Photochemical production of reactive oxygen species by C60 in the aqueous phase during UV irradiation

The objective of this study was to investigate photochemical production of singlet oxygen (1O2) and superoxide radical anion (02*-) by C60 in water. It was demonstrated that photoexcited C60 in the aqueous phase efficiently mediated transfer of absorbed energy to oxygen and produced singlet oxygen when associated with surfactant (Triton X100 and Brij 78) or polymer (polyvinylpyrrolidone), which is consistent with previously observed behavior in organic solvents. However, when C60 was present as colloidal aggregate suspension, prepared through solvent exchange or sonication, this intrinsic character was lost. Similarly, C60 associated with surfactant mediated electron transfer from electron donor (triethylamine) to oxygen producing superoxide radical, while C60 aggregates and C60 associated with polymer did not. These results suggestthat the ability of C60 to mediate energy and electron transfer may be affected by the degree of C60 aggregation in the aqueous phase as well as characteristics of associated stabilizing molecules. Dependence of photochemical reactivity of C60 on its dispersion status in the aqueous phase is critical in assessing environmental impact and cytotoxicity of this material, as C60 associated with model natural organic matter was found to exist in aggregate form and did not produce reactive oxygen species under UV irradiation.     

Quantitative assessment of antioxidant properties of natural colorants and phytochemicals: carotenoids,flavonoids, phenols and indigoids. The role of β‐carotene in antioxidant functions

Reactive oxygen species are potentially damaging molecules. An important function of antioxidants is to intercept harmful triplet states, in order to prevent the formation of singlet oxygen, or to quench singlet oxygen directly. However, antioxidants are also reactive towards other active oxygen species such as the hydroxyl radical, the superoxide anion and the non‐excited oxygen ground state in the presence of radical initiators. It is well known that flavonoids and carotenoids show strong antioxidant properties. Polyenes and carotenoids are the best known among the compounds that quench singlet oxygen by efficient energy transfer. A large number of modified, synthetic analogues and derivatives have been synthesised to prepare even better quenchers than the natural carotenoids. Phenols are also excellent chain‐breaking antioxidants. Recently, many indigoid dyes (including bacterial indigoids) were studied, with the remarkable result that most, but not all, members of this class of chromophores quench singlet oxygen at the diffusion limit and some of them are excellent radical traps. It has been shown in this study that a quantitative assessment of antioxidant properties of flavonoids, carotenoids, phenols and natural indigoids can be achieved using the following three assays: (1) oxygen pressure dependence; (2) peroxide formation; (3) singlet oxygen quenching. Reactivities towards both excited states and ground state radicals can be properly described by these assays. The remarkable role of β‐carotene as an ‘unusual antioxidant’ (Burton GW and Ingold KU, Science 224 : 569–573 (1984)) in reactions using various oxygen pressures becomes clearer. The so‐called ‘pro‐oxidant effects’ concern primarily the antioxidant itself and its degradation, since no or very little damage to the substrate occurs in this type of experiment. Three main categories of antioxidants may be classified: (1) excellent antioxidants that perfectly quench excited states as well as ground state radicals (eg actinioerythrol, astaxanthin); (2) good antioxidants that strongly inhibit peroxide formation but are less efficient in quenching excited states (eg flavonols, tocopherols) or lead to considerable degradation of the antioxidant itself (eg β‐carotene, lycopene); (3) moderate antioxidants that fail to excel in both reactivities (eg ζ‐carotene, flavone). © 2001 Society of Chemical Industry     

Assessing the contribution of free hydroxyl radical in organic matter-sensitized photohydroxylation reactions

Photochemical formation of reactive oxygen species from dissolved organic matter (DOM) is incompletely understood, especially in the case of hydroxyl radical (?OH) production. Many studies have used various probes to detect photochemically produced ?OH from DOM, but the fundamental reactions of these probes are not necessarily specific for free ?OH and may also detect lower-energy hydroxylation agents. In this study, two tests were applied that have previously been used as a diagnostic for the presence of free ?OH: methane quenching of ?OH and hydroxybenzoic acid (hBZA) product yields. Upon application of these two tests to a set of five DOM isolates, it was found that methane quenching and the hBZA ratio results were not necessarily consistent. Overall, the results provide compelling evidence that all isolates studied photochemically produce free ?OH. The hydroxylating acitivity of Elliot Soil Humic Acid and Pony Lake Fulvic Acid, however, also had a significant contribution from a photochemically generated hydroxylating agent that is lower in energy than free ?OH. Catalase quenching experiments were conducted to assess whether hydrogen peroxide was the immediate precursor to hydroxyl in these systems. In all cases, catalase addition slowed photohydroxylation of terephthalate, but the contribution of hydrogen peroxide photolysis was determined to be less than 50%.     

Mechanism of C60 photoreactivity in water: fate of triplet state and radical anion and production of reactive oxygen species

The mechanism involved with (1) energy and electron transfer by C60 in the aqueous phase during UV irradiation and (2) subsequent production of reactive oxygen species (ROS) such as singlet oxygen and superoxide radical anion was investigated. Electron paramagnetic resonance (EPR) study showed that C60 embedded in micelles of nonionic surfactant (Triton X 100) or anionic surfactant (sodium dodecylbenzenesulfonate) produced ROS, but aggregated C60 did not, consistent with our earlier findings made using indicator chemicals. Nanosecond and femtosecond laser flash photolysis showed that the aggregation of C60 significantly accelerates the decay of excited triplet state C60, which is a key intermediate for energy and electron transfer, thus blocking the pathway for ROS production. This finding suggests that C60 clusters will not contribute to oxidative damage or redox reactions in natural environment and biological systems in the same way molecular C60 in organic phase reportedly does. In contrast, C60 embedded in surfactant micelles produces ROS and the evidence is presented for the formation of C60 radical anion as an intermediate.     

Free radical destruction of N-nitrosodimethylamine in water

Absolute rate constants for the reactions of the hydroxyl radical, hydrated electron, and hydrogen atom with N-nitrosodimethylamine (NDMA) in water at room temperature have been determined using electron pulse radiolysis and transient absorption spectroscopy (*OH and e- aq) and EPR free induction decay attenuation (*H) measurements. Specific values of (4.30 +/- 0.12) x 10(8), (1.41 +/- 0.02) x 10(10), and (2.01 +/- 0.03) x 10(8) M(-1) s(-1) were measured, respectively. DMPO spin-trapping experiments demonstrated that the hydroxyl radical reaction with NDMA occurs by hydrogen atom abstraction from a methyl group, and the rate constant for the subsequent reaction of this radical transient with dissolved oxygen was measured as (5.3 +/- 0.6) x 10(6) M(-1) s(-1). This relatively slow rate constant implies that regeneration of the parent nitrosoamine from the oxidized transient could occur in natural waters containing dissolved organic compounds. The reaction of the hydrated electron with NDMA was to form a transient adduct anion, which could subsequently transfer this excess electron to regenerate the parent chemical. Such regeneration reactions would significantly reduce the effectiveness of any applied advanced oxidation technology remediation effort on NDMA-contaminated natural waters.     

葡萄籽原花青素的制备及活性研究

选用多种有机溶剂,经过梯度分离,从葡萄籽中得到葡萄籽原花青素提取物,并采用邻苯三酚和DPPH方法研究提取物的清除自由基活性。实验结果表明：通过显色反应,可初步判断和溶剂提取物中含有黄酮类和多酚类物质,利用邻苯三酚和DPPH检测方法,证明各提取物具有清除超氧阴离子自由基和1,1--二苯基苦肼基的活性,其中乙酸乙酯提取物清除自由基的效果最好。     

Transformation of sulfamethazine by manganese oxide in aqueous solution

The transformation of the sulfonamide antimicrobial sulfamethazine (SMZ) by a synthetic analogue of the birnessite-family mineral vernadite (δ-MnO(2)) was studied. The observed pseudo-first-order reaction constants (k(obs)) decreased as the pH increased from 4.0 to 5.6, consistent with the decline in δ-MnO(2) reduction potential with increasing pH. Molecular oxygen accelerated SMZ transformation by δ-MnO(2) and influenced the transformation product distribution. Increases in the Na(+) concentration produced declines in k(obs). Transformation products identified by tandem mass spectrometry and the use of (13)C-labeled SMZ included an azo dimer self-coupling product and SO(2) extrusion products. Product analysis and density functional theory calculations are consistent with surface precursor complex formation followed by single-electron transfer from SMZ to δ-MnO(2) to produce SMZ radical species. Sulfamethazine radicals undergo further transformation by at least two pathways: radical-radical self-coupling or a Smiles-type rearrangement with O addition and then extrusion of SO(3). Experiments conducted in H(2)(18)O or in the presence of (18)O(2)(aq) demonstrated that oxygen both from the lattice of as-synthesized δ-MnO(2) and initially present as dissolved oxygen reacted with SMZ. The study results suggest that the oxic state and pH of soil and sediment environments can be expected to influence manganese oxide-mediated transformation of sulfonamide antimicrobials.     

Activated oxygen species and oxidation of food constituents.

Activated oxygen species which may be important in initiating oxidative changes in foods include singlet oxygen, hydroxyl radical, ozone, superoxide anion (perhydroxyl radical at low pH), and hydrogen peroxide. Chemical and enzymic reactions known to occur in biological materials can generate singlet oxygen, hydroxyl radical, superoxide anion, and hydrogen peroxide. Ozone is primarily a product of photoreactions in polluted air. Reactions involving singlet oxygen, hydroxyl radical, and ozone with food constituents can ultimately yield peroxides which decompose to initiate oxidative chain reactions. Superoxide anion and hydrogen peroxide are relatively inert toward organic molecules but can decompose to produce the more reactive singlet oxygen and hydroxyl radical. Inhibition of reactions initiated by reactive oxygen species in foods should be very important in preserving the oxidative stability of foods. The generation, detection, measurement, reaction, and inhibition of reactions of active oxygen species are surveyed in this review.     

Activated oxygen species and oxidation of food constituents

Activated oxygen species which may be important in initiating oxidative changes in foods include singlet oxygen, hydroxyl radical, ozone, superoxide anion (perhydroxyl radical at low pH), and hydrogen peroxide. Chemical and enzymic reactions known to occur in biological materials can generate singlet oxygen, hydroxyl radical, superoxide anion, and hydrogen peroxide. Ozone is primarily a product of photoreactions in polluted air. Reactions involving singlet oxygen, hydroxyl radical, and ozone with food constituents can ultimately yield peroxides which decompose to initiate oxidative chain reactions. Superoxide anion and hydrogen peroxide are relatively inert toward organic molecules but can decompose to produce the more reactive singlet oxygen and hydroxyl radical. Inhibition of reactions initiated by reactive oxygen species in foods should be very important in preserving the oxidative stability of foods. The generation, detection, measurement, reaction, and inhibition of reactions of active oxygen species are surveyed in this review.     

Free radical scavenging activity of morin 2'-O(-) phenoxide anion

Due to intramolecular H-atom transfer, deprotonation of the most acidic 3-OH group of morin yields 2'-O(-) phenoxide anion. The reaction enthalpies related to mechanisms of free radical scavenging activity of this dominant species at a physiological pH of 7.4 were calculated by PM6 and DFT methods in gas-phase, water, benzene and DMSO. Results indicate the 4'-OH group of 2'-O(-) phenoxide anion is the active site for radical inactivation. The thermodynamically favoured mechanism depends on the polarity of the reaction media: in polar solvents (water and DMSO), the sequential proton loss electron transfer (SPLET) mechanism is preferred while in non-polar benzene (and in gas-phase), the hydrogen atom transfer (HAT) mechanism is responsible for the free radical scavenging activity of the morin phenoxide anion. Results show that the fast, semiempirical PM6 method fairly mimics more accurate, though time-consuming DFT methodologies.     

Evaluating behavior of oxygen,nitrate, and sulfate during recharge and quantifying reduction rates in a contaminated aquifer

This study evaluates the biogeochemical changes that occur when recharge water comes in contact with a reduced aquifer. It specifically addresses (1) which reactions occur in situ, (2) the order in which these reactions will occur if terminal electron acceptors (TEAs) are introduced simultaneously, (3) the rates of these reactions, and (4) the roles of the aqueous and solid-phase portions of the aquifer. Recharge events of waters containing various combinations of O2, NO3, and SO4 were simulated at a shallow sandy aquifer contaminated with waste fuels and chlorinated solvents using modified push-pull tests to quantify rates. In situ rate constants for aerobic respiration (14.4 day(-1)), denitrification (5.04-7.44 day(-1)), and sulfate reduction (4.32-6.48 day(-1)) were estimated. Results show that when introduced together, NO3 and SO4 can be consumed simultaneously at similar rates. To distinguish the role of aqueous phase from that of the solid phase of the aquifer, groundwater was extracted, amended with NO3 and SO4, and monitored overtime. Results indicate that neither NO3 nor SO4 was reduced during the course of the aqueous-phase study, suggesting that NO3 and SO4 can behave conservatively in highly reduced water. It is clear that sediments and their associated microbial communities are important in driving redox reactions.     

Indigo carmine degradation in the presence of maltose and ethanol

The majority of important reactions in beer are associated with the action of oxygen during malting and brewing. With respect to oxygen, beer is considered to possess a relatively strong reducing environment. The gradual reduction of oxygen gives rise to reactive oxygen species (ROS) capable of attacking beer components that are important to consumers. Indigo carmine (IC) bleaching was used to prove ROS formation formed by oxidation of beer energy sources such as maltose and ethanol. This paper examines oxygen radical oxidation of organic matter in the presence of the radical initiator potassium peroxodisulphate. A mixture of organic sources of energy such as maltose and ethanol, oxygen, radical initiator and cupric ions generates ROS formation in a broad range of concentrations even at room temperature. ROS irreversibly split blue dye IC into the colourless form and takes part in the radical transformation of organic matter, which is similar to its burning at low temperature or to radical oxidation of reducing agents. Other organic matter can suppress ROS formation and be incorporated into the radical chain of the pathway of its ‘cold burning’. Ethanol inhibited phenylalanine oxidation during its oxidative Maillard reaction. IC bleaching can serve as simple indicator of ROS generation in aerated food. Copyright © 2015 The Institute of Brewing & Distilling     

Effect of natural pigments on the oxidative stability of sausages stored under refrigeration

The objective was to use natural pigments to replace sodium erythorbate (NaEry), a synthetic compound used as an antioxidant in sausage formulations, and to evaluate the oxidative stability of the samples. Six assays were prepared in which sodium erythorbate (ERY) at 0.05 g/100 g was substituted by norbixin (NOR), lycopene (LYC), zeaxanthin (ZEA), β-carotene (CAR) or dextrose (used as a control (CON)). Physical, chemical, color, texture and sensory parameters were measured on the first day and after 45 days of storage at 4 °C. All pigments used in the sausage formulations were able to maintain the oxidative stability of the sausages (MDA equivalents <0.38 mg/kg). Zeaxanthin and norbixin were the most efficient antioxidants of those tested. This antioxidant effect might be associated with the intermediate polarities of these two compounds, which would allow them to concentrate in the membrane lipids or emulsion interface, where lipid oxidation is most prevalent. Other volatile secondary products of oxidation besides MDA should be evaluated in further studies involving natural pigments and sensory oxidative stability.     

Polyoxometalate-enhanced oxidation of organic compounds by nanoparticulate zero-valent iron and ferrous ion in the presence of oxygen

In the presence of oxygen, organic compounds can be oxidized by zerovalent iron or dissolved Fe(II). However, this process is not a very effective means of degrading contaminants because the yields of oxidants are usually low (i.e., typically less than 5% of the iron added is converted into oxidants capable of transforming organic compounds). The addition of polyoxometalate (POM) greatly increases the yield of oxidants in both systems. The mechanism of POM enhancement depends on the solution pH. Under acidic conditions, POM mediates the electron transfer from nanoparticulate zerovalent iron (nZVI) or Fe(II) to oxygen, increasing the production of hydrogen peroxide, which is subsequently converted to hydroxyl radical through the Fenton reaction. At neutral pH values, iron forms a complex with POM, preventing iron precipitation on the nZVI surface and in bulk solution. At pH 7, the yield of oxidant approaches the theoretical maximum in the nZVI/O2 and the Fe(II)/O2 systems when POM is present, suggesting that coordination of iron by POM alters the mechanism of the Fenton reaction by converting the active oxidant from ferryl ion to hydroxyl radical. Comparable enhancements in oxidant yields are also observed when nZVI or Fe(II) is exposed to oxygen in the presence of silica-immobilized POM.     

Time-weighted average SPME analysis for in planta determination of cVOCs

The potential of phytoscreening for plume delineation at contaminated sites has promoted interest in innovative, sensitive contaminant sampling techniques. Solid-phase microextraction (SPME) methods have been developed, offering quick, undemanding, noninvasive sampling without the use of solvents. In this study, time-weighted average SPME (TWA-SPME) sampling was evaluated for in planta quantification of chlorinated solvents. TWA-SPME was found to have increased sensitivity over headspace and equilibrium SPME sampling. Using a variety of chlorinated solvents and a polydimethylsiloxane/carboxen (PDMS/CAR) SPME fiber, most compounds exhibited near linear or linear uptake over the sampling period. Smaller, less hydrophobic compounds exhibited more nonlinearity than larger, more hydrophobic molecules. Using a specifically designed in planta sampler, field sampling was conducted at a site contaminated with chlorinated solvents. Sampling with TWA-SPME produced instrument responses ranging from 5 to over 200 times higher than headspace tree core sampling. This work demonstrates that TWA-SPME can be used for in planta detection of a broad range of chlorinated solvents and methods can likely be applied to other volatile and semivolatile organic compounds.     

Kinetic and mechanistic investigations of the oxidation of tramadol by ferrate and ozone

The kinetics and oxidation products (OPs) of tramadol (TRA), an opioid, were investigated for its oxidation with ferrate (Fe(VI)) and ozone (O(3)). The kinetics could be explained by the speciation of the tertiary amine moiety of TRA, with apparent second-order rate constants of 7.4 (±0.4) M(-1) s(-1) (Fe(VI)) and 4.2 (±0.3) × 10(4) M(-1) s(-1) (O(3)) at pH 8.0, respectively. In total, six OPs of TRA were identified for both oxidants using Qq-LIT-MS, LTQ-FT-MS, GC-MS, and moiety-specific chemical reactions. In excess of oxidants, these OPs can be further transformed to unidentified OPs. Kinetics and OP identification confirmed that the lone electron pair of the amine-N is the predominant site of oxidant attack. An oxygen transfer mechanism can explain the formation of N-oxide-TRA, while a one-electron transfer may result in the formation of N-centered radical cation intermediates, which could lead to the observed N-dealkylation, and to the identified formamide and aldehyde derivatives via several intermediate steps. The proposed radical intermediate mechanism is favored for Fe(VI) leading predominantly to N-desmethyl-TRA (ca. 40%), whereas the proposed oxygen transfer prevails for O(3) attack resulting in N-oxide-TRA as the main OP (ca. 90%).     

Effect of Different Solvents on the Measurement of Phenolics and the Antioxidant Activity of Mulberry (Morus atropurpurea Roxb.) with Accelerated Solvent Extraction

The effects of 9 different solvents on the measurement of the total phenolics and antioxidant activities of mulberry fruits were studied using accelerated solvent extraction (ASE). Sixteen to 22 types of phenolics (flavonols, flavan‐3‐ols, flavanol, hydroxycinnamic acids, hydroxybenzoic acids, and stilbenes) from different mulberry extracts were characterized and quantified using HPLC‐MS/MS. The principal component analysis (PCA) was used to determine the suitable solvents to distinguish between different classes of phenolics. Additionally, the phenolic extraction abilities of ASE and ultrasound‐assisted extraction (UAE) were compared. The highest extraction efficiency could be achieved by using 50% acidified methanol (50MA) as ASE solvents with 15.14 mg/gallic acid equivalents g dry weight of mulberry fruit. The PCA results revealed that the 50MA followed by 50% acidified acetone (50AA) was the most efficient solvent for the extraction of phenolics, particularly flavonols (627.12 and 510.31 μg/g dry weight, respectively), while water (W) was not beneficial to the extraction of all categories of phenolics. Besides, the results of 3 antioxidant capability assays (DPPH, ABTS free radical‐scavenging assay, and ferric‐reducing antioxidant power assay) showed that water‐based organic solvents increased the antioxidant capabilities of the extracts compared with water or pure organic solvents. ASE was more suitable for the extraction of phenolics than UAE.     

Visible-light-Mediated TiO2 photocatalysis of fluoroquinolone antibacterial agents

This study reports on the photocatalytic transformation of fluoroquinolone antibacterial agents (ciprofloxacin, enrofloxacin, norfloxacin, and flumequine) in aqueous titanium dioxide (TiO2) suspensions irradiated with ultraviolet (UV; lambda > 324 nm) or visible light (lambda > 400, > 420, or > 450 nm). Visible-light-mediated fluoroquinolone degradation is unexpected from direct photolysis or established TiO2 band gap photoexcitation mechanisms, which both require UV light. Visible-light-mediated photocatalysis requires an appropriate conduction band electron acceptor (e.g., O2, BrO3-), but is not dependent upon hydroxyl radical, superoxide, or other reactive oxygen species generated upon TiO2 band gap excitation. The process slows considerably when fluoroquinolone adsorption is inhibited. Whereas fluoroquinolone decomposition in UV-irradiated TiO2 suspensions is accompanied by mineralization, no changes in dissolved organic carbon occur during visible-light-photocatalyzed degradation. Results are consistent with a proposed charge-transfer mechanism initiated by photoexcitation of surface-complexed fluoroquinolone molecules. Complexation to the TiO2 surface causes a red shift in the fluoroquinolone absorption spectrum (via ligand-to-metal charge transfer), enabling photoexcitation by visible light. Fluoroquinolone oxidation then occurs by electron transfer into the TiO2 conduction band, which delivers the electron to an adsorbed electron acceptor. The lack of organic carbon mineralization indicates formation of stable organic byproducts that are resistant to further degradation by visible light. In UV-irradiated TiO2 suspensions, the charge-transfer mechanism acts in parallel with the semiconductor band gap photoexcitation mechanism.     

Effects of electron transfer mediators on the bioreduction of lepidocrocite (gamma-FeOOH) by Shewanella putrefaciens CN32

Electron transfer mediators (ETMs) such as low-molecular-mass quinones (e.g., juglone and lawsone) and humic substances are believed to play a role in many redox reactions involved in contaminant transformations and the biogeochemical cycling of many redox-active elements (e.g., Fe and Mn) in aquatic and terrestrial environments. This study examines the effects of a series of compounds representing major classes of natural and synthetic organic ETMs, including low-molecular-mass quinones, humic substances, phenazines, phenoxazines, phenothiazines, and indigo derivatives, on the bioreduction of lepidocrocite (gamma-FeOOH) by the dissimilatory Fe(III)-reducing bacterium Shewanella putrefaciens CN32. Although S. putrefaciens CN32 was able to reduce lepidocrocite in the absence of exogenous ETMs, the addition of exogenous ETMs enhanced the bioreduction of lepidocrocite. In general, the rate of Fe(II) production correlated well with the reduction potentials of the ETMs. The addition of humic acids or unfractionated natural organic matter at concentrations of 10 mg organic CL(-1) resulted in, at best, a minimal enhancement of lepidocrocite bioreduction. This observation suggests that electron shuttling by humic substances is not likely to play a major role in Fe(lll) bioreduction in oligotrophic environments such as subsurface sediments with low organic C contents.