Reevaluation of analytical methods for photogenerated singlet oxygen

Photodynamic treatment of Staphylococcus aureus with non-iron hemin analogs in the presence of hydrogen peroxide

Bacteria subjected to antiseptic or antibiotic stress often develop tolerance, a trait that can lead to permanent resistance. To determine whether photodynamic agents could be used to counter tolerance, we evaluated three non-iron hemin analogs (M-PpIX; M = Al, Ga, In) as targeted photosensitizers for antimicrobial photodynamic inactivation (aPDI) following exposure to sublethal H2O2. Al-PpIX is an active producer of ROS whereas Ga- and In-PpIX are more efficient at generating singlet oxygen. Al- and Ga-PpIX are highly potent aPDI agents against S. aureus and methicillin-resistant strains (MRSA) with antimicrobial activity (3 log reduction in colony-forming units) at nanomolar concentrations. The aPDI activities of Al- and Ga-PpIX against S. aureus were tested in the presence of 1 mM H2O2 added at different stages of growth. Bacteria exposed to H2O2 during log-phase growth were less susceptible to aPDI but bacteria treated with H2O2 in their postgrowth phase exhibited aPDI hypersensitivity, with no detectable colony growth after treatment with 15 nM Ga-PpIX.

Colorimetric Determination of Sulfoxy Radicals and Sulfoxy Radical Scavenging-Based Antioxidant Activity

Sulfoxy radicals (SORs) are oxygen- and sulfur-containing species such as SO3•-, SO4•-, and SO5•-. They can be physiologically generated by S(IV) autoxidation with transition metal catalysis. Due to their harmful effects, the detection of both SORs and their scavengers are important. Here, a simple and cost-effective method for the determination of SORs and the scavenging activity of different antioxidant compounds was proposed. A SOR was selectively generated by combining CoSO4·7H2O with Na2SO3. To detect SOR species as a whole, 3,3',5,5'-tetramethylbenzidine (TMB) was used as the chromogenic reagent, where SOR generated in the medium caused the formation of a blue-colored diimine from TMB. Additionally, the SOR scavenging effects of a number of antioxidant compounds (AOx) belonging to different classes were investigated, among which catechin derivatives were the most effective scavengers. The obtained results were compared with those of a reference rhodamine B decolorization assay. The radical scavenging effects of the tested AOx were ranked by both assays and then compared using the Spearman statistical test to yield a very strong correlation between the two rankings. The method was applied to real samples such as catechin-rich tea, that is, white, black, and green tea, among which white tea was determined as the most effective SOR scavenger.

Locally Administered Photodynamic Therapy for Cancer Using Nano-Adhesive Photosensitizer

Photodynamic therapy (PDT) is a great potential anti-tumor therapy owing to its non-invasiveness and high spatiotemporal selectivity. However, systemically administered photosensitizers diffuse in the skin and the eyes for a long duration, which cause phototoxicity to bright light and sunlight. Therefore, following PDT, patients must avoid exposure of to light and sunlight to avoid this phototoxicity. In this study, we have developed a locally administered PDT using nano-adhesive porphyrin with polycations consisting of quaternary ammonium salt groups (aHP) as a photosensitizer. The aHP, approximately 3.0 nm in diameter, adhered the negatively charged cell membrane via electrostatic interaction. The aHP localized to the endosome via cell adhesion and induced apoptosis upon 635 nm light irradiation. On being administered subcutaneously on the tumor, 30% of the injected aHP remained in the administered sites. However, low-molecular-weight hematoporphyrin dihydrochloride (HP) disappeared due to rapid diffusion. PDT with locally administered aHP showed a higher anti-tumor effect after light irradiation at 635 nm for three days compared to low-molecular-weight HP. Intraperitoneal administration of HP caused severe phototoxicity upon irradiation with ultraviolet A at 10 J cm-2, whereas aHP did not cause phototoxicity because its diffusion into the skin could be suppressed, probably due to the high-molecular weight of aHP. Therefore, locally administered PDT with aHP is a potential PDT having high therapeutic efficacy without phototoxicity.

Insight into novel anti-mucormycosis therapies: investigation of new anti-mucormycosis laser-induced photodynamic therapy based on a sulphone bis-compound loaded silica nanoemulsion

For drug delivery applications, silica nanoemulsion encapsulated with organic compounds are becoming increasingly more desirable. Therefore, the emphasis of this research was on the synthesis of a new potent antifungal drug-like candidate (1,1'-((sulfonylbis(4,1-phenylene)bis(5-methyl-1H-1,2,3-triazole-1,4-diyl))bis(3-(dimethylamino)prop-2-en-1-one), SBDMP), the chemical structure of which was confirmed on the basis of its spectral and microanalytical data. Then, silica nanoemulsion loaded with SBDMP was prepared using Pluronic F-68 as a potent surfactant. The particle shape, hydrodynamic size, and zeta potential of the produced silica nanoemulsion (with and without drug loading) were assessed. The antitumoral activity of the synthesized molecules showed the superiority of SBDMP and silica nanoemulsion with and without SBDMP loading against Rhizopus microsporous and Syncephalastrum racemosum. Subsequently, the laser-induced photodynamic inactivation (LIPDI) of Mucorales strains was determined using the tested samples. The optical properties of the samples were investigated using UV-vis optical absorption and the photoluminescence. The photosensitivity of the selected samples appeared to enhance the eradication of the tested pathogenic strains when exposed to a red (640 nm) laser light. The optical property results verified that the SBDMP-loaded silica nanoemulsion has a high depth of penetration into biological tissues due to a two-absorption photon (TAP) mechanism. Interestingly, the photosensitizing of the nanoemulsion loaded with a newly synthesized drug-like candidate, SBDMP, opens up a new route to apply new organic compounds as photosensitizers under laser-induced photodynamic therapy (LIPDT).

Evaluation of cold atmospheric pressure plasma irradiation of water as a method of singlet oxygen generation

We used cold atmospheric pressure plasma jet to examine in detail 1O2 generation in water. ESR with 2,2,5,5-tetramethyl-3-pyrroline-3-carboxamide, a secondary amine probe, was used for the detection of 1O2. Nitroxide radical formation was detected after cold atmospheric pressure plasma jet irradiation of a 2,2,5,5-tetramethyl-3-pyrroline-3-carboxamide solution. An 1O2 scavenger/quencher inhibited the ESR signal intensity induced by cold atmospheric pressure plasma jet irradiation, but this inhibition was not 100%. As 2,2,5,5-tetramethyl-3-pyrroline-3-carboxamide reacts with oxidizing species other than 1O2, it was assumed that the signal intensity inhibited by NaN3 corresponds to only the nitroxide radical generated by 1O2. The concentration of 1O2 produced by cold atmospheric pressure plasma jet irradiation for 60 s was estimated at 8 μM. When this 1O2 generation was compared to methods of 1O2 generation like rose bengal photoirradiation and 4-methyl-1,4-etheno-2,3-benzodioxin-1(4H)-propanoic acid (endoperoxide) thermal decomposition, 1O2 generation was found to be, in decreasing order, rose bengal photoirradiation ≥ cold atmospheric pressure plasma jet > endoperoxide thermal decomposition. Cold atmospheric pressure plasma jet is presumed to not specifically generate 1O2, but can be used to mimic states of oxidative stress involving multiple ROS.

Quenching effect of cerium oxide nanoparticles on singlet oxygen: validation of the potential for reaction with multiple reactive oxygen species

Here we studied cerium oxide nanoparticles (nanoceria) as an agent for the future treatment of oxidative damage by validating and evaluating its scavenging activity towards reactive oxygen species (ROS) in vitro. Nanoceria has been shown to mimic the activities of superoxide dismutase and catalase, degrading superoxide (O2•-) and hydrogen peroxide (H2O2). We examined the antioxidative activity of nanoceria, focusing on its ability to quench singlet oxygen (1O2) in an aqueous solution. Electron paramagnetic resonance (EPR) was used to determine the rates of second-order reactions between nanoceria and three ROS (1O2, O2•-, and H2O2) in aqueous solution, and its antioxidative abilities were demonstrated. Nanoceria shows a wide range of ultraviolet-light absorption bands and thus 1O2 was produced directly in a nanoceria suspension using high-frequency ultrasound. The quenching or scavenging abilities of nanoceria for 1O2 and hypoxanthine-xanthine oxidase reaction-derived O2•- were examined by EPR spin-trapping methods, and the consumption of H2O2 was estimated by the EPR oximetry method. Our results indicated that nanoceria interact not only with two previously reported ROS but also with 1O2. Nanoceria were shown to degrade O2•- and H2O2, and their ability to quench 1O2 may be one mechanism by which they protect against oxidative damage such as inflammation.

Detection of singlet oxygen by EPR: The instability of the nitroxyl radicals

The use of spin traps and redox probes coupled with electron paramagnetic resonance (EPR) is a method frequently applied in the evaluation of the efficiency of photosensitizers and photocatalysts in phototherapeutic and photocatalytic processes that involve reactive oxygen species. In this way, the method helps to clarify the mechanism behind photo-induced reactions. Hydroxy-TEMP is a very specific redox probe for selectively identifying and quantifying singlet oxygen (¹O₂). In this work, the kinetics of radical generated by the oxidation products of the Hydroxy-TEMP redox probe was analyzed from EPR spectra in aqueous solutions of several water-soluble porphyrins ([H₂T4MPyP](OTs)₄, Na₄[H₂T4SPP], [H₂T2MPyP](OTs)₄, [ZnT4MyPyP](OTs)₄, [MnT4MyPyP](OTs)₅, H₂T4CPP, and [H₂T4TriMAPP](OTs)4) under white light illumination. Different factors such as the concentration of the redox probe, pH of the medium, and photostability of the porphyrins were evaluated. A systematic study was carried out to reveal the factors associated with stable radical degradation (TEMPOL) by illumination in the visible spectral region in systems containing photosensitizer (porphyrin) and redox probe (Hydroxy-TEMP). With the aid of EPR and gas chromatography coupled with mass spectroscopy (GC-MS) techniques, the mechanism of the radical degradation and the photobleaching of porphyrins were investigated. After successive interactions with the porphyrin in its excited state, in alkaline aqueous solution (pH > 10), the free radical TEMPOL is transformed into TEMPONE until the final diamagnetic product Phorone. A protocol was elaborated to identify and quantify the generation of ¹O₂ by Hydroxy-TEMP reliably, to avoid possible errors in the interpretation of efficiency of photosensitizers.

Microfluidic Synthesis of Theranostic Nanoparticles with Near-Infrared Scintillation: Toward Next-Generation Dosimetry in X-ray-Induced Photodynamic Therapy

We developed a microfluidic synthesis to grow GdF₃:Eu theranostic scintillating nanoparticles to simultaneously monitor the X-ray dose delivered to tumors during treatments with X-ray activated photodynamic therapy (X-PDT). The flow reaction was optimized to enhance scintillation emission from the Eu³⁺ ions. The as-prepared ∼15 nm rhombohedral-shaped nanoparticles self-assembled into ∼100 nm mesoporous flower-like nanostructures, but the rhombohedral units remained intact and the scintillation spectra unaltered. The conjugation of the ScNPs with multilayers of methylene blue (MB) in a core-shell structure (GdF@MB) resulted in enhanced singlet oxygen (¹O₂) generation under X-ray irradiation, with maximum ¹O₂ production for nanoparticles with 4 MB layers (GdF@4MB). High ¹O₂ yield was further evidenced in cytotoxicity assays, demonstrating complete cell death only for the association of ScNPs with MB and X-rays. Because the scintillating Eu³⁺ emission at 694 nm is within the therapeutic window and was only partially absorbed by the MB molecules, it was explored for getting in vivo dosimetric information. Using porcine skin and fat to simulate the optical and radiological properties of the human tissues, we showed that the scintillation light can be detected for a tissue layer of ∼16 mm, thick enough to be employed in radiotherapy treatments of breast cancers, for instance. Therefore, the GdF₃:Eu ScNPs and the GdF@4MB nanoconjugates are strong candidates for treating cancer with X-PDT while monitoring the treatment and the radiation dose delivered, opening new avenues to develop a next-generation modality of real-time in vivo dosimetry.

Prospects for More Efficient Multi-Photon Absorption Photosensitizers Exhibiting Both Reactive Oxygen Species Generation and Luminescence

The use of two-photon absorption (TPA) for such applications as microscopy, imaging, and photodynamic therapy (PDT) offers several advantages over the usual one-photon excitation. This creates a need for photosensitizers that exhibit both strong two-photon absorption and the highly efficient generation of reactive oxygen species (ROS), as well as, ideally, bright luminescence. This review focuses on different strategies utilized to improve the TPA properties of various multi-photon absorbing species that have the required photophysical properties. Along with well-known families of photosensitizers, including porphyrins, we also describe other promising organic and organometallic structures and more complex systems involving organic and inorganic nanoparticles. We concentrate on the published studies that provide two-photon absorption cross-section values and the singlet oxygen (or other ROS) and luminescence quantum yields, which are crucial for potential use within PDT and diagnostics. We hope that this review will aid in the design and modification of novel TPA photosensitizers, which can help in exploiting the features of nonlinear absorption processes.

Multitherapeutic nanoplatform based on scintillating anthracene, silver@anthracene, and gold@anthracene nanoparticles for combined radiation and photodynamic cancer therapies

We have synthesized anthracene and metal@anthracene core-shell nanoparticles to combine radiation (RT) and photodynamic (PDT) therapies. Synthesis of anthracene nanoparticles in the presence of colloidal silver or gold reduced the nanoparticles hydrodynamic radius, caused core-shell nanostructures to grow, and led to plasmon-enhanced fluorescence. Singlet oxygen (¹O₂) generation was investigated by electron spin resonance (ESR) and fluorescence spectroscopies. In the presence of a porphyrin, anthracene nanoparticles and the core-shell nanoparticles acted as energy mediators and increased ¹O₂ generation under exposure to light, as evidenced by the ESR results. Fluorescence suppression experiments showed that the core-shell nanoparticles captured ¹O₂ at rates higher than anthracene nanoparticles, suggesting that overall production of ¹O₂ (¹O₂ captured by spin-trap + ¹O₂ captured by surface anthracene molecules) was higher for the core-shell nanoparticles. Moreover, the Ag@anthracene nanoparticles stood out as a new and more sensitive fluorescent probe for ¹O₂. During irradiation with X-rays, both anthracene and Ag@anthracene nanoparticles trapped ¹O₂; subsequently, they afforded sustained release of the trapped ¹O₂ for up 12 days after irradiation. This could be an interesting strategy to extend the radiation therapy treatment after the irradiation sessions. Furthermore, the presence of the metallic nanoparticle in the core of the core-shell nanostructure increased interaction with X-rays, raising the radiation dose around the nanoparticle. Therefore, metal@anthracene nanostructures may allow combination of cancer treatments by different approaches depending on the adopted nanoparticle configuration.

Singlet Oxygen Generation from a Water-Soluble Hypervalent Iodine(V) Reagent AIBX and H2O2: An Access to Artemisinin

Herein, we report an efficient method for the chemical generation of ¹O₂ by treatment of H₂O₂ with AIBX, a highly water-soluble, bench-stable, recyclable hypervalent iodine(V) reagent developed by our group. The generation of ¹O₂ was confirmed by the following results: (1) capture of ¹O₂ with the sodium salt of anthracene-9,10-bis(ethanesulfonate) produced the corresponding endoperoxide and (2) TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) produced by the oxidation of 2,2,6,6-tetramethylpiperidine with ¹O₂ generated using the AIBX/H₂O₂ system was detected by electron spin resonance spectroscopy. To illustrate the potential utility of this method for organic synthesis, we used the AIBX/H₂O₂ system to perform typical reactions of ¹O₂: [2 + 2]/[4 + 2] cycloadditions, Schenck ene reactions, and heteroatom oxidation reactions, which afforded the corresponding products in high yields. Moreover, we used the method to synthesize the antimalarial drug artemisinin. Finally, we demonstrated that AIBX could be regenerated after the reaction by means of a workup involving extraction and removal of water to obtain a precursor of AIBX, which could then be re-oxidized.

Environmental risk of nanomaterials and nanoparticles and EPR technique as an effective tool to study them-a review

Nanotechnologies and different types of nanomaterials belong in present day to intensively studied materials due to their unique properties and diverse potential applications in, e.g., electronics, medicine, or display technologies. Together with the investigation of their desired beneficial properties, a need to investigate and evaluate their influence on the environment and possible harmful effects towards living organisms is growing. This review summarizes possible toxic effects of nanomaterials on environment and living organisms, focusing on the possible bioaccumulation in organisms, toxicity, and its mechanisms. The main goal of this review is to refer to potential environmental risks rising from the use of nanomaterials and the necessity to deal with the possible toxic effects considering the growing interest in the wide-scale utilization of these materials. Electron paramagnetic resonance spectroscopy as the only analytical technique capable of detecting radical species enables detection, quantification, and monitoring of the generation of short-lived radicals often coupled with toxic effects of nanomaterials, which makes it an important method in the process of nanotoxicity mechanism determination.

Glyceraldehyde-derived advanced glycation end-products having pyrrolopyridinium-based crosslinks

Reducing sugars and reactive aldehydes, such as glyceraldehyde, non-enzymatically react with amino or guanidino groups of proteins to form advanced glycation end-products (AGEs) by the Maillard reaction that involves Schiff base formation followed by Amadori rearrangement. AGEs are found relatively in abundance in the human eye and to accumulate at a higher rate in diseases that impair vision such as cataract, diabetic retinopathy or age-related macular degeneration. We identified two novel AGEs of pyrrolopyridinium lysine dimer derived from glyceraldehyde, PPG1 and PPG2, in the Maillard reaction of N ^α-acetyl-l-lysine with glyceraldehyde under physiological conditions. Having fluorophores similar to that of vesperlysine A, which was isolated from the human lens, PPGs were found to act as photosensitizers producing singlet oxygen in response to blue light irradiation. Moreover, PPG2 interacts with receptor for AGE (RAGE) in vitro with a higher binding affinity than GLAP, a well-known ligand of the receptor. We also proposed a pathway to form PPGs and discussed how they would be formed in vitro. As glyceraldehyde-derived AGEs have been studied extensively in connection with various hyperglycemia-related diseases, further studies will be required to find PPGs in vivo such as in the lens or other tissues.

Antimicrobial photodynamic activity of gallium-substituted haemoglobin on silver nanoparticles

Methicillin-resistant Staphylococcus aureus (MRSA), a major scourge in skin and soft-tissue infections, expresses surface-bound haemoprotein receptors that can be exploited for the targeted delivery of photosensitizers. We have developed a nanosized agent for targeted antimicrobial photodynamic therapy (aPDT), comprised of GaPpIX (a hemin analog with potent photosensitizer activity) encapsulated in haemoglobin (GaHb), mounted on 10 nm Ag nanoparticles (AgNPs). The average GaHb-AgNP contains 28 GaPpIX units stabilized by Hb αβ-dimer units. Eradication (>6-log reduction) of S. aureus and MRSA can be achieved by a 10-second exposure to 405 nm irradiation from a light-emitting diode (LED) array (140 mW cm-2), with GaHb-AgNP loadings as low as 5.6 μg mL-1 for S. aureus and 16.6 μg mL-1 for MRSA, corresponding to nanomolar levels of GaPpIX. This reduction in bacterial count is several orders of magnitude greater than that of GaHb or free GaPpIX on a per mole basis. The GaHb-AgNP platform is also effective against persister MRSA and intracellular MRSA, and can provide comparable levels of aPDT with a 15-minute irradiation by an inexpensive compact fluorescent lightbulb. Collateral phototoxicity to keratinocytes (HaCaT cells) is low at the GaHb-AgNP concentrations and fluences used for aPDT. GaHb adsorbed on 10 nm AgNPs is much more potent than that on 40 nm AgNPs or 10 nm AuNPs, indicating that both size and plasmon-resonant coupling are important factors for enhanced aPDT. Electron microscopy analysis reveals that GaHb-AgNPs are not readily internalized by S. aureus but remain attached to the bacterial cell wall, the likely target of photo-oxidative damage.

Reactive Oxygen Species Penetrate Persister Cell Membranes of Escherichia coli for Effective Cell Killing

Persister cells are difficult to eliminate because they are tolerant to antibiotic stress. In the present study, using artificially induced Escherichia coli persister cells, we found that reactive oxygen species (ROS) have greater effects on persister cells than on exponential cells. Thus, we examined which types of ROS could effectively eliminate persister cells and determined the mechanisms underlying the effects of these ROS. Ultraviolet (UV) light irradiation can kill persister cells, and bacterial viability is markedly increased under UV shielding. UV induces the production of ROS, which kill bacteria by moving toward the shielded area. Electron spin resonance-based analysis confirmed that hydroxyl radicals are produced by UV irradiation, although singlet oxygen is not produced. These results clearly revealed that ROS sterilizes persister cells more effectively compared to the sterilization of exponential cells (^**p < 0.01). These ROS do not injure the bacterial cell wall but rather invade the cell, followed by cell killing. Additionally, the sterilization effect on persister cells was increased by exposure to oxygen plasma during UV irradiation. However, vapor conditions decreased persister cell sterilization by reducing the levels of hydroxyl radicals. We also verified the effect of ROS against bacteria in biofilms that are more resistant than planktonic cells. Although UV alone could not completely sterilize the biofilm bacteria, UV with ROS achieved complete sterilization. Our results demonstrate that persister cells strongly resist the effects of antibiotics and starvation stress but are less able to withstand exposure to ROS. It was shown that ROS does not affect the cell membrane but penetrates it and acts internally to kill persister cells. In particular, it was clarified that the hydroxy radical is an effective sterilizer to kill persister cells.

Absolute structure and anti-oxidative activity of chaetochiversin C isolated from fungal strain Neocosmospora sp. FKI-7792 by physicochemical screening

A new chaetochiversin analog, designated chaetochiversin C (1), was discovered from a cultured broth of fungal strain FKI-7792 by physicochemical screening. This strain was identified as a member of genus Neocosmospora based on morphology and DNA barcoding. The partially relative configuration of 1 was determined by ¹³C-NMR chemical shifts of the acetonide analog of 1. The absolute configuration was determined using an advanced Mosher's method. Compound 1 was assessed for anti-tumor, anti-microbial, and anti-malarial activities, and its ability to scavenge or quench reactive oxygen species (ROS), such as superoxide anion radicals, hydroxy radicals and singlet oxygen (¹O₂). Compound 1 showed a quenching effect on ¹O₂.

Development of a new photosafety test method based on singlet oxygen generation detected using electron spin resonance

Photosafety evaluations of chemicals used in consumer products, such as pharmaceuticals and cosmetics, are very important. Currently, two non-animal tests for photosafety evaluations, the in vitro 3T3 neutral red uptake phototoxicity test (NRU PT) and the reactive oxygen species (ROS) assay, are used to detect photoreactive chemicals. However, these two tests are difficult to apply to hydrophobic chemicals. In the present study, we attempted to develop a new photosafety test method, named the electron spin resonance-based photosafety test (ESR-PT), that would be applicable even to hydrophobic chemicals based on the detection of singlet oxygen generation after irradiation using ESR spectroscopy with 4-hydroxy-2,2,6,6-tetramethyl-piperidine as a spin trap reagent. To achieve a quantitative evaluation, the singlet oxygen formation (SOF) value, which can be calculated as the increment in relative intensity after irradiation of the test mixture normalized by the increment in relative intensity after irradiation of the vehicle control solution, was calculated. The performance of the ESR-PT was evaluated by testing all the proficiency chemicals of the ROS assay plus additional chemicals, including hydrophobic chemicals and chemicals that tested false negative in the 3T3-NRU PT and ROS assay. SOF values were successfully calculated for all the chemicals tested including the hydrophobic chemicals, and the accuracy of the ESR-PT using a tentative cutoff value of 2.8 against the photosafety information was 100%. Therefore, the SOF value could be an effective parameter for photosafety evaluations, suggesting that the newly developed ESR-PT is a promising non-animal test applicable even to hydrophobic chemicals.

Anti-inflammatory effect of photodynamic therapy using guaiazulene and red lasers on peripheral blood mononuclear cells

INTRODUCTION

Photodynamic therapy improves oral mucositis treatment. The reactive oxygen species (ROS) generated from this reaction could contribute to an anti-inflammatory effect by suppressing inflammatory cells.

OBJECTIVE

To evaluate the anti-inflammatory effect of photodynamic therapy using guaiazulene and a red laser in peripheral blood mononuclear cells (PBMCs).

METHODS

Guaiazulene solutions (1, 2, 5, 25, 35, and 100 μM in 99.8 % methanol) were irradiated with red laser light (625 nm, 146.2 mW/cm²) in continuous mode at 0, 4, and 8 J/cm² in black 96-well plates. ROS were measured using spin trapping technique with electron spin resonance (ESR) spectroscopy and fluorescence. The two highest concentrations were tested using cell viability (PrestoBlue®) and anti-inflammation (RANTES and PGE₂ ELISA) assay kits. Kruskal-Wallis and Dunn Bonferroni tests were used for statistical analyses with significant differences at p-value < 0.05.

RESULTS

Guaiazulene solutions between 2 and 5 μM exposed to red laser light at 4-8 J/cm² generated significantly more singlet oxygen compared to the no guaiazulene group (p < 0.01) and reduced RANTES and PGE₂ levels in TNF-α-inflamed peripheral blood mononuclear cells without affecting cell viability.

CONCLUSION

Photodynamic activation of guaiazulene generated singlet oxygen and suppressed inflammatory markers in PBMCs.

Highly Virulent Bactericidal Effects of Curcumin-Based μ-Cages Fabricated by Two-Photon Polymerization

A new antibacterial strategy is reported based on two-photon fabrication of three-dimensional curcumin-embedded μ-cages. Such devices were designed to entrap and kill Staphylococcus aureus bacteria upon visible light irradiation. The proposed concept mainly relies on the pivotal role of curcumin, which is sequentially used as a two-photon active free radical initiator and as a photogenerator of reactive oxygen species within the cage μ-volumes. We show that these μ-cages exhibit extremely high antimicrobial properties, leading to 95% bacteria mortality after only 10 min visible irradiation. A preconcentration mechanism of photogenerated oxygen species is proposed to account for this highly performing bactericidal effect whose virulence can be strikingly switched on by increasing the light exposure time from 5 to 10 min.

Oral mucosal irritation potential of antimicrobial chemotherapy involving hydrogen peroxide photolysis with high-power laser irradiation for the treatment of periodontitis

In the present study, we assessed the oral mucosal irritation potential of antimicrobial chemotherapy involving hydrogen peroxide (H₂O₂) photolysis with a 405-nm laser device at an output power of ≥100 mW in hamsters. Twenty-four cheek pouches from 12 male Syrian hamsters received 7-min treatment with pure water (PW), 3% H₂O₂, laser irradiation of PW at 100 mW, laser irradiation of 3% H₂O₂ at 100 mW, laser irradiation of PW at 200 mW, or laser irradiation of 3% H₂O₂ at 200 mW (n = 4 each). The diameter of the irradiation area was set at 3 mm; accordingly, the calculated irradiances (optical power densities) of the 100- and 200-mW laser lights were approximately 1400 and 2800 mW/cm², respectively. In addition, 12 cheek pouches from six animals received laser irradiation of 3% H₂O₂ at 100 mW for 1, 3, or 5 min (n = 4 each). Each treatment was repeated three times at 1-h intervals. Macroscopic and histological changes were evaluated 24 h after the last treatment. In addition, in vitro bactericidal activity of the treatment against periodontal pathogens was evaluated. We found that 405-nm laser irradiation of 3% H₂O₂ caused moderate to severe oral mucosal irritation when performed at powers of 100 and 200 mW for ≥3 min, while the same treatment performed at 100 mW for 1 min resulted in mild irritation. Moreover, 1-min H₂O₂ photolysis at 100 mW caused a >4-log decrease in viable bacterial counts. These findings suggest that 1-min H₂O₂ photolysis, which can effectively kill periodontal pathogens, may be acceptable when a 405-nm laser device is used at 100 mW. However, use of the laser at a lower power would be preferable for the prevention of unnecessary oral mucosal irritation.

Function of Epirubicin-Conjugated Polymeric Micelles in Sonodynamic Therapy

The combinatory use of high-intensity focused ultrasound (HIFU) and epirubicin (EPI)-conjugated polymeric micellar nanoparticles (NC-6300) is thought to be a less invasive and more efficient method of cancer therapy. To investigate the mechanism underlying the combination effect, we examined the effect of trigger-pulsed HIFU (TP-HIFU) and NC-6300 from the perspective of reactive oxygen species (ROS) generation, which is considered the primary function of sonodynamic therapy (SDT), and changes in drug characteristics. TP-HIFU is an effective sequence for generating hydroxyl radicals to kill cancer cells. EPI was susceptible to degradation by TP-HIFU through the production of hydroxyl radicals. In contrast, EPI degradation of NC-6300 was suppressed by the hydrophilic shell of the micelles. NC-6300 also exhibited a sonosensitizer function, which promoted the generation of superoxide anions by TP-HIFU irradiation. The amount of ROS produced by TP-HIFU reached a level that caused structural changes to the cellular membrane. In conclusion, drug-conjugated micellar nanoparticles are more desirable for SDT because of accelerated ROS production and drug protection from ROS. Furthermore, a combination of NC-6300 and TP-HIFU is useful for minimally invasive cancer therapy with cooperative effects of HIFU-derived features, antitumor activity of EPI, and increased ROS generation to cause damage to cancer cells.

Measurement of OH* Generation by Pulverized Minerals Using Electron Spin Resonance Spectroscopy and Implications for the Reactivity of Planetary Regolith

Mineral analogs to silicate phases common to planetary regolith, including olivine; the pyroxenes augite and diopside; the plagioclase feldspars labradorite, bytownite, and albite; the Johnson Space Center-1A lunar regolith simulant; as well as quartz (used as a reference), were subjected to mechanical pulverization by laboratory milling for times ranging from 5 to 45 min. Pulverized minerals were then incubated in an aqueous solution containing the free radical spin trapping compound 5,5-Dimethyl-1-Pyrroline-N-Oxide for times ranging from 5 to 30 min. These slurries were then analyzed by Electron Paramagnetic Resonance spectroscopy to quantify the amount of hydroxyl radical (the neutral charge form of the hydroxide ion, denoted as OH*) formed in solution. We find that all tested materials generate an Electron Paramagnetic Resonance spectrum indicating the formation of OH* with concentrations ranging between 0.1 and 1.5 μM. We also find that, in general, mineral pulverization time is inversely correlated to OH* generation, while OH* generation is positively correlated to mineral fluid incubation time for phases that have iron in their nominal chemical formulae, suggesting the possible action of Fenton reaction as a cofactor in increasing the reactivity of these phases. Our results add to a body of literature that indicates that the finely comminuted minerals and rocks present in planetary regolith are capable of generating highly reactive and highly oxidizing radical species in solution. The results provide the foundation for further in vitro and in vivo toxicological studies to evaluate the possible health risks that future explorers visiting the surfaces of planetary bodies may face from these reactive regolith materials.

Rapid Uptake and Photodynamic Inactivation of Staphylococci by Ga(III)-Protoporphyrin IX

Antimicrobial photodynamic therapy (aPDT) is a promising method for the topical treatment of drug-resistant staphylococcal infections and can be further improved by identifying mechanisms that increase the specificity of photosensitizer uptake by bacteria. Here we show that Ga(III)-protoporphyrin IX chloride (Ga-PpIX), a fluorescent hemin analog with previously undisclosed photosensitizing properties, can be taken up within seconds by Staphylococcus aureus including multidrug-resistant strains such as MRSA. The uptake of Ga-PpIX by staphylococci is likely diffusion-limited and is attributed to the expression of high-affinity cell-surface hemin receptors (CSHRs), namely iron-regulated surface determinant (Isd) proteins. A structure-activity study reveals the ionic character of both the heme center and propionyl groups to be important for uptake specificity. Ga-PpIX was evaluated as a photosensitizer against S. aureus and several clinical isolates of MRSA using a visible light source, with antimicrobial activity at 0.03 μM with 10 s of irradiation by a 405 nm diode array (1.4 J/cm²); antimicrobial activity could also be achieved within minutes using a compact fluorescent lightbulb. GaPpIX was not only many times more potent than PpIX, a standard photosensitizer featured in clinical aPDI, but also demonstrated low cytotoxicity against HEK293 cells and human keratinocytes. Ga-PpIX uptake was screened against a diverse panel of bacterial pathogens using a fluorescence-based imaging assay, which revealed rapid uptake by several Gram-positive species known to express CSHRs, suggesting future candidates for targeted aPDT.

Mechanisms of the pH- and Oxygen-Dependent Oxidation Activities of Artesunate

Artemisinin was discovered in 1971 as a constituent of the wormwood genus plant (Artemisia annua). This plant has been used as an herbal medicine to treat malaria since ancient times. The compound artemisinin has a sesquiterpene lactone bearing a peroxide group that offers its biological activity. In addition to anti-malarial activity, artemisinin derivatives have been reported to exert antitumor activity in cancer cells, and have attracted attention as potential anti-cancer drugs. Mechanisms that might explain the antitumor activities of artemisinin derivatives reportedly induction of apoptosis, angiogenesis inhibitory effects, inhibition of hypoxia-inducible factor-1α (HIF-1α) activation, and direct DNA injury. Reactive oxygen species (ROS) generation is involved in many cases. However, little is known about the mechanism of ROS formation from artemisinin derivatives and what types of ROS are produced. Therefore, we investigated the iron-induced ROS formation mechanism by using artesunate, a water-soluble artemisinin derivative, which is thought to be the underlying mechanism involved in artesunate-mediated cell death. The ROS generated by the coexistence of iron(II), artesunate, and molecular oxygen was a hydroxyl radical or hydroxyl radical-like ROS. Artesunate can reduce iron(III) to iron(II), which enables generation of ROS irrespective of the iron valence. We found that reduction from iron(III) to iron(II) was activated in the acidic rather than the neutral region and was proportional to the hydrogen ion concentration.

Mechanistic insights into plasmonic photocatalysts in utilizing visible light

The utilisation of sunlight as an abundant and renewable resource has motivated the development of sustainable photocatalysts that can collectively harvest visible light. However, the bottleneck in utilising the low energy photons has led to the discovery of plasmonic photocatalysts. The presence of noble metal on the plasmonic photocatalyst enables the harvesting of visible light through the unique characteristic features of the noble metal nanomaterials. Moreover, the formation of interfaces between noble metal particles and semiconductor materials further results in the formation of a Schottky junction. Thereby, the plasmonic characteristics have opened up a new direction in promoting an alternative path that can be of value to the society through sustainable development derived through energy available for all for diverse applications. We have comprehensively prepared this review to specifically focus on fundamental insights into plasmonic photocatalysts, various synthesis routes, together with their strengths and weaknesses, and the interaction of the plasmonic photocatalyst with pollutants as well as the role of active radical generation and identification. The review ends with a pinnacle insight into future perspectives regarding realistic applications of plasmonic photocatalysts.

"Dark" Singlet Oxygen and Electron Paramagnetic Resonance Spin Trapping as Convenient Tools to Assess Photolytic Drug Degradation

Forced degradation studies are an important tool for a systematic assessment of decomposition pathways and identification of reactive sites in active pharmaceutical ingredients (APIs). Two methodologies have been combined in order to provide a deeper understanding of singlet oxygen-related degradation pathways of APIs under light irradiation. First, we report that a "dark" singlet oxygen test enables the investigation of drug reactivity toward singlet oxygen independently of photolytic irradiation processes. Second, the photosensitizing properties of the API producing the singlet oxygen was proven and quantified by spin trapping and electron paramagnetic resonance analysis. A combination of these techniques is an interesting addition to the forced degradation portfolio as it can be used for (1) revealing unexpected degradation pathways of APIs due to singlet oxygen, (2) clarifying photolytic drug-drug interactions in fixed-dose combinations, and (3) synthesizing larger quantities of hardly accessible oxidative drug degradants.

A mechanistically-distinct approach to fluorescence visualization of singlet oxygen

¹O₂ oxidation of a non-fluorescent sulfoxide forms a fluorescent sulfone, with significant (>50-fold) increase in emission. The oxidation occurs via intramolecular oxygen atom transfer from a reactive persulfoxide intermediate. This represents a new, mechanistically-distinct approach to fluorescent ¹O₂ detection.

Singlet oxygen detection in biological systems: Uses and limitations

The study of singlet oxygen in biological systems is challenging in many ways. Singlet oxygen is a relatively unstable ephemeral molecule, and its properties make it highly reactive with many biomolecules, making it difficult to quantify accurately. Several methods have been developed to study this elusive molecule, but most studies thus far have focused on those conditions that produce relatively large amounts of singlet oxygen. However, the need for more sensitive methods is required as one begins to explore the levels of singlet oxygen required in signaling and regulatory processes. Here we discuss the various methods used in the study of singlet oxygen, and outline their uses and limitations.

Singlet Oxygen-Induced Membrane Disruption and Serpin-Protease Balance in Vacuolar-Driven Cell Death

Singlet oxygen plays a role in cellular stress either by providing direct toxicity or through signaling to initiate death programs. It was therefore of interest to examine cell death, as occurs in Arabidopsis, due to differentially localized singlet oxygen photosensitizers. The photosensitizers rose bengal (RB) and acridine orange (AO) were localized to the plasmalemma and vacuole, respectively. Their photoactivation led to cell death as measured by ion leakage. Cell death could be inhibited by the singlet oxygen scavenger histidine in treatments with AO but not with RB In the case of AO treatment, the vacuolar membrane was observed to disintegrate. Concomitantly, a complex was formed between a vacuolar cell-death protease, RESPONSIVE TO DESSICATION-21 and its cognate cytoplasmic protease inhibitor ATSERPIN1. In the case of RB treatment, the tonoplast remained intact and no complex was formed. Over-expression of AtSerpin1 repressed cell death, only under AO photodynamic treatment. Interestingly, acute water stress showed accumulation of singlet oxygen as determined by fluorescence of Singlet Oxygen Sensor Green, by electron paramagnetic resonance spectroscopy and the induction of singlet oxygen marker genes. Cell death by acute water stress was inhibited by the singlet oxygen scavenger histidine and was accompanied by vacuolar collapse and the appearance of serpin-protease complex. Over-expression of AtSerpin1 also attenuated cell death under this mode of cell stress. Thus, acute water stress damage shows parallels to vacuole-mediated cell death where the generation of singlet oxygen may play a role.

Characterization of radicals arising from oxidation of commercially-important essential oils

Inappropriate use of essential oils may entail risks to human health due to mutational events, carcinogenic effects, genetic damages and sensitizing effect caused by generation of reactive oxygen species. In order to detect radicals that are expected to form during their oxidation, we measured the electron spin resonance (ESR) spectra of a standard reaction mixture (I) containing 25 μM flavin mononucleotide, 0.018% several essential oils (or 0.015% geraniol), 1.9 M acetonitrile, 20 mM phosphate buffer (pH 7.4), 0.1 M α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) and 1.0 mM FeSO4(NH4)2SO4 irradiated with 436 nm visible light (7.8 J/cm(2)). The ESR peak heights of the standard reaction mixture (I) of the essential oils increased in the following order: tea tree > palmarosa >geranium > clary sage > petitgrain > lavender > bergamot > frankincense > ravintsara > ylang ylang > lemongrass > niaouli > eucalyptus globulus > peppermint. The ESR peak height of the standard reaction mixture (I) of geraniol, a main component of palmarosa, was comparable to the one of palmarosa (97 ± 19% of palmarosa). Furthermore, high performance liquid chromatography (HPLC)-ESR analyses of the standard reaction mixture (I) of palmarosa and geraniol gave the same peaks. The results suggest that the radicals formed in the standard reaction mixture (I) of palmarosa are derived from geraniol. HPLC-ESR-mass spectrometry analyses detected m/z 294 ions, 4-POBN/5-hydroxy-3-methyl-3-pentenyl radical adducts and m/z 320 ions, 4-POBN/C7O2H9 radical adducts in the standard reaction (I) of geraniol. The 5-hydroxy-3-methyl-3-pentenyl and C7O2H9 radicals may be implicated in the sensitizing effect of palmarosa.

Hybrid structures of polycationic aluminum phthalocyanines and quantum dots

Semiconductor nanocrystals (CdSe/ZnS quantum dots, QDs) were used as inorganic focusing antenna, allowing for the enhancement of fluorescence and photosensitizing activity of polycationic aluminum phthalocyanines (PCs). It was found that QDs form stable complexes with PCs in aqueous solutions due to electrostatic interactions. In such hybrid complexes, we observed highly efficient nonradiative energy transfer from QD to PC, leading to a sharp increase in the effective absorption cross section of PC in the absorption bands of the CdSe/ZnS quantum dots. When hybrid complexes are excited within these bands, the intensity of PC fluorescence and the rate of photosensitized singlet oxygen generation increases significantly (up to 500 and 350%, correspondingly) compared to free PC at the same concentration. The observed effect is of interest for modeling primary stages of photosynthesis and increasing photosensitizing activity of dyes used in photodynamic therapy.

Cytoprotective effects of grape seed extract on human gingival fibroblasts in relation to its antioxidant potential

Cytoprotective effects of short-term treatment with grape seed extract (GSE) upon human gingival fibroblasts (hGFs) were evaluated in relation to its antioxidant properties and compared with those of a water-soluble analog of vitamin E: trolox (Tx). GSE and Tx showed comparable antioxidant potential in vitro against di(phenyl)-(2,4,6-trinitrophenyl)iminoazanium (DPPH; a stable radical), hydroxyl radical (^•OH), singlet oxygen (¹O₂), and hydrogen peroxide (H₂O₂). Pretreatment or concomitant treatment with GSE for 1 min protected hGFs from oxidative stressors, including H₂O₂, acid-electrolyzed water (AEW), and ¹O₂, and attenuated the intracellular formation of reactive oxygen species induced by H₂O₂ and AEW. Tx also reduced the H₂O₂- and AEW-induced intracellular formation of reactive oxygen species, but showed no cytoprotective effects on hGFs exposed to H₂O₂, AEW, or ¹O₂. These results suggest that the cytoprotective effects of GSE are likely exerted independently of its antioxidant potential.

Enhanced singlet oxygen production by photodynamic therapy and a novel method for its intracellular measurement

The generation of singlet oxygen (SO) in the presence of specific photosensitizers (PSs) or semiconductor quantum dots (QDs) and its application in photodynamic therapy (PDT) is of great interest to develop cancer therapies with no need of surgery, chemotherapy, and/or radiotherapy. This work was focused on the identification of the main factors leading to the enhancement of SO production using Rose Bengal (RB), and Methylene Blue (MB) as PS species in organic and aqueous mediums. Subsequently, the capacity of zinc oxide (ZnO), zinc sulfide (ZnS), and ZnO/ZnS core-shell QDs as well as manganese (Mn(+2)) doped ZnO and ZnS nanoparticles (NPs) as potential PS was also investigated. Many variable parameters such as type of quencher, PSs, NPs, as well as its different concentrations, light source, excitation wavelength, reaction time, distance from light source, and nature of solvent were used. The degradation kinetics of the quenchers generated by SO species and the corresponding quantum yields were determined by monitoring the photo-oxidation of the chemical quencher and measuring its disappearance by fluorometry and spectrophotometry in the presence of NPs. Small intracellular changes of SO induced by these metal Zn (zinc) NPs and PDT could execute and accelerate deadly programs in these leukemic cells, providing in this way an innovative modality of treatment. In order to perform further more specific in vitro cytotoxic studies on B-chronic lymphocytic leukemia cells exposed to Zn NPs and PDT, we needed first to measure and ascertain those possible intracellular SO variations generated by this type of treatment; for this purpose, we have also developed and tested a novel method first described by us.

Potentiation of photoinactivation of Gram-positive and Gram-negative bacteria mediated by six phenothiazinium dyes by addition of azide ion

Antimicrobial photodynamic inactivation (APDI) using phenothiazinium dyes is mediated by reactive oxygen species consisting of a combination of singlet oxygen (quenched by azide), hydroxyl radicals and other reactive oxygen species. We recently showed that addition of sodium azide paradoxically potentiated APDI of Gram-positive and Gram-negative bacteria using methylene blue as the photosensitizer, and this was due to electron transfer to the dye triplet state from azide anion, producing azidyl radical. Here we compare this effect using six different homologous phenothiazinium dyes: methylene blue, toluidine blue O, new methylene blue, dimethylmethylene blue, azure A, and azure B. We found both significant potentiation (up to 2 logs) and also significant inhibition (>3 logs) of killing by adding 10 mM azide depending on Gram classification, washing the dye from the cells, and dye structure. Killing of E. coli was potentiated with all 6 dyes after a wash, while S. aureus killing was only potentiated by MB and TBO with a wash and DMMB with no wash. More lipophilic dyes (higher log P value, such as DMMB) were more likely to show potentiation. We conclude that the Type I photochemical mechanism (potentiation with azide) likely depends on the microenvironment, i.e. higher binding of dye to bacteria. Bacterial dye-binding is thought to be higher with Gram-negative compared to Gram-positive bacteria, when unbound dye has been washed away, and with more lipophilic dyes.

Radical intermediates in photoinduced reactions on TiO2 (an EPR spin trapping study)

The radical intermediates formed upon UVA irradiation of titanium dioxide suspensions in aqueous and non-aqueous environments were investigated applying the EPR spin trapping technique. The results showed that the generation of reactive species and their consecutive reactions are influenced by the solvent properties (e.g., polarity, solubility of molecular oxygen, rate constant for the reaction of hydroxyl radicals with the solvent). The formation of hydroxyl radicals, evidenced as the corresponding spin-adducts, dominated in the irradiated TiO₂ aqueous suspensions. The addition of ¹⁷O-enriched water caused changes in the EPR spectra reflecting the interaction of an unpaired electron with the ¹⁷O nucleus. The photoexcitation of TiO₂ in non-aqueous solvents (dimethylsulfoxide, acetonitrile, methanol and ethanol) in the presence of 5,5-dimethyl-1-pyrroline N-oxide spin trap displayed a stabilization of the superoxide radical anions generated via electron transfer reaction to molecular oxygen, and various oxygen- and carbon-centered radicals from the solvents were generated. The character and origin of the carbon-centered spin-adducts was confirmed using nitroso spin trapping agents.

Detection and identification of 1-methylethyl and methyl radicals generated by irradiating tea tree (Melaleuca alternifolia) oil with visible light (436 nm) in the presence of flavin mononucleotide and ferrous ion

Here, we determined the electron spin resonance (ESR) spectra of standard reaction mixtures (I) containing 25 μM flavin mononucleotide (FMN), 0.018% tea tree (Melaleuca alternifolia) oil, 1.9 M acetonitrile, 20 mM phosphate buffer (pH 7.4), 0.1 M α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN), and 1.0 mM FeSO₄(NH₄)₂SO₄ irradiated with 436 nm visible light (7.8 J/cm²). Prominent ESR signals (αN = 1.58 mT and αHβ = 0.26 mT) were detected, suggesting that free radicals form in the standard reaction. In order to know whether singlet oxygen (¹O₂) is involved in the radical formation or not, ESR measurement was performed for the standard D₂O reaction mixture (I) which contained 25 μM FMN, 0.0036% tea tree oil, 1.9 M acetonitrile-d3, 20 mM phosphate buffer (pH 7.4), 0.1 M 4-POBN and 1.0 mM FeSO₄ in D₂O. The ESR peak height of the standard D₂O reaction increased to 169 ± 24% of the control. Thus, ¹O₂ seems to be involved in the formation of the radicals because D₂O increases the lifetime of singlet oxygen. High-performance liquid chromatography-ESR-mass spectrometry analyses detected 1-methylethyl and methyl radicals in the standard reaction. The radicals appear to form through the reaction of ferrous ion with α-terpinene endoperoxide (ascaridole), which generated from the reaction of α-terpinene with ¹O₂. The 1-methylethyl and methyl radicals may exert a pro-oxidant effect under these conditions.

Caffeic acid inhibits the formation of 7-carboxyheptyl radicals from oleic acid under flavin mononucleotide photosensitization by scavenging singlet oxygen and quenching the excited state of flavin mononucleotide

We examined the effects of caffeic acid (CA) and related compounds on 7-carboxyheptyl radical formation. This analysis was performed using a standard D₂O reaction mixture containing 4.3 mM oleic acid, 25 μM flavin mononucleotide (FMN), 160 mM phosphate buffer (pH 7.4), 10 mM cholic acid, 100 mM α-(4-pyridyl-1-oxide)-N-tert-butylnitrone, and 1 mM Fe(SO₄)₂(NH₄)₂ during irradiation with 7.8 J/cm² at 436 nm. 7-Carboxyheptyl radical formation was inhibited by CA, catechol, gallic acid, chlorogenic acid, ferulic acid, noradrenalin, 2-hydroxybenzoic acid, 3-hydroxybenzoic acid, and 4-hydroxybenzoic acid. Quinic acid, benzoic acid, and p-anisic acid had no effect on radical formation. These results suggest that a phenol moiety is essential for these inhibitory effects. The fluorescence intensity of FMN decreased by 69% ± 2% after CA addition, suggesting that CA quenches the singlet excited state of FMN. When 1 mM CA was added to a standard reaction mixture containing 25 μM FMN, 140 mM phosphate buffer (pH 7.4), and 10 mM 4-oxo-2,2,6,6-tetramethylpiperidine, the electron spin resonance signal of 4-oxo-2,2,6,6-tetramethylpiperidinooxy disappeared. This finding suggests that singlet oxygen was scavenged completely by CA. Therefore, CA appears to inhibit 7-carboxyheptyl radical formation by scavenging singlet oxygen and quenching the excited state of FMN.

Fused-ring derivatives of quinoxalines: spectroscopic characterization and photoinduced processes investigated by EPR spin trapping technique

10-Ethyl-7-oxo-7,10-dihydropyrido[2,3-f]quinoxaline derivatives, synthesized as promising biologically/photobiologically active compounds were characterized by UV/vis, FT-IR and fluorescent spectroscopy. Photoinduced processes of these derivatives were studied by EPR spectroscopy, monitoring in situ the generation of reactive intermediates upon UVA (λmax=365 nm) irradiation. The formation of reactive oxygen species and further oxygen- and carbon-centered radical intermediates was detected and possible reaction routes were suggested. To quantify the investigated processes, the quantum yields of the superoxide radical anion spin-adduct and 4-oxo-2,2,6,6-tetramethylpiperidine N-oxyl generation were determined, reflecting the activation of molecular oxygen by the excited state of the quinoxaline derivative.