Taming of Singlet Oxygen: Towards Artificial Oxygen Carriers Based on 1,4-Dialkylnaphthalenes

Naphthalene endoperoxides are known as convenient sources of singlet oxygen (O₂, ¹Δ_g), which is the major product of endoperoxide cycloreversion reaction. However, their potential as carriers of ground-state molecular oxygen (O₂, ³Σ_g) similar to artificial oxygen carriers remains largely unexplored. This is due to the extreme reactivity and cytotoxic effects of the released singlet oxygen. We now report that a compound with a bimodular design, which incorporates an endoperoxide and an efficient physical quencher of singlet oxygen, 1,4-diazabicyclo[2.2.2]octane (DABCO), produces exclusively ground-state molecular oxygen. This result, coupled with the fact that oxygen release rates from endoperoxides are highly amenable to fine-tuning in a very broad range, and open to targeting by ligand attachment, raises the potential of these compounds far above any comparable chemical, or even biochemical sources. In cell culture experiments, we showed that the addition of the endoperoxide-quencher conjugate can enhance and sustain cell proliferation.     

Aromatic Endoperoxides†

The fundamental aspects of aromatic endoperoxide chemistry are reviewed including their synthesis and reactions. The discussion will focus on factors that will both enhance and prevent the formation of aromatic endoperoxides and on structural features that will provide control over their ability to release singlet oxygen. This approach recognizes the dual use of aromatic hydrocarbons as both precursors of endoperoxides and as valuable materials for incorporation in electronic and photonic devices. Improvement of the existing methods and development of new methods for the synthesis of endoperoxides is necessary as a result of the demand to improve existing and to create new applications for these valuable materials. On the contrary, prevention of endoperoxide formation is crucial to inhibit irreversible oxidative degradation of aromatic hydrocarbons and to extend their lifetimes as useful organic semiconductors.     

Release of Singlet Oxygen from Aromatic Endoperoxides by Chemical Triggers

The generation of reactive singlet oxygen under mild conditions is of current interest in chemistry, biology, and medicine. We were able to release oxygen from dipyridylanthracene endoperoxides (EPOs) by using a simple chemical trigger at low temperature. Protonation and methylation of such EPOs strongly accelerated these reactions. Furthermore, the methyl pyridinium derivatives are water soluble and therefore serve as oxygen carriers in aqueous media. Methylation of the EPO of the ortho isomer affords the parent form directly without increasing the temperature under very mild conditions. This exceptional behavior is ascribed to the close contact between the nitrogen atom and the peroxo group. Singlet oxygen is released upon this reaction, and can be used to oxygenate an acceptor such as tetramethylethylene in the dark with no heating. Thus, a new chemical source of singlet oxygen has been found, which is triggered by a simple stimulus.     

Investigation of photobleaching of hypocrellin B in non-polar organic solvent and in liposome suspension

Hypocrellin B (HB) is a natural pigment with a promising application in the photodynamic therapy (PDT) for anticancer treatment. The photobleaching of HB in non-polar organic solvents and in liposomes in aqueous solution were investigated by the measurements of absorption spectra, quenching experiments and determination of photoproducts. Control experiments indicated that the sensitizer, oxygen and light were all essential for the photobleaching of HB, which suggested that it was mainly self-sensitized photooxidation. The illumination of HB with visible light in aerobic non-polar solvent generated singlet oxygen efficiently [Phi(1O(2))=0.76] which then attacked the sensitizer HB with formation of an endoperoxide product. The endoperoxide of HB was unstable at room temperature and underwent predominantly loss of singlet oxygen with regeneration of parent HB. The singlet oxygen released from the endoperoxide of HB was detected with chemical trapping experiments. When HB was embedded in EPC liposomes, no endoperoxide product and no singlet oxygen release from the photobleaching process of HB were detected. The quenching experiments indicated that the singlet oxygen mechanism (type II) played an important role in the non-polar solvent and the free radical mechanism (type I) was predominant in liposomal aqueous solution for the photobleaching of HB.     

Chemistry of dioxine-annelated cycloheptatriene endoperoxides and their conversion into tropolone derivatives: an unusual non-benzenoid singlet oxygen source

The chemistry of two bicyclic endoperoxides, obtained by photooxygenation of 2,3-dihydro-7H-cyclohepta[1,4]dioxine and 2,3-dihydro-7H-cyclohepta[b][1,4]dioxin-7-one was investigated with the aim of synthesizing the respective tropolone derivatives. The reaction of these endoperoxides with base, thiourea and their thermolysis provided the desired tropolone derivatives in high yield. On the other hand, the thermolysis of the endoperoxide derived from 2,3-dihydro-7H-cyclohepta[b][1,4]dioxin-7-one underwent an unprecedented route and formed parent molecule and singlet oxygen instead of the expected troponoids. The formation mechanisms of all products are discussed.     

Protoporphyrin IX nanoparticle carrier: preparation, optical properties, and singlet oxygen generation

The present study is focused on developing a nanoparticle carrier for the photosensitizer protoporphyrin IX for use in photodynamic therapy. The entrapment of protoporphyrin IX (Pp IX) in silica spheres was achieved by modification of Pp IX molecules with an organosilane reagent. The immobilized drug preserved its optical properties and the capacity to generate singlet oxygen, which was detected by a direct method from its characteristic phosphorescence decay curve at near-infrared and by a chemical method using 1,3-diphenylisobenzofuran to trap singlet oxygen. The lifetime of singlet oxygen when a suspension of Pp IX-loaded particles in acetonitrile was excited at 532 nm was determined as 52 micros, which is in good agreement with the value determined for methylene blue in acetonitrile solution under the same conditions. The Pp IX-loaded silica particles have an efficiency of singlet oxygen generation (eta Delta) higher than the quantum yield of free porphyrins. This high efficiency of singlet oxygen generation was attributed to changes on the monomer-dimer equilibrium after photosentisizer immobilization.     

Using the rotaxane mechanical bond to enhance chemical reactivity

Rates of cycloreversion for squaraine rotaxane mono(endoperoxides) were enhanced by structural modifications that increased cross-component steric destabilization of the inward directed 9,10-anthracene endoperoxide group. The largest rate enhancements were obtained when the surrounding macrocycle contained two 2,6-pyridine dicarboxamide bridging units, which induced a cavity contraction effect. The precursor fluorescent, near-IR, squaraine rotaxanes are effectively photostable because the mono(endoperoxide) products, formed by reaction with photogenerated singlet oxygen, rapidly cyclorevert back to the original squaraine rotaxane.     

WAVELENGTH EFFECTS ON THE PHOTOCYCLOREVERSION QUANTUM YIELD OF SOME PHOTOCHROMIC ENDOPEROXIDES

Abstract— The photocycloreversion quantum yields Q of the endoperoxides of anthradichromene, benzodixanthene and dimethylhomoocoerdianthrone have been investigated in the 248 ≤γ≤ 405 nm wavelength range. Two types of wavelength dependencies can be distinguished phenomenologically. For the excitation of the S_n (n ≥ 3) states of dimethylhomöocoerdianthrone endoperoxide γ-independent Q-values have been observed throughout 365 ≥γ≥ 265 nm. The endoperoxides of anthradichromene and benzodixanthene in contrast exhibit a quite different and unusual dependence of Q on γ. For both compounds Q first increases with photon energy until a fairly sharp maximum in Q is reached at 302 nm. With further increasing energy Q falls off and approaches a wavelength independent plateau between 280 and 248 nm. The S₁-states of the endoperoxides investigated are inactive for photocycloreversion, which is generally found in endoperoxide photochemistry. The wavelength dependencies observed reveal that the photocycloreversion of endoperoxides deviates remarkably from the commonly accepted generalization that photochemical reactions in solution phase should only occur from the lowest excited singlet or triplet states.     

Photochemistry of A1E, a retinoid with a conjugated pyridinium moiety: competition between pericyclic photooxygenation and pericyclization

The photochemistry of the retinoid analogue A1E shows an oxygen and solvent dependence. Irradiation of A1E with visible light (lambda(irr) = 425 nm) in methanol solutions resulted in pericyclization to form pyridinium terpenoids. Although the quantum yield for this cyclization is low (approximately 10(-4)), nevertheless the photochemical transformation occurs with quantitative chemical yield with remarkable chemoselectivity and diastereoselectivity. Conversely, irradiation of A1E under the same irradiation conditions in air-saturated carbon tetrachloride or deuterated chloroform produced a cyclic 5,8-peroxide as the major product. Deuterium solvent effects, experiments utilizing endoperoxide, phosphorescence, and chemiluminescence quenching studies strongly support the involvement of singlet oxygen in the endoperoxide formation. It is proposed that, upon irradiation, in the presence of oxygen, A1E acts as a sensitizer for generation of singlet oxygen from triplet oxygen present in the solution; the singlet oxygen produced reacts with A1E to produce cyclic peroxide. Thus, the photochemistry of A1E is characterized by two competing reactions, cyclization and peroxide formation. The dominant reaction is determined by the concentration of oxygen, the concentration of A1E, and the lifetime of singlet oxygen in the solvent employed. If the lifetime of singlet oxygen in a given solvent is long enough, then oxidation (peroxide formation) is the major reaction. If the singlet oxygen produced is quenched by the protonated solvent molecules faster than singlet oxygen reacts with A1E, then cyclization dominates.     

Selective endoperoxide formation by heterogeneous TiO2 photocatalysis with dioxygen

We report the selective formation of endoperoxides by aerobic TiO₂ photocatalysis through the cyclic addition of dioxygen and a non-conjugated diene, the first heterogeneous catalytic system for endoperoxide synthesis. This green protocol does not require any additive and the photocatalyst is abundant and recyclable, providing a yield up to 64% and >20:1 diastereoselectivity. Mechanistic investigations were carried out by using product analysis, kinetic studies, O-18 labelling experiments, electron-spin resonance and a set of quenching experiments. Superoxide (but not singlet oxygen, triplet oxygen or peroxide) is directly involved in the reaction cascade to form the endoperoxide product. The new findings may be helpful for future for designing eco-friendly and energy sustainable strategies for selective oxygenation reactions using semiconductors, O₂ and sunlight.     

Capturing Transient Endoperoxide in the Singlet Oxygen Oxidation of Guanine

The chemistry of singlet O₂ toward the guanine base of DNA is highly relevant to DNA lesion, mutation, cell death, and pathological conditions. This oxidative damage is initiated by the formation of a transient endoperoxide through the Diels–Alder cycloaddition of singlet O₂ to the guanine imidazole ring. However, no endoperoxide formation was directly detected in native guanine or guanosine, even at ?100 °C. Herein, gas‐phase ion–molecule scattering mass spectrometry was utilized to capture unstable endoperoxides in the collisions of hydrated guanine ions (protonated or deprotonated) with singlet O₂ at ambient temperature. Corroborated by results from potential energy surface exploration, kinetic modeling, and dynamics simulations, various aspects of endoperoxide formation and transformation (including its dependence on guanine ionization and hydration states, as well as on collision energy) were determined. This work has pieced together reaction mechanisms, kinetics, and dynamics data concerning the early stage of singlet O₂ induced guanine oxidation, which is missing from conventional condensed‐phase studies.     

Characterization of endoperoxide and hydroperoxide intermediates in the reaction of pyridoxine with singlet oxygen

The photosensitized oxidation of vitamin B6, pyridoxine, is investigated by product and kinetic analysis. Singlet oxygen quenching rates, measured by time-resolved laser flash generation of singlet oxygen followed by monitoring singlet oxygen phosphorescence decay, confirm previous observations that pyridoxine is a moderate quencher. The quenching rate for 3-methoxypyridine is 100 times slower than that for 3-hydroxypyridine, indicating the hydroxy moiety is required for efficient quenching. The chemical quenching rate constant, kr, was estimated by comparison with a known singlet oxygen reaction. Results indicate that the chemical quenching rate of pyridoxine dominates the total quenching. The major reaction product in methanol was isolated and characterized by NMR and MS. The data are consistent with a solvent adduct of the substituted 2,5-pyridinedione. At low temperature, two semistable intermediates were characterized by NMR. The data are consistent with a hydroperoxide and endoperoxide. These intermediates suggest initial attack of singlet oxygen para to the hydroxy group followed by either proton transfer to form the hydroperoxide or addition of the peroxide to the imine to form the endoperoxide. In the presence of protic solvents, the solvent adducts to the imine and elimination of water yield the observed 2,5-pyridinedione product.     

Photochemistry of Bacteriochlorophylls in Human Blood Plasma: 1. Pigment Stability and Light-induced Modifications of Lipoproteins

Transmetalated derivatives of bacteriochlorophyll are promising sensitizers in photodynamic therapy. Protocols using short delay times between injection and irradiation cause interest in the photochemistry of these pigments in the blood. Using near-infrared irradiation where these pigments absorb strongly, we have studied the photochemistry of Zn- and Pd-bacteriopheophorbide (WST09), and of the highly polar taurinated Pd-derivative, WST11, in isolated fractions of human blood plasma. The stability of all pigments is increased in blood plasma, compared with monomeric solutions. Pd-bacteriopheophorbide is much more stable than the other two derivatives. It also has a higher capacity for inducing reactive oxygen species, yet the consumption of oxygen is comparable. There is furthermore evidence for photobleaching under anoxic conditions. The generation of hydroperoxides (ROOH) is faster with Pd- than with Zn-complexes; the formation of endoperoxides (ROOR′), measured as thiobarbituric acid reactive substances, is comparable with the two central metals. Formation of both ROOH and ROOR′ is increased in low-density lipoproteins (LDL) compared with high-density lipoproteins (HDL), which is probably related to the higher concentration of target molecules in the former. In HDL, extensive cross-linking is induced among the apolipoproteins; judged from the electrophoretic mobility of LDL and HDL particles, there is also a gross structural change. Photosensitized cross-linking is much less pronounced with high-density proteins.     

The Photolysis Of The Endoperoxide Of 9,10-Diphenylanthracene

During the photolysis of the endoperoxide of 9,10-diphenylanthracene, two different reactions are observed, depending on the irradiation wavelength: (i) Excitation of the S₁ band causes a homolytic cleavage of the peroxide bridge with a quantum yield Q₂ = 0.08. (ii) Irradiation of the S₂ band leads to an adiabatic photocleavage of the endoperoxide into 9,10-diphenylanthracene and singlet molecular oxygen with a quantum yield Q₁ = 0.28. Both reaction pathways confirm the theory of Kearns and Khan concerning the photolysis of endoperoxides     

A WATER-SOLUBLE RUBRENE DERIVATIVE: SYNTHESIS, PROPERTIES AND TRAPPING OF 1O2 IN AQUEOUS SOLUTION

Abstract A rubrene derivative has been synthesized as a new trap for singlet oxygen in aqueous solution. Photooxygenation and evolution of the endoperoxide under various conditions have been investigated. This trap may be used either as a colorimetric test or as a specific one by isolation of the endoperoxide.     

Theoretical study of the oxidation of histidine by singlet oxygen

Herein we present a theoretical study of the reaction of singlet oxygen with histidine performed both in the gas phase and in aqueous solution. The potential energy surface of the reactive system was explored at the B3LYP/cc-pVTZ level of theory and the electronic energies were refined by means of single-point CCSD(T)/cc-pVTZ(-f) calculations. Solvent effects were taken into account by using a solvent continuum model (COSMO) and by adding explicit water molecules. The results show that the first step in the reaction mechanism corresponds to a nearly symmetric Diels-Alder addition of the singlet oxygen molecule to the imidazole ring to yield an endoperoxide, in agreement with experimental evidence. The intermediate formed can evolve along two different reaction paths leading to two isomeric hydroperoxides and, eventually, to open-chain or internally cyclised oxidised products. Water plays a significant role in stabilising the reaction structures by solvation and by acting as a bifunctional catalyst in the elimination/addition reaction steps. Our results explain why substituents at the N1-imidazole ring can hamper the evolution of the initial endoperoxide and result in Gibbs energy barriers in solution similar to those experimentally measured and suggest a likely route to the formation of peptide aggregates during the oxidation of histidine by singlet molecular oxygen.     

Sulfanyl Porphyrazines with Morpholinylethyl Periphery—Synthesis,Electrochemistry, and Photocatalytic Studies after Deposition on Titanium(IV) Oxide P25 Nanoparticles

The syntheses, spectral UV–Vis, NMR, and electrochemical as well as photocatalytic properties of novel magnesium(II) and zinc(II) symmetrical sulfanyl porphyrazines with 2-(morpholin-4-yl)ethylsulfanyl peripheral substituents are presented. Both porphyrazine derivatives were synthesized in cyclotetramerization reactions and subsequently embedded on the surface of commercially available P25 titanium(IV) oxide nanoparticles. The obtained macrocyclic compounds were broadly characterized by ESI MS spectrometry, 1D and 2D NMR techniques, UV–Vis spectroscopy, and subjected to electrochemical studies. Both hybrid materials, consisting of porphyrazine derivatives embedded on the titanium(IV) oxide nanoparticles’ surface, were characterized in terms of particle size and distribution. Next, they were subjected to photocatalytic studies with 1,3-diphenylisobenzofuran, a known singlet oxygen quencher. The applicability of the obtained hybrid material consisting of titanium(IV) oxide P25 nanoparticles and magnesium(II) porphyrazine derivative was assessed in photocatalytic studies with selected active pharmaceutical ingredients, such as diclofenac sodium salt and ibuprofen.     

Synthesis, primary photophysical and antibacterial properties of naphthyl ester cinoxacin and nalidixic acid derivatives

We have synthesized two naphthyl ester quinolone derivates and determined their ability to generate reactive oxygen species (ROS) such as (1)O(2), ()OH, H(2)O(2) upon photolysis with UV-A light. The ability of cinoxacin (1) and nalidixic acid (2), and their naphthyl ester derivatives (3 and 4) to generate a dose-dependent amount of singlet oxygen and ROS (()(-)O(2), ()OH) in cell-free systems was detected by histidine assay and by luminol-enhanced chemiluminescence (LCL), respectively. Their electronic absorption and emission spectra were quantified and their photostability was determined. Their tendency to generate peroxidic derivative species showed the following order: 3>4; in contrast, their ability to generate singlet oxygen was 4>3 and these were better sensitizers than their parent quinolones 1 and 2. The antibacterial activity in darkness and under irradiation of compounds 3 and 4 was tested on Escherichia coli and compared with that of their parent compounds. An enhanced antibacterial activity by irradiation of the naphthyl esters of cinoxacin and nalidixic acid on E. coli was observed.     

Stereoselective [4+2] Cycloaddition of Singlet Oxygen to Naphthalenes Controlled by Carbohydrates

Stereoselective reactions of singlet oxygen are of current interest. Since enantioselective photooxygenations have not been realized efficiently, auxiliary control is an attractive alternative. However, the obtained peroxides are often too labile for isolation or further transformations into enantiomerically pure products. Herein, we describe the oxidation of naphthalenes by singlet oxygen, where the face selectivity is controlled by carbohydrates for the first time. The synthesis of the precursors is easily achieved starting from naphthoquinone and a protected glucose derivative in only two steps. Photooxygenations proceed smoothly at low temperature, and we detected the corresponding endoperoxides as sole products by NMR. They are labile and can thermally react back to the parent naphthalenes and singlet oxygen. However, we could isolate and characterize two enantiomerically pure peroxides, which are sufficiently stable at room temperature. An interesting influence of substituents on the stereoselectivities of the photooxygenations has been found, ranging from 51:49 to up to 91:9 dr (diastereomeric ratio). We explain this by a hindered rotation of the carbohydrate substituents, substantiated by a combination of NOESY measurements and theoretical calculations. Finally, we could transfer the chiral information from a pure endoperoxide to an epoxide, which was isolated after cleavage of the sugar chiral auxiliary in enantiomerically pure form.     

Generation of Singlet Molecular Oxygen by Lipid Hydroperoxides and Nitronium Ion†

Singlet molecular oxygen is a reactive species involved in biological oxidative processes. The major cellular targets of singlet molecular oxygen are unsaturated fatty acids in the membrane, as well as nucleic acids and proteins. The aim of this study was to investigate whether lipids and commercial hydroperoxides generate singlet molecular oxygen, in presence of nitronium and activated nitronium ion. For this purpose, monomol light emitted in the near-infrared region (λ = 1270 nm) was used to monitor singlet molecular oxygen decay in different solvents, with different hydroperoxides and in the presence of azide. Direct measurements of the singlet molecular oxygen spectrum at 1270 nm recorded during the reaction between lipids and commercial hydroperoxides and nitronium ions unequivocally demonstrated the formation of this excited species.