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.     

EPR and Spectrophotometric Studies of Free Radicals (O2°−, °OH, BPD-MA°−) and Singlet Oxygen (1O2) Generated by Irradiation of Benzoporphyrin Derivative Monoacid Ring A

Abstract— Benzoporphyrin derivative monoacid ring A (BPD-MA), a chlorin-type molecule, is a new photosensitizer currently in phase II clinical trials for the treatment by pho-todynamic therapy of cancerous lesions, psoriasis and pathologic neovascularization. The photochemistry (type I and/or II) of BPD-MA has been studied in homogeneous solution and in aqueous dispersions of unilamellar liposomes of dipalmitoylphosphatidylcholine (DPPC) using electron paramagnetic resonance and spectrophotometric methods. When oxygen-saturated solutions of BPD-MA were illuminated with 690 nm light, singlet oxygen (¹O₂), superoxide anion radical (O₂^?), hydroxyl radical (OH) and hydrogen peroxide (H₂O₂) were formed. The BPD-MA generates ¹O₂ with a quantum yield of ca 0.81 in ethanolic solution. The quantum yield does not change upon incorporation of BPD-MA into liposomes of DPPC. The superoxide anion radical was generated by the BPD-MA anion radical (BPD-MA^?) via electron transfer to oxygen, and this process was significantly enhanced by the presence of electron donors. The rate of production of 0₂ was also dependent on the concentration of BPD-MA used (3-100 μM). The quantum yield of O₂^?was found to be 0.011 and 0.025 in aqueous solution and DPPC liposomes, respectively. Moreover, O₂^_upon dis-proportionation can generate H₂O₂ and ultimately the highly reactive OH via the Fenton reaction. In anaerobic homogeneous solution, BPD-MA^?was predominantly photoproduced via the self-electron transfer between the excited- and ground-state species. The presence of an electron donor significantly promotes the reduced form of BPD-MA. These findings suggest that the photodynamic action of BPD-MA may proceed via both type I and type II mechanisms.     

Photoproduction and Direct Spectral Detection of Singlet Molecular Oxygen (102) in Keratinocytes Stained with Rose Bengal

In vivo, keratinocyte skin cells are exposed to photoox-idative processes, some of which can be mediated by singlet molecular oxygen (¹O₂), a species that is very difficult to detect spectrally in cells. We photosensitized ¹O₂ in cultured HaCaT keratinocytes stained with rose bengal (RB) that localizes exclusively inside the keratinocyte hydrophobic regions, as evidenced by strongly red-shifted absorbance and intense fluorescence. We used keratinocytes grown in a monolayer on a plastic coverslip and in suspension. The phosphorescence spectrum (1200–1350nm) from ¹O₂ was strongest when the coverslip containing RB-stained keratinocytes was irradiated in air. The spectral intensity decreased when the coverslip was immersed in D₂O during irradiation and was almost completely quenched when it was irradiated while immersed in water. Water not only shortens the ¹O₂ lifetime but also reabsorbs part of the ¹O₂ phosphorescence, processes that do not occur when ¹O₂ is produced in a keratinocyte layer exposed to air. Because the RB was inside keratinocytes, singlet oxygen must also be produced inside the keratinocytes. However, the sensitivity to the extracellular environment suggests that most of the detectable ¹O₂ phosphorescence originates from those ¹O₂molecules that escaped from the cell through its membrane into D₂O or into the air, where ¹O₂ has longer lifetimes. Our results confirm directly that ¹O₂ is indeed photosensitized in living cells by RB. They also suggest that keratinocyte monolayers may be a good cell model to examine in vitro the production of ¹O₂ by other photo-sensitizers of environmental and photomedical interest.     

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.     

LIGHT-INDUCED FORMATION OF O-2˙ OXYGEN RADICALS IN SYSTEMS CONTAINING CHLOROPHYLL

It has been shown recently that photosystem 1 particles, photosystem 1 lipid vesicles and chlorophyll-a lipid vesicles show identical photochemical reactions in the presence of oxygen e.g. H⁺-and O₂-uptake (Van Ginkel, 1979). Therefore, spin-trapping experiments were done to identify the oxygen radicals formed. The spintrap phenyltertiarybutylnitrone (PBN) failed to yield information about oxygen radicals. With the spintrap 5,5-dimethyl-1-pyrroline-1-oxide (DMPO), however, we obtained a mixed spectrum of O^-_2˙ and OH·-adducts generated in chloroplasts, photosystem 1 particles or chlorophyll-a lipid vesicles. These data indicate that chlorophyll-a in an artificial membrane can also catalyze O^-_2˙-formation. Chlorophyll-a lipid vesicles catalyze light-induced formation of the Tiron-semiquinone free radical, which has been proposed as a specific O^-_2˙-probe (Greenstock and Miller, 1975). However, OH· scavengers strongly reduce the formation of this radical, whereas superoxide dismutase does not. Pulse-radiolysis measurements showed that the rate constant for the reaction of Tiron with OH· is 8.2 · 10⁹M^-1 s^-1, which is considerably higher than the published Tiron/O^-_2˙ rate constants. Therefore, Tiron is a better spin probe for OH· than for O^-_2˙. We suggest that light-induced H⁺-and O^-_2˙-uptake in membranes containing chlorophyll-a in the presence of ascorbate is caused mainly by the very rapid reaction of OH· with ascorbate.     

UVB Shielding Role of FeCI3 and Certain Cyanobacterial Pigments

The shielding role of ferric iron (FeCl₃) and certain cyanobacterial pigments (a brown-colored pigment from Scytonema hofmanii culture filtrate and a pink extract from Nostoc spongiaeforme) against UVB-induced damage in the filamentous, nitrogen-fixing cyanobacterium Nostoc muscorum has been demonstrated. Addition of these colored compounds to agarose gels (1–3 mm thick) resulted in a considerable decrease in UVB transmittance through the gels. The lowest UVB transmittance (15%) occurred through a 3 mm gel containing 0.01% FeCl₃ followed by S. hofmanii culture filtrate (40%) and N. spongiaeforme extract (50%). These substances appear to act as very efficient UVB-absorbing screens. Percent survival and ¹⁴CO₂ uptake of N. muscorum increased significantly if UVB exposure was given on gels containing FeCl₃ or other UVB-shielding substances. The highest protection of N. muscorum was recorded with FeCl₃, followed by S. hofmanii culture filtrate and N. spongiaeforme extract. Such UV-shielding substances if present in required concentration range may enhance the survival of cyanobacteria exposed to high levels of UVB.     

NanoSOSG: A Nanostructured Fluorescent Probe for the Detection of Intracellular Singlet Oxygen

A biocompatible fluorescent nanoprobe for singlet oxygen (¹O₂) detection in biological systems was designed, synthesized, and characterized, that circumvents many of the limitations of the molecular probe Singlet Oxygen Sensor Green^® (SOSG). This widely used commercial singlet oxygen probe was covalently linked to a polyacrylamide nanoparticle core using different architectures to optimize the response to ¹O₂. In contrast to its molecular counterpart, the optimum SOSG‐based nanoprobe, which we call NanoSOSG, is readily internalized by E. coli cells and does not interact with bovine serum albumin. Furthermore, the spectral characteristics do not change inside cells, and the probe responds to intracellularly generated ¹O₂ with an increase in fluorescence.     

Induction of phr gene expression in E. coli strain KY706/pPL-1 by He–Ne laser (632.8 nm) irradiation

We have observed that He–Ne laser irradiation of E. coli strain KY706/pPL-1 leads to induction of photolyase gene, phr. The magnitude of induction was found to depend on the He–Ne laser fluence, fluence rate and post-irradiation incubation period in the nutrient medium. The optimum values for fluence and fluence rate were 7×10³ J/m² and 100 W/m², respectively, and the induction of phr gene was observed to saturate beyond an incubation period of 2 h. Experiments carried out with singlet oxygen quenchers and with D₂O suggest that the effect is mediated via singlet oxygen. Photoreactivation studies carried out after UVC exposure of both the He–Ne laser-exposed as well as unexposed cells showed a larger surviving fraction in the He–Ne laser pre-irradiated cells. This can be attributed to He–Ne laser irradiation-induced induction of phr expression. However, since even without photoreactivating light He–Ne laser pre-irradiated cells show higher survival against UVC radiation it appears that He–Ne laser irradiation induces both light-dependent as well as dark DNA repair processes.     

Excitation-Wavelength-Dependent Functionalities of Temporally Controlled Sensing and Generation of Singlet Oxygen by a Photoexcited State Engineered Rhodamine 6G-Anthracene Conjugate

The present study provides design guidance for unique multipotent molecules that sense and generate singlet oxygen (¹O₂). A rhodamine 6G-aminomethylanthracene-linked donor-acceptor molecule ( RA ) is designed and synthesized for demonstrating wavelength-dependent functionalities as follows; (i) RA acts as a conventional fluorogenic ¹O₂ sensor molecule like the commercially available reagent, singlet oxygen sensor green (SOSG), when it absorbs ultraviolet (UV)-visible light and reacts with ¹O₂. (ii) RA acts as a temporally controlled ¹O₂ sensing reagent under the longer wavelength (∼700 nm) photosensitization. RA enters an intermediate state after capturing ¹O₂ and does not become strongly fluorescent until it is exposed to UV, blue, or green light. (iii) RA acts as an efficient photosensitizer to generate ¹O₂ under green light illumination. The spin-orbit charge transfer mediated intersystem crossing (SOCT-ISC) process achieves this function, and RA shows a potential cancer-killing effect on pancreatic cancer cells. The wavelength-switchable functionalities in RA offer to promise molecular tools to apply ¹O₂ in a spatiotemporal manner.     

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.     

REACTIVITY OF SINGLET OXYGEN TOWARD AMINO ACIDS AND PEPTIDES

Quenching of singlet oxygen (¹O₂) in D₂O-ethanol by the amino acids tryptophan, tyrosine, histidine, methionine, cysteine and their derivatives was measured by exciting the sensitizers rose bengal or meso-tetra (N-methyl-4-pyridyl)porphyrin tetratosylate in the presence of oxygen and the above quenchers in solution. In our polar solvent, containing 75% D₂O on a molar basis it was found that (1) substitution of the aromatic ring in indole, phenol and imidazole by the electron-donating methyl group increases the total (i.e. nonreactive and reactive) quenching rate constant by a factor of five to eight. Free or blocked amino and carboxyl groups removed by two methylene groups from the ring counteract the above increase in the rate constant. The reactive quenching of singlet oxygen, which leads to oxidative destruction of the aromatic ring, correlates with the above substitution effects. It has been proposed that the quenching process takes place by formation of an exciplex between ¹O₂ and the quencher. Thus our results indicate that the better an electron donor the amino acid residue is the more pronounced is the charge transfer contribution in the exciplex formed with ¹O₂ and the more likely it is to lead to charge separation and hence to a chemical reaction. (2) Oligopeptides in solution or peptide bonds linked to the amino acid residue have only a minor effect on singlet oxygen. It can therefore be expected that the polypeptide chains per se in the protein network will not interact significantly with the single oxygen molecules present. The quenching of the latter should, to a first approximation, depend only on the presence of the above reactive amino acid residues and to their accessibility to ¹O₂ as well as on the effective dielectric constant within the protein structure.     

A Study of the Uptake of Toluidine Blue O by Porphyromonas gingivalis and the Mechanism of Lethal Photosensitization

The purpose of the study was to determine the distribution of the photosensitizer toluidine blue O (TBO) within Porphyromonas gingivalis and the possible mechanism(s) involved in the lethal photosensitization of this organism. The distribution of TBO was determined by incubating P. gingivalis with tritiated TBO (³H-TBO) and fractionating the cells into outer membrane (OM), plasma membrane (PM), cytoplasmic proteins, other cytoplasmic constituents and DNA. The percentage of TBO in each of the fractions was found to be, 86.7, 5.4, 1.9, 5.7 and 0.3%, respectively. The involvement of cytotoxic species in the lethal photosensitization induced by light from a helium-neon (HeNe) laser and TBO was investigated by using deuterium oxide (D₂O), which prolongs the lifetime of singlet oxygen, and the free radical and singlet oxygen scavenger L-tryptophan. There were 9.0 log₁₀ and 2 log₁₀ reductions in the presence of D₂O and H₂O (saline solutions), respectively, at a light dose of 0.44 J (energy density = 0.22 J/cm²), suggesting the involvement of singlet oxygen. Decreased kills were attained in the presence of increasing concentrations of L-tryptophan. The effect of lethal photosensitization on whole cell proteins was determined by measuring tryptophan fluorescence, which decreased by 30% using 4.3 J (energy density = 4.3 J/ cm²) of light. Effects on the OM and PM proteins were determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis. There was evidence of change in the molecular masses of several PM proteins and OM proteins compared to controls. There was evidence of damage to the DNA obtained from irradiated cells. Scanning electron microscopic studies showed that there was coaggre-gation of P. gingivalis cells when sensitized and then exposed to laser light. These results suggest that lethal photosensitization of P. gingivalis may involve changes in OM and/or PM proteins and DNA damage mediated by singlet oxygen.     

EFFECTIVE PHOTODYNAMIC ACTION BY RHODAMINE 123 LEADING TO PHOTOSENSITIZED KILLING OF CHINESE HAMSTER OVARY CELLS IN TISSUE CULTURE AND A PROPOSED MECHANISM

The effectiveness of rhodamine 123 (R123) as a photosensitizer of cell killing is relatively low and correlates with its inefficient production of singlet oxygen. The known selective retention of R123 in the mitochondria of epithelially derived carcinoma cells, however, is a selective feature that could lead to a more useful therapeutic ratio if photosensitizing effectiveness could be increased. Chinese hamster ovary (CHO) cells in tissue culture were therefore exposed to R123 shortly before and during illumination under conditions controlled for oxygen concentration and temperature. Effective photosensitization of cell killing, as judged by colony formation, was produced by 95% but not by 19% O₂ during illumination of cells at 5d?C or 37d?C, and this was additionally enhanced at the sublethal temperature of 42d?C. Two CHO cell lines were examined; one line, CHO-AA8, was proficient in the repair of DNA damage and the parent to the second line, CHO-EM9, that was deficient in the repair of DNA strand breaks. Cells of both lines incorporated R123 to a similar degree and were similarly photosensitized by the presence of igh oxygen concentration. Furthermore, plasma membrane damage as judged by teh exclusion of trypan blue was not observed immediately after illumination in the presence of R123, but was seen in the presence of meso-tetra-(4-sulfonatophenyl)-porphine (TPPS₄). The extent of damage to the plasma membrane by TPPS₄ was greater in the presence of 95% compared to 19% O₂ during illumination. Photodynamic action at the level of teh plasma membrane appears to contribute to photosensitization by TPPS₄ but not by R123 soon after exposure of cells to these sensitizers. It is hypothesized that photodynamic action by R123 is the primary mechanism causing the observed photosensitization of cell killing, and that mitochondria are teh site of photosensitized damage responsible for this killing.     

Photodynamic effects induced by meso-tetrakis[4-(carboxymethyleneoxy)phenyl] porphyrin on isolated Sarcoma 180 ascites mitochondria

Using mitochondria isolated from Sarcoma 180 ascites tumour in Swiss mice as a model system, we have evaluated the ability of a novel porphyrin, meso-tetrakis[4-(carboxymethyleneoxy)phenyl]porphyrin (H₂T4CPP), to induce damage on photosensitization. Oxidative damage to mitochondria, one of the primary and crucial targets of the photodynamic effect, is assessed by measuring products of lipid peroxidation such as thiobarbituric acid reactive substances (TBARS) and lipid hydroperoxides (LOOH), besides the loss of activity of the mitochondrial marker enzyme succinate dehydrogenase (SDH). Analysis of product formation, the effect of deuteration and selective inhibition by scavengers of reactive oxygen species (ROS) show that the damage observed is due mainly to singlet oxygen (¹O₂) and to a minor extent to hydroxyl radicals (^•OH). The ¹O₂ generation and triplet lifetime of this porphyrin have also been estimated. Fluorescence spectroscopy, used to ascertain the binding of this porphyrin to the mitochondrial proteins, shows a rapid association within 0–2 h and a decline thereafter. Confocal microscopy reveals intracellular localisation of this porphyrin in cells in vitro. Our overall results suggest that the porphyrin H₂T4CPP, due to its ability to bind to mitochondrial protein components and to generate ROS upon photoexcitation, may have potential applications in photodynamic therapy.     

介孔钛纳米复合团簇应用于光热-光动力疗法

采用水热法合成了一种新型的介孔二氧化钛/碳/亚甲蓝复合纳米团簇(TiO_2@C-MB),并应用于肿瘤细胞的光动力(PDT)和光热治疗(PTT)。系统中介孔二氧化钛作为有效的光敏剂,MB作为重要的光敏添加剂以改善二氧化钛纳米晶的光化学效应,并将其光响应区域拓宽至光动力学疗法的理想治疗窗(650~900 nm)。柠檬酸在水热条件下被还原成碳并裹覆在二氧化钛表面。碳层表现出良好的光热效果,也充当多功能的电子受体以加速生成单线态氧。该纳米团簇不仅可以保持肿瘤细胞内部高浓度的MB和二氧化钛以产生大量的单线态氧杀死肿瘤细胞,而且可以避免MB退化失活。     

Comparative Study of Ru(bpz)32+Ru(bipy)32+and Ru(bpz)2CI2 as Photosensitizers of DNA Cleavage and Adduct Formation

Abstract— The efficiency of ruthenium complexes for photosensitizing DNA damage depends on the oxidizing character of their ligands. Here we report on the difference in behavior of tris(2.2'-bipyrazyl)ruthenium(II) (Ru[bpz]₃²+), tris(2,2′-bipyridyl)ruthenium(II) (Ru[bipy]₃²+) and cis-dichlorobis(2,2′-bipyrazyl)ruthenium(II) (Ru[bpz]₂Cl₂). Upon irradiation at 436 nm, Ru(bpz)₃²+was far less stable than Ru(bipy)₃²+. Ru(bpz)₃²+in phosphate buffer containing NaCl undergoes a photoanation reaction leading to the formation of Ru(bpz)₂Cl₂, as previously reported also in organic media. In the presence of phage φX174 DNA, Ru(bpz)₃²+photosensitized the formation of single strand breaks with an efficiency that was, at the beginning of irradiation, similar to that of Ru(bipy)₃²+. After 8 min of irradiation, the cleavage efficiency of Ru(bpz)₃²+reached a plateau that may correspond to its photode-composition. For the same conditions, Ru(bpz)₂Cl₂ did not induce DNA breakage. Scavenging experiments showed that, in the presence of oxygen, DNA cleavage induced by Ru(bpz)₃²+partly resulted from the formation of singlet oxygen and hydroxyl radical while in the absence of oxygen an additionnal mechanism involving electron transfer between the excited state of the ruthenium complex and DNA is proposed. The ICP measurement showed that Ru(bpz)₃²+and Ru(bpz)₂Cl₂ gave rise to covalent binding onto DNA in contrast with Ru(bipy)₃²+, which did not bind to DNA under the experimental conditions. The results are discussed with regard to the potential use of these photosensitizers in phototherapy.     

Influence of solvent on the yield of singlet oxygen upon the chloride ion-catalyzed decomposition of dimethyldioxirane

The excitation yield of the singlet molecular oxygen ¹Δ _g [φ(¹O₂)] in the reaction of the dimethyldioxirane with the chloride ion in benzene, acetone, methylene chloride, and acetonitrile solutions has been determined. The φ(¹O₂) value depends on the solvent nature and is in the range 50–84%. Correlation between the value of the singlet oxygen yield and the Kamlet-Taft parameter (α) characterizing the acidity of the solvent as a hydrogen bond donor has been established: the higher the α value, the higher the ¹O₂ yield.     

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.     

Stability of Flavonoids in the Presence of Riboflavin-photogenerated Reactive Oxygen Species: A Kinetic and Mechanistic Study on Quercetin,Morin and Rutin

Kinetic and mechanistic aspects on the stability of the flavones (FL) quercetin (Que), morin (Mor) and rutin (Rut), in methanolic solution and in the presence of reactive oxygen species (ROS) generated by visible light-promoted riboflavin (Rf, vitamin B₂) photoirradiation were studied. The system was chosen as a model for the evaluation of the in vivo protective effect of biological targets by the flavones. The overall picture includes the vitamin as an endogenous natural photosensitizer. A systematic study on the effect of ROS on FL photostability shows that under work conditions Que is oxidized by singlet molecular oxygen (O₂(¹Δ_g)), superoxide radical anion (O₂˙⁻) and hydrogen peroxide; Mor is degraded by O₂(¹Δ_g) and O₂˙⁻ whereas Rut only reacts with O₂(¹Δ_g). Que and Rut, with an extremely poor overall rate constant, are mainly physical quenchers of O₂(¹Δ_g). Mor, with O₂(¹Δ_g)-interception ability slightly lower than the recognized synthetic antioxidant trolox (Tx), behaves as a typical sacrificial scavenger provided that ca 80% of the collisions with O₂(¹Δ_g) cause its own degradation, whereas this parameter reaches around 50% in the case of Tx.