Stable Nitroxide Radicals Research Articles

Keep Your TEMPO Up: Nitroxide Radicals as Sensors of Intermolecular Interactions.

This study examines experimental data on the influence of the surrounding medium and non-covalent interactions on the isotropic hyperfine coupling constant, Aiso(14N), of the stable nitroxide radical 2,2,6,6-Tetramethylpiperidin-1-yl)oxyl (TEMPO) in solution. The data were used to identify a density functional theory functional/basis set combination that accurately reproduces the experimental Aiso(14N) values. The variations in Aiso(14N) due to external factors are two orders of magnitude greater than the accuracy of its experimental measurements, making Aiso(14N) a highly sensitive experimental probe for quantifying these effects. Additionally, it was found that the proton-accepting ability of the N-O• moiety in TEMPO resembles that of the P=O moiety, enabling the simultaneous formation of two equally strong hydrogen bonds.

Combine calculation and experiment to study the undesired polymerization of styrene for screening of effective inhibitors

Regarding undeniable formation of the undesired polymer (UP) in the styrene purification process, it is of vital importance to inhibit its production. To address the mentioned issue, effects of injecting antipolymers (e.g., stable nitroxide radicals (i.e., SNRs) and antioxidants (e.g., alkylhydroxylamines (AHA)) on the controlling the UP formation have been studied through density functional theory (DFT) calculations as well as experimental method in this study. In fact, electrophilicity and growth percentage were evaluated by DFT calculations and experimental approach, correspondingly. Accordingly, it was revealed that the best antipolymers in terms of inhibiting performance were 4‑hydroxy-2,2,6,6-tetramethyl piperidine 1-Oxyl (4‑hydroxy-TEMPO) and 4-oxo-2,2,6,6-tetramethylpiperidine 1-Oxyl (4-oxo-TEMPO), and from the antioxidants, N-Ethyl-N-phenyl hydroxylamine (EPHA) and N-Benzyl-N-phenyl hydroxylamine (BPHA) exhibited the best performance. In addition, the growth percentage of 18.70, 20.55, 24.85 and 46.8, were obtained for the EPHA, BPHA, 4‑hydroxy-TEMPO, and 4-oxo-TEMPO after 4 hrs of operation, correspondingly. Besides, the synergetic effects of the used inhibitors were determined experimentally. Among the evaluated inhibitors, EPHA/4‑hydroxy-TEMPO demonstrated the best synergetic effects over the control of UP formation. Finally, the blend of 4‑hydroxy-TEMPO (40 wt.%) and EPHA (60 wt.%) was obtained to be the optimum (best) inhibitor with 7.2 % polymer growth after 4 hrs of operation.

Competition between n → π* interactions and H-bonding: a way to stabilize aminoxyl anions and to improve electron transfer kinetics

A new approach to stabilization of the reduced state of nitroxides in solution is suggested. Thermodynamic stabilization of the strongly basic aminoxyl anions toward protonation and hydrogen bond formation with the solvent was achieved by intramolecular n → π* interactions. A new stable nitroxide radical bearing ortho-acyl group was synthesized to demonstrate this effect by means of cyclic voltammetry.

Synthesis of Nitronyl Nitroxide Radical-Modified Multi-Walled Carbon Nanotubes and Oxidative Desulfurization in Fuel.

Novel and highly stable nitronyl nitroxide radical (NIT) derivatives were synthesized and coated on the surface of multi-walled carbon nanotubes (MWCNTs) to improve their desulfurization performance. They were characterized by FTIR, UV-vis, SEM, XRD, Raman spectroscopy and ESR. Thiophene in fuel was desulfurized by molecular O2, and the oxidation activity of these compounds was evaluated. At a normal temperature and pressure, the degradation rates of thiophene by four compounds in 4 h can reach 92.66%, 96.38%, 93.25% and 89.49%, respectively. The MWCNTs/NIT-F have a high special activity for the degradation of thiophene, and their desulfurization activity can be recycled for five times without a significant reduction. The mechanistic studies of MWCNTs/NIT composites show that the ammonium oxide ion is the key active intermediate in catalytic oxidative desulfurization, which provides a new choice for fuel oxidative desulfurization. The results show that NIT significantly improves the photocatalytic performance of MWCNTs.

Two-electron conversion nitroxide radicals-based electrode synergistically enhancing charge storage in water-in-salt electrolyte

Water-in-salt electrolytes (WISEs) are attractive in aqueous batteries due to their non-flammability, environmental friendliness, and wide electrochemical stability window. Stable nitroxide radicals with fast electron transfer rate and good cycling stability are promising electrode materials applied for WISEs, while their limited conductivity and high solubility in electrolytes are still challenges. Here, porous carbon with high specific area is used as conductive substrate for the deposition of 4-amino-TEMPO (TEMPO = 2,2,6,6-tetramethylpiperidine-1-oxyl), and MXene nanosheets are employed as their conductive binder to construct freestanding electrode with “hyacinth bean-like” structure. Nitroxide radicals undergo a two-step two-electron reversible redox reaction and MXene exhibits pseudocapacitive ion intercalation, leading to synergistically enhanced discharge capacity in WISE (19.8 m LiCl). The incorporated redox active nitroxide radicals in MXene/porous carbon-TEMPO electrode boost the surface pseudocapacity and the rationally designed three-dimensional structure further facilitates the ions diffusion and electrons transfer, endowing the composite electrode with fast charge storage kinetic. This work provides a promising potential for the design and fabrication of high-performance aqueous batteries.

Towards liquid EPR dosimetry using nitroxides in aqueous solution.

Objective. Water-equivalent dosimeters are desirable for dosimetry in radiotherapy. The present work investigates basic characteristics of novel aqueous detector materials and presents a signal loss approach for electron paramagnetic resonance (EPR) dosimetry.Approach. The proposed principle is based on the radiation dose dependent annihilation of EPR active nitroxides (NO·) in aqueous solutions. Stable nitroxide radicals (3-Maleimido-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (MmP), 3-Carbamoyl-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (CmP)) in aqueous solutions containing dimethyl sulfoxide (DMSO) as an additive were filled in glass capillaries for irradiation and EPR readout. Radiation doses ranging from 1 to 64 Gy were applied with a clinical 6 MV flattening filter free photon beam. EPR readout was then performed with a X-band benchtop spectrometer. The dose response, temporal stability and reproducibility of the samples' EPR signal amplitudes as well as the influence of the nitroxide concentration between 10 and 160μM on the absolute signal loss were investigated using MmP. CmP was used to examine the dependence of the dose response on DMSO concentration between 0 and 10 vol%. An indirect effect model was fitted to the experimental data assuming irradiation induced radical reactions as the underlying mechanism.Main results. For an initial MmP concentration of 20μM, absolute EPR signal loss is linear up to a dose of 16 Gy with a yield G(-NO·) of approximately 0.4μmol J-1. Within five weeks upon sample irradiation to doses between 0 and 32 Gy relative EPR signal fluctuations were on average (126 readouts) below 1% (1σ). For c(MmP) ≥ 20μM, absolute signal loss is only weakly dependent on c(MmP), whereas it increases strongly with increasing c(DMSO) in the range 0-5 vol%. An indirect effect model is applicable to describe the reaction mechanism resulting in the observed dose response curve.Significance. Liquids consisting of nitroxides in aqueous solution and small amounts of DMSO (2 vol%) show promising basic characteristics for application as water-equivalent EPR dosimeter materials in radiotherapy. The EPR signal loss is based on an indirect effect mediated by diffusing radicals originating from the radiolysis of the water/DMSO mixture.

Hair Follicle Damage after 100 mGy Low-Dose Fractionated X-Ray Irradiation and the Protective Effects of TEMPOL, a Stable Nitroxide Radical, against Radiation.

The effects of long-term low-dose X-ray irradiation on the outer root sheath (ORS) cells of C3H/He mice were investigated. Mice were irradiated with a regime of 100 mGy/day, 5 days/week, for 12 weeks (Group X) and the results obtained were compared to those in a non-irradiated control (Group C). Potential protection against ORS cells damage induced by this exposure was investigated by adding the stable nitroxide radical 4-hydroxyl-2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPOL) at 1 mM to the drinking water of mice (Group X + TEMPOL). The results obtained were compared with Group C and a non-irradiated group treated with TEMPOL (Group C + TEMPOL). After fractionated X-ray irradiation, skin was removed and ORS cells were examined by hematoxylin and eosin staining and electron microscopy for an abnormal nuclear morphology and nuclear condensation changes. Fractionated X-irradiated mice had an increased number of ORS cells with an abnormal nuclear morphology as well as nuclear condensation changes. Sections were also immunohistochemically examined for the presence of TdT-mediated dUTP nick-end labeling (TUNEL), 8-hydroxy-2'-deoxyguanosine (8-OHdG), 4-hydroxynonenal (4-HNE), vascular endothelial growth factor (VEGF), nitrotyrosine, heme oxygenase 1 (HO-1), and protein gene product 9.5 (PGP 9.5). Significant increases were observed in TUNEL, 8-OHdG, and 4-HNE levels in ORS cells from mice in Group X. Electron microscopy also showed irregular shrunken ORS cells in Group X. These changes were prevented by the presence of TEMPOL in the drinking water of the irradiated mice. TEMPOL alone had no significant effects. These results suggest that fractionated doses of radiation induced oxidative damage in ORS cells; however, TEMPOL provided protection against this damage, possibly as a result of the rapid reaction of this nitroxide radical with the reactive oxidants generated by fractionated X-ray irradiation.

A radical containing micellar probe for assessing esterase enzymatic activity with ultra-low field Overhauser-enhanced magnetic resonance imaging.

The ability to track altered enzyme activity using a non-invasive imaging protocol is crucial for the early diagnosis of many diseases but is often challenging. Herein, we show that Overhauser magnetic resonance imaging (OMRI) can be used to monitor enzymatic conversion at an ultra-low field (206 μT) using a highly sensitive "off/on" probe with a nitroxide stable radical containing ester, named T2C12-T80. This TEMPO derivative containing probe forms stable electron paramagnetic resonance (EPR) silent micelles in water that are hydrolysed by esterases, thus yielding narrow EPR signals whose intensities correlate directly with specific enzymatic activity. The responsiveness of the probe to tumours, facilitated by increased esterase activity, was initially determined by comparing EPR signals measured upon incubation with 3T3 (healthy fibroblasts used as control), HepG2 (human hepatoma) and Hs766T (human pancreatic cancer cells) cell lysates and then with Hs766T and 3T3 living cells. Next, Overhauser MR images were detected on a phantom containing the probe and the esterases to show that the approach is well suited for being translated to the in vivo detection at the earth's magnetic field. Regarding detection sensitivity, ultra-low field OMRI (ULF-OMRI) is beneficial over OMRI at higher fields (e.g. 0.2 T) since Overhauser enhancements are significantly higher and the technique is safe in terms of the specific absorption rate.

A water-soluble aza-adamantyl nitroxide radical and its complexes with β-cyclodextrin derivatives

A water-soluble, stable aza-adamantyl nitroxide radical has been synthesized. Complexation with selected β-cyclodextrin derivatives increases its water-solubility and maintains well the stability of the radical.

Enhancing Coherence Times of Chromophore-Radical Molecular Qubits and Qudits by Rational Design.

An important criterion for quantum operations is long qubit coherence times. To elucidate the influence of molecular structure on the coherence times of molecular spin qubits and qudits, a series of molecules featuring perylenediimide (PDI) chromophores covalently linked to stable nitroxide radicals were synthesized and investigated by pulse electron paramagnetic resonance spectroscopy. Photoexcitation of PDI in these systems creates an excited quartet state (Q) followed by a spin-polarized doublet ground state (D0), which hold promise as spin qudits and qubits, respectively. By tailoring the molecular structure of these spin qudit/qubit candidates by selective deuteration and eliminating intramolecular motion, coherence times of Tm = 9.1 ± 0.3 and 4.2 ± 0.3 μs at 85 K for D0 and Q, respectively, are achieved. These coherence times represent a nearly 3-fold enhancement compared to those of the initial molecular design. This approach offers a rational structural design protocol for effectively extending coherence times in molecular spin qudits/qubits.

4-Amino-TEMPO loaded liposomes as sensitive EPR and OMRI probes for the detection of phospholipase A2 activity

This work aims at developing a diagnostic method based on Electron Paramagnetic Resonance (EPR) measurements of stable nitroxide radicals released from “EPR silent” liposomes. The liposome destabilisation and consequent radical release is enzymatically triggered by the action of phospholipase A2 (PLA2) present in the biological sample of interest. PLA2 are involved in a broad range of processes, and changes in their activity may be considered as a unique valuable biomarker for early diagnoses. The minimum amount of PLA2 measured “in vitro” was 0.09 U/mL. Moreover, the liposomes were successfully used to perform Overhauser-enhanced Magnetic Resonance Imaging (OMRI) in vitro at 0.2 T. The amount of radicals released by PLA2 driven liposome destabilization was sufficient to generate a well detectable contrast enhancement in the corresponding OMRI image.

Magnetically Cured Macroradical Epoxy as Antimicrobial Coating.

The radical-bearing epoxy monomer could be the ideal embodiment of multifunctionality in epoxy-based materials. This study demonstrates the potential of macroradical epoxies as surface coating materials. A diepoxide monomer derivatized with a stable nitroxide radical is polymerized with a diamine hardener under the influence of a magnetic field. The magnetically oriented and stable radicals in the polymer backbone render the coatings antimicrobial. The unconventional use of magnets during polymerization proved crucial in correlating the structure-property relationships with antimicrobial performance inferred from oscillatory rheological technique, polarized macro-attenuated total reflectance - infrared (macro-ATR-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The magnetic thermal curing influenced the surface morphology, resulting in a synergy of the coating's radical nature with microbiostatic performance assessed using the Kirby-Bauer test and liquid chromatography - mass spectroscopy (LC-MS). Further, the magnetic curing of blends with a traditional epoxy monomer demonstrates that radical alignment is more critical than radical density in imparting biocidal behavior. This study shows how the systematic use of magnets during polymerization could pave for probing more significant insights into the mechanism of antimicrobial action in radical-bearing polymers.

Direct Application of 3-Maleimido-PROXYL for Proving Hypoalbuminemia in Cases of SARS-CoV-2 Infection: The Potential Diagnostic Method of Determining Albumin Instability and Oxidized Protein Level in Severe COVID-19.

Oxidative stress and the albumin oxidized form can lead to hypoalbuminemia, which is a predisposing factor for reduced treatment effectiveness and an increased mortality rate in severe COVID-19 patients. The aim of the study is to evaluate the application of free radical 3-Maleimido-PROXYL and SDSL-EPR spectroscopy in the in vitro determination of ox/red HSA in serum samples from patients with SARS-CoV-2 infection. Venous blood was collected from patients intubated (pO2 < 90%) with a positive PCR test for SARS-CoV-2 and controls. At the 120th minute after the incubation of the serum samples from both groups with the 3-Maleimido-PROXYL, the EPR measurement was started. The high levels of free radicals were determined through the nitroxide radical TEMPOL, which probably led to increased oxidation of HSA and hypoalbuminemia in severe COVID-19. The double-integrated spectra of 3-Maleimido-PROXYL radical showed a low degree of connectivity due to high levels of oxidized albumin in COVID-19 patients. The low concentrations of reduced albumin in serum samples partially inhibit spin-label rotation, with Amax values and ΔH0 spectral parameters comparable to those of 3-Maleimido-PROXYL/DMSO. Based on the obtained results, we suggest that the stable nitroxide radical 3-Maleimido-PROXYL can be successfully used as a marker to study oxidized albumin levels in COVID-19.

Novel Orientation-Sensitive Spin Probes for Graphene Oxide Membranes Study.

Spin probe EPR spectroscopy is currently the only method to quantitatively report on the orientational ordering of graphene oxide membranes. This technique is based on the analysis of EPR spectra of a membrane containing stable radicals sorbed on oxidized graphene planes. The efficiency of the method depends on the spin probe structure; therefore, it is important to find stable paramagnetic substances that are most sensitive to the alignment of graphene oxide membranes. In the present work, three novel stable nitroxide radicals containing aromatic fragments with two nitrogen atoms were tested as spin probes to study graphene oxide membranes. The spin-Hamiltonian parameters of the radicals in graphite oxide powder and orientational order parameters of the probes inside graphene oxide membrane were determined. The sensitivity of one of these radicals to membrane orientational ordering was found to be higher than for any of spin probes used previously. A likely reason for this higher sensitivity is the presence of heteroatoms which can facilitate interaction between paramagnetic molecules and oxygen-containing groups on the inner surface of the membrane. The new high-sensitivity spin probe may significantly increase the potential of EPR spectroscopy for studying the internal structure of graphene oxide membranes.

Robust compound elastomers of integrated polymer cathode and electrolyte for high-performance all-solid-state lithium battery

A novel type of all-solid-state lithium-organic batteries based on redox active multicomponent polyethers comprising stable nitroxide radicals, ethylene oxide groups and cross-linkable moieties has been demonstrated. The radical copolymer with a low glass transition temperature exhibits enhanced mixed conductivities of electron and ion. Furthermore, the in-situ cross-linking of the radical copolymer cathode and a comb-branched polyether electrolyte results in a flexible yet strong compound elastomer, which demonstrates a low interfacial resistance and fast ion transport. The fabricated lithium cell shows a high cycling stability with a low polarization and durable rate capability at room temperature, benefiting from the fast redox reactions in the organic cathode and the integrated structure. This work is expected to advance the development of flexible solid batteries with high electrochemical performance.

Silver Nanoparticles Densely Grafted with Nitroxides as a Recyclable Green Catalyst in the Selective Oxidation of Alcohols.

The selective oxidation of alcohols, leading to appropriate aldehydes, is widely recognised as one of the most important reactions in organic synthesis. With ever-increasing environmental concerns, much attention has been directed toward developing catalytic protocols that use molecular oxygen as an oxidant. An ideal green oxidation process should employ a highly active, selective and recyclable catalyst that can work with oxygen under mild conditions. This paper presents a successful application of densely grafted silver nanostructures with stable nitroxide radicals (N-AgNPs) as an effective, easily-recovered and regenerable catalyst for the selective oxidation of alcohols. The fabricated ultra-small and narrow dispersive silver nanoparticles have been fully characterised using physicochemical methods (TEM, DLS, XPS, TGA). N-AgNPs have been successfully applied to oxidise several model alcohols: benzyl alcohol, 4-pyridinemethanol, furfuryl alcohol, 1-phenyl ethanol, n-heptanol and allyl alcohol under mild conditions using oxygen as a stoichiometric oxidant. Notably, the fabricated nitroxide grafted silver nanoparticles (N-AgNPs) were reused more than ten times in the oxidation of a series of primary alcohols to corresponding aldehydes under mild conditions with very high yields and a selectivity close to 100%.

A novel nitronyl nitroxide radical HPN-C6 attenuates brain damage in an acute hypobaric hypoxia mouse model through inhibition of the oxidative stress

Hypobaric hypoxia (HH) results in the increased formation of ROS and induces a wide range of deleterious effects on the brain. Nitronyl nitroxide radicals are a kind of stable organic nitroxide radicals with a good ability to scavenge ROS. Herein, a series of new nitronyl nitroxide radicals was synthesized according to the previous method. Among them, a compound with six carbon atoms alkyl chain (HPN-C6) was screened for estimating the protective effect against HH-induced brain injury in the acute HH mouse model. The results showed that HPN-C6 ameliorated oxidative stress by decreasing the activity of LDH and the content of H2O2, NO, and MDA, increasing the activities of SOD, CAT and GSH-PX in the brain tissue of HH injured mice. Moreover, HPN-C6 exhibited an anti-inflammatory effect by reducing the level of IL-1β, TNF-α, and IL-6 in the brains of mice after HH exposure through regulating the P38 MAPK signaling pathway. In addition, HPN-C6 exhibited the protection effect by inhibiting the release of mitochondrial cytochrome C, locking the activation of caspase-3, and downregulating the expression of Bax and upregulating the expression of Bcl-2. HPN-C6 pretreatment could inhibit the caspase-dependent mitochondria apoptosis pathway. These findings indicate that HPN-C6 is a potential therapeutic agent to prevent brain damage induced by HH.

Cyclic Hydroxylamines as Monitors of Peroxynitrite and Superoxide-Revisited

There is a considerable need for methods that allow quantitative determination in vitro and in vivo of transient oxidative species such as peroxynitrite (ONOOH/ONOO−) and superoxide (HO2•/O2•−). Cyclic hydroxylamines, which upon oxidation yield their respective stable nitroxide radicals, have been suggested as spin probes of peroxynitrite and superoxide. The present study investigated this approach by following the kinetics of peroxynitrite decay in the absence and presence of various 5-membered and 6-membered ring hydroxylamines, and comparing the yield of their respective nitroxides using electron paramagnetic spectroscopy. The results demonstrate that hydroxylamines do not react directly with peroxynitrite, but are oxidized to their respective nitroxides by the radicals formed during peroxynitrite self-decomposition, namely •OH and •NO2. The accumulated nitroxides are far below their expected yield, had the hydroxylamines fully scavenged all these radicals, due to multiple competing reactions of the oxidized forms of the hydroxylamines with •NO2 and ONOO−. Therefore, cyclic hydroxylamines cannot be used for quantitative assay of peroxynitrite in vitro. The situation is even more complex in vivo where •OH and •NO2 are formed also via other oxidizing reactions systems. The present study also compared the yield of accumulated nitroxides under constant flux of superoxide in the presence of various cyclic hydroxylamines. It is demonstrated that certain 5-membered ring hydroxylamines, which their respective nitroxides are poor SOD-mimics, might be considered as stoichiometric monitors of superoxide in vitro at highest possible concentrations and pH.

Overhauser Dynamic Nuclear Polarization Parameters of a Nitroxide Radical in Liquid

Solution state Overhauser-effect-type dynamic nuclear polarization (ODNP) has been studied in the last years. The basis of ODNP is a polarization mechanism that presents nuclei with electron spin polarization. ODNP enhancements studies are presented 4-fluorobenzyl alcohol (4FBA) with the stable nitroxide radical as the polarizing agent for 15, 30 and 60 mM concentrations at 1.53 mT. Major ODNP parameters were obtained. The significant parameter is the coupling parameter, since it determines the maximum NMR signal enhancements and the interactions between the nuclear spin and the electron spin. The coupling parameter can vary from -0.152 to 0.001. The obtained ODNP parameters show that both scalar and dipolar interactions occur. The results show that nitroxide radical can be taken as polarizing agent for ODNP studies of 4-fluorobenzyl alcohol solvent. The effect of dissolved molecular oxygen on the enhancement factor for the sample with the smallest concentration was experimentally investigated.

Magnetic Resonance and Computational Studies on Crossover Reactions in Li-Air Batteries and Redox-Flow Batteries Using TEMPO

Redox active solution species are important in a variety of electrochemical energy storage systems. In lithium-air batteries, redox mediators are added as soluble catalysts that mitigate charge overpotentials as well as promote solution-phase reactions that improve the cycle life of a cell. Redox flow batteries are also dependent on solution redox species to carry charge and can attain high energy capacities by increasing the concentrations of these species. In both of these systems, crossover reactions, where the redox active species diffuse from one side of the cell to the other, result in capacity loss and parasitic side reactions. Here, we report a method to monitor crossover reactions using online nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy. 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) is a stable nitroxide radical used as a redox mediator in lithium-air batteries and charge species redox flow batteries. Radical concentrations can be probed and quantified during electrochemical cycling by flowing the electrolyte through the NMR and EPR spectrometers. The experimental results reveal severe crossover and self-reduction during electrochemical oxidation of TEMPO. Diffusion models performed support the in-situ data, highlighting that crossover is a major phenomenon in these systems. Figure 1