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

Abstract

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

Lipid and lipoprotein oxidation: basic mechanisms and unresolved questions in vivo

Abstract

The kinetics and mechanistic aspects of the riboflavin-photosensitised oxidation of the topically administrable ophthalmic drugs Timolol (Tim) and Pindolol (Pin) were investigated in water–MeOH (9:1, v/v) solution employing light of wavelength > 400 nm. riboflavin, belonging to the vitamin B₂ complex, is a known human endogenous photosensitiser. The irradiation of riboflavin in the presence of ophthalmic drugs triggers a complex picture of competitive reactions which produces the photodegradation of both the drugs and the pigment itself. The mechanism was elucidated employing stationary photolysis, polarographic detection of dissolved oxygen, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. Ophthalmic drugs quench riboflavin-excited singlet and triplet states. From the quenching of excited triplet riboflavin, the semireduced form of the pigment is generated, through an electron transfer process from the drug, with the subsequent production of superoxide anion radical (O₂^?–) by reaction with dissolved molecular oxygen. Through the interaction of dissolved oxygen with excited triplet riboflavin, the species singlet oxygen (O₂(¹Δ_g)) is also generated to a lesser extent. Both O₂^?– and O₂(¹Δ_g) induce photodegradation of ophthalmic drugs, Tim being ~3-fold more easily photooxidisable than Pin, as estimated by oxygen consumption experiments.     

不同处理条件对介质阻挡放电低温等离子体杀菌效果及影响机理研究

【目的】研究不同处理条件下介质阻挡放电低温等离子体对单增李斯特菌和肠炎沙门氏菌的杀菌效果,以及抑菌活性物质含量随处理时间的变化。【方法】以单增李斯特菌和肠炎沙门氏菌为对象,研究不同电压、时间及氧气浓度对介质阻挡放电低温等离子体杀菌效果的影响,通过气态活性物质测定管测定O_3和NO_2浓度随处理时间的变化,利用荧光强度表征菌液中产生的活性物质浓度,采用荧光分光光度法测定·OH、H_2O_2和~1O_2随处理时间的变化。【结果】采用70 kV、150 s或80 kV、1_20 s能够完全杀灭肠炎沙门氏菌,而单增李斯特菌只在80kV、1_20s条件下才能被完全杀死。采用50%O_2+50%N_2的混合气体能够在90 s内完全杀死细菌。随处理时间的增加,两种菌体细胞内活性氧物质含量呈逐渐增加趋势。【结论】电压、时间及氧气浓度均能显著增强等离子体的杀菌效果,其杀菌作用主要与H_2O_2、·OH、1O_2等活性氧物质的含量有关,其中·OH和H_2O_2是介质阻挡放电等离子体杀菌的主要物质。     

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

Abstract

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

34S/32S and 18O/16O Fractionation During Sulfur Disproportionation by Desulfobulbus propionicus

In the present study, coupled stable sulfur and oxygen isotope fractionation during elemental sulfur disproportionation according to the overall reaction: 4H₂O + 4S^? → 3H₂S + SO₄ ^{2 ?} + 2H⁺, was experimentally investigated for the first time using a pure culture of the sulfate reducer Desulfobulbus propionicus at 35^?C. Bacterial disproportionation of elemental sulfur is an important process in the sulfur cycle of natural surface sediments and leads to the simultaneous formation of sulfide and sulfate. A dual-isotope approach considering both sulfur and oxygen isotope discrimination has been shown to be most effective in evaluating specific microbial reactions. The influence of iron- and manganese bearing-solids (Fe(II)CO₃, Fe(III)OOH, Mn(IV)O₂) acting in natural sediments as scavengers for hydrogen sulfide, was considered, too. Disproportionation of elemental sulfur was observed in the presence of iron solids at a cell-specific sulfur disproportionation rate of about 10^{? 9.5± 0.4} μ mol S^? cell^{? 1} h^{? 1}. No disproportionation, however, was observed with MnO₂. In the presence of iron solids, newly formed sulfate was enriched in ¹⁸ O compared to water by about +21‰ (≡ ? _H2O ), in agreement with a suggested oxygen isotope exchange via traces of intra- or extracellular sulfite that is formed as a disproportionation intermediate. Dissolved sulfate was also enriched in ³⁴S compared to elemental sulfur by up to +35%. Isotope fractionation by Desulfobulbus propionicusis highest for all disproportionating bacteria investigated, so far, and may impact on the development of isotope signals at the redox boundary of surface sediments.     

Metal-Enhanced Photosensitization of Singlet Oxygen (ME1O2) from Brominated Carbon Nanodots on Silver Nanoparticle Substrates

The deleterious effects of reactive oxygen species (ROS), including singlet oxygen (¹O₂), on biological systems have cultivated widespread interest in fields ranging from therapeutic techniques to sterilization materials. Researchers have, for example, sought to capitalize on the oxidative damage from singlet oxygen to treat tumors as well as to kill antibiotic resistant bacteria. To generate ¹O₂ in a controllable manner, photosensitizers are optimized to generate ¹O₂ from ground state oxygen (³O₂) when excited by light. When considering applications of photosensitization, favorable properties include high ¹O₂ yield, low synthetic complexity, and minimal cost. Previously, studies have shown that plasmonic nanoparticles are able to amplify the photosensitization of ¹O₂ from small molecule photosensitizers in a mechanism similar to metal-enhanced fluorescence (MEF), thereby improving yield. A recent study from our lab has demonstrated that brominated carbon nanodots, which are an inexpensive and simple-to-collect as a hydrocarbon combustion byproduct, generate reactive oxygen species that can be used for antimicrobial photodynamic inactivation of bacteria. Herein we investigate the combination of these advantageous properties. Using the turn-on fluorescent probe Singlet Oxygen Sensor Green™ to detect ¹O₂, we report the metal-enhanced photosensitization of ¹O₂ by brominated dots in silvered Quanta Plate™ wells. These results provide a promising direction for the potential optimization of carbon nanodot-based agents in light-activated antimicrobial materials.

     

A novel hydrogen peroxide sensor based on immobilization of haemoglobin on nano-TiO2/DTAB composite film

Abstract

The direct electron transfer of immobilized haemoglobin (Hb) on nano-TiO₂ and dodecyltrimethylammonium bromide (DTAB) film modified carbon paste electrode (CPE) and its application as a hydrogen peroxide (H₂O₂) biosensor were investigated. On nano-TiO₂/DTAB/Hb/CPE, Hb displayed a rapid electron transfer process with participation of one proton and with an electron transfer rate constant which estimated as 0.29 s^??1. Thus, the proposed biosensor exhibited a high sensitivity and excellent electrocatalytic activity for the reduction of H₂O₂. The catalytic reduction current of H₂O₂ was proportional to H₂O₂ concentration in the range of 0.2–4.0 mM with a detection limit of 0.07 mM. The apparent Michaelis–Menten constant (K_m^app) of the biosensor was calculated to be 0.127 mM, exhibiting a high enzymatic activity and affinity. This sensor for H₂O₂ can potentially be applied in determination of other reactive oxygen species as well.     

Distribution of Sprattus sprattus phalericus (Risso, 1827) and zooplankton near the Black Sea redoxcline

Understanding what environmental drivers influence marine predator–prey relationships can be key to managing and protecting ecosystems, especially in the face of future climate change risks. This is especially important in environments such as the Black Sea, where strong biogeochemical gradients can drive marine habitat partitioning and ecological interactions. We used underwater video recordings in the north-eastern Black Sea in November 2013 to observe the distribution and behaviour of the Black Sea sprat (Sprattus sprattus phalericus, Risso 1827) and its zooplankton prey. Video recordings have shown that the Black Sea sprat S. sprattus phalericus tolerates severely hypoxic waters near the redoxcline. The school was distributed in the 33–96 m layer [oxygen concentration (O₂) 277–84 μmol L⁻¹]. Some individuals were observed to leave the school and descended 20 m deeper for foraging on copepods in the 119–123 m layer (O₂ 12–10 μmol L⁻¹). Zooplankton appeared concentrated on the upper boundary of the suboxic zone (O₂ < 10 μmol L⁻¹). No zooplankton were observed below O₂ 6–7 μmol L⁻¹ (128 m). Understanding the ability of this species to tolerate low oxygen waters is crucial to predicting future responses to natural and anthropogenic changes in hypoxia.     

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

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

Aerial and Aquatic Respiration in the River Crab Potamonautes Warreni Calman with Notes on Gill Structure

Summary

The oxygen consumption rate (?O₂) for Potamonauteus warreni Calman (= Potamon warreni (Calman) kept in 25 °C water was 34,4 μmol 1^?1 O₂ kg^?1 and after 72 hours in 98% R.H. air the rate was 31,9 μmol 1^?1 O₂ kg^?1 min^?1. The ?O₂ values for each of the two groups are not significantly different (P > 0,05). The partial oxygen tension of pre-branchial (v = venous) haemolymph (P_vCO₂) is 15,3 mm Hg in water and 13,0 mm Hg in air); partial carbon dioxide tension of pre-branchial (v) haemolymph (P_vCO₂) is 13,2 mm Hg in water and 13,0 mm Hg in air); the total carbon dioxide concentration in pre-branchial (v) haemolymph (C_vCO₂) tot. is 12,3 mmol 1^?1 in air and 13,9 mmol 1^?1 in water) are not significantly different for the two groups (P > 0,05). The haemolymph pH and the lactate concentration for crabs in water was found to be 7,51 and 0,38 mmol 1^?1 respectively. No significant differences were found in pre-branchial haemolymph oxygen tension, carbon dioxide tension, total carbon dioxide content, haemolymph pH, lactate level, chloride concentration, P₅₀ and haemocyanin-oxygen cooperativity in control crabs kept in water, and experimental crabs held in air for 72 hours. The chloride concentration, (327,0 mmol 1^?1) for crabs kept in water does not differ from that of crabs held in air for 72 hours but is at least 15% higher than the sodium concentration (255 mmol 1^?1) for crabs kept in water. The gill surface area is 520 mm² g^?1 wet body mass; on average 9,2 gill platelets (lamellae) can be found on a gill length of one millimetre. Each lamella is spaced 60–70 μm apart, each with a thickness of 30–40 μm. It is concluded that P. warreni may be described as a truly amphibious fresh-water crab.     

Methemoglobin and the Activities of Catalase and Superoxide Dismutase in Nucleated Erythrocytes of Scorpaena porcus (Linnaeus, 1758) under Experimental Hypoxia (in vitro)

The effect of hypoxia on nucleated red blood cells of the black scorpionfish (Scorpaena porcus) was studied in vitro. Deep hypoxia (the oxygen concentration was less than 1 mg O₂ L^–1; the norm was 7–8 mg O₂ L^–1) led to the transition of a part of the hemoglobin molecules to the ferric state (methemoglobin). The maximum increase in the concentration of methemoglobin was 32%. The accumulation of methemoglobin in red blood cells was accompanied by an increase in the activity of catalase and superoxide dismutase and a decrease in the content of reactive oxygen species in the cytoplasm of cells. It was shown that the formation of methemoglobin did not cause damage to the cytoplasmic membranes of red blood cells. The percentage of red blood cell lysis in deoxygenated (less than 1.0 mg O₂ L^–1) suspensions quantitatively coincided with the control values.

     

Photooxidative stress‐induced and abundant small RNAs in Rhodobacter sphaeroides

Exposure to oxygen and light generates photooxidative stress by the bacteriochlorophyll a mediated formation of singlet oxygen (¹O₂) in Rhodobacter sphaeroides. Our study reports the genome‐wide search for small RNAs (sRNAs) involved in the regulatory response to ¹O₂. By using 454 pyrosequencing and Northern blot analysis, we identified 20 sRNAs from R. sphaeroides aerobic cultures or following treatment with ¹O₂ or superoxide (O^–₂). One sRNA was specifically induced by ¹O₂ and its expression depends on the extracytoplasmic function sigma factor RpoE. Two sRNAs induced by ¹O₂ and O^–₂ were cotranscribed with upstream genes preceded by promoters with target sequences for the alternative sigma factors RpoH_I and RpoH_II. The most abundant sRNA was processed in the presence of ¹O₂ but not by O^–₂. From this and a second sRNA a conserved 3′‐segment accumulated from a larger precursor. Absence of the RNA chaperone Hfq changed the half‐lives, abundance and processing of ¹O₂‐affected sRNAs. Orthologues of three sRNA genes are present in different alpha‐proteobacteria, but the majority was unique to R. sphaeroides or Rhodobacterales species. Our discovery that abundant sRNAs are affected by ¹O₂ exposure extends the knowledge on the role of sRNAs and Hfq in the regulatory response to oxidative stress.     

Scavenging of reactive oxygen species by N‐substituted indole‐2 and 3‐carboxamides

Indole‐2 and 3‐carboxamides (IDs) are proposed to be selective cyclooxygenase inhibitors. Since cyclooxygenase‐1 may be involved in reactive oxygen species (ROS) production, we hypothesize that these indole derivatives have antioxidative properties. We have employed chemiluminescence (CL) and electron spin resonance (ESR) spin trapping to examine this hypothesis. We report here the results of a study of reactivity of 10 selected indole derivatives towards ROS. The following generators of ROS were applied: potassium superoxide (KO₂) as a source of superoxide radicals (O₂^·?), the Fenton reaction (Co‐EDTA/H₂O₂) for hydroxyl radicals (HO^·), and a mixture of alkaline aqueous H₂O₂ and acetonitrile for singlet oxygen (¹O₂). Hydroxyl radicals were detected as 5,5‐dimethyl‐1‐pyrroline‐N‐oxide (DMPO) spin adduct, whereas 2,2,6,6‐tetramethyl‐piperidine (TEMP) was used as a detector of ¹O₂. Using the Fenton reaction, 0.5 mmol/L IDs were found to inhibit DMPO‐?H radical formation in the range 7–37%. Furthermore the tested compounds containing the thiazolyl group also inhibited the ¹O₂‐dependent TEMPO radical, generated in the acetonitrile + H₂O₂ system. About 20% inhibition was obtained in the presence of 0.5 mmol/L IDs. 1 mmol/L IDs caused an approximately 13–70% decrease in the CL sum from the O₂^·? generating system (1 mmol/L). The aim of this paper is to evaluate these indole derivatives as antioxidants and their abilities to scavenge ROS. Copyright © 2004 John Wiley & Sons, Ltd.     

Prothrombin and fibrinogen carbonylation: How that can affect the blood clotting

Objectives: The aim of the work was the development of a simple method for measuring the plasma prothrombin carbonylation and the study the impact of prothrombin and fibrinogen oxidation on the rate of plasma clotting.

Methods: A new method was based on the ability of prothrombin to be adsorbed by the barium sulfate. It consists of four steps: prothrombin mixing with the water suspension of BaSO₄; reaction of 2,4-dinitrophenylhydrazine with the BaSO₄-bound prothrombin; desorption of prothrombin-2,4-dinitrophenylhydrazone complex from BaSO₄ in an alkaline medium; neutralization and reading of the optical absorbance of the complex (λ?=?370?nm). The prothrombin/fibrinogen carbonylation and plasma clotting rate in vitro in the presence of reactive oxygen species (ROS)-generating agents (0.05–0.8?mM Fe²⁺/H₂O₂) were monitored.

Results: The plasma volume required for measurement of carbonylated prothrombin was 0.4?ml. High level of linearity and reproducibility was observed (r?=?0.9995, P?=?0.0005 – for the protein; r?=?0.9971, P?=?0.0029 – for carbonyls). In the intact rats, the concentration of blood plasma prothrombin was 0.355?±?0.009?mg/ml, and that of carbonyls was 4.94?±?0.09?nmol/mg.

Discussion: Prothrombin and plasma clotting rate was not affected by low concentrations of ROS (0.05–0.2?mM Fe²⁺/H₂O₂). The fibrinogen was susceptible to ROS-related effect over all the used range of concentration (0.05–0.8?mM Fe²⁺/H₂O₂). Carbonylation of fibrinogen did not affect the plasma clotting activity at low ROS concentration (0.05–0.2?mM Fe²⁺/H₂O₂), however it retarded the clotting at higher ROS (0.2–0.8?mM Fe²⁺/H₂O₂).     

Alternatives to the ‘water oxidation pathway’ of biological ozone formation

Recent studies have shown that ozone (O₃) is endogenously generated in living tissues, where it makes both positive and negative physiological contributions. A pathway for the formation of both O₃ and hydrogen peroxide (H₂O₂) was previously proposed, beginning with the antibody or amino acid-catalyzed oxidation of water by singlet oxygen (¹O₂₎ to form hydrogen trioxide (H₂O₃) as a key intermediate. A key pillar of this hypothesis is that some of the H₂O₂ molecules incorporate water-derived oxygen atoms. However, H₂O₃ decomposes extremely readily in water to form ¹O₂ and water, rather than O₃ and H₂O₂. This article highlights key literature indicating that the oxidation of organic molecules such as the amino acids methionine, tryptophan, histidine, and cysteine by ¹O₂ is involved in ozone formation. Based on this, an alternative hypothesis for ozone formation is developed involving a further reaction of singlet oxygen with various oxidized organic intermediates. H₂O₂ having water-derived oxygen atoms is subsequently formed during ozone decomposition in water by known reactions.     

Reactivity of hypotaurine and cysteine sulfinic acid toward carbonate radical anion and nitrogen dioxide as explored by the peroxidase activity of Cu,Zn superoxide dismutase and by pulse radiolysis

Abstract

Hypotaurine and cysteine sulfinic acid are known to be readily oxidized to the respective sulfonates, taurine and cysteic acid, by several oxidative agents that may be present in biological systems. In this work, the relevance of both the carbonate anion and nitrogen dioxide radicals in the oxidation of hypotaurine and cysteine sulfinic acid has been explored by the peroxidase activity of Cu,Zn superoxide dismutase (SOD) and by pulse radiolysis. The extent of sulfinate oxidation induced by the system SOD/H₂O₂ in the presence of bicarbonate (CO₃^?– generation), or nitrite (^?NO₂ generation) has been evaluated. Hypotaurine is efficiently oxidized by the carbonate radical anion generated by the peroxidase activity of Cu,Zn SOD. Pulse radiolysis studies have shown that the carbonate radical anion reacts with hypotaurine more rapidly (k = 1.1 × 10⁹ M^?1s^?1) than nitrogen dioxide (k = 1.6 × 10⁷ M^?1s^?1). Regarding cysteine sulfinic acid, it is less reactive with the carbonate radical anion (k = 5.5 × 10⁷ M^?1s^?1) than hypotaurine. It has also been observed that the one-electron transfer oxidation of both sulfinates by the radicals is accompanied by the generation of transient sulfonyl radicals (RSO₂^?). Considering that the carbonate radical anion could be formed in vivo at high level from bicarbonate, this radical can be included in the oxidants capable of performing the last metabolic step of taurine biosynthesis. Moreover, the protective effect exerted by hypotaurine and cysteine sulfinate on the carbonate radical anion-mediated tyrosine dimerization indicates that both sulfinates have scavenging activity towards the carbonate radical anion. However, the formation of transient reactive intermediates during sulfinate oxidation by carbonate anion and nitrogen dioxide radical may at the same time promote oxidative reactions.     

Quantitative generation of singlet (1Δg) oxygen from acidified aqueous peroxynitrite produced by the reaction of nitric oxide and superoxide anion

Simple acidification of aqueous alkaline peroxynitrite quantitatively generates singlet (¹Δ_g) molecular oxygen, detected and quantitated spectroscopically (1270 nm). This observation provides a chemical basis for physiological cytotoxicity of ONOO^? generated in the diffusion - controlled reaction of cellular NO^? and O. The experiments consist of (i) chemical generation of ONOO^? from NO^? gas and KO₂ powder in alkaline aqueous solution; (ii) absorption spectral identification of ONOO^? in the near-UV with maximum at 302 nm; (iii) spectroscopic identification of ¹O₂ by its emission band at 1200–1340 nm with maximum at 1275 nm; and (iv) quantitation of ¹O₂ generated in ONOO^?/H⁺ reaction by comparison of the chemiluminescence intensity at 1270 nm with that from H₂O₂/OCl^? reaction that generates ¹O₂ with unit efficiency at alkaline pH. ¹O₂ was generated with unit efficiency with respect to ONOO^? concentration by the ONOO^?/H⁺ reaction.     

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

Abstract

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

Photogeneration of singlet oxygen (1O2) and free radicals (Sen·-, O·-2) by tetra-brominated hypocrellin B derivative

To improve photodynamic activity of the parent hypocrellin B (HB), a tetra-brominated HB derivative (compound 1) was synthesized in high yield. Compared with HB, compound 1 has enhanced red absorption and high molar extinction coefficients. The photodynamic action of compound 1, especially the generation mechanism and efficiencies of active species (Sen^·-, O^·-₂ and ¹O₂) were studied using electron paramagnetic resonance (EPR) and spectrophotometric methods. In the deoxygenated DMSO solution of compound 1, the semiquinone anion radical of compound 1 is photogenerated via the self-electron transfer between the excited and ground state species. The presence of electron donor significantly promotes the reduction of compound 1. When oxygen is present, superoxide anion radical (O^·-₂) is formed via the electron transfer from Sens^·- to the ground state molecular oxygen. The efficiencies of Sens^·- and O^·-₂ generation by compound 1 are about three and two times as much as that of HB, respectively. Singlet oxygen (¹O₂) can be produced via the energy transfer from triplet compound 1 to ground state oxygen molecules. The quantum yield of singlet oxygen (¹O₂) is 0.54 in CHCl₃ similar to that of HB. Furthermore, it was found that the accumulation of Sens^·- would replace that of O^·-₂ or ¹O₂ with the depletion of oxygen in the sealed system.