Oxidation of hydroxylamine to nitrite as an assay for the combined presence of superoxide anions and hydroxyl radicals.

The pulse-radiolytic oxidation of hydroxylamine by either hydroxyl radicals (OH), superoxide anions (O₂^?), or a combination of both radicals was investigated. It was found that only OH radicals efficiently attack the substrate, while O₂^? is necessary for the subsequent formation of nitrite. Determination of the latter reaction thus allows the detection of the combined presence of both oxygen radical species.     

Pulse-radiolytic investigations of catechols and catecholamines II. Reactions of Tiron with oxygen radical species

Transient spectra and kinetic data of Tiron (1,2-dihydroxybenzene-3,5-disulphonic acid) are reported, obtained after pulse-radiolytic oxidation by hydroxyl radicals (°OH), superoxide anions (O₂^?) or a combination of both oxygen radicals. The rate constant with °OH radicals was determined at 1.0·10⁹ M^?1·s^?1. Contrary to a previous report (Greenstock, C.L. and Miller, R.W. (1975) Biochim. Biophys. Acta 396, 11–16), the rate constant with O₂^? of 1.0·10⁷ M^?1·s^?1 is lower by one order of magnitude; also the semiquinone absorbs at 300 nm rather than at 400 nm. The ratio of the rate constants with °OH and O₂^? of 100 again demonstrates that any oxidation reaction by the latter radical is unspecific due to the more efficient reaction of °OH radicals, leading to the same products with catechol compounds.     

Effects of Free Radicals on the Fluidity of Myocardial Membranes

Free radicals, including superoxide anions (O₂^?_?), hydroxyl radical (HO'), and hypohalite radical (OCl'), as well as oxidants such as hydrogen peroxide (H₂O₂) and hypochlorous acid (HOCl), have been indicated in the pathogenesis of myocardial ischemic and reperfusion injury. In this report, we compared the integrity of the myocardial membrane when exposed to these free radicals/oxidants. Isolated rat heart membrane preparations were exposed to chemically generated free radicals with or without their respective scavengers. Membrane fluidity was monitored by fluorescence polarization using the diphenylhexatriene probe, as well as by electron spin resonance (ESR) spectroscopy using 2,2,6,6-tetramethyl piperidine-n-oxyl as the spin labeling agent. HO', H₂O₂, and OCl' + HOCl increased the fluorescence polarization (FP) and microvis-cosity significantly by 1.7-fold, 1.8-fold, and 1.7-fold, respectively, as compared to an only 1.2– fold increase in FP by O₂^?_? O₂^?_? did not alter the fatty acid profiles of the membrane phospholipids. However, HO' and H₂O₂ reduced the arachidonic acid contents in phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylinositol (PI). These radicals also stimulated the lipid peroxidation by several-fold, while that by O₂^?_? was only insignificant. These results suggest that HO' and H₂O₂ decreased the membrane fluidity and induced lipid peroxidation by releasing the arachidonic acid from PC, PE. and PI.     

Detection and identification of oxidants formed during •NO/O2•– reaction: A multi-well plate CW-EPR spectroscopy combined with HPLC analyses

Abstract

New techniques and probes are routinely emerging for detecting short-lived free radicals such as superoxide radical anion (O₂^?–), nitric oxide (^?NO), and transient oxidants derived from peroxynitrite (ONOO^–/ONOOH). Recently, we reported the profiles of oxidation products (2-hydroxyethidium, ethidium, and various dimeric products) of the fluorogenic probe hydroethidine (HE) in the ^?NO/O₂^?– system (Zielonka et al. 2012). In this study, we used HPLC analyses of HE oxidation products in combination with continuous wave electron paramagnetic resonance (CW-EPR) spin trapping with 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide (BMPO) to define the identity of the oxidizing species formed in the ^?NO/O₂^?– system. EPR spin-trapping technique is still considered as the gold standard for characterization of free radicals and their intermediates. We monitored formation of BMPO-superoxide (BMPO-^?OOH) and BMPO-hydroxyl (BMPO-^?OH) radical adducts. Simultaneous analyses of results from EPR spin-trapping and HPLC measurements are helpful in the interpretation of the mechanism of formation of products of HE oxidation.     

Hydroxyl radical production from hydrogen peroxide and enzymatically generated paraquat radicals: Catalytic requirements and oxygen dependence

The ability of paraquat radicals (PQ^+.) generated by xanthine oxidase and glutathione reductase to give H₂O₂-dependent hydroxyl radical production was investigated. Under anaerobic conditions, paraquat radicals from each source caused chain oxidation of formate to CO₂, and oxidation of deoxyribose to thiobarbituric acid-reactive products that was inhibited by hydroxyl radical scavengers. This is in accordance with the following mechanism derived for radicals generated by γ-irradiation [H. C. Sutton and C. C. Winterbourn (1984) Arch. Biochem. Biophys.235, 106–115] PQ^+. + Fe³⁺ (chelate) → Fe²⁺ (chelate) + PQ⁺⁺ H₂O₂ + Fe²⁺ (chelate) → Fe³⁺ (chelate) + OH^? + OH.. Iron-(EDTA) and iron-(diethylenetriaminepentaacetic acid) (DTPA) were good catalysts of the reaction; iron complexed with desferrioxamine or transferrin was not. Extremely low concentrations of iron (0.03 μm) gave near-maximum yields of hydroxyl radicals. In the absence of added chelator, no formate oxidation occurred. Paraquat radicals generated from xanthine oxidase (but not by the other methods) caused H₂O₂-dependent deoxyribose oxidation. However, inhibition by scavengers was much less than expected for a reaction of hydroxyl radicals, and this deoxyribose oxidation with xanthine oxidase does not appear to be mediated by free hydroxyl radicals. With O₂ present, no hydroxyl radical production from H₂O₂ and paraquat radicals generated by radiation was detected. However, with paraquat radicals continuously generated by either enzyme, oxidation of both formate and deoxyribose was measured. Product yields decreased with increasing O₂ concentration and increased with increasing iron(DTPA). These results imply a major difference in reactivity between free and enzymatically generated paraquat radicals, and suggest that the latter could react as an enzyme-paraquat radical complex, for which the relative rate of reaction with Fe³⁺ (chelate) compared with O₂ is greater than is the case with free paraquat radicals.     

Mechanism of the Photocatalytic Inactivation of Lactobacillus casei Phage PL-1 by Titania Thin Film

The mechanism of the inactivation of Lactobacillus casei phage PL-1 suspended in a phosphate buffer by black-light (BL) -catalytic titanium dioxide (TiO₂) thin film was studied. Generation of both superoxide anions (O₂ ⁻) and hydroxyl radicals ( · OH) was confirmed in the aqueous medium in which TiO₂ film was settled with BL irradiation under gentle shaking. With BL-irradiation alone without TiO₂ film, only O₂ ⁻ was generated to some extent. The genome DNA inside the phage particles was found to be fragmented by the treatment of PL-1 phages with BL-catalytic TiO₂ film. The phage inactivation by BL-catalytic TiO₂ film was inhibited by the addition of albumin in a concentration-dependent manner. BL-catalytic TiO₂ film was considered to cause primarily the damage to the capsid protein through the generation of active oxygen species such as · OH, followed by damage to the genome DNA inside the phage particles. Received: 11 August 2000 / Accepted: 30 August 2000     

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.     

Interaction between fulvic acids of different origins and active oxygen radicals

Using the spin trapping technique, the interaction between fulvic acids (FAs) of different origins and the active oxygen radicals was studied. The active oxygen radicals under study included superoxide anion (O2 · -) produced by xanthine oxidase (XOD) and stimulated polymorphonuclear leukocytes (PMN) of human being and hydroxyl radical ( ·OH) produced from Fenton's reaction. It has been found that the FAs from both Kaschin-Beck disease (KBD) region and non-KBD region can accelerate the production of ·OH and scavenge O2 ·- . FA from peat can scavenge both O2·- and ·OH. The results show that the behavior of KBD and non-KBD FAs differs clearly from peat FA. It has been concluded that the superoxidation damage of KBD induced by FA is mainly due to hydroxyl radical reaction initiated in biological system.     

The mechanism of the activity-dependent luminescence of xanthine oxidase.

The weak luminescence that accompanies the aerobic xanthine oxidase reaction is inhibited by superoxide dismutase, by catalase, and by scavengers of hydroxyl radicals. It is also entirely dependent upon the presence of carbonate. It thus appears that the O₂ and H₂O₂ produced during the aerobic action of xanthine oxidase interact to generate OH which, in turn, reacts with carbonate to yield the carbonate radical (CO₃^?). The species that is directly responsible for light emission appears to be produced by a dimerization of carbonate radicals, since the light intensity was a function of the square of the carbonate concentration. The data provide no reason to suppose that the light-emitting species is singlet oxygen.     

Radical scavenging ability of some compounds isolated from Piper cubeba towards free radicals

The purpose of this study was to identify the antioxidant activity of 16 compounds isolated from Piper cubeba (CNCs) through the extent of their capacities to scavenge free radicals, hydroxyl radical (HO^?), superoxide anion radical () and 2,2‐diphenyl‐1‐picrylhydrazyl radical (DPPH^?), in different systems. Electron paramagnetic resonance (EPR) and 5,5‐dimethyl‐1‐pyrroline‐N‐oxide, DMPO, as the spin trap, and chemiluminescence techniques were applied. Using the Fenton‐like reaction [Fe(II) + H₂O₂], CNCs were found to inhibit DMPO? OH radical formation ranging from 5 to 57% at 1.25 mmol L^?1 concentration. The examined CNCs also showed a high DPPH antiradical activity (ranging from 15 to 99% at 5 mmol L^?1 concentration). Furthermore, the results indicated that seven of the 16 tested compounds may catalyse the conversion of superoxide radicals generated in the potassium superoxide/18‐crown‐6 ether system, thus showing superoxide dismutase‐like activity. The data obtained suggest that radical scavenging properties of CNCs might have potential application in many plant medicines. Copyright © 2010 John Wiley & Sons, Ltd.     

New Aspects in the Free-Radical Chemistry of Pyrimidine Nucleobases

The contribution will cover three aspects:

i) It has been known for some time that OH radicals and H atoms react with the pyrimidines by adding to the C(5)-C(6) double bond, but only the u.v.-spectra of the sum of these radicals have been reported so far. It will be shown how to arrive at the individual spectra of the C(5) and the C(6) adduct radicals.

ii) α-Hydroxyalkyl radicals are known to inactivate biologically active DNA. In contrast to the electrophilic radicals H and OH they are nucleophilic and the hydroxymethyl radicals add exclusively at the C(6) position of 1,3-dimethyluracil (k ～ 10⁴dm³ mol^?1 s^?1). In the corresponding thymine derivative this reaction also occurs, but one third of the hydroxymethyl radicals abstract an H-atom from the C(5)-methyl group thereby forming an allylic radical. In the course of these reactions pyrimidines with an exocyclic double bond are formed. These products react much more rapidly with hydroxymethyl radicals than the starting material leading to highly hydroxymethylated material at very low doses.

iii) The direct effect of ionizing radiation which would produce a pyrimidine base radical cation can be mimicked by reacting the pyrimidine with SO₄^?, a very good electron acceptor. In water, the radical cation of 1,3-dimethyluracil is rapidly (t_1/2 2μs) converted into the C(5) OH adduct radical. In the presence of peroxodisulphate a chain reaction sets in which leads to the cis-glycol.

The relevance of these findings to radiobiological aspects of nucleic acid research will be discussed.     

Perhydroxyl Radical (HO<Subscript>2</Subscript><Superscript>•</Superscript>) as Inducer of the Isoprostane Lipid Peroxidation in Mitochondria

The nonenzymatic isoprostane pathway of lipid peroxidation of polyunsaturated fatty acids results in formation of products, termed isoprostanes, which have very large positional and stereo isomerism, possess various biological activities, produce adducts with proteins, and thus contribute to pathogeneses of the agedependent diseases. However, it was unclear what mechanism drives this type of lipid autoxidation, and why the products have very large isomerism. We propose a mechanism when perhydroxyl radicals (HO₂^?) react with polyunsaturated fatty acids in the hydrophobic milieu of membranes. In the membrane HO₂^? initiates a chain of reactions with formation first H₂O₂, which undergoes homolytic fission producing two ^?OH radicals, thus very rapidly abstracting three H atoms from a polyunsaturated fatty acid. As a result, the HO₂^? molecule is converted to two molecules of water, and the molecule of a polyunsaturated fatty acid loses two double bonds, becomes highly unstable and undergoes peroxidation and random intramolecular re-arrangements causing a very large isomerism of the final products. The extremely high reactivity of ^?HО₂ with polyunsaturated fatty acids is the cause of very subtle and slow accumulation of damages in the membrane and membrane associated proteins, even though the concentration of ^?HО₂ relative to superoxide radical may be very low.     

Radical scavenging and electron-transfer reactions in Polyporus Versicolor laccase: A pulse radiolysis study

The interaction of the radicals OH^?, t-BuO^?, e_aq^?, CO₂^XXX and O₂^XXX with the copper oxidase. laccase. from Polyporus, has been studied by the pulse-radiolysis technique. Each of these radicals formed transient adducts with a broad absorption maximum around 310 nm. Analysis of the optical properties and of the very fast rates of formation of these compounds shows that each radical interacts with a limited number of sites on the polypeplide part of the protein amongst R-S-S-R. histidine and aromatic residues. Interaction with the carbonyl group of some of the peptide bonds is also possible. The few target sites are probably hit simultaneously and electron transfer between these sites may also occur. In all cases, in a subsequent step, intramolecular electron transfer from the polypeptide radical adducts leads to a partial reduction of the blue type-1 Cu²⁺ with rates varying between 10³ and 10⁴ s^?1. Further reduction of the type-1 Cu²⁺ occurs through a slow intermolecular reaction between two laccase radical transient adducts. In the case of CO^XXX₂ and O^XXX₂, this slow reduction could alternatively be due to an intermolecular reaction between laccase and CO^XXX₂ or O^XXX₂. The oxidant radicals OH^?. Br^XXX₂ and (SCN)^XXX₂, which formed radical adducts with fully ascorbate-reduced laccase, did not induce any type-1 copper reoxidation.     

The role of interactions between O2, H2O2, .OH,e- and O2- in free radical damage to biological systems

The occurrence of the Haber-Weiss reaction and other interactions between free radicals has been investigated in the effects of mixtures of free radicals on the permeability of resealed erythrocyte ghosts and on the activity of membrane-bound glyceraldehyde-3-phosphate dehydrogenase. The following mixtures were found to induce damage greater than that which could be accounted for by the independent actions of the constituent free radicals: (i) · OH + H₂O₂, and (ii) · OH + H₂O₂ + O₂^?. In contrast, the following mixtures were found to induce less damage than that predicted on the basis of independent actions of constituent free radicals: (i) H₂O₂ + O₂^?, and (ii) oxidizing radicals ( · OH, H₂O₂) + reducing radicals (e^?, H · ). These results suggest a Haber-Weiss-like interaction between H₂O₂ and O₂^?and an interaction between H₂O₂ and · OH to produce a species more potent than either in causing increased permeability. The decrease in damage observed in the simultaneous presence of oxidizing and reducing radicals suggests an antagonistic effect by which each tends to moderate damage by the other. Inactivation of glyceraldehyde-3-phosphate dehydrogenase was found to be more sensitive to radiation than permeability by an order of magnitude, while permeability was more sensitive to the enhancement of damage by oxygen. Comparison of the effectiveness of free radical scavengers in inhibiting the increase in permeability caused by free radicals showed the following order of effectiveness, expressed in terms of percentage protection: formate (90%) > nitrogen (65%) > catalase (60%) > dismutase (32%), and with respect to enzymatic inactivation, nitrogen (100%) > formate (77%) > dismutase (48%) > catalase (44%). The relative rates observed anaerobically and aerobically in the presence and absence of the above scavengers suggest that (at least in the case of radiation damage to the membranes of erythrocyte ghost cells) the “oxygen effect” is due to the interaction of oxygen with e^? and H., producing O₂^? which aggravates damage under conditions which allow consequent Haber-Weiss-like reactions. The further increase in damage when oxygen concentration is raised yet higher is due to the interaction of oxygen with the sites of initial damage.     

Solvent Effects in the Spin Trapping Of Lipid Oxyl Radicals

Studies documenting spin trapping of lipid radicals in defined model systems have shown some surprising solvent effects with the spin trap DMPO. In aqueous reactions comparing the reduction of H₂O₂ and methyl linoleate hydroperoxide (MLOOH) by Fe^z+, hydroxyl (HO·) and lipid alkoxyl (LO·) radicals produce identical four-line spectra with line intensities 1:2:2:1. Both types of radicals react with commonly-used HO· scavengers, e.g. with ethanol to produce ·C(CH₃)HOH and with dirnethylsulfoxide (DMSO)togive ·CH₃. However, DMSO radicals (either ·CH₃or ·OOCH₃) react further with lipids, and when radicals are trapped in these MLOOH systems, multiple adducts are evident. When acetonitrile is added to the aqueous reaction systems in increasing concentrations, ·CH₂CN radicals resulting from HO· attack on acetonitrile are evident, even with trace quantities of that solvent. In contrast, little, if any, reaction of LO· with acetonitrile occurs, even in 100% acetonitrile. A single four-line signal persists in the lipid systems as long as any water is present, although the relative intensity of the two center lines decreases as solvent-induced changes gradually dissociate the nitrogen and β-hydrogen splitting constants. Extraction of the aqueous-phase adducts into ethyl acetate shows clearly that the identical four-line spectra in the H₂0₂ and MLOOH systems arise from different radical species in this study, but the lack of stability of the adducts to phase transfer may limit the use of this technique for routine adduct identification in more complex systems. These results indicate that the four-line 1:2:2:1. a_N = a_H = 14.9G spectrum from DMPO cannot automatically be assigned to the HO· adduct in reaction systems where lipid is present, even when the expected spin adducts from ethanol or DMSO appear confirmatory for HO-. Conclusive distinction between HO· and LO· ultimately will require use of ¹³C-labelled DMPO or HPLC-MS separation and specific identification of adducts when DMPO is used as the spin trap.     

Free Radical Scavenging by Myocardial Fatty Acid Binding Protein

Recent investigations have indicated the presence of a fatty acid binding protein (FABP) in mammalian heart. This protein binds free fatty acids and their esters with high affinity, however, its physiological role remains unknown. Since FABP constitutes a significant amount of cystolic protein, it is likely that it would be a target for free radical attack. To test this hypothesis, FABP was examined for scavenging against free radicals such as the superoxide anion (O^?₂,). hydroxyl radical (OH') and hypochlorite radical (OCl') which may be present in an ischemic reperfused heart. Our results suggest that FABP scavenges O^?₂, OH' and OCl' as indicated by the FABP inhibition of O^?₂-dependent reduction of cytochrome c, OH'-dependent hydroxybenzoic acid formation and OCl'-mediated chemiluminescence response. FABP was found to be a more potent scavenger of these free radicals compared to bovine serum albumin. Furthermore, FABP was more effective in scavenging OH' than O^?₂, and inhibited OH' mediated lipid peroxidation process. These results indicate that FABP can scavenge free radicals which may be present in an ischemic/reperfused heart and, thus, may play a significant physiological role in the heart during ischemia and reperfusion.     

Hydroxyl Radical Generation Depending on O2 or H2O by a Photocatalyzed Reaction in an Aqueous Suspension of Titanium Dioxide

Photocatalytic production of the electron (e^-) and positive hole (h⁺) in an aqueous suspension of TiO₂ (anatase form) under illumination by near-UV light (295-390 nm) generated the superoxide (O₂ ^-) and hydroxyl radical (?OH), which both proceeded linearly with reaction time, while H₂O₂ accumulated non-linearly. Under anaerobic conditions (introduced Ar gas), the yields of three active species of oxygen were decreased to 10-20% of those detected in the air-saturated reaction. The electron spin resonance (ESR) signal characteristics of ?OH were obtained when a spin trap of 5,5-dimthyl-1-pyrroline-N-oxide (DMPO) was included in the illuminating mixture. The intensity of the ESR signal was increased by Cu/Zn superoxide dismutase, and decreased under anaerobic conditions, amounting to only 20% of the intensity detected in the aerobic reaction. The addition of H₂O₂ to the reaction mixture resulted in about an 8-fold increase of ?OH production in the anaerobic reaction, but only about 1.5-fold in the aerobic reaction, indicating that e^- generated by the photocatalytic reaction reduced H₂O₂ to produce ?OH plus OH^-. On the other hand, D₂O lowered the yield of ?OH generation to 18% under air and 40% under Ar conditions, indicating the oxidation of H₂O by h⁺. The addition of Fe(III)-EDTA as an electron acceptor effectively increased ?OH generation, 2.3-fold in the aerobic reaction and 8.4-fold in the anaerobic reaction, the yield in the latter exceeding that in the air-saturated reaction.     

Evaluation of the Antioxidant Actions of Ferulic Acid and Catechins

We have evaluated the abilities of ferulic acid, (±) catechin, (+) catechin and (-) epicatechin to scavenge the reactive oxygen species hydroxyl radical (OH±), hypochlorous acid (HOCl) and peroxyl radicals (RO₂).

Ferulic acid tested at concentrations up to 5 mM inhibited the peroxidation of phospholipid liposomes. Both (±) and (+) catechin and (-) epicatechin were much more effective. All the compounds tested reacted with trichloromethyl peroxyl radical (CCl₃O₂) with rate constants > 1 × 10⁶M^?1s^?1.

A mixture of FeCl₃-EDTA, hydrogen peroxide (H₂O₂) and ascorbic acid at pH 7.4, has often been used to generate hydroxyl radicals (OH.) which are detected by their ability to cause damage to the sugar deoxyribose. Ferulic acid, (+) and (±) catechin and (-) epicatechin inhibited deoxyribose damage by reacting with OH. with rate constants of 4.5 × 10⁹M^?1s^?1, 3.65 × 10⁹M^?1s^?1, 2.36 × 10⁹M^?1s^?1 and 2.84 × 10⁹M^?1s^?1 respectively. (-) Epicatechin, ferulic acid and the (+) and (±) catechins exerted pro-oxidant action, accelerating damage to DNA in the presence of a bleomycin-iron complex. On a molar basis, ferulic acid was less effective in causing damage to DNA compared with the catechins.

A mixture of hypoxanthine and xanthine oxidase generates O₂ which reduces cytochrome c to ferrocytochrome c. (+) Catechin and (-) epicatechin inhibited the reduction of cytochrome c in a concentration dependent manner. Ferulic acid and (±) catechin had only weak effects.

All the compounds tested were able to scavenge hypochlorous acid at a rate sufficient to protect alpha-1-antiproteinase against inactivation. Our results show that catechins and ferulic acid possess antioxidant properties. This may become important given the current search for “natural” replacements for synthetic antioxidant food additives.     

New Insight into the Synthesis of Aromatic Azo Compounds Assisted by Surface Plasmon Resonance

The application of surface plasmon resonance (SPR)-assisted catalysis to synthesize aromatic azo compounds was first reported in 2010. The feasibility of SPR-assisted catalytic decomposition of aromatic azo compounds has also been confirmed, both experimentally and theoretically. Compared with traditional chemical synthesis methods, SPR-assisted catalysis has many advantages, such as high efficiency, low energy consumption, and high selectivity. The synthetic route to aromatic azo compounds and the kinetics thereof can be efficiently monitored by Raman spectroscopy. In this way, it has been confirmed that SPR-assisted catalysis occurs on the surface of noble metal nanoparticles (NPs). Mechanistically, the process involves transfer of electrons excited by the incident laser from noble metal NPs to ³O₂ in air to form ²O₂ ^? for the generation of SPR on the surface of the noble metal nanoparticles. The ²O₂ ^? can then react with the metal to form metal oxides or hydroxides, which in turn can react with the substrate molecule. The substrate molecule can gain a proton from a proton donor or lose a proton to form a radical, which can react further. This mechanism accounts for the conversion of 4-aminothiophenol (PATP) into 4, 4-dimercaptoazobenzene (DMAB). The metal oxide or hydroxide formed reacts with PATP in an acid-base neutralization process. PATP radicals (PATP ^· ) are formed by the loss of a proton, and pairing of two PATP ^· leads to the intermediate product DMHAB. Deprotonation DMHAB then gives the final product DMAB.     

Mechanism of alcohol‐enhanced lucigenin chemiluminescence in alkaline solution

The chemiluminescence (CL) of lucigenin (Luc²⁺) can be enhanced by different alcohols in alkaline solution. The effect of different fatty alcohols on the CL of lucigenin was related to the carbon chain length and the number of hydroxyl groups. Glycerol provides the greatest enhancement. UV/Vis absorption spectra and fluorescence spectra showed that N‐methylacridone (NMA) was produced in the CL reaction in the presence of different alcohols. The peak of the CL spectrum was located at 470 nm in all cases, indicating that the luminophore was always the excited‐state NMA. The quenching of lucigenin CL by superoxide dismutase (SOD) and the electron spin resonance (ESR) results with the spin trap of 5,5‐dimethyl‐1‐pyrroline N‐oxide (DMPO) demonstrated that superoxide anions (O₂^?–) were generated from dissolved oxygen in the CL reaction and that glycerol and dihydroxyacetone (DHA) can promote O₂^?? production by the reduction of dissolved oxygen in alkaline solution. It was assumed that the enhancement provided by different alcohols was related to the solvent effect and reducing capacity. Glycerol and DHA can also reduce Luc²⁺ into lucigenin cation radicals (Luc^?+), which react with O₂^?? to produce CL, and glycerol can slowly transform into DHA, which is oxidized quickly in alkaline solution. Copyright © 2015 John Wiley & Sons, Ltd.