitiser. The irradiation of riboflavin in the presence of ophthalmic drugs triggers a complex picture of competitive reactions which produces the photodegradation of both the drugs and the pigment itself. The mechanism was elucidated employing stationary photolysis, polarographic detection of dissolved oxygen, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. Ophthalmic drugs quench riboflavin-excited singlet and triplet states. From the quenching of excited triplet riboflavin, the semireduced form of the pigment is generated, through an electron transfer process from the drug, with the subsequent production of superoxide anion radical (O(2)(*-)) by reaction with dissolved molecular oxygen. Through the interaction of dissolved oxygen with excited triplet riboflavin, the species singlet oxygen (O(2)((1)Delta(g))) is also generated to a lesser extent. Both O(2)(*-) and O(2)((1)Delta(g)) induce photodegradation of ophthalmic drugs, Tim being approximately 3-fold more easily photooxidisable than Pin, as estimated by oxygen consumption experiments.     

Visible-light promoted degradation of the commercial antioxidants butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT): a kinetic study

Abstract

Visible-light photo-irradiation of the commercial phenolic antioxidants (PhAs) butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), in the presence of vitamin B₂ (riboflavin, Rf), in methanolic solutions and under aerobic conditions, results in the photo-oxidation of the PhAs. The synthetic dye photosensitiser Rose Bengal was also employed for auxiliary experiments. With concentrations of riboflavin and PhAs of ca. 0.02 mM and < 1 mM, respectively, the excited triplet state of the vitamin (³Rf*) is quenched by BHT in a competitive fashion with dissolved ground state triplet oxygen. From the quenching of ³Rf*, the semireduced form of the pigment is generated through an electron transfer process from BHT, with the subsequent production of superoxide anion radical (O₂^??) by reaction with dissolved molecular oxygen. In parallel, the species singlet molecular oxygen, O₂(¹Δ_g), is also generated. Both reactive oxygen species produce the photodegradation of BHT. In the case of BHA, the lack of any effect exerted by superoxide dismutase drives out a significant participation of a O₂^??-mediated mechanism. BHA mainly interacts with O₂(¹Δ_g) and exhibits a desirable property as an antioxidant – a relatively high capacity for O₂(1Δ_g) de-activation and a low photodegradation efficiency by the oxidative species. Electrochemical determinations support the proposed photodegradative mechanism.     

Kinetic study on the photostability of riboflavin in the presence of barbituric acid

Abstract

The photochemical fate of riboflavin (vitamin B₂) in the presence of barbituric acid was examined employing polarographic detection of dissolved oxygen and steady-state and time-resolved spectroscopy. Under visible light, riboflavin reacts with barbituric acid – the latter being transparent to this type of photo-irradiation – via radicals and reactive oxygen species, such as singlet molecular oxygen [O₂(¹Δ_g)] and superoxide radical anion, which are generated from the excited triplet state of the vitamin. As a result, both the vitamin and barbituric acid are photodegraded. Kinetic and mechanistic studies on the photoreactions of riboflavin in the presence of barbituric acid indicate the excellent quenching ability of the latter towards O₂(¹Δ_g).     

Visible-light-promoted degradation of the antioxidants propyl gallate and t-butylhydroquinone: Mechanistic aspects

Abstract

The kinetic and mechanistic aspects of the visible-light-mediated photodegradation of the phenolic antioxidants (PA), propyl gallate (PG), and t-butylhydroquinone (TBHQ), employing riboflavin (Rf) as photosensitizer, have been studied by time-resolved and stationary techniques. The photosensitizer Rose Bengal (RB) was used for auxiliary experiments. Results show the occurrence of chemical transformations on PA with the participation of electronically excited states of Rf and different reactive oxygen species (ROS) generated from these states. With 0.02 mM Rf and 1.0 mM PA, the electronically excited triplet state of Rf is quenched by PA, in a competitive manner with the dissolved oxygen. As a consequence, a cascade of photoprocesses produces singlet oxygen (O₂(¹Δ_g)) and H₂O₂ in the case of PG and, O₂(¹Δ_g), H₂O₂ and HO^? in the case of TBHQ. The participation of these species is supported by experiments of oxygen consumption carried out in the presence of specific ROS scavengers. TBHQ has a relatively high capacity for O₂(¹Δ_g) physical deactivation and a low photodegradation efficiency by the oxidative species. Comparatively, it can be asserted that TBHQ has a higher antioxidant capacity than PG.     

Scavenging of photogenerated oxidative species by antimuscarinic drugs: atropine and derivatives

The quenching ability of photogenerated oxidative species by some antimuscarinic drugs generically named atropines (e.g. atropine [I] eucatropine [II], homatropine [III] and scopolamine [IV]) have been investigated employing stationary photolysis, polarographic detection of dissolved oxygen, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. Using Rose Bengal as a dye sensitiser for singlet molecular oxygen, O(2)((1)Delta(g)), generation, compounds I-IV behave as moderate chemical plus physical quenchers of the oxidative species. Correlation between kinetic and electrochemical data indicates that the process is possibly driven by a charge-transfer interaction. The situation is somewhat more complicated employing the natural pigment riboflavin (Rf) as a sensitiser. Compounds I and II complex Rf ground state, diminishing the quenching ability towards singlet and triplet excited state of the pigment. On the other hand, compounds III and IV effectively quench Rf excited states, protecting the pigment against photodegradation. Under anaerobic conditions, semireduced Rf (Rf(.-)) is formed through quenching of excited triplet Rf. Nevertheless, although Rf(.-) is a well-known generator of the reactive species superoxide radical anion by reductive quenching in the presence of oxygen, the process of O(2)((1)Delta(g)) production prevails over superoxide radical generation, due to the relatively low rate constants for the quenching of triplet Rf by the atropines (in the order of 10(7) M(-1)s(-1) for compounds III and IV) in comparison to the rate constant for the quenching by ground state oxygen, approximately two orders of magnitude higher, yielding O(2)((1)Delta(g)). Compound I is the most promising O(2)((1)Delta(g)) physical scavenger, provided that it exhibits the higher value for the overall quenching rate constant and only 11% of the quenching process leads to its own chemical damage.     

Visible-light promoted degradation of the commercial antioxidants butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT): a kinetic study

Visible-light photo-irradiation of the commercial phenolic antioxidants (PhAs) butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT), in the presence of vitamin B2 (riboflavin, Rf), in methanolic solutions and under aerobic conditions, results in the photo-oxidation of the PhAs. The synthetic dye photosensitiser Rose Bengal was also employed for auxiliary experiments. With concentrations of riboflavin and PhAs of ca. 0.02 mM and < 1 mM, respectively, the excited triplet state of the vitamin (3Rf*) is quenched by BHT in a competitive fashion with dissolved ground state triplet oxygen. From the quenching of 3Rf*, the semireduced form of the pigment is generated through an electron transfer process from BHT, with the subsequent production of superoxide anion radical (O2*-) by reaction with dissolved molecular oxygen. In parallel, the species singlet molecular oxygen, O2(1delta(g)), is also generated. Both reactive oxygen species produce the photodegradation of BHT. In the case of BHA, the lack of any effect exerted by superoxide dismutase drives out a significant participation of a O2(*-)-mediated mechanism. BHA mainly interacts with O2(1delta(g)) and exhibits a desirable property as an antioxidant--a relatively high capacity for O2(1delta(g)) de-activation and a low photodegradation efficiency by the oxidative species. Electrochemical determinations support the proposed photodegradative mechanism.     

Vancomycin-sensitized photooxidation in the presence of the natural pigment vitamin B2: Interaction with excited states and photogenerated ROS

Sensitized photooxidation processes in the presence of natural pigments may provide an alternative to antibiotics degradation since these compounds are transparent to natural light irradiation, therefore, they can be degraded by the action of photosensitizers which absorb light and produce highly reactive species, especially those derived from molecular oxygen (ROS). Most antibiotics used currently belong to a group of pharmaceutical substances that have been considered a new type of contaminants due to their persistence and bioaccumulation in the environment.

Objective: In this context, we decided to investigate the kinetic and mechanistic aspects of Vancomycin (Vanco) photosensitized degradation in the presence of the natural pigment Riboflavin (Vitamin B2, Rf) and the artificial dye Rose Bengal (RB) for comparative purposes.

Methods: The study have been done by using Stationary photolysis, Laser flash photolysis, Time-resolved phosphorence detection of O₂(1Δg) experiments and Bactericidal activity evaluation. The experiments were carried out in aqueous solution at different pH values in order to establish relationships between the structure of the compound and its susceptibility to ROS-mediated photooxidation.

Results: Experimental evidence indicates that in the presence of Rf there is considerable contribution of the radical-mediated mechanism, while in the presence of RB the photooxidation process occurs exclusively through O₂(1Δg) and the reactivity to this excited species increases with increasing pH of the environment.

Discussion: The results obtained, have been shown that Rf can raise the photodegradation of Vanco by both the radical pathway and the O₂(¹Δ_g) mediated. Furthermore, the antibiotic is able to interact with the excited electronic states of Rf as well as O₂(¹Δ_g) generated by energy transfer between the excited triplet state of the photosensitizer and the oxygen ground state. The predominant mechanism for photodegradation of Vanco in the presence of the Rf is the radical via because of the considerable interaction with the excited triplet state of the photosensitizer demonstrated by laser flash photolysis experiments.

Microbiological test on Staphylococcus aureus ATCC25923 showed that the bactericidal activity of the antibiotic on the strain studied was affected by the sensitized photodegradation process, suggesting that photoproducts generated eventually do not retain the bactericidal properties of the original antibiotic.     

Cathodic electrochemiluminescence of the peroxydisulphate–ciprofloxacin system and its analytical applications

The cathodic electrochemiluminescence (ECL) of peroxydisulphate (S₂O₈^2?)–ciprofloxacin (CPF) system at a wax‐impregnated graphite electrode was studied. When CPF was absent, S₂O₈^2? was electrochemically reduced to sulphate free radical (SO₄^??), and dissolved oxygen absorbed on the electrode surface was reduced to protonated superoxide anion radical (HO₂^?). The HO₂^? was oxidized by SO₄^?? to produce molecular oxygen in both singlet and triplet states. Some of the singlet molecular oxygen (¹O₂) further combined through collision to be an energy‐rich precursor singlet molecular oxygen pair (¹O₂)₂. A weak ECL was produced when ¹O₂ or (¹O₂)₂ was converted to ground‐state molecular oxygen (³O₂). When CPF was present, a stronger ECL was produced, which originated from two emitting species. The main emitting species was excited state CPF (CPF*), which was produced by accepting energy from (¹O₂)₂. The other emitting species was excited singlet molecular oxygen pair [(¹O₂)₂*], which originated from the chemical oxidation of CPF by SO₄^?? and dissolved oxygen. Based on the stronger ECL phenomenon, an ECL method for the determination of either S₂O₈^2? or CPF was proposed. The proposed ECL method has been applied to the determination of CPF in pharmaceutical preparations. Copyright © 2011 John Wiley & Sons, Ltd.     

Evidence for an O2-barrier in the light-harvesting chlorophyll-a/b-protein complex LHC II

An O₂-barrier in the intact light-harvesting complex LHC II protects chlorophylls (Chl) and xanthophylls (Car) from photooxidation. Direct evidence for the limited access of O₂ to pigment sites comes from the decay kinetics of the first excited triplet state of Car (³Car-). The LHC-bound ³Car- in air-saturated solution decays mono-exponentially with a lifetime of 6.7-7.1 µs as compared to the approx. 1.2 µs of the -carotene triplet in hexane and the 8.8-9 µs observed for both systems under anaerobiosis. Further properties of the photostable complex are the limited access of protons to pigment sites and the efficient energy transfer from ¹Car- to Chl-a and from ³Chl- to Car. Fatty acids with increasing chain length increasingly lower both, the efficiency of the O₂ barrier and the photo- and acid stability of the LHC-bound pigments while singlet and triplet energy transfer between the pigments is maintained. Therefore, the close proximity of Chl and Car is not sufficient to protect the pigments from photooxidation; in addition, an O₂-barrier limiting the access of O₂ to pigment sites is required for efficient photoprotection. Structural properties of the photostable LHC II possibly underlying its O₂-barrier function are discussed.     

Generation of reactive oxygen species upon strong visible light irradiation of isolated phycobilisomes from Synechocystis PCC 6803

The mechanism of photodegradation of antenna system in cyanobacteria was investigated using spin trapping ESR spectroscopy, SDS-PAGE and HPLC-MS. Exposure of isolated intact phycobilisomes to illumination with strong white light (3500 μmol m^− 2 s^− 1 photosynthetically active radiation) gave rise to the formation of free radicals, which subsequently led to specific protein degradation as a consequence of reactive oxygen species-induced cleavage of the polypeptide backbone. The use of specific scavengers demonstrated an initial formation of both singlet oxygen (¹O₂) and superoxide (O₂⁻), most likely after direct reaction of molecular oxygen with the triplet state of phycobiliproteins, generated from intersystem crossing of the excited singlet state. In a second phase carbon-based radicals, detected through the appearance of DMPO-R adducts, were produced either via O₂⁻ or by direct ¹O₂ attack on amino acid moieties. Thus photo-induced degradation of intact phycobilisomes in cyanobacteria occurs through a complex process with two independent routes leading to protein damage: one involving superoxide and the other singlet oxygen. This is in contrast to the mechanism found in plants, where damage to the light-harvesting complex proteins has been shown to be mediated entirely by ¹O₂ generation.     

Glow discharge in singlet oxygen

Currently, there is no experimental data on the plasma balance in gas mixtures with a high content of singlet delta oxygen O₂(¹Δ_g). These data can be obtained by studying the parameters of an electric discharge in singlet oxygen produced by a chemical generator. The O₂(¹Δ_g) molecules significantly change the kinetics of electrons and negative ions in plasma. Hence, the discharge conditions at low and high. O₂(¹Δ_g) concentrations are very different. Here, the parameters of the positive column of a glow discharge in a gas flow from a chemical singlet-oxygen generator are studied. It is experimentally shown that, at an O₂(¹Δ_g) concentration of 50% and at pressures of 1.5 and 2 torr, the electric field required to sustain the discharge is considerably lower than in the case when all of the oxygen molecules are in the ground state. A theoretical model of the glow discharge is proposed whose predictions are in good agreement with the experimental data.     

On the mechanism of photosensitized luminescence of singlet oxygen dimols in air-saturated pigment solutions

Luminescence of singlet oxygen dimols (¹O₂)₂ was studied in aerobic solutions of a nonfluorescent photosensitizer phenalenone in CCl₄ and C₆F₆ using a setup with a mechanical phosphoroscope and relatively low rates of photosensitizer excitation. The luminescence spectrum was found to resemble those reported in our previous papers dealing with dimol luminescence in solutions of porphyrins and other organic dyes. The main maximum was located at 703–706 nm, and two much weaker bands at 640 and 770–780 nm. These data suggest that dimol luminescence arises owing to interaction of two ¹O₂ molecules and one ground-state pigment molecule. Light is emitted by the dimol-pigment contact complexes, which are formed as a result of ¹O₂ collisions with metastable, probably triplet, intermediates appearing in ¹O₂ reaction with pigment molecules. It is proposed that this mechanism of dimol luminescence might be of general importance for photochemical, chemical, and biological systems where singlet oxygen is generated. However, the luminescence of this type dominates at relatively low rates of ¹O₂ generation. According to the literature data, at high ¹O₂ generation rates the prevalent type of dimol luminescence has the main maximum at 635–637 nm and is caused by direct collisions of two ¹O₂ molecules.     

Production of singlet oxygen in a non-self-sustained discharge

The production of O₂(a¹Δ_g) singlet oxygen in non-self-sustained discharges in pure oxygen and mixtures of oxygen with noble gases (Ar or He) was studied experimentally. It is shown that the energy efficiency of O₂(a¹Δ_g production can be optimized with respect to the reduced electric field E/N. It is shown that the optimal E/N values correspond to electron temperatures of 1.2–1.4 eV. At these E/N values, a decrease in the oxygen percentage in the mixture leads to an increase in the excitation rate of singlet oxygen because of the increase in the specific energy deposition per O₂ molecule. The onset of discharge instabilities not only greatly reduces the energy efficiency of singlet oxygen production but also makes it impossible to achieve high energy deposition in a non-self-sustained discharge. A model of a non-self-sustained discharge in pure oxygen is developed. It is shown that good agreement between the experimental and computed results for a discharge in oxygen over a wide range of reduced electric fields can be achieved only by taking into account the ion component of the discharge current. The cross section for the electron-impact excitation of O₂(a¹Δ_g and the kinetic scheme of the discharge processes with the participation of singlet oxygen are verified by comparing the experimental and computed data on the energy efficiency of the production of O₂(a¹Δ_g and the dynamics of its concentration. It is shown that, in the dynamics of O₂(a¹Δ_g molecules in the discharge afterglow, an important role is played by their deexcitation in a three-body reaction with the participation of O(³P) atoms. At high energy depositions in a non-self-sustained discharge, this reaction can reduce the maximal attainable concentration of singlet oxygen. The effect of a hydrogen additive to an Ar: O₂ mixture is analyzed based on the results obtained using the model developed. It is shown that, for actual electron beam current densities, a significant energy deposition in a non-self-sustained discharge in the mixtures under study can be achieved due to the high rate of electron detachment from negative ions. In this case, however, significant heating of the mixture can lead to a rapid quenching of O₂(a¹Δ_g molecules by atomic hydrogen.     

四苯基卟啉在苯溶液中光解产生自由基的ESR研究

用自旋捕捉-ESR方法对四苯基卟啉(H₂TPP)在苯中光解后产生的活泼自由基进行研究。当PBN存在时,光解中可给出6重ESR峰,表明自旋加合物为[HTPP-PBN]^·。而氧和碘化物的猝灭作用可知H₂TPP的光解是经由三重激发态途径进行的。而PBN的光敏分解则是由尚未转入三重态的H₂TPP所引起的。     

Kinetic study on the photostability of riboflavin in the presence of barbituric acid

The photochemical fate of riboflavin (vitamin B2) in the presence of barbituric acid was examined employing polarographic detection of dissolved oxygen and steady-state and time-resolved spectroscopy. Under visible light, riboflavin reacts with barbituric acid--the latter being transparent to this type of photo-irradiation--via radicals and reactive oxygen species, such as singlet molecular oxygen [O2(1delta(g))] and superoxide radical anion, which are generated from the excited triplet state of the vitamin. As a result, both the vitamin and barbituric acid are photodegraded. Kinetic and mechanistic studies on the photoreactions of riboflavin in the presence of barbituric acid indicate the excellent quenching ability of the latter towards O2(1delta(g)).     

Triplet states of bacteriochlorophyll and carotenoids in chromatophores of photosynthetic bacteria

Chromatophores from photosynthetic bacteria were excited with flashes lasting approx. 15 ns. Transient optical absorbance changes not associated with the photochemical electron-transfer reactions were interpreted as reflecting the conversion of bacteriochlorophyll or carotenoids into triplet states. Triplet states of various carotenoids were detected in five strains of bacteria; triplet states of bacteriochlorophyll, in two strains that lack carotenoids. Triplet states of antenna pigments could be distinguished from those of pigments specifically associated with the photochemical reaction centers. Antenna pigments were converted into their triplet states if the photochemical apparatus was oversaturated with light, if the primary photochemical reaction was blocked by prior chemical oxidation of P-870 or reduction of the primary electron acceptor, or if the bacteria were genetically devoid of reaction centers. Only the reduction of the electron acceptor appeared to lead to the formation of triplet states in the reaction centers.In the antenna bacteriochlorophyll, triplet states probably arise from excited singlet states by intersystem crossing. The antenna carotenoid triplets probably are formed by energy transfer from triplet antenna bacteriochlorophyll. The energy transfer process has a half time of approx. 20 ns, and is about 1 × 10³ times more rapid than the reaction of the bacteriochlorophyll triplet states with O₂. This is consistent with a role of carotenoids in preventing the formation of singlet O₂ in vivo. In the absence of carotenoids and O₂, the decay half times of the triplet states are 70 μs for the antenna bacteriochlorophyll and 6–10 μs for the reaction center bacteriochlorophyll. The carotenoid triplets decay with half times of 2–8 μs.With weak flashes, the quantum yields of the antenna triplet states are in the order of 0.02. The quantum yields decline severely after approximately one triplet state is formed per photosynthetic unit, so that even extremely strong flashes convert only a very small fraction of the antenna pigments into triplet states. The yield of fluorescence from the antenna bacteriochlorophyll declines similarly. These observations can be explained by the proposal that singlet-triplet fusion causes rapid quenching of excited singlet states in the antenna bacteriochlorophyll.     

Triplet excited states of polycyclic aromatic compounds as probes of their microenvironment in serum albumin complexes.

Using flash photolysis techniques, the triplet excited states of benzo(a)pyrene, pyrene, benz(a)anthracene and other aromatic hydrocarbons have been detected in complexes of bovine (and human) serum albumin dissolved in aqueous solutions at room temperature. The triplet lifetimes can be adjusted to any value within the microsecond-millisecond time domains by varying the partial pressure of oxygen from zero to one atmosphere, thus providing a useful probe on these time scales. Local oxygen concentrations as low as ～ 2 × 10^?7M can be detected. In air saturated solutions, the triplet lifetimes are sensitive to pH dependent conformational changes of the host bovine serum albumin molecules.     

Determination of tannic acid in industrial wastewater based on chemiluminescence system of KIO4–H2O2–Tween40

The oxidation reaction of H₂O₂ with KIO₄ can produce chemiluminescence (CL) in the presence of the surfactant Tween40 and the CL intensity of the CL system KIO₄–H₂O₂–Tween40 can be strikingly enhanced after injection of tannic acid. On this basis, a flow injection method with CL detection was established for the determination of tannic acid. The method is simple, rapid and effective to determine tannic acid in the range of 7.0 × 10^?9 to 1.0 × 10^?5 mol/L with a determination limit of 2.3 × 10^?9 mol/L. The relative standard deviation is 2.6% for the determination of 5.0 × 10^?6 mol/L tannic acid (n = 11). The method has been applied to determine the content of tannic acid in industrial wastewater with satisfactory results. It is believed that the CL reaction formed singlet oxygen ¹O₂* and the emission was from an excited oxygen molecular pair O₂(¹Δ_g)O₂(¹∑^?_g) in the KIO₄–H₂O₂–Tween40 reaction. Tween40 played an important role in enhancing stabilization of the excited oxygen molecular pair O₂(¹Δ_g)O₂(¹∑^?_g) and in increasing CL intensity. Copyright © 2010 John Wiley & Sons, Ltd.     

Specific features of the kinetics of H2-O2-O2(a 1Δ g ) mixtures: I. formation and quenching of electronically and vibrationally excited (A′) molecules in H2-O2-O2(A 1Δ g ) mixtures at a temperature of 300 K

Available data on the kinetic processes in H₂-O₂-O₂(a ¹Δ _g ) mixtures are analyzed theoretically, and the ranges in which the rate constants of these processes can vary are determined. The processes of energy transformation in an O₂(a ¹Δ _g )-H₂-H-HO₂ system in the approximations of the fast and slow (in comparison with the vibrational relaxation time of the HO₂ radical) quenching of the electronically excited state are considered. The experiments on the quenching of singlet delta oxygen (SDO) molecules O₂(a ¹Δ _g ) excited in a microwave discharge at a temperature of 300 K and pressure of 6 Torr in a lean hydrogen-oxygen mixture are simulated (by a lean fuel mixture is meant a mixture in which the ratio of the fuel to the oxidizer mass fraction is less than the stoichiometric ratio, which is 2: 1 for a hydrogen-oxygen mixture). It is shown that, over the experimental observation times, the SDO quenching frequency depends on the concentration of molecular hydrogen, the residual odd oxygen fraction in the gas flow, and the ratio between the rate constants of kinetic processes involving HO₂ and HO₂* radicals. Simulations of the microwave discharge and the transport of excited oxygen along the drift tube make it possible to determine the residual odd oxygen concentration in the gas flow. Recommendations on the choice of the rate constants for the O₂(a ¹Δ _g ) + HO₂)A″, v₃″ = 0) ? O₂ + HO₂*(A′, v₃′ = 1), HO₂*(A′v₃′ ≤ 1) + O₂(a ¹Δ _g ) → HO₂*(A′,v₃′ ≥ 6) + O₂, and HO₂*(A′,v₃′ ≤ 1) + O₂(a ¹Δ _g ) → H + O₂ + O₂ processes are given. It is shown that, in the case of slow quenching in a H₂-O₂-O₂(a ¹Δ _g ) mixture at a low temperature, the intensity of hydrogen oxidation is enhanced due to the reaction + HO₂*(A′,v₃′ ≤ 1) + O₂(¹Δ) → H + O₂ + O₂.     

Chemiluminescence Emission during Reactions between Superoxide and Selected Aliphatic and Aromatic Halocarbons in Aprotic Media

The reactions between superoxide free radical anion (.O⁻₂) with the halocarbons CCl₄, CHCl₃, BrCH₂CH₂Br(EDB), decachloro-biphenyl (DCBP), and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in dimethyl sulphoxide (DMSO) results in the emission of chemiluminescence (CL). The chemiluminescence reactions are characterized as having biphasic second order kinetics, CL wavelengths between 350 nm and 650 nm, and exhibiting perturbation by chemicals reactive with singlet oxygen. These data suggest that singlet oxygen species are the excited state responsible for the light emissions. Polarographic studies confirm .O⁻₂ consumption and halide release in the reactions, while gas liquid chromatography and NBT reduction demonstrate the decomposition of the halocarbons into products. A chemiluminescent reaction mechanism is proposed involving reductive dehalogenation of the halocarbons and the generation of singlet oxygen. The significance of singlet oxygen generation is discussed with respect to a general mechanism for explaining the rapid initiation of lipid peroxidative membrane damage in halocarbon toxigenicity in animal and plant tissues.