2-ethoxycarbonyl-2-methyl-3,4-dihydro-2H-pyrrole-1-oxide: evaluation of the spin trapping properties

A nanozyme-based enhanced system for total removal of organic mercury and SERS sensing

Total removal of organic mercury in industrial wastewater is a crucially important task facing environmental pollution in the current world. Herein, we demonstrate the fabrication of Au-NiFe layered double hydroxide (LDH)/rGO nanocomposite as not only an efficient nanozyme with oxidase-like activity but also an efficient surface-enhanced Raman spectroscopy (SERS) substrate to determine organic mercury, with the minimum detection concentration as low as 1 × 10^-8 M. According to the binding energy of X-Ray photoelectron spectrometer (XPS) and the free radicals of electron paramagnetic resonance (EPR) spectra, the mechanism of catalytic enhanced degradation is the production of Au-amalgam on Au surface, accelerating the electron transfer and the generation of O₂^•- radicals from oxygen molecules and •CH₃ radicals from the methyl group in MeHg to participate the oxidase-like reaction. Furthermore, the Au-NiFe LDH/rGO nanocomposite is able to degrade and remove 99.9% of organic mercury in two hours without the secondary pollution by Hg²⁺. In addition, the material can be used for the multiple degradation-regeneration cycles in actual applications, which is significant in terms of the environmental and economic point of view. This work may open a new horizon for both highly sensitive detection and thorough degradation of organic mercury in environmental science and technology.

Para-Substituted α-Phenyl-N-tert-butyl Nitrones: Spin-Trapping, Redox and Neuroprotective Properties

In this work, a series of para-substituted α-phenyl-N-tert-butyl nitrones (PBN) were studied. Their radical-trapping properties were evaluated by electron paramagnetic resonance, with 4-CF₃-PBN being the fastest derivative to trap the hydroxymethyl radical (^•CH₂OH). The redox properties of the nitrones were further investigated by cyclic voltammetry, and 4-CF₃-PBN was the easiest to reduce and the hardest to oxidize. This is due to the presence of the electron-withdrawing CF₃ group. Very good correlations between the Hammett constants (σ_p) of the substituents and both spin-trapping rates and redox potentials were observed. These correlations were further supported by computationally determined ionization potentials and atom charge densities. Finally, the neuroprotective effect of these derivatives was studied using two different in vitro models of cell death on primary cortical neurons injured by glutamate exposure or on glial cells exposed to ^t BuOOH. Trends between the protection afforded by the nitrones and their lipophilicity were observed. 4-CF₃-PBN was the most potent agent against ^t BuOOH-induced oxidative stress on glial cells, while 4-Me₂N-PBN showed potency in both models.

5-Aryl-2-(3,5-dialkyl-4-hydroxyphenyl)-4,4-dimethyl-4H-imidazole 3-Oxides and Their Redox Species: How Antioxidant Activity of 1-Hydroxy-2,5-dihydro-1H-imidazoles Correlates with the Stability of Hybrid Phenoxyl-Nitroxides

Cyclic nitrones of the imidazole series, containing a sterically hindered phenol group, are promising objects for studying antioxidant activity; on the other hand, they can form persistent hybrid phenoxyl-nitroxyl radicals (HPNs) upon oxidation. Here, a series of 5-aryl-4,4-dimethyl-4H-imidazole 3-oxides was obtained by condensation of aromatic 2-hydroxylaminoketones with 4-formyl-2,6-dialkylphenols followed by oxidation of the initially formed N-hydroxy derivatives. It was shown that the antioxidant activity of both 1-hydroxy-2,5-dihydroimidazoles and 4H-imidazole 3-oxides increases with a decrease in steric volume of the alkyl substituent in the phenol group, while the stability of the corresponding HPNs generated from 4H-imidazole 3-oxides reveals the opposite tendency.

Heterogeneous Fenton-like degradation of EDTA in an aqueous solution with enhanced COD removal under neutral pH

By providing the key carbon and nitrogen elements needed for eutrophication, the potential toxicity of ethylenediaminetetraacetic acid (EDTA) prompts the exploration of effective treatment methods to minimize the amount of EDTA released into the environment. In this study, Fe₃O₄ magnetic nanoparticles (MNPs) were prepared and used as catalysts to study the mineralization of EDTA in Fenton-like reactions under neutral pH. Fe₃O₄ MNPs were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), and Brunauer-Emmett-Teller (BET). The effects of pH, ferric ion leaching, and H₂O₂ concentration on chemical oxygen demand (COD) removal of EDTA were investigated. The morphological characterization of the nanoparticles suggests a quasi-spherical structure with small particle size and a surface area of 49.9 m²/g. The results show that Fe₃O₄ MNPs had good catalytic activity for the mineralization of EDTA under pH 5.0-9.0. The optimum conditions for the COD removal of 45% at pH 7.0 were: 40 mM H₂O₂, 10 mM Fe₃O₄, and 1 g/L EDTANa₂·2H₂O at 303 K. Fe₃O₄ MNPs maintained high catalytic activity after five cycles of continuous degradation of EDTA. According to reactive oxidizing species measurements obtained by electron spin resonance (ESR), it was confirmed that HO· free radicals, presented in the H₂O₂/Fe₃O₄ MNPs heterogeneous Fenton-like reaction, were the primary active group in the removal of EDTA. These features can be considered beneficial to the application of Fe₃O₄ MNPs towards industrial wastewater treatment.

Synthesis and evaluation of new heteroaryl nitrones with spin trap properties

A new series of heteroaryl nitrones were synthesized and evaluated as free radical traps due to the results showed in our previous report. The physicochemical characterization of these new nitrones by electron spin resonance (ESR) demonstrated their high capability to trap and stabilize different atom centered free radicals generated by the Fenton reaction. Additionally, we intensely studied them in terms of their physicochemical properties. Kinetic studies, including the use of a method based on competition and the hydroxyl adduct decay, gave the corresponding rate constants and half-lives at the physiological pH of these newly synthesized nitrones. New nitrones derived from quinoxaline 1,4-dioxide heterocycles were more suitable than DMPO to trap hydroxyl free radicals with a half-life longer than two hours. We explain some of the results using computational chemistry through density functional theory (DFT).

A new series of heteroaryl nitrones were synthesized and evaluated as free radical traps due to the results showed in our previous report.

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.

Synthesis and properties of a series of β-cyclodextrin/nitrone spin traps for improved superoxide detection

Three new DEPMPO-based spin traps have been designed and synthesized for improved superoxide detection, each carrying a cyclodextrin (CD) moiety but with a different alkyl chain on the phosphorus atom or with a long spacer arm. EPR spectroscopy allowed us to estimate the half-life of the superoxide spin adducts which is close to the value previously reported for CD-DEPMPO (t_1/2 ≈ 50-55 min under the conditions investigated). The spectra are typical of superoxide adducts (almost no features of the HO˙ adduct that usually forms with time for other nitrone spin traps such as DMPO) and we show that at 250 μM, the new spin trap enables the reliable detection of superoxide by 1 scan at the position opposite to the corresponding spin trap without the CD moiety. The resistance of the spin adducts to a reduction process has been evaluated, and the superoxide spin adducts are sensitive to ascorbate and glutathione (GSH), but not to glutathione peroxidase/GSH, reflecting the exposed nature of the nitroxide moiety to the bulk solvent. To understand these results, 2D-ROESY NMR studies and molecular dynamics pointed to a shallow or surface self-inclusion of the nitrone spin traps and of nitroxide spin adducts presumably due to the high flexibility of the permethyl-β-CD rim.

Amidinoquinoxaline N-oxides: spin trapping of O- and C-centered radicals

Amidinoquinoxaline N-oxides represent a novel family of heterocyclic spin traps. In this work, their ability to trap O- and C-centered radicals was tested using selected derivatives with different structural modifications. All the studied nitrones were able to trap radicals forming persistent spin adducts, also in the case of OH and OOH radicals which are of wide biological interest as examples of ROS. The stability of the adducts was mainly attributed to the wide delocalization of the unpaired electron over the whole quinoxaline moiety. The nitroxide spectral parameters (hfccs and g-factors) were analyzed and the results were supported by DFT calculations. The N-19 hfccs and g-factors were characteristic of each aminoxyl and could aid in the identification of the trapped radical. The enhanced stability of the OH adducts under the employed reaction conditions could be ascribed to their possible stabilization by IHBs with two different acceptors: the N-O˙ moiety or the amidine functionality. DFT calculations indicate that the preferred IHB is strongly conditioned by the amidine ring size. While five membered homologues show a clear preference for the IHB with the N-O˙ group, in six membered derivatives this stabilizing interaction is preferentially established with the amidine nitrogen as an IHB acceptor.

Use of a cocktail of spin traps for fingerprinting large range of free radicals in biological systems

It is well established that the formation of radical species centered on various atoms is involved in the mechanism leading to the development of several diseases or to the appearance of deleterious effects of toxic molecules. The detection of free radical is possible using Electron Paramagnetic Resonance (EPR) spectroscopy and the spin trapping technique. The classical EPR spin-trapping technique can be considered as a "hypothesis-driven" approach because it requires an a priori assumption regarding the nature of the free radical in order to select the most appropriate spin-trap. We here describe a "data-driven" approach using EPR and a cocktail of spin-traps. The rationale for using this cocktail was that it would cover a wide range of biologically relevant free radicals and have a large range of hydrophilicity and lipophilicity in order to trap free radicals produced in different cellular compartments. As a proof-of-concept, we validated the ability of the system to measure a large variety of free radicals (O-, N-, C-, or S- centered) in well characterized conditions, and we illustrated the ability of the technique to unambiguously detect free radical production in cells exposed to chemicals known to be radical-mediated toxic agents.

The Hydroxyl Radical is a Critical Intermediate in the Voltammetric Detection of Hydrogen Peroxide

Cyclic voltammetry is a widely used and powerful tool for sensitively and selectively measuring hydrogen peroxide (H2O2). Herein, voltammetry was combined with electron paramagnetic resonance spectroscopy to identify and define the role of an oxygen-centered radical liberated during the oxidation of H2O2. The spin-trap reagents, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and 2-ethoxycarbonyl-2-methyl-3,4-dihydro-2H-pyrrole-1-oxide (EMPO), were employed. Spectra exhibit distinct hyperfine patterns that clearly identify the DMPO(•)-OH and EMPO(•)-OH adducts. Multiple linear regression analysis of voltammograms demonstrated that the hydroxyl radical is a principal contributor to the voltammetry of H2O2, as signal is attenuated when this species is trapped. These data incorporate a missing, fundamental element to our knowledge of the mechanisms that underlie H2O2 electrochemistry.

Reactive oxygen species: Reactions and detection from photosynthetic tissues

Reactive oxygen species (ROS) have long been recognized as compounds with dual roles. They cause cellular damage by reacting with biomolecules but they also function as agents of cellular signaling. Several different oxygen-containing compounds are classified as ROS because they react, at least with certain partners, more rapidly than ground-state molecular oxygen or because they are known to have biological effects. The present review describes the typical reactions of the most important ROS. The reactions are the basis for both the detection methods and for prediction of reactions between ROS and biomolecules. Chemical and physical methods used for detection, visualization and quantification of ROS from plants, algae and cyanobacteria will be reviewed. The main focus will be on photosynthetic tissues, and limitations of the methods will be discussed.

Amidinoquinoxaline N-oxides as novel spin traps

A series of nitrones were synthesized and tested as novel spin traps. The adducts generated by CH₃ addition showed remarkably persistent signals. Their EPR features and kinetics were rationalised by DFT and MP2 calculations.

Fused-ring derivatives of quinoxalines: spectroscopic characterization and photoinduced processes investigated by EPR spin trapping technique

10-Ethyl-7-oxo-7,10-dihydropyrido[2,3-f]quinoxaline derivatives, synthesized as promising biologically/photobiologically active compounds were characterized by UV/vis, FT-IR and fluorescent spectroscopy. Photoinduced processes of these derivatives were studied by EPR spectroscopy, monitoring in situ the generation of reactive intermediates upon UVA (λmax=365 nm) irradiation. The formation of reactive oxygen species and further oxygen- and carbon-centered radical intermediates was detected and possible reaction routes were suggested. To quantify the investigated processes, the quantum yields of the superoxide radical anion spin-adduct and 4-oxo-2,2,6,6-tetramethylpiperidine N-oxyl generation were determined, reflecting the activation of molecular oxygen by the excited state of the quinoxaline derivative.

Mitochondria-targeted spin traps: synthesis, superoxide spin trapping, and mitochondrial uptake

Development of reliable methods and site-specific detection of free radicals is an active area of research. Here, we describe the synthesis and radical-trapping properties of new derivatives of DEPMPO and DIPPMPO, bearing a mitochondria-targeting triphenylphosphonium cationic moiety or guanidinium cationic group. All of the spin traps prepared have been observed to efficiently trap superoxide radical anions in a cell-free system. The superoxide spin adducts exhibited similar spectral properties, indicating no significant differences in the geometry of the cyclic nitroxide moieties of the spin adducts. The superoxide adduct stability was measured and observed to be highest (t1/2 = 73 min) for DIPPMPO nitrone linked to triphenylphosphonium moiety via a short carbon chain (Mito-DIPPMPO). The experimental results and DFT quantum chemical calculations indicate that the cationic property of the triphenylphosphonium group may be responsible for increased superoxide trapping efficiency and adduct stability of Mito-DIPPMPO, as compared to the DIPPMPO spin trap. The studies of uptake of the synthesized traps into isolated mitochondria indicated the importance of both cationic and lipophilic properties, with the DEPMPO nitrone linked to the triphenylphosphonium moiety via a long carbon chain (Mito10-DEPMPO) exhibiting the highest mitochondrial uptake. We conclude that, of the synthesized traps, Mito-DIPPMPO and Mito10-DEPMPO are the best candidates for potential mitochondria-specific spin traps for use in biologically relevant systems.

Detection of superoxide production in stimulated and unstimulated living cells using new cyclic nitrone spin traps

Reactive oxygen species (ROS), including superoxide anion and hydrogen peroxide (H2O2), have a diverse array of physiological and pathological effects within living cells depending on the extent, timing, and location of their production. For measuring ROS production in cells, the ESR spin trapping technique using cyclic nitrones distinguishes itself from other methods by its specificity for superoxide and hydroxyl radical. However, several drawbacks, such as the low spin trapping rate and the spontaneous and cell-enhanced decomposition of the spin adducts to ESR-silent products, limit the application of this method to biological systems. Recently, new cyclic nitrones bearing a triphenylphosphonium (Mito-DIPPMPO) or a permethylated β-cyclodextrin moiety (CD-DIPPMPO) have been synthesized and their spin adducts demonstrated increased stability in buffer. In this study, a comparison of the spin trapping efficiency of these new compounds with commonly used cyclic nitrone spin traps, i.e., 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and analogs BMPO, DEPMPO, and DIPPMPO, was performed on RAW 264.7 macrophages stimulated with phorbol 12-myristate 13-acetate. Our results show that Mito-DIPPMPO and CD-DIPPMPO enable a higher detection of superoxide adduct, with a low (if any) amount of hydroxyl adduct. CD-DIPPMPO, especially, appears to be a superior spin trap for extracellular superoxide detection in living macrophages, allowing measurement of superoxide production in unstimulated cells for the first time. The main rationale put forward for this extreme sensitivity is that the extracellular localization of the spin trap prevents the reduction of the spin adducts by ascorbic acid and glutathione within cells.

Synthesis and spin-trapping properties of a trifluoromethyl analogue of DMPO: 5-methyl-5-trifluoromethyl-1-pyrroline N-oxide (5-TFDMPO)

The 5-diethoxyphosphonyl-5-methyl-1-pyrroline N-oxide superoxide spin adduct (DEPMPO-OOH) is much more persistent (about 15 times) than the 5,5-dimethyl-1-pyrroline N-oxide superoxide spin adduct (DMPO-OOH). The diethoxyphosphonyl group is bulkier than the methyl group and its electron-withdrawing effect is much stronger. These two factors could play a role in explaining the different half-lifetimes of DMPO-OOH and DEPMPO-OOH. The trifluoromethyl and the diethoxyphosphonyl groups show similar electron-withdrawing effects but have different sizes. We have thus synthesized and studied 5-methyl-5-trifluoromethyl-1-pyrroline N-oxide (5-TFDMPO), a new trifluoromethyl analogue of DMPO, to compare its spin-trapping performance with those of DMPO and DEPMPO. 5-TFDMPO was prepared in a five-step sequence by means of the Zn/AcOH reductive cyclization of 5,5,5-trifluoro-4-methyl-4-nitropentanal, and the geometry of the molecule was estimated by using DFT calculations. The spin-trapping properties were investigated both in toluene and in aqueous buffer solutions for oxygen-, sulfur-, and carbon-centered radicals. All the spin adducts exhibit slightly different fluorine hyperfine coupling constants, thereby suggesting a hindered rotation of the trifluoromethyl group, which was confirmed by variable-temperature EPR studies and DFT calculations. In phosphate buffer at pH 7.4, the half-life of 5-TFDMPOOOH is about three times shorter than for DEPMPO-OOH and five times longer than for DMPO-OOH. Our results suggest that the stabilization of the superoxide adducts comes from a delicate balance between steric, electronic, and hydrogen-bonding effects that involve the β group, the hydroperoxyl moiety, and the nitroxide.

Metabolic stability of superoxide adducts derived from newly developed cyclic nitrone spin traps

Reactive oxygen species are by-products of aerobic metabolism involved in the onset and evolution of various pathological conditions. Among them, the superoxide radical is of special interest as the origin of several damaging species such as H2O2, hydroxyl radical, or peroxynitrite (ONOO(-)). Spin trapping coupled with ESR is a method of choice to characterize these species in chemical and biological systems and the metabolic stability of the spin adducts derived from reaction of superoxide and hydroxyl radicals with nitrones is the main limit to the in vivo application of the method. Recently, new cyclic nitrones bearing a triphenylphosphonium or permethylated β-cyclodextrin moiety have been synthesized and their spin adducts demonstrated increased stability in buffer. In this article, we studied the stability of the superoxide adducts of four new cyclic nitrones in the presence of liver subcellular fractions and biologically relevant reductants using an original setup combining a stopped-flow device and an ESR spectrometer. The kinetics of disappearance of the spin adducts were analyzed using an appropriate simulation program. Our results highlight the interest of the new spin trapping agents CD-DEPMPO and CD-DIPPMPO for specific detection of superoxide with high stability of the superoxide adducts in the presence of liver microsomes.

Detection of nitric oxide and superoxide radical anion by electron paramagnetic resonance spectroscopy from cells using spin traps

Reactive nitrogen/oxygen species (ROS/RNS) at low concentrations play an important role in regulating cell function, signaling, and immune response but in unregulated concentrations are detrimental to cell viability. While living systems have evolved with endogenous and dietary antioxidant defense mechanisms to regulate ROS generation, ROS are produced continuously as natural by-products of normal metabolism of oxygen and can cause oxidative damage to biomolecules resulting in loss of protein function, DNA cleavage, or lipid peroxidation, and ultimately to oxidative stress leading to cell injury or death. Superoxide radical anion (O2•-) is the major precursor of some of the most highly oxidizing species known to exist in biological systems such as peroxynitrite and hydroxyl radical. The generation of O2•- signals the first sign of oxidative burst, and therefore, its detection and/or sequestration in biological systems is important. In this demonstration, O2•- was generated from polymorphonuclear neutrophils (PMNs). Through chemotactic stimulation with phorbol-12-myristate-13-acetate (PMA), PMN generates O2•- via activation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase. Nitric oxide (NO) synthase which comes in three isoforms, as inducible-, neuronal- and endothelial-NOS, or iNOS, nNOS or eNOS, respectively, catalyzes the conversion of L- arginine to L-citrulline, using NADPH to produce NO. Here, we generated NO from endothelial cells. Under oxidative stress conditions, eNOS for example can switch from producing NO to O2•- in a process called uncoupling, which is believed to be caused by oxidation of heme or the co-factor, tetrahydrobiopterin (BH4). There are only few reliable methods for the detection of free radicals in biological systems but are limited by specificity and sensitivity. Spin trapping is commonly used for the identification of free radicals and involves the addition reaction of a radical to a spin trap forming a persistent spin adduct which can be detected by electron paramagnetic resonance (EPR) spectroscopy. The various radical adducts exhibit distinctive spectrum which can be used to identify the radicals being generated and can provide a wealth of information about the nature and kinetics of radical production. The cyclic nitrones, 5,5-dimethyl-pyrroline-N-oxide, DMPO, the phosphoryl-substituted DEPMPO, and the ester-substituted, EMPO and BMPO, have been widely employed as spin traps--the latter spin traps exhibiting longer half-lives for O2•- adduct. Iron (II)-N-methyl-D-glucamine dithiocarbamate, Fe(MGD)2 is commonly used to trap NO due to high rate of adduct formation and the high stability of the spin adduct.

Spin trapping of hydroperoxyl radical by a cyclic nitrone conjugated to β-cyclodextrin: a computational study

Spin trapping of hydroperoxyl radical (HOO^.) by the amide-linked conjugate of 5-carbamoyl-5-methyl-1-pyrroline N-oxide (AMPO) to β-cyclodextrin (β-CD) was studied computationally using a two-layered ONIOM method. From a conformational perspective, the "internal" conformation of 5R-β-CD-AMPO is more favored than the "external" conformation in which the nitrone is located outside of the cavity of the β-CD. When the HOO^. addition product is formed, the most stable isomer has the nitroxyl (N₁-O₁) moiety pointing inside the cavity of the β-CD. Thus, this "internal" conformation might protect the N₁-O₁ moiety of the resulting spin adduct from access by reducing agents, thereby improving the lifetime of the radical adduct. The computed energetic barrier for HOO^. addition to the 5R-β-CD-AMPO is 8.7 kcal/mol, which is marginally smaller than spin trapping by the non-conjugated AMPO (that is, without the β-CD). To optimize the reactivity of the β-CD-AMPO conjugate, the effect of a spacer unit between the AMPO segment and the β-CD moiety with varying methylene units, (CH₂) _n (n = 1, 2, 3), on the energetics of HOO^. addition was evaluated. The structure with only one methylene spacer (n = 1) appears to be optimal as determined by the smaller activation barrier (6.2 kcal/mol) for HOO^. addition to the nitrone moiety. Compared with very time-consuming quantum mechanical methods, the ONIOM method appears to offer significant advantages for evaluation of the best β-CD-AMPO conjugate for trapping of such reactive oxygen species and providing for the rational design of novel nitrones as spin traps.

An effective strategy for the synthesis of biocompatible gold nanoparticles using danshensu antioxidant: prevention of cytotoxicity via attenuation of free radical formation

To suppress the cytotoxicity of gold nanoparticles (AuNPs), danshensu, a naturally occurring polyphenol antioxidant isolated from Chinese herb, was used to provide a fundamental protection layer for AuNPs, to alleviate oxidative stress and as a reducing agent to react with chloroauric acid. Besides danshensu, gum arabic was chosen as an auxiliary stabilising agent to improve the stability of AuNPs against aggregation. As expected, the prepared GA-DS-AuNPs (gum arabic-danshensu-gold nanoparticle) was remarkably stable in various buffer solutions. More interestingly, the GA-DS-AuNPs not only did not show any appreciable cytotoxicity, but also could alleviate the oxidative damage induced by AuNPs. Meanwhile, the ROS/RNS scavenging activities of GA-DS-AuNPs was evaluated by electron spin resonance spectroscopy (ESR), potentiometric nitric oxide (NO) sensor and cell confocal imaging. The results suggest that GA-DS-AuNPs might have effectively reduced the AuNPs-induced cytotoxicity and oxidative stress by downregulation of ROS/NOS production. The GA-DS-AuNPs may provide potential opportunities for the application in nanomedicine and nanobiology.

Reactivities of superoxide and hydroperoxyl radicals with disubstituted cyclic nitrones: a DFT study

The unique ability of nitrone spin traps to detect and characterize transient free radicals by electron paramagnetic resonance (EPR) spectroscopy has fueled the development of new spin traps with improved properties. Among a variety of free radicals in chemical and biological systems, superoxide radical anion (O(2)(•-)) plays a critical role as a precursor to other more oxidizing species such as hydroxyl radical (HO(•)), peroxynitrite (ONOO(-)), and hypochlorous acid (HOCl), and therefore the direct detection of O(2)(•-) is important. To overcome the limitations of conventional cyclic nitrones, that is, poor reactivity with O(2)(•-), instability of the O(2)(•-) adduct, and poor cellular target specificity, synthesis of disubstituted nitrones has become attractive. Disubstituted nitrones offer advantages over the monosubstituted ones because they allow bifunctionalization of spin traps, therefore accommodating all the desired spin trap properties in one molecular design. However, because of the high number of possible disubstituted analogues as candidate, a systematic computational study is needed to find leads for the optimal spin trap design for biconjugation. In this paper, calculation of the energetics of O(2)(•-) and HO(2)(•) adduct formation from various disubstituted nitrones at PCM/B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level of theory was performed to determine the most favorable disubstituted nitrones for this reaction. In addition, our results provided general trends of radical reactivity that is dependent upon but not exclusive to the charge densities of nitronyl-C, the position of substituents including stereoselectivities, and the presence of intramolecular H-bonding interaction. Unusually high exoergic ΔG(298K,aq)'s for O(2)(•-) and HO(2)(•) adduct formation were predicted for (3S,5S)-5-methyl-3,5-bis(methylcarbamoyl)-1-pyrroline N-oxide (11-cis) and (4S,5S)-5-dimethoxyphosphoryl-5-methyl-4-ethoxycarbonyl-1-pyrroline N-oxide (29-trans) with ΔG(298K,aq) = -3.3 and -9.4 kcal/mol, respectively, which are the most exoergic ΔG(298K,aq) observed thus far for any nitrone at the level of theory employed in this study.

Electron paramagnetic resonance spectroscopy in radiation research: Current status and perspectives

Exposure to radiation leads to a number of health-related malfunctions. Ionizing radiation is more harmful than non-ionizing radiation, as it causes both direct and indirect effects. Irradiation with ionizing radiation results in free radical-induced oxidative stress. Free radical-mediated oxidative stress has been implicated in a plethora of diseased states, including cancer, arthritis, aging, Parkinson's disease, and so on. Electron Paramagnetic Resonance (EPR) spectroscopy has various applications to measure free radicals, in radiation research. Free radicals disintegrate immediately in aqueous environment. Free radicals can be detected indirectly by the EPR spin trapping technique in which these forms stabilize the radical adduct and produce characteristic EPR spectra for specific radicals. Ionizing radiation-induced free radicals in calcified tissues, for example, teeth, bone, and fingernail, can be detected directly by EPR spectroscopy, due to their extended stability. Various applications of EPR in radiation research studies are discussed in this review.

Synthesis and characterization of 5-alkoxycarbonyl-4-hydroxymethyl-5-alkyl-pyrroline N-oxide derivatives

The syntheses, analytical properties, and spin trapping behavior of four novel EMPO derivatives, namely 5-ethoxycarbonyl-4-hydroxymethyl-5-methyl-pyrroline N-oxide (EHMPO), 5-ethoxycarbonyl-5-ethyl-4-hydroxymethyl-pyrroline N-oxide (EEHPO), 4-hydroxymethyl-5-methyl-5-propoxycarbonyl-pyrroline N-oxide (HMPPO), and 4-hydroxymethyl-5-methyl-5-iso-propoxycarbonyl-pyrroline N-oxide (HMiPPO), towards different oxygen- and carbon-centered radicals are described.

Synthesis and characterization of 5-hydroxymethyl-5-methyl-pyrroline N-oxide and its derivatives

Synthesis and spin trapping behavior of three novel DMPO derivatives, namely 5-hydroxymethyl-5-methyl-pyrroline N-oxide (HMMPO), 5-(2-furanyl)-oxymethyl-5-methyl-pyrroline N-oxide (FMMPO), and 5-(2-pyranyl)-oxymethyl-5-methyl-pyrroline N-oxide (PMMPO) towards different oxygen- and carbon-centered radicals are described. The stabilizing effect of a series of cyclodextrins on the superoxide adducts was tested.

High-light induced superoxide radical formation in cytochrome b₆f complex from Bryopsis corticulans as detected by EPR spectroscopy

The generation of superoxide radical (O₂·⁻) in Cyt b₆f of Bryopsis corticulans under high light illumination was studied using electron paramagnetic resonance (EPR) spectroscopy. This could be evidenced by the addition of SOD which specifically reacted with O₂·⁻. The generation of O₂·⁻ was lost in the absence of oxygen and was found to be suppressed in the presence of NaN₃ and be scavenged by extraneous antioxidants such as ascorbate, β-carotene and glutathione which could also scavenged ¹O₂*. These results indicated that O₂·⁻ which produced under high light illumination in Cyt b₆f of B. corticulans might rise from a reaction which ¹O₂* could participated in. Also the photo-protection mechanism to Cyt b₆f complex by antioxidants which might contain in thylakoid was speculated.

Hydroxyl radical release from dental resins: electron paramagnetic resonance evidence

It is well known that polymeric free radicals remain trapped inside dental resins for a long time after photopolymerization. Moreover, although these high molecular mass compounds have very limited mobility, there is evidence to suggest that they disappear progressively over time. The purpose of this study was to provide new experimental data to help understand this phenomenon. To determine whether low molecular mass free radicals are released by dental composites stored in hydrophilic media, we used electron paramagnetic resonance spectroscopy to perform spin-trapping experiments on experimental and commercial samples stored in ethanol. Under these conditions, ethoxy radicals were produced. Further experiments demonstrated that (1) hydroxyl radicals were released from the methacrylated resin and (2) they reacted with ethanol molecules to produce "secondary" ethoxy free radicals. In addition to the well-known monomer toxicity of methacrylated resins, we may have identified a new source of concern for these biomaterials.

Effect of 2, 5-substituents on the stability of cyclic nitrone superoxide spin adducts: A density functional theory approach

In the present study, five cyclic nitrone superoxide spin adducts, i.e. DMPO-OOH, M(3)PO-OOH, EMPO-OOH, DEPMPO-OOH and DEPDMPO-OOH, were chosen as model compounds to investigate the effect of 2,5-subsitituents on their stability, through structural analysis and decay thermodynamics using density functional theory (DFT) calculations. Analysis of the optimized geometries reveals that none of the previously proposed stabilizing factors, including intramolecular H-bonds, intramolecular non-bonding interactions, bulky steric protection nor the C(2)-N(1) bond distance can be used to clearly explain the effect of 2,5-substituents on the stability of the spin adducts. Subsequent study found that spin densities on the nitroxyl nitrogen and oxygen are well correlated with the half-lives of the spin adducts and consequently are the proper parameters to characterize the effect of 2,5-substituents on their stability. Examination of the decomposition thermodynamics further supports the effect of the substituents on the persistence of cyclic nitrone superoxide spin adducts.

Superoxide radical anion adduct of 5,5-dimethyl-1-pyrroline N-oxide. 6. Redox properties

Cyclic nitrones have been employed for decades as spin trapping reagents for the detection and identification of transient radicals, and have been employed as pharmacological agent against ROS-mediated toxicity. The short half-life of the nitrone-superoxide adducts limits the application of nitrones in biological millieu, and therefore investigaton of the redox properties of the superoxide adducts is important. Moreover, computational investigation of the redox properties of the nitrones and their corresponding spin adducts may provide new insights into the nature of their pharmacological activity against ROS-induced toxicity. In general, electron-withdrawing group substitution at the C-5 position results in higher EAs and IPs making these substituted nitrones more susceptible to reduction but more difficult to oxidize compared to DMPO. One-electron reduction and oxidation of nitrones both resulted in elongated N-C(2) bonds indicating the tendency of radical anion and cation forms of nitrone to undergo ring-opening. The EAs and IPs of various O(2)(*-) adducts indicate that DEPMPO-O(2)H is the most difficult to reduce and oxidize compared to the O(2)(*-) adducts of DMPO, EMPO, and AMPO. In general, nitroxides gave higher EAs compared to nitrones making them more suceptible to reduction. One-electron oxidation of nitroxides leads to elongation of the N-C(2) bond but not for their reduction. The energetics of redox reaction of O(2)(*-) adducts was also explored. Results indicate that the reduction of O(2)(*-) adducts with O(2)(*-) is preferred followed by their oxidation by O(2) and then by O(2)(*-), but the maximum difference between these free energies of redox reactions in aqueous solution is only 0.21 kcal/mol. The preferred decomposition pathways for the one-electron oxidation and reduction of nitroxides was also explored, and formation of potentially biologically active products such as NO, H(2)O(2), and hydroxamic acid was predicted.

Improvement of the sensitivity of EPR spin trapping in biological systems by cyclodextrins: A model study with thylakoids and photosystem II particles

Electron paramagnetic resonance (EPR) spin trapping spectroscopy is an important method used in free radical research; however, its application in biological systems is hindered by EPR silencing of spin adducts. Previous studies in superoxide-generating chemical systems have shown that spin adducts can be partially stabilized by cyclodextrins. In this work, for the first time, this proposed protective effect of cyclodextrins is investigated in a real biological sample-in isolated thylakoid membranes and photosystem II (PSII) particles with EMPO as a spin trap. It is shown that (i) randomly methylated beta-cyclodextrin and 2-hydroxypropyl-beta-cyclodextrin form inclusion complexes with EMPO-superoxide adducts (EMPO-OOH), (ii) both cyclodextrins increase the intensity of the EMPO-OOH EPR signal in PSII particles up to five times, (iii) higher EMPO-OOH EPR signal intensity is a result of increased stability of EMPO-OOH, and (iv) the extent of the protection of EMPO-OOH adduct provided by cyclodextrins is different in thylakoids and PSII particles. Along with the spin trapping data, the toxicity of cyclodextrins is also discussed with particular focus on photosynthetic preparations. The presented data show that both tested cyclodextrins can be used as valuable tools to improve the sensitivity of spin trapping in biological samples.

New heteroaryl nitrones with spin trap properties: Identification of a 4-furoxanyl derivative with excellent properties to be used in biological systems

A new series of heteroaryl nitrones, 1-7, bearing furoxanyl and thiadiazolyl moieties, were evaluated for their free radical-trapping properties. The physicochemical characterization by electron paramagnetic resonance (EPR) demonstrated its capability to trap and stabilize oxygen-, carbon-, sulfur-, and nitrogen-centered free radicals. The 4-furoxanyl nitrone 3 (FxBN), alpha(Z)-(3-methylfuroxan-4-yl)-N-tert-butylnitrone, showed appropriate solubility in aqueous solution and taking into account that this physicochemical property is very important for biological applications, we studied it deeply in terms of its trapping and kinetic behaviors. For this, kinetic studies of the hydroxyl adduct decay gave rate constants k(ST) of 1.22x10(10)dm(3)mol(-1)s(-1) and half-live up to 7200s at physiological pH, without any artifactual signals. The ability of FxBN to directly traps and stabilizes superoxide free radical, with a half-life of 1620s at physiological pH, was also demonstrated. Besides, FxBN-hydroxyl and -superoxide adducts exhibited distinct and characteristic EPR spectral patterns. Finally, we confirmed the ability of FxBN to act as spin trap in a specific biological system, that is, in the free radical production of experimental anti-trypanosomatid drugs using Trypanosoma cruzi microsomes as biological system. Moreover, previous observations of low FxBN toxicity transform it in a good candidate for in vivo spin trapping.