Formation Of Reactive Radicals Research Articles

Sonolytic, photocatalytic and sonophotocatalytic degradation of malachite green in aqueous solutions

The degradation of malachite green (MG) in water by means of ultrasound irradiation and its combination with heterogeneous (TiO2) and homogeneous photocatalysis (photo-Fenton) was investigated. Emphasis was given on the effect of key operating conditions on MG conversion and mineralization rates and the elucidation of major reaction by-products. Eighty-kilohertz of ultrasound irradiation was provided by a horn-type sonicator, while a 9W lamp was used for UV-A irradiation. The extent of sonolytic degradation increased with increasing ultrasound power (in the range 75–135W) and decreasing initial concentration (in the range 2.5–12.5mgL−1), while the presence of TiO2 in the dark generally had little effect on degradation. Sonolysis under argon was substantially faster than under air, oxygen or helium leading to complete MG degradation after 120min at 10mgL−1 initial concentration and 135W ultrasound power. On the other hand, TiO2 photocatalysis or photo-Fenton led to complete MG degradation in 15–60min with the rate increasing with increasing catalyst loading (in the range 0.1–0.5gL−1 for TiO2 and 7–20mgL−1 for Fe3+) and also depending on the gas used. TiO2 sonophotocatalysis was always faster than the respective individual processes due to the enhanced formation of reactive radicals as well as the possible ultrasound-induced increase of the active surface area of the catalyst. For instance, the pseudo-first order rate constant for the sonophotocatalytic degradation at 0.5mgL−1 TiO2 under air was 136.7×10−3min−1 with the respective values for photocatalysis and sonolysis being 112.6×10−3 and 11.6×10−3min−1. Irrespective of the process employed, mineralization was slower than MG decomposition implying the formation of stable by-products accompanied by the release of nitrates in the solution. GC/MS analysis verified the identity of primary intermediates and a reaction pathway based on them was proposed. Depending on the conditions employed, ecotoxicity of MG to marine bacteria was partly or fully eliminated.

The influence of inorganic oxidants and metal ions on semiconductor sensitized photodegradation of 4-fluorophenol

Abstract This paper mainly focuses on the influence of inorganic oxidants and metal ions on the TiO 2 , ZnO sensitized photodegradation of 4-fluorophenol (4-FP) by UV-A light. TiO 2 -P25 is more efficient than ZnO. Among the oxidants periodate ion was found to be more efficient than others in improving the degradation of 4-FP via formation of reactive radicals. The effect of oxidants on the degradation of 4-FP was found to be in the order of IO 4 − > BrO 3 − > S 2 O 8 2− H 2 O 2 > ClO 3 − . The effect of metal ions on degradation of 4-FP was found to be in the order of Mg 2+ > Fe 3+ > Fe 2+ > Cu 2+ . The degradation of 4-FP follows pseudo -first order kinetics according to the Langmuir–Hinshelwood model. The photomineralisation has also been confirmed by COD, gas chromatography and fluoride ion measurements.

Free Radicals in the Photolysis and Radiolysis of Polymers: IV. Radicals in γ- and UV-Irradiated Wood and Lignin

Free radicals formed in the radiolysis and photolysis of wood and lignin were studied using X-band and D-band EPR measurements. It was found that singlet spectra at g ≈ 2, which appeared upon the low-temperature (77 K) γ- and UV irradiation of wood and lignin, or singlets detected in a posteffect on heating the irradiated samples belong to radicals having conjugated carbon–carbon bonds. Formyl radicals in γ-irradiated wood and peroxide radicals in γ- and UV-irradiated wood were detected for the first time using EPR spectra. The radiation-chemical and quantum yields of radical formation reactions were determined. In wood at 77 K, GR ≈ 3.2 1/100 eV and ϕR ≈ 2 × 10–3.

Degradation Study of MEA for PEMFCs under Low Humidity Conditions

The degradation study of the membrane electrode assembly (MEA) for proton exchange membrane fuel cells (PEMFCs) under low humidity conditions was conducted. The MEA degradation was observed during low humidity open-circuit voltage tests. To investigate the degradation mechanism, we conducted electron spin resonance studies of the degenerated MEA, and confirmed the formation of radicals in the catalyst layers. The MEA degradation mechanism that includes the radical formation reaction was verified. © 2004 The Electrochemical Society. All rights reserved.

Labile plasma iron in iron overload: redox activity and susceptibility to chelation

AbstractPlasma non-transferrin-bound-iron (NTBI) is believed to be responsible for catalyzing the formation of reactive radicals in the circulation of iron overloaded subjects, resulting in accumulation of oxidation products. We assessed the redox active component of NTBI in the plasma of healthy and β-thalassemic patients. The labile plasma iron (LPI) was determined with the fluorogenic dihydrorhodamine 123 by monitoring the generation of reactive radicals prompted by ascorbate but blocked by iron chelators. The assay was LPI specific since it was generated by physiologic concentrations of ascorbate, involved no sample manipulation, and was blocked by iron chelators that bind iron selectively. LPI, essentially absent from sera of healthy individuals, was present in those of β-thalassemia patients at levels (1-16 μM) that correlated significantly with those of NTBI measured as mobilizer-dependent chelatable iron or desferrioxamine chelatable iron. Oral treatment of patients with deferiprone (L1) raised plasma NTBI due to iron mobilization but did not lead to LPI appearance, indicating that L1-chelated iron in plasma was not redox active. Moreover, oral L1 treatment eliminated LPI in patients. The approach enabled the assessment of LPI susceptibility to in vivo or in vitro chelation and the potential of LPI to cause tissue damage, as found in iron overload conditions. (Blood. 2003;102:2670-2677)

Laccases and their occurrence in prokaryotes.

Laccases are copper-containing proteins that require O(2) to oxidize phenols, polyphenols, aromatic amines, and different non-phenolic substrates by one-electron transfer, resulting in the formation of reactive radicals. Although their specific physiological functions are not completely understood, there are several indications that laccases are involved in the morphogenesis of microorganisms (e.g., fungal spore development, melanization) and in the formation and/or degradation of complex organic substances such as lignin or humic matter. Owing to their high relative non-specific oxidation capacity, laccases are useful biocatalysts for diverse biotechnological applications. To date, laccases have been found only in eukaryotes (fungi, plants); however, databank searches and experimental data now provide evidence for their distribution in prokaryotes. This survey shows that laccase-like enzymes occur in many gram-negative and gram-positive bacteria. Corresponding genes have been found in prokaryotes that are thought to have branched off early during evolution, e.g., the extremely thermophilic Aquifex aeolicus and the archaeon Pyrobaculum aerophilum. Phylogenetically, the enzymes are members of the multi-copper protein family that have developed from small-sized prokaryotic azurins to eukaryotic plasma proteins.

Effects of ambient pressure on flame structure of CO/H2/N2 counterflow diffusion flame

A numerical study has been conducted for the understanding of flame structure with the fuel composition of 40 per cent CO, 30 per cent H2, 30 per cent N2 and an oxidizer composition of 79 per cent N2 and 21 per cent O2 in a counterflow diffusion flame. Numerical results are obtained for flames over wide-ranged conditions of axial velocity gradients and ambient pressures. Main concern is specially given to effects of axial velocity gradient and ambient pressure on flame structure. It is seen that the role of axial velocity gradient on combustion processes is globally opposite to that of ambient pressure, on the basis of the Damköhler number considering the balance of convection, diffusion, and chemical reaction. That is, chemical non-equilibrium effects become dominant with increasing axial velocity gradient, but are suppressed with increasing ambient pressure. Flame strength is globally weakened by the increase of axial velocity gradient but is augmented by the increase of ambient pressure. However, flame extinction is described better on the basis of only chemical reaction and in this case axial velocity gradient and ambient pressure play a similar role conceptually such that the increase of axial velocity gradient and ambient pressure cause the flame not to be extinguished and extend the extinction limit, respectively. It is suggested that a combustion process like flame extinction is mainly influenced by the competition between the radical formation reaction and the third-body recombination reaction. Copyright © 2001 John Wiley & Sons, Ltd.

Modelling of silicon hydride clustering in a low-pressure silaneplasma

A new silicon hydride clustering model was developed to study thenucleation of particles in a low-temperature silane plasma.The model contains neutral silanes, silylenes, silenes and silylradicals as well as silyl and silylene anions. Reaction rateswere estimated from available data. Simulations were carried outfor typical discharge parameters in a capacitive plasma. It was shownthat the main pathway leading to silicon hydride clustering wasgoverned by anion-neutral reactions. SiH2 radical insertion wasfound to be important only in the initial stages of clustering,whereas electron-induced dissociations were seen to lead todehydrogenation. Increased ion density (radiofrequency power density) leads to fasterclustering due to increased formation of reactive radicals.

Resonance Raman studies of the stoichiometric catalytic turnover of a substrate-stearoyl-acyl carrier protein delta(9) desaturase complex.

Resonance Raman spectroscopy has been used to study the effects of substrate binding (stearoyl-acyl carrier protein, 18:0-ACP) on the diferric centers of Ricinus communis 18:0-ACP Delta(9) desaturase. These studies show that complex formation produces changes in the frequencies of nu(s)(Fe-O-Fe) and nu(as)(Fe-O-Fe) consistent with a decrease in the Fe-O-Fe angle from approximately 123 degrees in the oxo-bridged diferric centers of the as-isolated enzyme to approximately 120 degrees in oxo-bridged diferric centers of the complex. Analysis of the shifts in nu(s)(Fe-O-Fe) and nu(as)(Fe-O-Fe) as a function of 18:0-ACP concentration also suggests that 4e(-)-reduced Delta9D containing two diferrous centers has a higher affinity for 18:0-ACP than resting Delta9D containing two diferric centers. Catalytic turnover of a stoichiometric complex of 18:0-ACP and Delta9D was used to investigate whether an O-atom from O(2) would be incorporated into a bridging position of the resultant mu-oxo-bridged diferric centers during the desaturation reaction. Upon formation of approximately 70% yield of 18:1-ACP product in the presence of (18)O(2), no incorporation of an (18)O atom into the mu-oxo bridge position was detected. The result with 18:0-ACP Delta(9) desaturase differs from that obtained during the tyrosyl radical formation reaction of the diiron enzyme ribonucleotide reductase R2 component, which proceeds with incorporation of an O-atom from O(2) into the mu-oxo bridge of the resting diferric site. The possible implications of these results for the O-O bond cleavage reaction and the nature of intermediates formed during Delta9D catalysis are discussed.

Increased vulnerability of human erythrocytes to hydroperoxide damage after exposure to cigarette smoke or 1-chloro-2,4-dinitrobenzene in vitro.

Glutathione depletion, a major effect of cigarette smoke on biological tissues exposed to high concentrations of smoke, substantially slowed the consumption of tert-butyl hydroperoxide (tBH) in human erythrocytes in vitro, as shown by electron spin resonance (ESR) analyses of the rate of disappearance of extracellular tBH. Glutathione depletion by the reagent 1-chloro-2,4-dinitrobenzene induced a structural alteration of intracellular hemoglobin by tBH, which was inferred from an increase in hydrophobicity of erythrocyte proteins. Protein hydrophobicity was analyzed with a new ESR assay comprising detection of an increased binding of both anionic and cationic amphiphilic paramagnetic probes in membrane-depleted hemolysates. An increased affinity of oxidant-damaged proteins for amphiphilic probes was also observed in myoglobin and in protein fractions of erythrocytes treated with tBH subsequent to hemolysis. Smoke exposure enhanced the formation of reactive free radicals from tBH by chelated iron and ascorbate. Reactive radical formation, as monitored by spin-trapping methods, was substantially prolonged in erythrocyte suspensions that had been exposed to cigarette smoke. The results of this study suggest that the susceptibility of cells to peroxide-mediated damage, including damage associated with iron-mediated free radical production, is increased after exposure to high concentrations of cigarette smoke.

FATIGUE AND FRACTURE IN POLYACETAL RESINS

Abstract— The fatigue behaviour of polyoxymethylene (POM) of different molar mass (6.1 × 104–1.5 × 105) was studied at stress levels from 35 to 50 MPa (about 55 and 75% of the yield strength, respectively). It was found that cyclic loadings promoted bond breaking of the polymeric chains which considerably reduced the molar mass both in the crack tip region of cracked specimens and in the bulk in the case of uncracked samples. This process probably included the formation of reactive radicals which subsequently favoured the formation of microvoids that weakened the material. The static fracture toughness of fatigued samples decreased on increasing the stress level, the process accelerating as the cyclic loads approached the yield strength. As a result of material modifications during the tests the low molar mass resins did not exhibit stable crack propagation.

Formation of the benzyl radical upon UV light irradiation of toluene in benzene at 4.2 K

A study on the formation of the benzyl radical by photolysis of toluene in a benzene host at 77 and 4.2 K is presented. The analysis of the dependence of the radical product yield upon irradiation light intensity proves that the radical formation reaction is unimolecular and biphotonic via the lowest excited triplet state of the toluene molecule.

Evaluation of the role of reactive oxygen species in the interactive toxicity of carbide-cobalt mixtures on macrophages in culture.

The lung toxicity of a carbide-cobalt mixture is more important than that of each individual component; the mechanism of this interaction is not understood. The capacity of cobalt metal particles alone and mixed with different carbides to generate hydroxyl radicals was examined with the deoxyribose assay. In a chemical system, cobalt ions and cobalt metal particles (Co) were found to catalyse the degradation of deoxyribose in the presence of hydrogen peroxide. Carbides were able to directly oxidize deoxyribose, but their respective activities did not support such a mechanism to explain the carbide-cobalt interactive toxicity, since there was no direct relationship between deoxyribose degradation ability and cytotoxicity toward macrophages. Tungsten, niobium, titanium and chromium carbides (interactive carbides) were only weak oxidants and conversely molybdenum, vanadium and silicon carbides (non-interactive carbides) were the most potent ones. The ability of cobalt metal to produce hydroxyl radicals in the presence of hydrogen peroxide was not increased by tungsten carbide. The role of reactive radical formation in the toxicity of these particles was further assessed in a macrophage culture model. Catalase (4000 U/ml), superoxide dismutase (300 U/ml), sodium azide (1 mM), sodium benzoate, mannitol, taurine and methionine (all 20 mM) were all unable to protect against the cytotoxic effects of cobalt ions and cobalt metal alone or mixed with tungsten carbide. In conclusion, no evidence was found that production of reactive oxygen species contributes to the elective toxicity of carbide-cobalt mixtures.

Reactive oxygen species and the central nervous system.

Radicals are species containing one or more unpaired electrons, such as nitric oxide (NO.). The oxygen radical superoxide (O2.-) and the nonradical hydrogen peroxide (H2O2) are produced during normal metabolism and perform several useful functions. Excessive production of O2.- and H2O2 can result in tissue damage, which often involves generation of highly reactive hydroxyl radical (.OH) and other oxidants in the presence of "catalytic" iron or copper ions. An important form of antioxidant defense is the storage and transport of iron and copper ions in forms that will not catalyze formation of reactive radicals. Tissue injury, e.g., by ischemia or trauma, can cause increased metal ion availability and accelerate free radical reactions. This may be especially important in the brain because areas of this organ are rich in iron and CSF cannot bind released iron ions. Oxidative stress on nervous tissue can produce damage by several interacting mechanisms, including increases in intracellular free Ca2+ and, possibly, release of excitatory amino acids. Recent suggestions that free radical reactions are involved in the neurotoxicity of aluminum and in damage to the substantia nigra in patients with Parkinson's disease are reviewed. Finally, the nature of antioxidants is discussed, it being suggested that antioxidant enzymes and chelators of transition metal ions may be more generally useful protective agents than chain-breaking antioxidants. Careful precautions must be used in the design of antioxidants for therapeutic use.

Pulse radiolysis studies on [Fe(CN)6]4−−BrO 3 − −CN− system in ethylene glycol — Water solutions

Abstract Formation of oxidizing and reducing radicals has been studied by pulse radiolysis of [Fe(CN)6]4––BrO 3 – –CN– system in ethylene glycol — water solvent mixture. Oxidizing ·BrO2 and BrO radicals formed by electron scavenging with ·BrO 3 – were identified and their reactions were investigated. The reaction of hydroxyl radicals with ethylene glycol leads to formation of reactive radicals with reducing properties and of compounds which reduce slowly in dark the ferricyanide formed in the reaction of ·BrO2 radical with ferrocyanide.

Oxidants and the central nervous system: some fundamental questions. Is oxidant damage relevant to Parkinson's disease, Alzheimer's disease, traumatic injury or stroke?

Radicals are species containing one or more unpaired electrons. The oxygen radical superoxide (O2-) and the non-radical oxidant hydrogen peroxide (H2O2) are produced during normal metabolism and perform several useful functions. Excessive production of O2- and H2O2 can result in tissue damage, which often involves generation of highly-reactive hydroxyl radical (.OH) and other oxidants in the presence of "catalytic" iron ions. A major form of antioxidant defence is the storage and transport of iron ions in forms that will not catalyze formation of reactive radicals. Tissue injury, eg. by ischaemia or trauma, can cause increased iron availability and accelerate free radical reactions. This may be especially important in the brain, since areas of this organ are rich in iron and cerebrospinal fluid cannot bind released iron ions. Oxidant stress upon nervous tissue can produce damage by several interacting mechanisms, including rises in intracellular free Ca2+ and, possibly, release of excitatory amino acids. Recent suggestions that iron-dependent free radical reactions are involved in the neurotoxicity of aluminium and in damage to the substantia nigra in Parkinson's disease are reviewed. Finally, the nature of antioxidants is discussed, it being suggested that antioxidant enzymes and chelators of iron ions may be more generally-useful protective agents than chain-breaking antioxidants.

Rapid formation kinetics of a long-lived electron-hole drop in Ge under pulsed excitation : J. E. Furneaux, J. P. Wolfe and C. D. Jeffries. Solid-St. Commun.20, 317 (1976)

The photodegradation of anthracene (Ant) and benz[a]anthracene (BaA) were studied in air-equilibrated hexane, methanol and pure water solutions in the absence and presence of phenolic compounds (4-methylcatechol, guaiacol and syringol). In the absence of phenolic compound, the pseudo first-order degradation rate constants of Ant and BaA increased with the polarity of solvent, demonstrating the influence of solvent effects on the photoreactivity. The differences can be attributed to factors such as the proton donating potential of the solvent, and the formation potentials of PAH radical cation and hydroxyl radical in solvent. In the presence of phenolic compound, Ant and BaA photodegradation rates were accelerated in hexane and methanol while inhibited in water solutions. Photodegradation kinetics of the phenolic compounds were also measured; and visible colored products were observed in all three solvents. The observed accelerations of Ant and BaA photodecay in the presence of the phenolic compounds can be explained by energy transfer from the phenolic compound to PAH, phenolic hydrogen abstraction by excited PAH and more reactive radical formation from the phenolic compound. However, the inhibition effects of Ant and BaA photodecay in water were more influenced by the quenching of photo-produced reactive oxygen species (ROS) by the phenolic compound and/or its photoproducts. The results indicate that the photodegradation of PAHs will be highly dependent on the aerosol water content. i.e., the chemical composition of the environmental aerosols.

The Charge Transfer Photochemistry of Coordination Complexes in Aqueous Solution

AbstractIrradiation of charge transfer absorption bands of coordination complexes can result in net oxidation reduction or ligand exchange reactions. The photoredox processes generally involve the formation of reactive radicals in the primary photochemical act. The identification of these primary radicals is essential to the specification of the primary photochemical act since radical reactions with substrate, other radicals, etc. can make the net products and yields appear vastly different from the nature of the primary act. For example, in a few documented cases, the metal ion fragment formed in a photoredox process has characteristics of a radical and can couple with ligand radicals giving rise to a net photochemical process which appears to be ligand exchange rather than photoredox. A variety of radicals, organic as well as inorganic, can be generated and studied in aqueous solution. Some information has also been obtained concerning the nature of the photochemical precursors to the radicals formed in photoredox processes.

Studies on the Radical Species in Solid Methylene Blue as Revealed from the ESR Spectra

Abstract The ESR signals of polycrystalline methylene blue (MB) salts have been investigated in respect to a) temperature effect, b) anion effect, c) influence of the addition of donor and acceptor and d) response to visible light irradiation, a) The signal intensity in general increases with the rising temperature, but a sharp difference in the ESR behavior has been found above and below room temperature. Response to the temperature change at low temperature is reversible and the apparent activation energy is ca. 0.03 eV (L-type behavior) whereas the high temperature behavior has a marked tendency of irreversibility and the activation energy is at least one order larger (H-type behavior), b) No essential anion effect has been observed on g-value, line shape and spin concentration for chloride, iodide and phosphate, c) The addition of trinitrobenzene and paraphenylenediamine in 10 : 1 to MB chloride also had no essential effect, d) Corresponding to the L-type and H-type behavior observed in a), visible light irradiation has been found to cause a decrease or an increase in signal intensity respectively at low and high temperature. From these findings it has been concluded that there are at least two kinds of defect (L-type and H-type) and that the formation of radical species occurs only on such defect sites. Some suggestions have been made on the nature of these two types of defects and on the molecular processes occurring at or near these defect sites. The radical formation reaction is most likely to be connected with dyedye interaction, D++D+\ightleftharpoonsD+++D.