Detection and characterisation of radicals using electron paramagnetic resonance (EPR) spin trapping and related methods

Versatile Tools for Understanding Electrosynthetic Mechanisms

Electrosynthesis is a popular, green alternative to traditional organic methods. Understanding the mechanisms is not trivial yet is necessary to optimize reaction processes. To this end, a multitude of analytical tools is available to identify and quantitate reaction products and intermediates. The first portion of this review serves as a guide that underscores electrosynthesis fundamentals, including instrumentation, electrode selection, impacts of electrolyte and solvent, cell configuration, and methods of electrosynthesis. Next, the broad base of analytical techniques that aid in mechanism elucidation are covered in detail. These methods are divided into electrochemical, spectroscopic, chromatographic, microscopic, and computational. Technique selection is dependent on predicted reaction pathways and electrogenerated intermediates. Often, a combination of techniques must be utilized to ensure accuracy of the proposed model. To conclude, future prospects that aim to enhance the field are discussed.

Liquid-Phase Cyclohexene Oxidation with O2 over Spray-Flame-Synthesized La1-x Srx CoO3 Perovskite Nanoparticles

La_1−xSr_xCoO₃ (x=0, 0.1, 0.2, 0.3, 0.4) nanoparticles were prepared by spray‐flame synthesis and applied in the liquid‐phase oxidation of cyclohexene with molecular O₂ as oxidant under mild conditions. The catalysts were systematically characterized by state‐of‐the‐art techniques. With increasing Sr content, the concentration of surface oxygen vacancy defects increases, which is beneficial for cyclohexene oxidation, but the surface concentration of less active Co²⁺ was also increased. However, Co²⁺ cations have a superior activity towards peroxide decomposition, which also plays an important role in cyclohexene oxidation. A Sr doping of 20 at. % was found to be the optimum in terms of activity and product selectivity. The catalyst also showed excellent reusability over three catalytic runs; this can be attributed to its highly stable particle size and morphology. Kinetic investigations revealed first‐order reaction kinetics for temperatures between 60 and 100 °C and an apparent activation energy of 68 kJ mol⁻¹ for cyclohexene oxidation. Moreover, the reaction was not affected by the applied O₂ pressure in the range from 10 to 20 bar. In situ attenuated total reflection infrared spectroscopy was used to monitor the conversion of cyclohexene and the formation of reaction products including the key intermediate cyclohex‐2‐ene‐1‐hydroperoxide; spin trap electron paramagnetic resonance spectroscopy provided strong evidence for a radical reaction pathway by identifying the cyclohexenyl alkoxyl radical.

Enhanced degradation of tetrabromobisphenol A by Fe3+/sulfite process under simulated sunlight irradiation

Degradation of tetrabromobisphenol A (TBBPA), an emerging micropollutant, by photo/Fe³⁺/sulfite process was investigated under different operational conditions and water matrices. 91% of TBBPA was efficiently degraded within 30 min in the Fe³⁺/sulfite system under sunlight irradiation when the initial pH was 6.0, which is much higher than that of TBBPA without irradiation (52%). The acceleration of radical generation and direct photolysis by photo irradiation were responsible for the enhanced TBBPA degradation. Although this process showed better performance on TBBPA degradation in weak acid conditions, the high removal efficiency was also achieved at near-neutral pH. HO, SO₄^- and direct photolysis contributed to TBBPA degradation. Direct photolysis and SO₄^- presented the dominant contribution. The degradation rate increased with elevating the Fe³⁺ dose (10-40 μM), but slightly decreased when the Fe³⁺ dose was further raised to 100 μM. Similarly, the degradation efficiency initially increased with increasing the sulfite dose (100-400 μM), but decreased when the sulfite concentration reached 1000 μM. Dissolved oxygen played a crucial role in TBBPA degradation, the presence of water matrices such as humic acid (0.8-4.0 mg/L), bicarbonate (0.5-10 mM) and chloride (0.5-10 mM) retarded TBBPA degradation. This study proposed a new efficient strategy to enhance TBBPA degradation in the Fe³⁺/sulfite process.

Metal-Free Solvent Promoted Oxidation of Benzylic Secondary Amines to Nitrones with H2O2

An environmentally benign protocol for the generation of nitrones from benzylic secondary amines via catalyst-free oxidation of secondary amines using H2O2 in MeOH or CH3CN is described. This methodology provides a selective access to a variety of C-aryl nitrones in yields of 60 to 93%. Several studies have been performed to shed light on the reaction mechanism and the role of the solvent.

EPR and NMR study of molecular components mobility and organization in goat milk under ultrasound treatment

The effect of ultrasound treatment on molecular mobility and organization of the main components in raw goat milk was studied by EPR and NMR spectroscopies. NMR relaxation studies showed an increase in the spin-lattice T1 and spin-spin T2 relaxation times in goat milk products (cream, anhydrous fat) and change in the diffusion of proton-containing molecules during ultrasound treatment. The diffusion became more uniform and could be rather accurately approximated by one effective diffusion coefficient Deff, which indicates homogenization of goat milk components, dispersion of globular and supermicellar formations under sonication. EPR studies have shown that molecular mobility and organization of hydrophobic regions in goat milk are similar to those observed in micellar formations of surfactants with a hydrocarbon chain length C12-C16. Ultrasound treatment did not affect submicellar and protein globule organization. Free radicals arising under ultrasound impact of milk reacted quickly with components of goat milk (triglycerides, proteins, fatty acids) and were not observed by spin trapping method.

Radical Intermediates in the Degradation of Hop Acids

Radical formation in isohumulones was investigated under different types of stress, including temperature, transition metal ions, and hydrogen peroxide. Including dihydroisohumulones and tetrahydroisohumulones, as relevant analogues, allowed us to evaluate critical functionalities in radical formation. Using spin-trapping methodology with 5,5-dimethyl-1-pyrroline N-oxide and N-tert-butyl-α-phenylnitrone as relevant traps, followed by simulation of corresponding spin adducts, identification of incipient radicals was attempted. The isohexenoyl side chain in isohumulones, but not present in dihydro- and tetrahydroisohumulones, was most sensitive to radical formation. Kinetic profiles further demonstrated that radical formation in this moiety was accelerated in the presence of ferrous ions. Reactivity of parent six-membered-ring humulones in radical formation was different, as scavenging of free radical species was more important. Lupulones, despite similarity with humulones, showed a different behavior with an obvious radical decay pathway during ageing, mainly ascribed to radical formation on the ring structure. Quantification of final spin adducts allowed us to determine absolute importance of the different degradation pathways. Eventually, mechanisms are presented explaining why isohumulones are more prone to radical processes in (aut)oxidation and thermal decay than close relatives such as dihydroisohumulones.

Amidinoquinoxaline-Based Nitrones as Lipophilic Antioxidants

The potential of nitrones (N-oxides) as therapeutic antioxidants is due to their ability to counteract oxidative stress, mainly attributed to their action as radical scavengers toward C- and O-centered radicals. Among them, nitrones from the amidinoquinoxaline series resulted in interesting derivatives, due to the ease with which it is possible to introduce proper substituents within their structure in order to modulate their lipophilicity. The goal is to obtain lipophilic antioxidants that are able to interact with cell membranes and, at the same time, enough hydrophilic to neutralize those radicals present in a water compartment. In this work, the antioxidant efficacy of a series of amidinoquinoxaline nitrones has been evaluated regarding the oxidation of 2-deoxyribose and lipid peroxidation. The results have been rationalized on the basis of the different possible mechanisms involved, depending on some of their properties, such as lipophilicity, the ability to scavenge free radicals, and to undergo single electron transfer (SET) reactions.

Different activation methods in sulfate radical-based oxidation for organic pollutants degradation: Catalytic mechanism and toxicity assessment of degradation intermediates

With the continuous development of industrialization, a growing number of refractory organic pollutants are released into the environment. These contaminants could cause serious risks to the human health and wildlife, therefore their degradation and mineralization is very critical and urgent. Recently sulfate radical-based advanced oxidation technology has been widely applied to organic pollutants treatment due to its high efficiency and eco-friendly nature. This review comprehensively summarizes different methods for persulfate (PS) and peroxymonosulfate (PMS) activation including ultraviolet light, ultrasonic, electrochemical, heat, radiation and alkali. The reactive oxygen species identification and mechanisms of PS/PMS activation by different approaches are discussed. In addition, this paper summarized the toxicity of degradation intermediates through bioassays and Ecological Structure Activity Relationships (ECOSAR) program prediction and the formation of toxic bromated disinfection byproducts (Br-DBPs) and carcinogenic bromate (BrO₃^-) in the presence of Br^-. The detoxification and mineralization of target pollutants induced by different reactive oxygen species are also analyzed. Finally, perspectives of potential future research and applications on sulfate radical-based advanced oxidation technology in the treatment of organic pollutants are proposed.

Reactive Oxygen Species in Emulated Martian Conditions and Their Effect on the Viability of the Unicellular Alga Scenedesmus dimorphus

Formation of oxygen-based free radicals from photochemical decomposition of hydrogen peroxide (H₂O₂) on Mars may be a key factor in the potential survival of terrestrial-like organisms on the red planet. Martian conditions that generate reactive oxygen species involve the decomposition of H₂O₂ at temperatures of around 278 K under relatively high doses of C-band ultraviolet radiation (UVC). This process is further amplified by the presence of iron oxides and perchlorates. Photosynthetic organisms exhibit a number of evolutionary traits that allow them to withstand both oxidative stress and UVC radiation. Here, we examine the effect of free radicals produced by the decomposition of H₂O₂ under emulated martian conditions on the viability of Scenedesmus dimorphus, a unicellular alga that is resistant to UVC radiation and varying levels of perchlorate and H₂O₂, both of which are present on Mars. Identification and quantification of free radicals formed under these conditions were performed with Electron Paramagnetic Resonance spectroscopy. These results were correlated with the viability of S. dimorphus, and the formation of oxygen-based free radicals and survival of the alga were found to be strongly dependent on the amount of H₂O₂ available. For H₂O₂ amounts close to those present in the rarefied martian environment, the products of these catalytic reactions did not have a significant effect on the algal population growth curve.

Menaquinone Biosynthesis: New Strategies to Trap Radical Intermediates in the MqnE-Catalyzed Reaction

Aminofutalosine synthase (MqnE) is a radical SAM enzyme that catalyzes the conversion of 3-((1-carboxyvinyl)oxy)benzoic acid to aminofutalosine during the futalosine-dependent menaquinone biosynthesis. In this Communication, we report the trapping of a radical intermediate in the MqnE-catalyzed reaction using sodium dithionite, molecular oxygen, or 5,5-dimethyl-1-pyrroline-N-oxide. These radical trapping strategies are potentially of general utility in the study of other radical SAM enzymes.

Degradation of malathion in the solution of acetyl peroxyborate activated by carbonate: Products, kinetics and mechanism

The degradation process of malathion in the acetyl peroxyborate (APB) solution of different APB/malathion molar ratio and in the carbonate-activated APB (APB/CO₃^2-) solution of different pH was studied by ³¹P NMR technology. In the APB solution, all malathion could be degraded in 47.5 min when the molar ratio of APB/malathion was 60. CO₃^2- could effectively activate APB to degrade all malathion in 10 min at pH of 10 when APB/malathion was 10, which was obviously higher than in APB solution. ¹O₂, •O₂^-, •OH and carbon-centered radicals (RC•) could be produced in the APB/CO₃^2- solution, and the degradation of malathion was mainly affected by RC•. The degradation mechanism of malathion in the APB/CO₃^2- solution was proposed based on the research results of malathion degradation process by ³¹P NMR and active species quenching test, which involves two steps: the first step is the oxidation of malathion to malaoxon by RC•, and the second step is the hydrolysis of malaoxon to dimethyl phosphate via hydroxyl anions nucleophilic addition.

Investigating the reactive oxygen species production of Rose Bengal and Merocyanine 540-loaded radioluminescent nanoparticles

Radioluminescent nanomaterials have garnered significant attention in the past decade due to their potential to perform X-ray mediated photodynamic therapy (X-PDT). Many of these materials are assumed to produce singlet oxygen based on a single assay. Herein we demonstrate that multiple assays are required to confidently determine whether singlet oxygen or other reactive oxygen species are being produced through type I or type II PDT mechanisms. Rose Bengal and Merocyanine 540 photosensitizers were loaded into mesoporous silica-coated NaLuF4:Dy3+,Gd3+ nanoparticles and the combination of ABDA, DPBF, and NaN3 assays along with electron paramagnetic resonance were employed to determine that superoxide and hydroxyl radicals were exclusively produced from this system under X-ray excitation. Knowledge of the correct PDT mechanism is crucial for informing what types of disease may be best suited for treatment using PDT nanosystems.

Plant Nitric Oxide Signaling under Drought Stress

Water deficit caused by drought is a significant threat to crop growth and production. Nitric oxide (NO), a water- and lipid-soluble free radical, plays an important role in cytoprotection. Apart from a few studies supporting the role of NO in drought responses, little is known about this pivotal molecular amendment in the regulation of abiotic stress signaling. In this review, we highlight the knowledge gaps in NO roles under drought stress and the technical challenges underlying NO detection and measurements, and we provide recommendations regarding potential avenues for future investigation. The modulation of NO production to alleviate abiotic stress disturbances in higher plants highlights the potential of genetic manipulation to influence NO metabolism as a tool with which plant fitness can be improved under adverse growth conditions.

Hybrid humic acid/titanium dioxide nanomaterials as highly effective antimicrobial agents against gram(-) pathogens and antibiotic contaminants in wastewater

Humic acids (HAs) provide an important bio-source for redox-active materials. Their functional chemical groups are responsible for several properties, such as metal ion chelating activity, adsorption ability towards small molecules and antibacterial activity, through reactive oxygen species (ROS) generation. However, the poor selectivity and instability of HAs in solution hinder their application. A promising strategy for overcoming these disadvantages is conjugation with an inorganic phase, which leads to more stable hybrid nanomaterials with tuneable functionalities. In this study, we demonstrate that hybrid humic acid/titanium dioxide nanostructured materials that are prepared via a versatile in situ hydrothermal strategy display promising antibacterial activity against various pathogens and behave as selective sequestering agents of amoxicillin and tetracycline antibiotics from wastewater. A physicochemical investigation in which a combination of techniques were utilized, which included TEM, BET, ¹³C-CPMAS-NMR, EPR, DLS and SANS, shed light on the structure-property-function relationships of the nanohybrids. The proposed approach traces a technological path for the exploitation of organic biowaste in the design at the molecular scale of multifunctional nanomaterials, which is useful for addressing environmental and health problems that are related to water contamination by antibiotics and pathogens.

Peracetic acid-based advanced oxidation processes for decontamination and disinfection of water: A review

Peracetic acid (PAA) has attracted growing attention as an alternative oxidant and disinfectant in wastewater treatment due to the increased demand to reduce chlorine usage and control disinfection byproducts (DBPs). These applications have stimulated new investigations on PAA-based advanced oxidation processes (AOPs), which can enhance water disinfection and remove micropollutants. The purpose of this review is to conduct a comprehensive analysis of scientific information and experimental data reported in recent years on the applications of PAA-based AOPs for the removal of chemical and microbiological micropollutants from water and wastewater. Various methods of PAA activation, including the supply of external energy and metal/metal-free catalysts, as well as their activation mechanisms are discussed. Then, a review on the usage of PAA-based AOPs for contaminant degradation is given. The degradation mechanisms of organic compounds and the influence of the controlling parameters of PAA-based treatment systems are summarized and discussed. Concurrently, the application of PAA-based AOPs for water disinfection and the related mechanisms of microorganism inactivation are also reviewed. Since combining UV light with PAA is the most commonly investigated PAA-based AOP for simultaneous pathogen inactivation and micropollutant oxidation, we have also focused on PAA microbial inactivation kinetics, together with the effects of key experimental parameters on the process. Moreover, we have discussed the advantages and disadvantages of UV/PAA as an AOP against the well-known and established UV/H₂O₂. Finally, the knowledge gaps, challenges, and new opportunities for research in this field are discussed. This critical review will facilitate an in-depth understanding of the PAA-based AOPs for water and wastewater treatment and provide useful perspectives for future research and development for PAA-based technologies.

Delivery of a mitochondria-targeted antioxidant from biocompatible, polymeric nanofibrous scaffolds

Cardiovascular disease has been associated with increased levels of reactive oxygen species (ROS). Recently, we have shown that a critical balance between cytosolic ROS and mitochondrial ROS is crucial in cardiovascular health and that modulation of mitochondrial ROS helps prevent detrimental effects of cytosolic ROS on endothelial cells (EC) in transgenic animals. Here, we report the development of a controlled delivery system for a mitochondria‐targeted antioxidant, JP4‐039, from an electrospun scaffold made of FDA‐approved biocompatible polymeric nanofibers. We demonstrate that the active antioxidant moiety was preserved in released JP4‐039 for over 72 h using electron paramagnetic resonance. We also show that both the initial burst release of the drug within the first 20 min and the ensuing slow and sustained release that occurred over the next 24 h improved tube formation in human coronary artery ECs (HCAEC) in vitro. Taken together, these findings suggest that electrospinning methods can be used to upload mitochondrial antioxidant (JP4‐039) onto a biocompatible nanofibrous PLGA scaffold, and the uploaded drug (JP4‐039) retains nitroxide antioxidant properties upon release from the scaffold, which in turn can reduce mitochondrial ROS and improve EC function in vitro.

Formation of free radicals by direct photolysis of halogenated phenols (HPs) and effects of DOM: A case study on monobromophenols

The free radicals play an important role to understand direct/indirect transformation mechanisms of organic pollutants. However, very few efforts have been made to elucidate the radicals produced by direct photolysis. In this study, the short-lived radicals generated under simulated sunlight irradiation from representative halogenated phenols (HPs), monobromophenols, were investigated by electron paramagnetic resonance (EPR). The results showed that three radicals, carbon-centered radical (C), hydrogen radical (H) and hydroxyl radical (OH), were generated from the direct irradiation of HPs. Compared to other substitutions, halogenated atom at para-position led to the highest production of these radicals which is in accordance with the energies calculated by density functional theory. Based on the analyses of the reactive species and corresponding intermediate adducts, the possible reaction pathways for these radicals were tentatively proposed. Dissolved organic matters (DOM) could enhance the photodegradation of HPs by directly affecting the radicals' formation, mainly due to generation of excited triplet DOM (³DOM*). A positive correlation was found between the concentrations of hydrated electron and the steady state ³DOM* from different DOM. Our findings provided insights into environmental photochemical fate of HPs through their direct photolysis and will help more accurately understand their phototransformation mechanisms in the environment.

Formation and Inhibition of Lipid Alkyl Radicals in Roasted Meat

Free radicals are ubiquitous in roasted foods. In this work, lipid-derived carbon-centered alkyl radical formation was first studied in roasted meat by electron spin resonance (ESR). The influence of antioxidants on the inhibition of free radicals was investigated. The results showed that the high temperature, high heat transfer rate, and high polyunsaturated fatty acid (PUFA) content resulted in high radical content in roasted meat, while the high water content in meat retarded radical formation. The 0.03% addition of tea polyphenols (TPP) significantly reduced radical formation during roasting (p < 0.05), whereas the 0.03% rosemary extract (RE) had no significant inhibitory effect (p > 0.05). These results suggested that water retention and the addition of TPP would decrease radical generation during the roasting of meat.

Substituted α-Phenyl and α-Naphthlyl-N-tert-butyl Nitrones: Synthesis, Spin-Trapping, and Neuroprotection Evaluation

New derivatives of α-phenyl-N-tert-butyl nitrone (PBN) bearing a hydroxyl, an acetate, or an acetamide substituent on the N-tert-butyl moiety and para-substituted phenyl or naphthlyl moieties were synthesized. Their ability to trap hydroxymethyl radical was evaluated by electron paramagnetic resonance spectroscopy. The presence of two electron-withdrawing substituents on both sides of the nitronyl function improves the spin-trapping properties, with 4-HOOC-PBN-CH₂OAc and 4-HOOC-PBN-CH₂NHAc being ∼4× more reactive than PBN. The electrochemical properties of the derivatives were further investigated by cyclic voltammetry and showed that the redox potentials of the nitrones are largely influenced by the nature of the substituents both on the aromatic ring and on the N-tert-butyl function. The acetamide derivatives PBN-CH₂NHAc, 4-AcNHCH₂-PBN-CH₂NHAc, and 4-MeO-PBN-CH₂NHAc were the easiest to oxidize. A computational approach was used to rationalize the effect of functionalization on the free energies of nitrone reactivity with hydroxymethyl radical as well as on the electron affinity and ionization potential. Finally, the neuroprotection of the derivatives was evaluated in an in vitro model of cellular injury on cortical neurons. Five derivatives showed good protection at very low concentrations (0.1-10 μM), with PBN-CH₂NHAc and 4-HOOC-PBN being the two most promising agents.

Selectedin vitro methods to determine antioxidant activity of hydrophilic/lipophilic substances

The topic of free radicals and related antioxidants is greatly discussed nowadays. Antioxidants help to neutralize free radicals before damaging cells. In the absence of antioxidants, a phenomenon called oxidative stress occurs. Oxidative stress can cause many diseases e.g. Alzheimer’s disease and cardiovascular diseases. Therefore, antioxidant activity of various compounds and the mechanism of their action have to be studied. Antioxidant activity and capacity are measured by in vitro and in vivo methods; in vitro methods are divided into two groups according to chemical reactions between free radicals and antioxidants. The first group is based on the transfer of hydrogen atoms (HAT), the second one on the transfer of electrons (ET). The most frequently used methods in the field of antioxidant power measurement are discussed in this work in terms of their principle, mechanism, methodology, the way of results evaluation and possible pitfalls.

Application of Cobalt/Peracetic Acid to Degrade Sulfamethoxazole at Neutral Condition: Efficiency and Mechanisms

An advanced oxidation process of combining cobalt and peracetic acid (Co/PAA) was developed to degrade sulfamethoxazole (SMX) in this study. The formed acetylperoxy radical (CH₃CO₃^•) through the activation of PAA by Co (Co²⁺) was the dominant radical responsible for SMX degradation, and acetoxyl radical (CH₃CO₂^•) might also have played a role. The efficient redox cycle of Co³⁺/Co²⁺ allows good removal efficiency of SMX even at quite low dosage of Co (<1 μM). The presence of H₂O₂ in the Co/PAA process has a negative effect on the degradation of SMX due to the competition for reactive radicals. The SMX degradation in the Co/PAA process is pH dependent, and the optimum reaction pH is near-neutral. Humic acid and HCO₃^- can inhibit SMX degradation in the Co/PAA process, while the presence of Cl^- plays a little role in the degradation of SMX in this system. Although transformation products of SMX in the Co/PAA system show higher acute toxicity, the low Co dose and SMX concentration in aquatic solution can efficiently weaken the acute toxicity. After reaction in the Co/PAA process, numerous carbon sources that could be provided for bacteria and algae growth can be produced, suggesting that the proposed Co/PAA process has good potential when combined with the biotreatment processes.

2-Amino-3'-dialkylaminobiphenyl-based fluorescent intracellular probes for nitric oxide surrogate N2O3

Fluorescent probes for nitric oxide (NO), or more frequently for its oxidized surrogate dinitrogen trioxide (N2O3), have enabled scientists to study the contributions of this signaling molecule to many physiological processes. Seeking to improve upon limitations of other probes, we have developed a family of fluorescent probes based on a 2-amino-3'-dialkylaminobiphenyl core. This core condenses with N2O3 to form benzo[c]cinnoline structures, incorporating the analyte into the newly formed fluorophore, which results in product fluorescence with virtually no background contribution from the initial probe. We varied the substituents in the core in order to optimize both the reactivity of the probes with N2O3 and their cinnoline products' fluorescence wavelengths and brightness. The top candidates were then applied to cultured cells to verify that they could respond to NO within cellular milieus, and the top performer, NO530, was compared with a "gold standard" commercial probe, DAF-FM, in a macrophage-derived cell line, RAW 264.7, stimulated to produce NO. NO530 demonstrated similar or better sensitivity and higher selectivity for NO than DAF, making it an attractive potential alternative for NO tracking in various applications.

Detection, identification, and quantification of oxidative protein modifications

Exposure of biological molecules to oxidants is inevitable and therefore commonplace. Oxidative stress in cells arises from both external agents and endogenous processes that generate reactive species, either purposely (e.g. during pathogen killing or enzymatic reactions) or accidentally (e.g. exposure to radiation, pollutants, drugs, or chemicals). As proteins are highly abundant and react rapidly with many oxidants, they are highly susceptible to, and major targets of, oxidative damage. This can result in changes to protein structure, function, and turnover and to loss or (occasional) gain of activity. Accumulation of oxidatively-modified proteins, due to either increased generation or decreased removal, has been associated with both aging and multiple diseases. Different oxidants generate a broad, and sometimes characteristic, spectrum of post-translational modifications. The kinetics (rates) of damage formation also vary dramatically. There is a pressing need for reliable and robust methods that can detect, identify, and quantify the products formed on amino acids, peptides, and proteins, especially in complex systems. This review summarizes several advances in our understanding of this complex chemistry and highlights methods that are available to detect oxidative modifications-at the amino acid, peptide, or protein level-and their nature, quantity, and position within a peptide sequence. Although considerable progress has been made in the development and application of new techniques, it is clear that further development is required to fully assess the relative importance of protein oxidation and to determine whether an oxidation is a cause, or merely a consequence, of injurious processes.

A coaxial nanocable textured by a cerium oxide shell and carbon core for sensing nitric oxide

A corn-like CeO₂/C coaxial cable textured by a cerium oxide shell and a carbon core was designed to sense NO. The carbon core possesses high electrical conductivity, and the CeO₂ surface delivers excellent electrocatalytic activity. The sensor, typically operated at 0.8 V (vs. Ag/AgCl), exhibits a detection limit of 1.7 nM, which is 4-times lower than that of CeO₂ nanotubes based one (at S/N = 3). It also displays wide linear response (up to 83 μM), a sensitivity of 0.81 μA μM^-1 cm^-2, and fast response (2 s). These values are highly competitive to that of a CeO₂ tube (0.92 μA μM^-1 cm^-2 and 2 s). The sensor was used to quantify NO that is released by Aspergillus flavus. Graphical abstractSchematic representation of corn-like CeO₂/C which can more sensitively and effectively detect NO released from A. flavus than when using CeO₂ nanotubes, benefitting from its unique coaxial cable structure.

Effects of biological buffer solutions on the peroxidase-like catalytic activity of Fe3O4 nanoparticles

Iron oxide nanoparticles (IONPs) are frequently used in biomedical applications due to their magnetic properties and putative chemical stability. Nevertheless, their well-known ability to mimic some features of the peroxidase enzyme activity under specific conditions of pH and temperature could lead to the formation of potentially harmful free radical species. In addition to the intrinsic enzyme-like activity of IONPs, the buffer solution is an important external factor that can alter dramatically the IONP activity because the buffer species can interact with the surface of the particles. In our study, IONP activity was evaluated in different buffering solutions under different experimental conditions and predominant free radical species were measured by electron paramagnetic resonance using the spin-trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The formation kinetics of the reactive oxygen species were studied by UV-visible spectroscopy with TMB and DAB peroxidase substrates. We found that the highest catalytic oxidation of peroxidase substrates and free radical generation were achieved in acetate buffer, while phosphate buffer inhibited the peroxidase-like activity of IONPs in a concentration dependent manner. When emulating the physiological conditions, a lower catalytic activity was observed at pH 7.4 when compared to that at pH 5.0. Also, in phosphate buffered saline (PBS), we observed an enhancement in the peroxidase substrate oxidation rate that was not accompanied by an increase in DMPO/adduct formation which could be related to a non-specific oxidation catalyzed by the chloride ion. Similar observations were found after the addition of a bicarbonate to HEPES buffer. TMB oxidation did not occur when the reaction was conducted with free iron ions from metal salts with the same concentration of the IONPs (0.33 Fe2+ and 0.66 Fe3+). However, we observed even higher catalytic activities than those when doubling the IONP concentration when they are combined with the free iron salts. These results indicate that biological buffering solutions need to be carefully considered when evaluating IONP catalytic activity and their potential toxicological effects since under physiological conditions of pH, salinity and buffering species, the peroxidase-like activity of IONPs is dramatically reduced.

Environmental remediation processes by zero valence copper: reaction mechanisms

Recent studies have shown Cu(0) as a promising material for the removal of organic and inorganic pollutants. However, there is no review addressing the studies performed. This fact may be related to the toxicity of the particles and the copper released in solution that has not motivated researchers, which entails in a reduced number of publications. However, studies point out how to solve the problem of Cu deposition in support materials. In this work, a detailed review of Cu(0) applications was performed. The specific focus was the reaction mechanisms related to adsorption, oxidation, and reduction processes. Initially, the resources that allow the understanding of the reaction mechanism, such as characterization techniques and the experimental conditions for investigation of the species involved in the process, were presented. The studies were evaluated separately, showing the mechanisms involved only with the application of Cu(0) in pure and isolated form and in association with oxidizing or reductive agents, combined with irradiation sources and ultrasonic waves and in the form supported in polymer matrices. It was verified that by the proposed reaction mechanisms, the exclusive participation of Cu(0), being the removal process, explained only by the redox behavior of copper. Therefore, the review showed the need for future research regarding the redox behavior of the contaminants.

Metal/Semiconductor Nanocomposites for Photocatalysis: Fundamentals, Structures, Applications and Properties

Due to the capability of utilizing light energy to drive chemical reactions, photocatalysis has been widely accepted as a green technology to help us address the increasingly severe environment and energy issues facing human society. To date, a large amount of research has been devoted to enhancing the properties of photocatalysts. As reported, coupling semiconductors with metals is one of the most effective methods to achieve high-performance photocatalysts. The excellent properties of metal/semiconductor (M/S) nanocomposite photocatalysts originate in two aspects: (i) improved charge separation at the metal-semiconductor interface; and (ii) increased absorption of visible light due to the surface plasmon resonance of metals. So far, many M/S nanocomposite photocatalysts with different structures have been developed for the application in environmental remediation, selective organic transformation, hydrogen evolution, and disinfection. Herein, we will give a review on the M/S nanocomposite photocatalysts, regarding their fundamentals, structures (as well as their typical synthetic approaches), applications and properties. Finally, we will also present our perspective on the future development of M/S nanocomposite photocatalysts.

Photoactivatable platinum anticancer complex can generate tryptophan radicals

l-Tryptophan (Trp), melatonin (MLT) and the Trp-peptide pentagastrin quenched the formation of azidyl radicals generated on irradiation of the anticancer complex trans,trans,trans-[Pt(pyridine)₂(N₃)₂(OH)₂] with visible light, giving rise to C3-centred indole radicals which were characterized for Trp and MLT using an EPR spin-trap.

l-Tryptophan (Trp), melatonin (MLT) and the Trp-peptide pentagastrin quenched the formation of azidyl radicals generated on irradiation of the anticancer complex trans,trans,trans-[Pt(pyridine)₂(N₃)₂(OH)₂] with visible light, giving rise to C3-centred indole radicals which were characterized for Trp and MLT using an EPR spin-trap; indole, together with azidyl and hydroxyl radicals, have potential roles in a multitargeting mechanism of action against resistant cancers.

A novel combinatorial technique for simultaneous quantification of oxygen radicals and aggregation reveals unexpected redox patterns in the activation of platelets by different physiopathological stimuli

The regulation of platelets by oxidants is critical for vascular health and may explain thrombotic complications in diseases such as diabetes and dementia, but remains poorly understood. Here, we describe a novel technique combining electron paramagnetic resonance spectroscopy and turbidimetry, which has been utilized to monitor simultaneously platelet activation and oxygen radical generation. This technique has been used to investigate the redox-dependence of human and mouse platelets. Using selective peptide inhibitors of NADPH oxidases (NOXs) on human platelets and genetically modified mouse platelets (NOX1^−/− or NOX2^−/−), we discovered that: 1) intracellular but not extracellular superoxide anion generated by NOX is critical for platelet activation by collagen; 2) superoxide dismutation to hydrogen peroxide is required for thrombin-dependent activation; 3) NOX1 is the main source of oxygen radicals in response to collagen, while NOX2 is critical for activation by thrombin; 4) two platelet modulators, namely oxidized low density lipoproteins (oxLDL) and amyloid peptide β (Aβ), require activation of both NOX1 and NOX2 to pre-activate platelets. This study provides new insights into the redox dependence of platelet activation. It suggests the possibility of selectively inhibiting platelet agonists by targeting either NOX1 (for collagen) or NOX2 (for thrombin). Selective inhibition of either NOX1 or NOX2 impairs the potentiatory effect of tested platelet modulators (oxLDL and Aβ), but does not completely abolish platelet hemostatic function. This information offers new opportunities for the development of disease-specific antiplatelet drugs with limited bleeding side effects by selectively targeting one NOX isoenzyme.

Embedding cyclic nitrone in mesoporous silica particles for EPR spin trapping of superoxide and other radicals

Mesoporous silica functionalised with a cyclic spin trap enabled the identification of a wide range of radicals in organic and aqueous media, including superoxide radical anion.

Reactivities of MeO-substituted PBN-type nitrones

MeO-derivatives of phenyl nitrones were synthesized and their electrochemical and spin-trapping properties were studied.

Targeting mitochondrial dysfunction and oxidative stress in heart failure: Challenges and opportunities

Mitochondrial dysfunction characterized by impaired bioenergetics, oxidative stress and aldehydic load is a hallmark of heart failure. Recently, different research groups have provided evidence that selective activation of mitochondrial detoxifying systems that counteract excessive accumulation of ROS, RNS and reactive aldehydes is sufficient to stop cardiac degeneration upon chronic stress, such as heart failure. Therefore, pharmacological and non-pharmacological approaches targeting mitochondria detoxification may play a critical role in the prevention or treatment of heart failure. In this review we discuss the most recent findings on the central role of mitochondrial dysfunction, oxidative stress and aldehydic load in heart failure, highlighting the most recent preclinical and clinical studies using mitochondria-targeted molecules and exercise training as effective tools against heart failure.

Comparison of different methods for measuring the superoxide radical by EPR spectroscopy in buffer, cell lysates and cells

As superoxide anion is of keen interest in biomedical research, it is highly desirable to have a technique allowing its detection sensitively and specifically in biological media. If electron paramagnetic resonance (EPR) techniques and probes have been individually described in the literature, there is actually no comparison of these techniques in the same conditions that may help guiding researchers for selecting the most appropriate approach. The aim of the present study was to compare different EPR strategies in terms of sensitivity and specificity to detect superoxide (vs. hydroxyl radical). Three main classes of EPR probes were used, including paramagnetic superoxide scavengers (such as nitroxides TEMPOL and mitoTEMPO as well as trityl CT-03), a spin trap (DIPPMPO), and diamagnetic superoxide scavengers (such as cyclic hydroxylamines CMH and mitoTEMPO-H). We analysed the reactivity of the different probes in the presence of a constant production of superoxide or hydroxyl radical in buffers and in cell lysates. We also assessed the performances of the different probes to detect superoxide produced by RAW264.7 macrophages stimulated by phorbol 12-myristate 13-acetate. In our conditions and models, we found that nitroxides were not specific for superoxide. CT-03 was specific, but the sensitivity of detection was low. Comparatively, we found that nitrone DIPPMPO and cyclic hydroxylamine CMH were good candidates to sensitively and specifically detect superoxide in complex biological media, CMH offering the best sensitivity.

Chemical structure and biological properties of sulfated fucan from the sequential extraction of subAntarctic Lessonia sp (Phaeophyceae)

This work is related to the structural characterization of the sulfated polysaccharide from Lessonia sp and the study of its antioxidant and antiparasitic properties. Sequential extraction afforded D-mannitol as the only low MW sugar alcohol. Extraction with 2% CaCl2 afforded in 3.0% yield, a sulfated fucan (SF). Its major fraction (48.5% yield), isolated by ion-exchange chromatography corresponds to a linear polymer of α-l-fucopyranosil residues linked 1→3, sulfated at the O-4 and partially at O-2 positions. By alkaline extraction, sodium alginate (10.3% yield) was obtained. The antioxidant capacity of SF by ESR showed high elimination index (IC50, mg/mL) of hydroxyl (0.27), alkoxy (10.05), and peroxyl (82.88) radicals in relation to commercial mannitol. SF showed activity against the epimastigote form of Trypanosoma cruzi parasite (250 μg/mL) and low cytotoxicity in murine cells (367 μg/mL). The elimination capacity of radicals in aqueous medium of SF would allow its potential biomedical application.

Structural and functional changes in RNAse A originating from tyrosine and histidine cross-linking and oxidation induced by singlet oxygen and peroxyl radicals

Oxidation can be induced by multiple processes in biological samples, with proteins being important targets due to their high abundance and reactivity. Oxidant reactions with proteins are not comprehensively understood, but it is known that structural and functional changes may be a cause, or a consequence, of disease. The mechanisms of oxidation of the model protein RNAse A by singlet oxygen (¹O₂) were examined and compared to peroxyl radical (ROO^•) oxidation, both common biological oxidants. This protein is a prototypic member of the RNAse family that exhibits antiviral activity by cleaving single-stranded RNA. RNAse A lacks tryptophan and cysteine residues which are major oxidant targets, but contains multiple histidine, tyrosine and methionine residues; these were therefore hypothesized to be the major sites of damage. ¹O₂ and ROO^• induce different patterns and extents of damage; both induce cross-links and side-chain oxidation, and ¹O₂ exposure modulates enzymatic activity. Multiple products have been characterized including methionine sulfoxide and sulfone, alcohols, DOPA, 2-oxohistidine, histidine-derived ring-opened species and inter- and intra-molecular cross-links (di-tyrosine, histidine-lysine, histidine-arginine, tyrosine-lysine). In addition to methionine modification, which appears not to be causative to activity loss, singlet oxygen also induces alteration to specific histidine, tyrosine and proline residues, including modification and cross-linking of the active site histidine, His12. The high homology among the RNAse family suggests that similar modifications may occur in humans, and be associated with the increased risk of viral infections in people with diabetes, as markers for ¹O₂ have been found in early stages of this pathology.

Low-Temperature EPR Spectroscopy as a Probe-Free Technique for Monitoring Oxidants Formed in Tumor Cells and Tissues: Implications in Drug Resistance and OXPHOS-Targeted Therapies

Oxidants formed from oxidative and nitrative metabolism include reactive oxygen species (ROS) such as superoxide, hydrogen peroxide/lipid hydroperoxides and reactive nitrogen species (RNS) (e.g., peroxynitrite [ONOO^-] and nitrogen dioxide), and reactive halogenated species (e.g., hypochlorous acid [HOCl]). Increasingly, ROS and RNS are implicated in tumorigenesis as well as tumor growth, progression, and metastasis. Recently, ROS were implicated in drug resistance, metabolic reprogramming, and T-cell metabolism in immunotherapy. Mostly, fluorescent probes have been used in cell culture systems. The identity of species is obtained by LC-MS analyses of diagnostic marker products. However, extrapolation of these assays to cancer xenografts is difficult if not impossible. Thus, development of a probe-free assay for monitoring and assessing oxidant formation in tumor cells and tumor xenografts is critical and timely. Here, we describe the use of ex vivo electron paramagnetic resonance (EPR) spectroscopy at cryogenic temperatures as a uniquely useful probe-free technique for assessing intracellular oxidation and oxidants via EPR signals from redox centers, particularly iron-sulfur clusters, in mitochondrial and cytosolic redox proteins. Examples of cancer cells subjected to inhibition of mitochondrial oxidative phosphorylation are presented. This ex vivo methodology can be readily extended to monitor oxidant formation in tumor tissues isolated from mice and humans.

Cold atmospheric-pressure plasma induces DNA-protein crosslinks through protein oxidation

Reactive oxygen and nitrogen species (ROS and RNS) generated by cold atmospheric-pressure plasma could damage genomic DNA, although the precise types of these DNA damage induced by plasma are poorly characterized. Understanding plasma-induced DNA damage will help to elucidate the biological effect of plasma and guide the application of plasma in ROS-based therapy. In this study, it was shown that ROS and RNS generated by physical plasma could efficiently induce DNA-protein crosslinks (DPCs) in bacteria, yeast, and human cells. An in vitro assay showed that plasma treatment resulted in the formation of covalent DPCs by activating proteins to crosslink with DNA. Mass spectrometry and hydroperoxide analysis detected oxidation products induced by plasma. DPC formation were alleviated by singlet oxygen scavenger, demonstrating the importance of singlet oxygen in this process. These results suggested the roles of DPC formation in DNA damage induced by plasma, which could improve the understanding of the biological effect of plasma and help to develop a new strategy in plasma-based therapy including infection and cancer therapy.

Detection and Characterization of Reactive Oxygen and Nitrogen Species in Biological Systems by Monitoring Species-Specific Products

SIGNIFICANCE

Since the discovery of the superoxide dismutase enzyme, the generation and fate of short-lived oxidizing, nitrosating, nitrating, and halogenating species in biological systems has been of great interest. Despite the significance of reactive oxygen species (ROS) and reactive nitrogen species (RNS) in numerous diseases and intracellular signaling, the rigorous detection of ROS and RNS has remained a challenge. Recent Advances: Chemical characterization of the reactions of selected ROS and RNS with electron paramagnetic resonance (EPR) spin traps and fluorescent probes led to the establishment of species-specific products, which can be used for specific detection of several forms of ROS and RNS in cell-free systems and in cultured cells in vitro and in animals in vivo. Profiling oxidation products from the ROS and RNS probes provides a rigorous method for detection of those species in biological systems.

CRITICAL ISSUES

Formation and detection of species-specific products from the probes enables accurate characterization of the oxidative environment in cells. Measurement of the total signal (fluorescence, chemiluminescence, etc.) intensity does not allow for identification of the ROS/RNS formed. It is critical to identify the products formed by using chromatographic or other rigorous techniques. Product analyses should be accompanied by monitoring of the intracellular probe level, another factor controlling the yield of the product(s) formed.

FUTURE DIRECTIONS

More work is required to characterize the chemical reactivity of the ROS/RNS probes, and to develop new probes/detection approaches enabling real-time, selective monitoring of the specific products formed from the probes. Antioxid. Redox Signal. 28, 1416-1432.

In Vivo and In Situ Detection of Macromolecular Free Radicals Using Immuno-Spin Trapping and Molecular Magnetic Resonance Imaging

SIGNIFICANCE

In vivo free radical imaging in preclinical models of disease has become a reality. Free radicals have traditionally been characterized by electron spin resonance (ESR) or electron paramagnetic resonance (EPR) spectroscopy coupled with spin trapping. The disadvantage of the ESR/EPR approach is that spin adducts are short-lived due to biological reductive and/or oxidative processes. Immuno-spin trapping (IST) involves the use of an antibody that recognizes macromolecular 5,5-dimethyl-pyrroline-N-oxide (DMPO) spin adducts (anti-DMPO antibody), regardless of the oxidative/reductive state of trapped radical adducts. Recent Advances: The IST approach has been extended to an in vivo application that combines IST with molecular magnetic resonance imaging (mMRI). This combined IST-mMRI approach involves the use of a spin-trapping agent, DMPO, to trap free radicals in disease models, and administration of an mMRI probe, an anti-DMPO probe, which combines an antibody against DMPO-radical adducts and an MRI contrast agent, resulting in targeted free radical adduct detection.

CRITICAL ISSUES

The combined IST-mMRI approach has been used in several rodent disease models, including diabetes, amyotrophic lateral sclerosis (ALS), gliomas, and septic encephalopathy. The advantage of this approach is that heterogeneous levels of trapped free radicals can be detected directly in vivo and in situ to pin point where free radicals are formed in different tissues.

FUTURE DIRECTIONS

The approach can also be used to assess therapeutic agents that are either free radical scavengers or generate free radicals. Smaller probe constructs and radical identification approaches are being considered. The focus of this review is on the different applications that have been studied, advantages and limitations, and future directions. Antioxid. Redox Signal. 28, 1404-1415.

Detection of mitochondria-generated reactive oxygen species in cells using multiple probes and methods: Potentials, pitfalls, and the future

Reactive oxygen and nitrogen species (ROS/RNS) such as superoxide (O₂^̇̄), hydrogen peroxide, lipid hydroperoxides, peroxynitrite, and hypochlorous and hypobromous acids play a key role in many pathophysiological processes. Recent studies have focused on mitochondrial ROS as redox signaling species responsible for promoting cell division, modulating and regulating kinases and phosphatases, and activating transcription factors. Many ROS also stimulate cell death and senescence. The extent to which these processes occur is attributed to ROS levels (low or high) in cells. However, the exact nature of ROS remains unknown. Investigators have used redox-active probes that, upon oxidation by ROS, yield products exhibiting fluorescence, chemiluminescence, or bioluminescence. Mitochondria-targeted probes can be used to detect ROS generated in mitochondria. However, because most of these redox-active probes (untargeted and mitochondria-targeted) are oxidized by several ROS species, attributing redox probe oxidation to specific ROS species is difficult. It is conceivable that redox-active probes are oxidized in common one-electron oxidation pathways, resulting in a radical intermediate that either reacts with another oxidant (including oxygen to produce O₂^̇̄) and forms a stable fluorescent product or reacts with O₂^̇̄ to form a fluorescent marker product. Here, we propose the use of multiple probes and complementary techniques (HPLC, LC-MS, redox blotting, and EPR) and the measurement of intracellular probe uptake and specific marker products to identify specific ROS generated in cells. The low-temperature EPR technique developed to investigate cellular/mitochondrial oxidants can easily be extended to animal and human tissues.