Oxidation of Chiral 1,4-Cyclohexadienes: A Comparison of Chemically and Photochemically Generated Singlet Oxygen1O2(1Δg)

Chiral alkyl-substituted 2,5-cyclohexadiene-l-carboxyIic acids la-c have been oxidized in water and in methanol with singlet oxygen, ¹O₂ (¹Δ_g), generated either photochemically or chemically from the catalytic system hydrogen peroxide/sodium molybdate. These methods were compared in terms of chemo-, regio- and diastereoselec-tivities and the chemical (k_T) and physical (k_q) quenching rate constants of ¹O₂ were determined. The ratio of the cis and trans isomers of the hydroperoxides 2a-c is not influenced by the source of ¹O₂ but, on the other hand, it depends slightly on the solvent and greatly on the steric hindrance of the substituents linked to the chiral carbon. The results may be interpreted on the basis of the successive formation of an exciplex and a perepoxide that evolves either by giving the final allylic hydroperoxide or by dissociating into the starting substrate and singlet or triplet oxygen.     

A study of sensitized photooxygenation of 3,4-dihydro-2H-pyran

The photooxygenation of 3,4-dihydro-2H-pyran sensitized by cyanoanthracenes (9,10-dicyanoanthracene and 9-cyanoanthracene) in different solvents (CH₃CN, CH₃Cl_2,C₆H₆ and CCl₄) has been studied in this paper. The products, product distribution as well as solvent isotope effect are the same as those in the reaction of singlet oxygen. Fluorescence quenching, exciplex formation and free energy change also reveal that electron transfer occurs between sensitizer-excited singlet state and substrate and then singlet oxygen is formed subsequently as the reactive intermediate in the process.     

Oxidation of diazo compounds by triphenyl phosphite ozonide. Quenching of singlet oxygen by diazo compounds

The reaction of triphenyl phosphite ozonide with various types of diazo compounds results in their oxidation, which is accomplished by singlet oxygen (¹O₂) evolved during thermal decomposition of the ozonide. A decrease in the ionization potential of the substrate results in an increase in the overall rate constant of quenching of¹O₂. In the case of 9-diazofluorene, the main channel of¹O₂ quenching is physical quenching.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1567–1571, September, 1994.The work was carried out with the financial support of the Russian Foundation for Basic Research (Project No. 93-03-5231).     

Amino acid-pyrrole N-conjugates; a new class of antioxidants II. Effectiveness of singlet oxygen quenching by luminescence measurements

The pyrrole-amino acid and peptide N-conjugates synthesized from tyrosine, histidine and glutathione very effectively quench the 1270 nm singlet oxygen luminescence, at rates ranging from 10⁸ to 10⁹ M⁻¹ s⁻¹. Nuclear magnetic resonance spectroscopy suggests that the electron-donating properties of the methyl groups after 2,5-dimethyl substitution on the pyrrole ring are probably an important determinant of the reactivity of singlet oxygen with the N-conjugate of glutathione. However, intramolecular interactions between the pyrrole ring and the side chain may also modulate the reactivity of the antioxidant as suggested by absorption and fluorescence spectroscopies carried out on tyrosine derivatives. Efficient fluorescence quenching of the phenol ring by the pyrrole ring occurs in the tyrosine derivatives. The reactivities of these antioxidants with ¹O₂ are comparable in methanol, ethanol and D₂O.     

Solvent Isotope Effect on the Rate Constants of Singlet-Oxygen Quenching by edta and Its Metal Complexes

The reactivity of singlet oxygen (O₂(¹Δ_g)) with edta and its metal complexes with Al³⁺, Cu²⁺, Fe³⁺, and Mn²⁺ was investigated. The emission of singlet oxygen at 1270 nm in D₂O was measured in order to determine the quenching efficiency of edta and edta-metal complexes for different metal/edta ratios. The sum of the rate constant (k_r + k_q) of the chemical reaction between singlet oxygen and the acceptor (k_r) and of the physical quenching of singlet oxygen by the acceptor (k_q) was obtained by a Stern-Volmer analysis. Measurements of the oxygen consumption in H₂O were used to determine quantum yields of the sensitized photooxidation, and the combined results of these experiments allowed the determination of k_r and k_q separately. A strong isotope effect was observed between the deuterated and the hydrogenated solvents. This effect was shown to be independent of the analytical procedure used. The isotope effect, as well as the reactivity of edta and its metal complexes, depend markedly on the complexed metal ion.     

A Targeting Singlet Oxygen Battery for Multidrug-Resistant Bacterial Deep-Tissue Infections

Traditional photodynamic therapy (PDT) is dependent on externally applied light and oxygen, and the depth of penetration of these factors can be insufficient for the treatment of deep infections. The short half-life and short diffusion distance of reactive oxygen species (ROS) also limit the antibacterial efficiency of PDT. Herein, we designed a targeting singlet oxygen delivery system, CARG-Py, for irradiation-free and oxygen-free PDT. This system was converted to the “singlet oxygen battery” CARG-¹O₂ and released singlet oxygen without external irradiation or oxygen. CARG-¹O₂ is composed of pyridones coupled to a targeting peptide that improves the utilization of singlet oxygen in deep multidrug-resistant bacterial infections. CARG-¹O₂ was shown to damage DNA, protein, and membranes by increasing the level of reactive oxygen inside bacteria; the attacking of multiple biomolecular sites caused the death of methicillin-resistant Staphylococcus aureus (MRSA). An in vivo study in a MRSA-infected mouse model of pneumonia demonstrated the potential of CARG-¹O₂ for the efficient treatment of deep infections. This work provides a new strategy to improve traditional PDT for irradiation- and oxygen-free treatment of deep infections while improving convenience of PDT.     

Photoinduced Degradation of the Herbicide Clomazone Model Reactions for Natural and Technical Systems

The photodegradation of the herbicide clomazone in the presence of S₂O₈^2? or of humic substances of different origin was investigated. A value of (9.4 ± 0.4) × 10⁸ m ^?1 s^?1 was measured for the bimolecular rate constant for the reaction of sulfate radicals with clomazone in flash‐photolysis experiments. Steady state photolysis of peroxydisulfate, leading to the formation of the sulfate radicals, in the presence of clomazone was shown to be an efficient photodegradation method of the herbicide. This is a relevant result regarding the in situ chemical oxidation procedures involving peroxydisulfate as the oxidant. The main reaction products are 2‐chlorobenzylalcohol and 2‐chlorobenzaldehyde. The degradation kinetics of clomazone was also studied under steady state conditions induced by photolysis of Aldrich humic acid or a vermicompost extract (VCE). The results indicate that singlet oxygen is the main species responsible for clomazone degradation. The quantum yield of O₂(a¹Δ_g) generation (λ = 400 nm) for the VCE in D₂O, Φ_Δ = (1.3 ± 0.1) × 10^?3, was determined by measuring the O₂(a¹Δ_g) phosphorescence at 1270 nm. The value of the overall quenching constant of O₂(a¹Δ_g) by clomazone was found to be (5.7 ± 0.3) × 10⁷ m ^?1 s^?1 in D₂O. The bimolecular rate constant for the reaction of clomazone with singlet oxygen was k_r = (5.4 ± 0.1) × 10⁷ m ^?1 s^?1, which means that the quenching process is mainly reactive.     

INFLUENCE OF THE PEPTIDE BOND ON THE SINGLET MOLECULAR OXYGEN-MEDIATED (O2[g]) PHOTOOXIDATION OF HISTIDINE and METHIONINE DIPEPTIDES. A KINETIC STUDY

The dye-sensitized photooxidation of l -histidine (His) and l -methionine (Met) and their simplest dipeptides with glycine (Gly) (His-Gly, Gly-His, Gly-Met) and Met-methyl ester (Met-ME) mediated by singlet molecular oxygen (O₂[_g]) was studied. The overall rate constants in acetonitrile-H₂O (K_t) for O₂(¹_g) quenching were measured by time-resolved phosphorescence detection. In H₂O a competitive kinetic method was employed. In both solvents the reactive rate constants (K_t,) were determined to discriminate between the overall and physical contributions to the quenching. The kinetic and mechanistic aspects of the interaction are discussed. For His-Gly, the peptide bond has practically no effect on the kinetics of photooxidation. For Gly-His the overall rate constant is much higher than that for His and His-Gly, in both H₂O and acetonitrile-H₂O. The main contribution to k₁ (for Gly-His) is the physical quenching of O₂(¹_g)- In water the k_t/k_r ratio for free His and His-Gly is 1.0, reaching a value of 2.0 in the organic solvent-H₂O mixture. The rates of-NH₂ loss upon sensitized photooxidation in all cases parallel the trend of k_r values. The main results for the His series indicate that: (1) a polar environment favors autoprotection (i.e. an increase in the contribution of physical quenching) against photodynamic effects; (2) only the rate constant for reactive interaction with O₂[_g] does not depend on the location of the peptide bond involving His. For Met derivatives the k_t, values are higher in both solvents than that for free Met. Only for the free amino acid in H₂O is the interaction with O₂(¹_g) totally reactive. For Gly-Met and Met-ME the physical quenching prevails: k_t, is, in both solvents, about one order of magnitude higher than k_r. According to our results on -NH₂ loss and on the basis of previous investigations by others, the photooxidative products distribution in the Met series indicates that Gly-Met yields only dehydroMet, whereas Met and Met-ME produce a mixture of Met-sulfoxide and the Met-dehydro compound.     

Role of the V(V)/ 1O2Complex in Oxidative Reactions in the H2O2/V(V)/AcOH System: Oxidation of Alkenes and Anthracenes

Anthracene and its alkyl derivatives undergo oxidation in the V^(V)/H₂O₂/AcOH system via a nonradical mechanism through the intermediate formation of the vanadium(V) complex with singlet dioxygen as a ligand. The ¹O₂molecule is transferred from this complex to an unsaturated substrate. The free singlet dioxygen ¹O₂(¹ _g ) is almost inactive toward anthracene in AcOH solution. Consequently, the vanadium(V) complex with singlet dioxygen is the only oxidant species active in the reaction. The ratio between the rate constant of the reaction of this complex with 2-ethylanthracene and the rate constant of its deactivation is an order of magnitude greater than the ratio between the rate constant of the reaction of dissolved free singlet dioxygen with the same substrate and the rate constant of its deactivation (physical quenching).     

Photooxidation of Troglitazone,a New Antidiabetic Drug *

Troglitazone (CS-045) is a new oral antidiabetic drug reported to be effective in insulin-resistant diabetes and to show antihypertensive effects. Photooxidation of troglitazone gave the quinone and quinone epoxide as the major final stable products. An intermediate observed by NMR spectroscopy was shown to be the hydroperoxydi-enone, which is moderately stable at room temperature. The rate constant of singlet oxygen quenching by troglitazone is 2.14 × 10⁸M^?1s^?1 and the reaction rate constant in acetone-d, is 8.64 × 10, M^?1 s^?1. Only the chroman ring of troglitazone reacts with and quenches singlet oxygen significantly, and its reactivity and products are analogous to those of a-tocopherol. The reactivity of CS-045 toward singlet oxygen is much larger than that of the related compounds lacking the chroman ring.     

Reactivity of Singlet Oxygen Toward Proteins: The Effect of Structure in Basic Pancreatic Trypsin Inhibitor and in Ribonuclease A

The reactions of singlet oxygen, ¹O₂, with large peptides have been described previously. It was found that even in these systems, which in their native form are generally not supposed to possess a stable structure in solution, the polypeptide does impede the access of ¹O₂ to the amino acids that react readily with ¹O₂. Here we describe the ¹0₂ reaction with two proteins of well-defined structure. The quenching of ¹O₂ by bovine pancreatic trypsin inhibitor (BPTI) and by ribonuclease A (RNase A) was compared to that of a solution at the same concentration as those of its constituent amino acids that react readily with ¹O₂. The proteins were studied in their native form, when partly denatured by splitting their S-S bonds and when fully denatured. It was found that while in the native form the quenching rate constant was seven times lower in BPTI (2.2 vs 15.2 times 10⁷WM^-1 s^-1) and three times lower in RNase A (11.0 vs 32 times 10⁷M^-l s^-1) than in a mixture of its constituent amino acid residues, it increased upon denaturation reaching in the fully denatured state the value of the corresponding amino acid mixture. More striking is the effect of the protein structure when comparing the fraction of the encounters between ¹O₂ and protein, which cause damage to the protein, as reflected in the decrease of its biological activity. This decrease is assumed to be due to the chemical (oxidative) reactions of ¹O₂ in the protein. In the exceptionally stable BPTI the fraction of such encounters was 0.05 and in RNase A it was 0.2, whereas for the amino acid tryptophan in solution, 0.7 of the collisions with ¹O₂ led to a chemical reaction.     

Kinetics Study on Reaction of Atenolol with Singlet Oxygen by Directly Monitoring the 1O2 Phosphorescence

The pharmaceutically active compound atenolol, a kind of $\beta$-blockers, may result in adverse effects both for human health and ecosystems if it is excreted to the surface water resources. To effectively remove atenolol in the environment, both direct and indirect photodegradation, driven by sunlight play an important role. Among indirect photodegradation, singlet oxygen (¹O₂), as a pivotal reactive species, is likely to determine the fates of atenolol. Nevertheless, the kinetic information on the reaction of atenolol with singlet oxygen has not been well investigated and the reaction rate constant is still ambiguous. Herein, the reaction rate constant of atenolol with singlet oxygen is investigated directly through observing the decay of the ¹O₂ phosphorescence at 1270 nm. It is determined that the reaction rate constant between atenolol and ¹O₂ is 7.0×10⁵ (mol/L)$^{-1}\cdot$s^-1 in D₂O, 8.0×10⁶ (mol/L)$^{-1}\cdot$s^-1 in acetonitrile, and 8.4×10⁵ (mol/L)$^{-1}\cdot$s^-1 in EtOH, respectively. Furthermore, the solvent effects on the title reaction were also investigated. It is revealed that the solvents with strong polarity and weak hydrogen donating ability are suitable to achieve high rate constant values. These kinetics information on the reaction of atenolol with singlet oxygen may provide fundamental knowledge to the indirect photodegradation of $\beta$-blockers.     

Quenching of singlet oxygen by hindered amine light stabilisers. A flash photolytic study

Rate constants for the quenching of singlet oxygen (¹O₂, ¹δ_g) for a series of piperidines, piperidine-N-oxyl free radicals and some commercially used hindered amine light stabilisers (HALS) have been measured by a laser flash photolysis method. Quenching rate constants are in the order: piperidine-N-oxyl free radicals ≤ secondary piperidines < tertiary piperidines. For some commercial HALS, ¹O₂ quenching rate constants and the light protective effect towards polypropylene photo-oxidation have been compared. No correlation has been found between the stabilizing action and the quenching efficiency towards ¹O₂. The data obtained point to little contribution of singlet oxygen to the key steps of polyolefin photo-oxidation.     

DIRECT EXPOSURE OF MANNALIAN CELLS TO PURE EXOGENOUS SINGLET OXYGEN (ΔgO2

Mammalian cells attached to membrane filters or deposited on filters without attachment were exposed to gas-phase singlet oxygen (¹O₂) in the absence of any other reactants. Cells were exposed in a monolayer or less, in the absence of external medium, during steady-state ¹O₂ generation, ensuring that singlet oxygen impinged directly and equally on all cells simultaneously. The current methodology for cell exposure ensures that ¹O₂ is initially the only reactive species to which the cells are exposed. Results seen with this system can therefore be attributed solely and unambiguously to events initiated by ¹O₂. Further, all cells in the sample receive the same magnitude of exposure per surface area per time interval, which supports calculations of the amount of ¹O₂ required for irreversible cell damage, based on measured ¹O₂ flux and exposed cell surface area. Exposure to pure ¹O₂ irreversibly damaged a variety of cell types, including rat basophilic leukemia, human squamous carcinoma and Chinese hamster lung fibroblast cell lines, and murine primary hepatocytes. Cell survival curves following exposure to ¹O₂ followed apparent first-order kinetics. A large number of singlet oxygen collisions (? 10¹²-10¹³) were required to inactivate a cell, on average, indicating a low probability that singlet oxygen collision will reduce cell survival. Regardless of cell type or the survival endpoint measured, lethal toxicity required a fairly constant number of ¹O₂ collisions per cell. This poses a serious caveat in the assignment of causality in correlating ¹O₂-initiated cellular damage with mechanism of death, i.e. most damage observed will not be related to death. The importance of various toxic effects of ¹O₂, whether lethal or nonlethal, will depend on the magnitude of exposure and therefore on the context in which exposure occurs.     

Singlet-Oxygen Quenching by Carotenoids: Steady-state luminescence experiments

The near-infrared luminescence of singlet oxygen (¹O₂) has been measured in order to determine the efficiency of ¹O₂ quenching by two carotenoid compounds, β-carotene and canthaxanthin. 1H-Phenalen-1-one and rose bengal have been used as photosensitizers in those steady-state luminescence experiments. Stern-Volmer analysis of the ¹O₂ luminescence in solutions of CCl₄ and CD₃OD, containing different concentrations of the carotenoids, has shown a very efficient quenching by canthaxanthin. The rate constants are about a factor of 2 below the diffusion limited values for the given solvents, confirming earlier results in benzene. In comparison, the efficiency of ¹O₂ quenching by β-carotene is slightly lower than that by canthaxanthin in non-polar solvents and is reduced by an order of magnitude in CD₃OD, due to the aggregation of this quencher.     

Fluoroquinolone Antimicrobials: Singlet Oxygen, Superoxide and Phototoxicity

The fluoroquinolone antibacterial agents possess photo-sensitizing properties that lead to phototoxic responses in both human and animal subjects. The phototoxicity order reported in humans is: fleroxacin > lomefloxacin, pefloxacin > ciprofloxacin ? enoxacin, norfloxacin and ofloxacin. Studies both in vivo and in vitro have related this phototoxicity to the generation of reactive oxygen species including hydrogen peroxide and the hydroxyl radical. We determined the quantum yields of singlet oxygen generation (φ_Δ,) by detection of the singlet oxygen (¹O₂) luminescence at 1270 tun for several fluoroquinolones, naphthyridines and other structurally related compounds. All the fluoroquinolones examined have low φ_Δ values ranging from 0.06 to 0.09 in phosphate buffer at pD 7.5. We also determined the ¹O₂ quenching constants for these compounds and their values were on the order of 10⁶M^?1 s¹, except for lomefloxacin whose rate constant was 1.8 × 10⁷M^?1 s^?1. The φ_Δ values were significantly decreased in a solvent of lower polarity such as methanol (0.007 ≤φ_Δ≤ 0.02). The production of ¹O₂ by these antibiotics did not correlate with the order reported for their phototoxicity. We also measured the photogeneration (λ > 300 nm) of superoxide by these antibacterials in dimethylsulfoxide using electron paramagnetic resonance and the spin trap 5,5-dimethyl-l-pyrroiine N-oxide. Although there is not a one-to-one correspondence between the relative rates of superoxide generation and the phototoxicity ranking of the fluoroquinolones, the more phototoxic compounds tended to produce superoxide at a faster rate. Nevertheless, the magnitudes of the observed differences do not appear sufficient to explain the range of fluoroquinolone phototoxicity potencies in human and animal subjects in general and the high activity of fleroxacin and lomefloxacin in particular. For these latter drugs the photoinduced loss of the F₈ atom as fluoride and the concomitant generation of a highly reactive carbene at C-8 provide a more plausible mechanism for their potent phototoxic and photocarcinogenic properties.     

The Triplet State of 6‐thio‐2′‐deoxyguanosine: Intrinsic Properties and Reactivity Toward Molecular Oxygen

Thiopurine prodrugs are currently among the leading treatment options for leukemia, immunosuppression, and arthritis. Patients undergoing long‐term thiopurine treatment are at a higher risk of developing sunlight‐induced skin cancers than the general population. This side effect originates from the cellular metabolization of thiopurine prodrugs to form 6‐thio‐2′‐deoxyguanosine, which can absorb UVA radiation, populating its reactive triplet state and leading to oxidatively generated damage. However, the photo‐oxidation mechanism is not fully understood. In this contribution, the oxidation potential and the adiabatic triplet energy of 6‐thio‐2′‐deoxyguanosine are estimated computationally, whereas the intrinsic rate of triple‐state decay and the rate constant for triplet quenching by molecular oxygen are determined using time‐resolved spectroscopic techniques. A singlet oxygen quantum yield of 0.24 ± 0.02 is measured in aqueous solution (0.29 ± 0.02 in acetonitrile). Its magnitude correlates with the relatively low percentage of triplet‐O₂ collision events that generate singlet oxygen (S_Δ = 37%). This behavior is rationalized as being due to the exergonic driving force for electron transfer between the triplet state of 6‐thio‐2′‐deoxyguanosine and molecular oxygen (ΔG_ET = ?69.7 kJ mol^?1), resulting in the formation of a charge‐transfer complex that favors nonradiative decay to the ground state over triplet energy transfer.     

Generation of Reactive Oxygen Species during the Photolysis of 6‐(Hydroxymethyl)pterin in Alkaline Aqueous Solutions

Photochemical studies of the reactivity of 6‐(hydroxymethyl)pterin (=2‐amino‐6‐(hydroxymethyl)pteridin‐4(1H)‐one; HPT) in alkaline aqueous solutions (pH 10.2–10.8) at 350 nm and room temperature were performed. The photochemical reactions were followed by UV/VIS spectrophotometry, thin‐layer chromatography (TLC), high‐performance liquid chromatography (HPLC), and an enzymatic method for H₂O₂ determination. In the presence of O₂, 6‐formylpterin (=2‐amino‐3,4‐dihydro‐4‐oxopteridine‐6‐carboxaldehyde; FPT) was the only photoproduct detected. In the absence of O₂, we observed a compound with an absorbance maximum at 480 nm, which was oxidized very rapidly by O₂ in a dark reaction to yield FPT. The quantum yields of substrates disappearance and of photoproducts formation were determined. The formation of H₂O₂ during photooxidation was monitored, and the number of mol of H₂O₂ released per mol of HPT consumed corresponded to a 1 : 1 stoichiometry. HPT was also investigated for efficiency of singlet‐oxygen (¹O₂) production and quenching in aqueous solution. The quantum yield of ¹O₂ production (Φ_Δ=0.21±0.01) was determined by measurements of the ¹O₂ luminescence in the near‐IR (1270 nm) upon continuous excitation of the sensitizer. The rate constant of ¹O₂ total quenching by HPT was determined (k_t=3.1?10⁶ M ^?1 s^?1), indicating that this compound was able to quench ¹O₂. However, ¹O₂ did not participate in the photooxidation of HPT to FPT.     

Singlet Oxygen Formation during the Charging Process of an Aprotic Lithium–Oxygen Battery

Aprotic lithium–oxygen (Li–O₂) batteries have attracted considerable attention in recent years owing to their outstanding theoretical energy density. A major challenge is their poor reversibility caused by degradation reactions, which mainly occur during battery charge and are still poorly understood. Herein, we show that singlet oxygen (¹Δ_g) is formed upon Li₂O₂ oxidation at potentials above 3.5 V. Singlet oxygen was detected through a reaction with a spin trap to form a stable radical that was observed by time‐ and voltage‐resolved in operando EPR spectroscopy in a purpose‐built spectroelectrochemical cell. According to our estimate, a lower limit of approximately 0.5 % of the evolved oxygen is singlet oxygen. The occurrence of highly reactive singlet oxygen might be the long‐overlooked missing link in the understanding of the electrolyte degradation and carbon corrosion reactions that occur during the charging of Li–O₂ cells.     

Fullerene C60 Bound to Insoluble Hydrophilic Polymer: Synthesis,Photophysical Behavior,and Generation of Singlet Oxygen in Water Suspensions

Fullerene C₆₀ has been covalently bound to an insoluble hydrophilic polymeric matrix: Sephadex ^® G‐200. The new polymeric equivalent of C₆₀ swells in H₂O to form gel‐like suspensions. The transient photochemical behavior of this polymeric fullerene has been studied in dry and H₂O‐suspended samples. Both samples show a transient absorption similar to the absorption of the parent C₆₀ solution. There is a lack of triplet‐triplet annihilation and of a O₂‐quenching process in the dry sample. On the contrary, the O₂‐quenching process is very efficient in the H₂O‐suspended samples (k_q(O₂)=(1.9±0.5)×10⁸ dm³ mol⁻¹ s⁻¹) and results in the formation of singlet oxygen, which is detected by its emission at 1270 nm. These results point to this hydrophilic polymeric equivalent of C₆₀ as a good candidate for use as a singlet‐oxygen solid sensitizer in H₂O suspensions.