Superoxide Radical Anion Adduct of 5,5-Dimethyl-1-pyrroline N-Oxide (DMPO). 2. The Thermodynamics of Decay and EPR Spectral Properties

Mechanisms of iopamidol transformation catalyzed by a copper corrosion product (c-Cu2O) during peroxymonosulfate disinfection

Peroxymonosulfate (PMS) is an alternative disinfectant for drinking water. This study aimed to investigate the transformation of iopamidol (IPM) catalyzed by a main copper corrosion product (c-Cu2O) with PMS as a disinfectant. The observed pseudo-first-order constant (kobs) for the IPM degradation in the c-Cu2O/PMS system (0.033 min-1) was 3 times that in the CuO/PMS system (0.011 min-1). The quenching tests and the electron paramagnetic resonance (EPR) experiments indicate that O2•- and 1O2 contributed to IPM degradation in the c-Cu2O/ PMS system. The complexation of metastable Cu(II) with a PMS molecule polarized the OO bond and then facilitated the electron transfer from the PMS molecule to other PMS and O2 molecules, which directly and indirectly promoted the yield of O2•- and 1O2. The iodine balance indicated that 26.0% of initial TOI was converted to IO3-, and CHI3 only accounted for 0.6% of the residual TOI. In the c-Cu2O/PMS system, IPM conversion was started with amide C-N bond breakage, deiodination reaction and hydrogen abstraction. This study helps to better understand the conversion mechanisms of iodine-containing organic micropollutants when PMS is deployed as a disinfectant in copper pipes.

Visible-Light-Enabled Catalytic Approach to N,O-Spirocycles through Amidyl Radical Addition/Cyclization

A rational combination of photoredox catalyst anthraquinone and hydrogen atom transfer (HAT) catalyst methyl thioglycolate allows for the rapid and straightforward conversion of a range of 2-amidated acetylenic alcohols to multifunctional N,O-spirocycles under visible light irradiation. With oxygen as the sole terminal oxidant, these reactions can be carried out efficiently at room temperature without the involvement of transition metals or strong oxidants. The successful application of this mild catalytic strategy in the late-stage functionalization of bioactive skeletons further highlights its practical value.

Europium-Functionalized Graphitic Carbon Nitride for Efficient Chemiluminescence Detection of Singlet Oxygen

Enhancing the sensitivity and selectivity of chemiluminescence (CL) sensors for detecting chemical species in complex samples poses a significant challenge in nanoparticle surface engineering. Graphitic carbon nitride (CN) shows promise but suffers from weak CL intensity and unknown luminescence mechanisms. In this study, we propose a nitrogen defect strategy to enhance the CL efficiency of europium-functionalized graphitic carbon nitride (Eu-CNNPs). By controlling the dosage of the europium modification, we can adjust the nitrogen defect content to reduce the energy gap and improve the CL performance. Remarkably, Eu-CNNPs with rich nitrogen defects exhibit strong chemiluminescence emission specifically for singlet oxygen (1O2) without responding to other reactive oxygen species (ROS). Building upon this finding, we developed a direct, selective, and sensitive CL sensing platform for 1O2 in PM2.5 and monitored 1O2 production in photosensitizers without interference from metal ions. Through extensive experiments, we attribute the 1O2-driven CL response to the presence of abundant nitrogen defects in the CN material, accelerating electron transfer and yielding a high generation of 1O2. Furthermore, chemiluminescence resonance energy transfer (CRET) between (1O2)2* (1O2 dimeric aggregate) and Eu-CNNPs contributes to strong CL emission. This work provides insights into enhancing the CL performance of CN and offers new possibilities for advancing the practical analysis of nanomaterials using the intriguing mechanism of nitrogen defects.

Fighting Antibiotic-Resistant Bacterial Infections by Surface Biofunctionalization of 3D-Printed Porous Titanium Implants with Reduced Graphene Oxide and Silver Nanoparticles

Nanoparticles (NPs) have high multifunctional potential to simultaneously enhance implant osseointegration and prevent infections caused by antibiotic-resistant bacteria. Here, we present the first report on using plasma electrolytic oxidation (PEO) to incorporate different combinations of reduced graphene oxide (rGO) and silver (Ag) NPs on additively manufactured geometrically ordered volume-porous titanium implants. The rGO nanosheets were mainly embedded parallel with the PEO surfaces. However, the formation of ‘nano-knife’ structures (particles embedded perpendicularly to the implant surfaces) was also found around the pores of the PEO layers. Enhanced in vitro antibacterial activity against methicillin-resistant Staphylococcus aureus was observed for the rGO+Ag-containing surfaces compared to the PEO surfaces prepared only with AgNPs. This was caused by a significant improvement in the generation of reactive oxygen species, higher levels of Ag+ release, and the presence of rGO ‘nano-knife’ structures. In addition, the implants developed in this study stimulated the metabolic activity and osteogenic differentiation of MC3T3-E1 preosteoblast cells compared to the PEO surfaces without nanoparticles. Therefore, the PEO titanium surfaces incorporating controlled levels of rGO+Ag nanoparticles have high clinical potential as multifunctional surfaces for 3D-printed orthopaedic implants.

Accurate identification of radicals by in-situ electron paramagnetic resonance in ultraviolet-based homogenous advanced oxidation processes

Accurate identification of radicals in advanced oxidation processes (AOPs) is important to study the mechanisms on radical production and subsequent oxidation-reduction reaction. The commonly applied radical quenching experiments cannot provide direct evidences on generation and evolution of radicals in AOPs, while electron paramagnetic resonance (EPR) is a cutting-edge technology to identify radicals based on spectral characteristics. However, the complexity of EPR spectrum brings uncertainty and inconsistency to radical identification and mechanism clarification. This work presented a comprehensive study on identification of radicals by in-situ EPR analysis in four typical UV-based homogenous AOPs, including UV/H₂O₂, UV/peroxodisulfate (and peroxymonosulfate), UV/peracetic acid and UV/IO₄^- systems. Radical formation mechanism was also clarified based on EPR results. A reliable EPR method using organic solvents was proposed to identify alkoxy and alkyl radicals (CH₃C(=O)OO·, CH₃C(=O)O· and ·CH₃) in UV/PAA system. Two activation pathways for radical production were proposed in UV/IO₄^- system, in which the produced IO₃·, IO₄·, ·OH and hydrated electron were precisely detected. It is interesting that addition of specific organic solvents can effectively identify oxygen-center and carbon-center radicals. A key parameter in EPR spectrum for 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin adduct, A_H, is ranked as: ·CH₃ (23 G) >·OH (15 G) >IO₃· (12.9 G) >O₂·^- (11 G) ≥·OOH (9-11 G) ≥IO₄· (9-10 G) ≥SO₄·^- (9-10 G) >CH₃C(=O)OO· (8.5 G) > CH₃C(=O)O· (7.5 G). This study will give a systematic method on identification of radicals in AOPs, and shed light on the insightful understanding of radical production mechanism.

In Situ EPR Spin Trapping and Competition Kinetics Demonstrate Temperature-Dependent Mechanisms of Synergistic Radical Production by Ultrasonically Activated Persulfate

Ultrasound coupled with activated persulfate can synergistically degrade aqueous organic contaminants. Here, in situ electron paramagnetic resonance spin trapping was used to compare radicals produced by ultrasonically activated persulfate (US-PS) and its individual technologies, ultrasound alone (US) and heat-activated persulfate (PS), with respect to temperature. Radicals were trapped using 5,5-dimethyl-1-pyrroline-N-oxide, DMPO, to form detectable nitroxide adducts. Using initial rates of radical adduct formation, and compared to US and PS, US-PS at 40 and 50 °C resulted in the largest synergistic production of radicals. Radicals generated from US were reasonably consistent from 40 to 70 °C, indicating that temperature had little effect on cavitational bubble collapse over this range. However, synergy indexes calculated from initial rates showed that ultrasonic activation of persulfate at the bubble interface changes with temperature. From these results, we speculate that higher temperatures enhance persulfate uptake into cavitation bubbles via nanodroplet injection. DMPO-OH was the predominant adduct detected for all conditions. However, competition modeling and spin trapping in the presence of nitrobenzene and atrazine probes showed that SO₄^•- predominated. Therefore, the DMPO-OH signal is derived from SO₄^•- trapping with subsequent DMPO-SO₄^- hydrolysis to DMPO-OH. Spin trapping is effective in quantifying total radical adduct formation but limited in measuring primary radical speciation in this case.

Visible Light-Induced Catalyst-Free Activation of Peroxydisulfate: Pollutant-Dependent Production of Reactive Species

Activation of peroxydisulfate (PDS, S₂O₈^2-) via various catalysts to degrade pollutants in water has been extensively investigated. However, catalyst-free activation of PDS by visible light has been largely ignored. This paper reports effective visible light activation of PDS without any additional catalyst, leading to the degradation of a wide range of organic compounds of high environmental and human health concerns. Importantly, the formation of reactive species is distinctively different in the PDS visible light system with and without pollutants [e.g., atrazine (ATZ)]. In addition to SO₄^•- generated via S₂O₈^2- dissociation under visible light irradiation, O₂^•- and ¹O₂ are also produced in both systems. However, in the absence of ATZ, H₂O₂ and O₂^•- are key intermediates and precursors for ¹O₂, whereas in the presence of ATZ, a different pathway was followed to produce O₂^•- and ¹O₂. Both radical and nonradical processes contribute to the degradation of ATZ in the PDS visible light system. The active role of ¹O₂ in the degradation of ATZ besides SO₄^•- is manifested by the enhanced degradation of contaminants and electron paramagnetic resonance spectroscopy measurements in D₂O.

Visible Light-Mediated Thiolation of Substituted 1,4-Naphthoquinones Using Eosin Y as a Photoredox Catalyst

In the presence of eosin Y, a visible light-induced one-step procedure (isolated yield of ≥75%) for thiolation of substituted 1,4-naphthoquinones using various aromatic and aliphatic thiols at room temperature is described herein. The rate-determining step of the reaction is thiyl radical generation, and the radical was characterized by high-resolution mass spectrometry. Cost effectiveness, operational simplicity, a short reaction time, high atom economy, and a very good yield make this photoredox-mediated process a useful alternative to the transition metal (e.g., Cu, Ag, and Pd)-catalyzed coupling reaction of quinones with thiols or disulfides.

Photo-Oxidation of Therapeutic Protein Formulations: From Radical Formation to Analytical Techniques

UV and ambient light-induced modifications and related degradation of therapeutic proteins are observed during manufacturing and storage. Therefore, to ensure product quality, protein formulations need to be analyzed with respect to photo-degradation processes and eventually protected from light exposure. This task usually demands the application and combination of various analytical methods. This review addresses analytical aspects of investigating photo-oxidation products and related mediators such as reactive oxygen species generated via UV and ambient light with well-established and novel techniques.

Visible-Light-Mediated Synthesis of Substituted Phenazine and Phenoxazinone Using Eosin Y as a Photoredox Catalyst

This paper describes an efficient, sustainable, one-step procedure for synthesizing substituted phenazines and phenoxazinones from commercially available ortho-substituted aromatic amines with very good yield (≥80%) in water. The procedure uses eosin Y (EY) as a photoredox catalyst at room temperature (RT). The highly reactive o-quinone-diimine or o-quinone-imine intermediate was characterized by the HR-MS technique.

Electron Spin Resonance Evidence for Electro-generated Hydroxyl Radicals

Electro-generated hydroxyl radicals (•OH) are of fundamental importance to the electrochemical advanced oxidation process (EAOP). Radical-specific electron spin resonance (ESR) evidence is still lacking in association with the direct electron transfer (DET) reaction of spin trap (e.g., 5,5-dimethyl-1-pyrroline-N-oxide; DMPO) and side reactions of the DMPO-OH adduct in the strongly oxidative environment offered by anodic polarization. Herein, we showed ESR identification of electro-generated •OH in EAOP based on the principle of kinetic selection. Excessive addition of a DMPO agent and fast spin trapping allowed suitable kinetic conditions to be set for effective spin trapping of electro-generated •OH and subsequent ESR identification. Otherwise, interferential triplet signals would emerge due to formation of paramagnetic dimer via dehydrogenation, DET oxidation, and dimerization reactions of the DMPO-OH adduct. The results demonstrate that •OH formation during spin-trapping on the titanium suboxide (TiSO) anode could be quantified as 47.84 ± 0.44 μM at current density of 10 mA cm^-2. This value revealed a positive dependence on electrolysis time, current density, and anode potential. The effectiveness of ESR measurements was verified by the results obtained with the terephthalic acid probe. The ESR identification not only provides direct evidence for electro-generated •OH from a fundamental point of view, but also suggests a strategy to screen effective anode materials.

Direct Evidence of Solution-Mediated Superoxide Transport and Organic Radical Formation in Sodium-Oxygen Batteries

Advanced large-scale electrochemical energy storage requires cost-effective battery systems with high energy densities. Aprotic sodium-oxygen (Na-O2) batteries offer advantages, being comprised of low-cost elements and possessing much lower charge overpotential and higher reversibility compared to their lithium-oxygen battery cousins. Although such differences have been explained by solution-mediated superoxide transport, the underlying nature of this mechanism is not fully understood. Water has been suggested to solubilize superoxide via formation of hydroperoxyl (HO2), but direct evidence of these HO2 radical species in cells has proven elusive. Here, we use ESR spectroscopy at 210 K to identify and quantify soluble HO2 radicals in the electrolyte-cold-trapped in situ to prolong their lifetime-in a Na-O2 cell. These investigations are coupled to parallel SEM studies that image crystalline sodium superoxide (NaO2) on the carbon cathode. The superoxide radicals were spin-trapped via reaction with 5,5-dimethyl-pyrroline N-oxide at different electrochemical stages, allowing monitoring of their production and consumption during cycling. Our results conclusively demonstrate that transport of superoxide from cathode to electrolyte leads to the nucleation and growth of NaO2, which follows classical mechanisms based on the variation of superoxide content in the electrolyte and its correlation with the crystallization of cubic NaO2. The changes in superoxide content upon charge show that charge proceeds through the reverse solution process. Furthermore, we identify the carbon-centered/oxygen-centered alkyl radicals arising from attack of these solubilized HO2 species on the diglyme solvent. This is the first direct evidence of such species, which are likely responsible for electrolyte degradation.

Stability of 5,5-dimethyl-1-pyrroline-N-oxide as a spin-trap for quantification of hydroxyl radicals in processes based on Fenton reaction

Fenton reaction was used to produce hydroxyl radicals under conditions similar to AOPs with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap agent in electron paramagnetic resonance (EPR) analysis. A theoretical kinetics model was developed to determine conditions under which the spin-adduct DMPO-OH is not further oxidized by Fe(3+) and excessive radicals, so that hydroxyl radicals concentration could be accurately inferred. Experiments were designed based upon the model and H2O2 and Fe(2+) concentrations were varied from 1 to 100 mM and from 0.1 to 10 mM, respectively, with a constant H2O2: Fe(2+) ratio of 10:1. Results confirmed that DMPO concentration should be at least 20 times higher than the concentration of H2O2 and 200 times higher than iron concentration to produce stable DMPO-OH EPR signal. When DMPO: H2O2 ratio varied from 1 to 10, DMPO-OH could generate intermediates and be further oxidized leading to the apparition of an additional triplet. This signal was attributed to a paramagnetic dimer: its structure and a formation mechanism were proposed. Finally, the utilization of sodium sulfite and catalase to terminate Fenton reaction was discussed. Catalase appeared to be compatible with DMPO. However, sodium sulfite should be avoided since it reacted with DMPO-OH to form DMPO-SO3.

Theoretical studies on the spin trapping of the 2-chloro-5-hydroxy-1,4-benzoquinone radical by 5,5-dimethyl-1-pyrroline N-oxide (DMPO): the identification of the C–O bonding spin adduct

The C–O bonding spin adduct has been identified in the spin trapping of 2-chloro-5-hydroxy-1,4-benzoquinone radical by DMPO.

Zn(ii) porphyrin based nano-/microscale metal–organic frameworks: morphology dependent sensitization and photocatalytic oxathiolane deprotection

Zn(ii) porphyrin based nano-/microscale MOFs showed morphology dependent ¹O₂ sensitization efficiency and were used as selective photocatalysts for oxathiolane deprotection.

Quantitative Structure--Activity Relationship (QSAR) for the Oxidation of Trace Organic Contaminants by Sulfate Radical

The sulfate radical anion (SO4•–) based oxidation of trace organic contaminants (TrOCs) has recently received great attention due to its high reactivity and low selectivity. In this study, a meta-analysis was conducted to better understand the role of functional groups on the reactivity between SO4•– and TrOCs. The results indicate that compounds in which electron transfer and addition channels dominate tend to exhibit a faster second-order rate constants (kSO4•–) than that of H–atom abstraction, corroborating the SO4•– reactivity and mechanisms observed in the individual studies. Then, a quantitative structure activity relationship (QSAR) model was developed using a sequential approach with constitutional, geometrical, electrostatic, and quantum chemical descriptors. Two descriptors, ELUMO and EHOMO energy gap (ELUMO–EHOMO) and the ratio of oxygen atoms to carbon atoms (#O:C), were found to mechanistically and statistically affect kSO4•– to a great extent with the standardized QSAR model: ln kSO4•– = 26.8–3.97 × #O:C – 0.746 × (ELUMO–EHOMO). In addition, the correlation analysis indicates that there is no dominant reaction channel for SO4•– reactions with various structurally diverse compounds. Our QSAR model provides a robust predictive tool for estimating emerging micropollutants removal using SO4•– during wastewater treatment processes.

Degradation of trichloroethene by siderite-catalyzed hydrogen peroxide and persulfate: Investigation of reaction mechanisms and degradation products

A binary catalytic system, siderite-catalyzed hydrogen peroxide (H₂O₂) coupled with persulfate (S₂O₈^2-), was investigated for the remediation of trichloroethene (TCE) contamination. Batch experiments were conducted to investigate reaction mechanisms, oxidant decomposition rates, and degradation products. By using high performance liquid chromatography (HPLC) coupled with electron paramagnetic resonance (EPR), we identified four radicals (hydroxyl (HO·), sulfate (SO₄^-·), hydroperoxyl (HO₂·), and superoxide (O₂^-·)) in the siderite-catalyzed H₂O₂-S₂O₈^2- system. In the absence of S₂O₈^2- (i.e., siderite-catalyzed H₂O₂), a majority of H₂O₂ was decomposed in the first hour of the experiment, resulting in the waste of HO·. The addition of S₂O₈^2- moderated the H₂O₂ decomposition rate, producing a more sustainable release of hydroxyl radicals that improved the treatment efficiency. Furthermore, the heat released by H₂O₂ decomposition accelerated the activation of S₂O₈^2-, and the resultant SO₄^-· was the primary oxidative agent during the first two hours of the reaction. Dichloroacetic acid was firstly detected by ion chromatography (IC). The results of this study indicate a new insight to the reaction mechanism for the catalytic binary H₂O₂-S₂O₈^2- oxidant system, and the delineation of radicals and the discovery of the chlorinated byproduct provide useful information for efficient treatment of chlorinated-solvent contamination in groundwater.

Theoretical and experimental studies of the spin trapping of inorganic radicals by 5,5-dimethyl-1-pyrroline N-oxide (DMPO). 3. Sulfur dioxide, sulfite, and sulfate radical anions

Radical forms of sulfur dioxide (SO(2)), sulfite (SO(3)(2-)), sulfate (SO(4)(2-)), and their conjugate acids are known to be generated in vivo through various chemical and biochemical pathways. Oxides of sulfur are environmentally pervasive compounds and are associated with a number of health problems. There is growing evidence that their toxicity may be mediated by their radical forms. Electron paramagnetic resonance (EPR) spin trapping using the commonly used spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), has been employed in the detection of SO(3)(•-) and SO(4)(•-). The thermochemistries of SO(2)(•-), SO(3)(•-), SO(4)(•-), and their respective conjugate acids addition to DMPO were predicted using density functional theory (DFT) at the PCM/B3LYP/6-31+G**//B3LYP/6-31G* level. No spin adduct was observed for SO(2)(•-) by EPR, but an S-centered adduct was observed for SO(3)(•-)and an O-centered adduct for SO(4)(•-). Determination of adducts as S- or O-centered was made via comparison based on qualitative trends of experimental hfcc's with theoretical values. The thermodynamics of the nonradical addition of SO(3)(2-) and HSO(3)(-) to DMPO followed by conversion to the corresponding radical adduct via the Forrester-Hepburn mechanism was also calculated. Adduct acidities and decomposition pathways were investigated as well, including an EPR experiment using H(2)(17)O to determine the site of hydrolysis of O-centered adducts. The mode of radical addition to DMPO is predicted to be governed by several factors, including spin population density, and geometries stabilized by hydrogen bonds. The thermodynamic data supports evidence for the radical addition pathway over the nucleophilic addition mechanism.

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 integrated computational protocol for the accurate prediction of EPR and PNMR parameters of aminoxyl radicals in solution

Magnetic spectroscopic techniques such as electron paramagnetic resonance (EPR) and paramagnetic NMR (PNMR) are valuable tools for understanding the structure and dynamics of complex systems such as, for example, biomolecules or nanomaterials labeled with suitable free radicals. Unfortunately, such spectra do not give direct access to the radical structure because of the subtle interplay between several different effects not easily separable and evaluable by experimentalists alone. In this respect, computational spectroscopy is becoming an essential and versatile tool for the assignment and interpretation of experimental spectra. In this article, the new integrated computational approaches developed in the recent years in our research group are reviewed. Such approaches have been applied to two widely used spin probes showing that proper account of stereo-electronic, environmental and dynamical effects leads to magnetic properties in remarkable agreement with experimental results.

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.

Potent inhibition of peroxynitrite-induced DNA strand breakage and hydroxyl radical formation by dimethyl sulfoxide at very low concentrations

Dimethyl sulfoxide (DMSO) is frequently used as a solvent for many water-insoluble drugs in biological studies at concentrations often up to 1%. However, little is known about its effects on oxidatively generated DNA damage at very low concentrations (0.005-0.5%). This study was undertaken to investigate the effects of DMSO on peroxynitrite-induced DNA strand breaks, a critical event leading to peroxynitrite-elicited cytotoxicity. Incubation of varphiX-174 plasmid DNA, with 3-morpholinosydnonimine (SIN-1), a peroxynitrite generator, led to the formation of DNA strand breaks in a concentration- and time-dependent manner. The presence of DMSO at concentrations of 0.005-0.5% was found to significantly inhibit SIN-1-induced DNA strand breaks in a concentration-dependent manner. However, DMSO at the above concentrations showed no affect on SIN-1-mediated oxygen consumption, indicating that DMSO did not affect the auto-oxidation of SIN-1 to form peroxynitrite. It is observed that incubation of the plasmid DNA with authentic peroxynitrite resulted in significant formation of DNA strand breaks, which could also be dramatically inhibited by the presence of DMSO at 0.005-0.5%. Electron paramagnetic resonance spectroscopy, using 5,5-dimethylpyrroline-N-oxide (DMPO) as a spin trap demonstrated the formation of DMPO-hydroxyl radical adduct from the SIN-1 and authentic peroxynitrite. DMSO at the concentrations ranging from 0.01% to 0.5% significantly inhibited the adduct signal. Taken together, these studies demonstrate, for the first time, that DMSO at extremely low concentrations (0.005-0.5%) can potently inhibit peroxynitrite-mediated DNA strand breakage and hydroxyl radical formation. The results of this study suggest that, where DMSO is applied as a solvent, caution should be observed when evaluating the actions of drugs in experiments involving DNA damage.

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.

Lipophilic beta-cyclodextrin cyclic-nitrone conjugate: synthesis and spin trapping studies

Nitrone spin traps are commonly employed as probes for the identification of transient radicals in chemical and biological systems using electron paramagnetic resonance (EPR) spectroscopy. Nitrones have also found applications as therapeutic agent in the treatment of radical-mediated diseases. Therefore, a spin trap that incorporates high reactivity to superoxide radical anion (O2(*-)), more persistent superoxide adduct, enhanced bioavailability, and selective targeting in one molecular design is desirable. In this work, the synthesis of a nitrone spin trap, 4, that is tethered via amide bonds to a beta-cyclodextrin (beta-CD) and a dodecyl chain was achieved with the expectation that the beta-cyclodextrin would lead to increased reactivity to O2(*-) and persistent O2(*-) adduct while the lipophilic chain would impart membrane targeting property. The two constitutional racemic isomers, 4a and 4b, were separated using preparative HPLC, and structural analysis and self-aggregation properties were carried out using NMR, induced circular dichroism, dynamic light scattering, transmission electron microscopy, and computational approach. EPR spin trapping of O2(*-) by 4a and 4b was only successful in DMSO and not in an aqueous system, due most likely to the amphiphilic character of 4 that can favor conformations (or aggregation) hindering radical addition to nitrone. Kinetics of formation and decay of the 4a-O2H adduct in polar aprotic solvents show faster reactivity to O2(*-) and more persistent O2(*-) adduct compared to nitrones not conjugated to beta-CD. Computational analysis of 4a and 4b as well as 4a-OOH and 4b-OOH adducts were carried out, and results show that isomerism, both constitutional and stereochemical, affects the orientations of aminoxyl-NO and/or hydroperoxyl groups relative to the beta-CD annulus for optimal H-bond interaction and stability.

Reactivity of superoxide radical anion and hydroperoxyl radical with alpha-phenyl-N-tert-butylnitrone (PBN) derivatives

Nitrones have exhibited pharmacological activity against radical-mediated pathophysiological conditions and as analytical reagents for the identification of transient radical species by electron paramagnetic resonance (EPR) spectroscopy. In this work, competitive spin trapping, stopped-flow kinetics, and density functional theory (DFT) were employed to assess and predict the reactivity of O(2)(*-) and HO(2)(*) with various para-substituted alpha-phenyl-N-tert-butylnitrone (PBN) spin traps. Rate constants of O(2)(*-) trapping by nitrones were determined using competitive UV-vis stopped-flow method with phenol red (PR) as probe, while HO(2)(*) trapping rate constants were calculated using competition kinetics with 5,5-dimethylpyrroline N-oxide (DMPO) by employing EPR spectroscopy. The effects of the para substitution on the charge density of the nitronyl-carbon and on the free energies of nitrone reactivity with O(2)(*-) and HO(2)(*) were computationally rationalized at the PCM/B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level of theory. Theoretical and experimental data show that the rate of O(2)(*-) addition to PBN derivatives is not affected by the polar effect of the substituents. However, the reactivity of HO(2)(*) follows the Hammett equation and is increased as the substituent becomes more electron withdrawing. This supports the conclusion that the nature of HO(2)(*) addition to PBN derivatives is electrophilic, while the addition of O(2)(*-) to PBN-type compounds is only weakly electrophilic.

Superoxide radical anion adduct of 5,5-dimethyl-1-pyrroline N-oxide. 4. Conformational effects on the EPR hyperfine splitting constants

Spin trapping has been commonly employed in the detection of superoxide radical anion in chemical and biological systems; hence, accurate interpretation of the hyperfine splitting constants (hfsc's) arising from the O(2)(*-) adducts (also referred to as hydroperoxyl (HO(2)(*)) radical adducts) of various nitrones is important. In this work, the nature of the relevant hfsc's was investigated by examining the effect of conformational changes in the hydroperoxyl moiety of the O(2)(*-) adducts of 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide (EMPO), 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO), 5-carbamoyl-5-methyl-1-pyrroline N-oxide (AMPO), and 7-oxa-1-azaspiro[4.4]non-1-en-6-one N-oxide, (CPCOMPO) on the magnitude of a(N), a(beta-H), and a(gamma-H). Conformational change around the substituents and their effect on the hfsc's were also explored. Results indicate that a(beta-H) is most sensitive to conformational changes of the hydroperoxyl and substituent groups relative to hfsc's of other nuclei. The orbital overlap between the C-H sigma-orbital and the SOMO of the nitroxyl nitrogen plays a crucial factor in determining the magnitude of the a(beta-H). The hfsc values for the O(2)(*-) adducts were predicted with high accuracy by using a low-cost computational method at the PCM(water)/BHandHLYP/EPR-III//B3LYP/6-31G* level of theory without taking into account the explicit water interaction.

Electronic basis of the comparable hydrogen bond properties of small H2CO/(H2O)n and H2NO/(H2O)n systems (n = 1, 2)

The electronic and structural properties of dihydronitroxide/water clusters are investigated and compared to the properties of formaldehyde/water clusters. Exploring the stationary points of their potential energy surfaces (structurally, vibrationally, and energetically) and characterizing their hydrogen bonds (by both atoms in molecules and natural bond orbitals methods) clearly reveal the strong similarity between these two kind of molecular systems. The main difference involves the nature of the hydrogen bond taking place between the X-H bond and the oxygen atom of a water molecule. All the properties of the hydrogen bonds occurring in both kind of clusters can be easily interpreted in terms of competition between intermolecular and intramolecular hyperconjugative interactions.

Reactivity of superoxide radical anion with cyclic nitrones: role of intramolecular H-bond and electrostatic effects

Limitations exist among the commonly used cyclic nitrone spin traps for biological free radical detection using electron paramagnetic resonance (EPR) spectroscopy. The design of new spin traps for biological free radical detection and identification using EPR spectroscopy has been a major challenge due to the lack of systematic and rational approaches to their design. In this work, density functional theory (DFT) calculations and stopped-flow kinetics were employed to predict the reactivity of functionalized spin traps with superoxide radical anion (O2*-). Functional groups provide versatility and can potentially improve spin-trap reactivity, adduct stability, and target specificity. The effect of functional group substitution at the C-5 position of pyrroline N-oxides on spin-trap reactivity toward O2*- was computationally rationalized at the PCM/B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) and PCM/mPW1K/6-31+G(d,p) levels of theory. Calculated free energies and rate constants for the reactivity of O2*- with model nitrones were found to correlate with the experimentally obtained rate constants using stopped-flow and EPR spectroscopic methods. New insights into the nucleophilic nature of O2*- addition to nitrones as well as the role of intramolecular hydrogen bonding of O2*- in facilitating this reaction are discussed. This study shows that using an N-monoalkylsubstituted amide or an ester as attached groups on the nitrone can be ideal in molecular tethering for improved spin-trapping properties and could pave the way for improved in vivo radical detection at the site of superoxide formation.

Theoretical and experimental studies of the spin trapping of inorganic radicals by 5,5-dimethyl-1-pyrroline N-oxide (DMPO). 2. Carbonate radical anion

Previous studies have shown that the enzyme-mediated generation of carbonate radical anion (CO(3)(.-)) may play an important role in the initiation of oxidative damage in cells. This study explored the thermodynamics of CO(3)(.-) addition to 5,5-dimethyl-1-pyrroline N-oxide (DMPO) using density functional theory at the B3LYP/6-31+G(**)//B3LYP/6-31G* and B3LYP/6-311+G* levels with the polarizable continuum model to simulate the effect of the bulk dielectric effect of water on the calculated energetics. Theoretical data reveal that the addition of CO(3)(.-) to DMPO yields an O-centered radical adduct (DMPO-OCO2) as governed by the spin (density) population on the CO(3)(.-). Electron paramagnetic resonance spin trapping with the commonly used spin trap, DMPO, has been employed in the detection of CO(3)(.-). UV photolysis of H(2)O(2) and DMPO in the presence of sodium carbonate (Na(2)CO(3)) or sodium bicarbonate (NaHCO(3)) gave two species (i.e., DMPO-OCO(2) and DMPO-OH) in which the former has hyperfine splitting constant values of a(N) = 14.32 G, a(beta)-Eta = 10.68 G, and a(gamma-H) = 1.37 G and with a shorter half-life compared to DMPO-OH. The origin of the DMPO-OH formed was experimentally confirmed using isotopically enriched H(2)(17)O(2) that indicates direct addition of HO(.) to DMPO. Theoretical studies on other possible pathways for the formation of DMPO-OH from DMPO-OCO(2) in aqueous solution and in the absence of free HO(.) such as in the case of enzymatically generated CO(3)(.-), show that the preferred pathway is via nucleophilc substitution of the carbonate moiety by H(2)O or HO(-). Nitrite formation has been observed as the end product of CO(3)(.-) trapping by DMPO and is partly dependent on the basicity of solution. The thermodynamic behavior of CO(3)(.-) in the aqueous phase is predicted to be similar to that of the hydroperoxyl (HO(2)(.)) radical.

Combined ESR and thermodynamic studies of the superoxide adduct of 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO): hindered rotation around the O-O bond evidenced by two-dimensional simulation of temperature-dependent spectra

Experiments were performed to elucidate the origin of the superhyperfine structure and line width alternation (LWA) seen in the ESR spectrum of the major diastereoisomer (1) of DEPMPO-OOH, the remarkably persistent superoxide adduct of 5-(diethoxyphosphoryl)-5-methyl-1-pyrroline N-oxide (DEPMPO). Using selectively deuterated DEPMPO derivatives, we demonstrated that the superhyperfine pattern can be unambiguously attributed to long-range couplings. The recording in pyridine of highly resolved spectra in a wide temperature range, combined with two-dimensional simulation, allowed us to characterize an inverted LWA in 1 and revealed a uniform line broadening in the spectrum of the minor DEPMPO-OOH diastereoisomer (2), with both effects originating from a chemical exchange between conformers. When the individual spectra of 1 presenting LWA in the fast-exchange regime were simulated, four equally good fits were obtained and this ambiguity could be resolved by using a two-dimensional simulation technique. The thermodynamic and kinetic constants of this exchange were consistent with a rotation around the O-O bond. We propose that line broadening effects in 1 and 2 result from this O-O rotation concerted with the pseudo-rotation of the pyrrolidine ring.

DMPO-OH Radical Formation from 5,5-Dimethyl-1-pyrroline N-Oxide (DMPO) in Hot Water

When an aqueous solution of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) was heated at 70 degrees C for 30 min, formation of DMPO-OH was observed by ESR. This DMPO-OH radical formation was suppressed under an argon atmosphere. When water was replaced with ultra-pure water for ICP-MS experiments, DMPO-OH radical formation was also diminished. Under an argon atmosphere in ultra-pure water, the intensity of the DMPO-OH signal decreased to about 1/20 of that observed under aerobic conditions with regular purified water. The addition of hydroxyl radical scavengers such as mannitol did not affect the formation of DMPO-OH, but the signal turned faint in the presence of EDTA. We suggest that DMPO reacted with dissolved oxygen to form DMPO-OH.

Theoretical and Experimental Studies of the Spin Trapping of Inorganic Radicals by 5,5-Dimethyl-1-Pyrroline N-Oxide (DMPO). 1. Carbon Dioxide Radical Anion

The carbon dioxide radical anion (CO2*-) is known to be generated in vivo through various chemical and biochemical pathways. Electron paramagnetic resonance (EPR) spin trapping with the commonly used spin trap, 5,5-dimethyl-1-pyrroline N-oxide (DMPO), has been employed in the detection of CO2*-. The thermodynamics of CO2*- addition to DMPO was predicted using density functional theory (DFT) at the B3LYP/6-31++G**//B3LYP/6-31G* and B3LYP/6-311+G* levels with the polarizable continuum model (PCM) to simulate the effect of the bulk dielectric effect of water on the calculated energetics. Three possible products of CO2*- addition to DMPO were predicted: (1) a carboxylate adduct, (2) pyrroline-alcohol and (3) DMPO-OH. Experimentally, UV photolysis of H2O2 in the presence of sodium formate (NaHCO2) and DMPO gave an EPR spectrum characteristic of a C-centered carboxylate adduct and is consistent with the theoretically derived hyperfine coupling constants (hfcc). The pKa of the carboxylate adduct was estimated computationally to be 6.4. The mode of CO2*- addition to DMPO is predicted to be governed predominantly by the spin (density) population on the radical, whereas electrostatic effects are not the dominant factor for the formation of the persistent adduct. The thermodynamic behavior of CO2*- in the aqueous phase is predicted to be similar to that of mercapto radical (*SH), indicating that formation of CO2*- in biological systems may have an important role in the initiation of oxidative damage in cells.

Effect of the Phosphoryl Substituent in the Linear Nitrone on the Spin Trapping of Superoxide Radical and the Stability of the Superoxide Adduct: Combined Experimental and Theoretical Studies

A new phosphorylated linear nitrone N-(4-hydroxybenzyliene)-1-diethoxyphosphoryl-1-methylethylamine N-oxide (4-HOPPN) was synthesized, and its X-ray structure was determined. The spin trapping ability of various kinds of free radicals by 4-HOPPN was evaluated. Kinetic study of decay of the superoxide spin adduct (4-HOPPN-OOH) shows the half-life time of 8.8 min. On the basis of the X-ray structural coordinates, theoretical analyses using density functional theory (DFT) calculations at the B3LYP/6-31+G(d,p)//B3LYP/6-31G(d) level were performed on spin-trapping reactions of superoxide radical with 4-HOPPN and PBN and three possible decay routes for their corresponding superoxide adducts. The comparative calculations on the spin-trapping reactions with superoxide radical predicted that both spin traps share an identical reaction type and have comparable potency when spin trapping superoxide radical. Analysis of the optimized geometries of 4-HOPPN-OOH and PBN-OOH reveals that an introduction of the phosphoryl group can efficiently stabilize the spin adduct through the intramolecular H-bonds, the intramolecular nonbonding attractive interactions, as well as the bulky steric protection. Examination of the decomposition thermodynamics of 4-HOPPN-OOH and PBN-OOH further supports the stabilizing role of the phosphoryl group to a linear phosphorylated spin adduct.

Nitric oxide release from the unimolecular decomposition of the superoxide radical anion adduct of cyclic nitrones in aqueous medium

Nitrones such as 5,5-dimethyl-1-pyrroline N-oxide (DMPO), 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO) and 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide (EMPO) have become the spin-traps of choice for the detection of transient radical species in chemical and biological systems using electron paramagnetic resonance (EPR) spectroscopy. The mechanism of decomposition of the superoxide radical anion (O2(.-)) adducts of DMPO, DEPMPO and EMPO in aqueous solutions was investigated. Our findings suggest that nitric oxide (NO) was formed during the decomposition of the O2(.-) adduct as detected by EPR spin trapping using Fe(II)N-methyl-d-glucamine dithiocarbamate (MGD). Nitric oxide release was observed from the O2(.-) adduct formed from hypoxanthine-xanthine oxidase, PMA-activated human neutrophils, and DMSO solution of KO2. Nitric oxide formation was not observed from the independently generated hydroxyl radical adduct. Formation of nitric oxide was also indirectly detected as nitrite (NO2(.-)) utilizing the Griess assay. Nitrite concentration increases with increasing O2(.-) concentration at constant DMPO concentration, while NO2(.-) formation is suppressed at anaerobic conditions. Moreover, large excess of DMPO also inhibits NO2(.-) formation which can be attributed to the oxidation of DMPO to hydroxamic acid nitroxide (DMPO-X) by nitrogen dioxide (NO2), a precursor to NO2(.-). Product analysis was also conducted to further elucidate the mechanism of adduct decay using gas chromatography-mass spectrometry (GC-MS) technique.