The nitroxide TEMPO is an efficient scavenger of protein radicals: cellular and kinetic studies

Redox factors in the antioxidant activity of nitroxides toward DNA guanyl and 2-deoxyribose-peroxyl radicals

A series of eight nitroxide compounds (four substituted piperidines, three pyrrolidines and one oxo-piperidine) are found to undergo electron transfer to 2'-deoxyribose-peroxyl and the guanyl radical. One-electron oxidation potentials of the nitroxides to oxoammonium cations (oxoammonium reduction potential), E0', have been measured against a common redox indicator, chlorpromazine, and found to span the range 751 ± 15 mV to 973 ± 15 mV. Fast chemical reduction of the 2'-deoxyribose-peroxyl radical to the hydroperoxide, generated by •OH radical attack on 2-deoxyribose, dR, in oxygenated aqueous solution, is a redox-dependent reaction, with rate constants of 0.8-3.5 x 107 M-1 s-1.The guanyl radicals, produced upon one-electron oxidation of 2'-deoxyguanosine monophosphate, dG, by the selenite radical, SeO3•-, react with the nitroxides in a redox-independent reaction with diffusion rate constants of 1-2 x 108 M-1 s-1. These findings represent a possible antioxidant role for nitroxides in the fast chemical repair of DNA radicals, which is supported by an in vitro strand break study using a plasmid.

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.

Effect of Low Concentration of Nitroxides on SH-SY5Y Cells Transfected with the Tau Protein

Nitroxides, stable synthetic free radicals, are promising antioxidants, showing many beneficial effects both at the cellular level and in animal studies. However, the cells are usually treated with high millimolar concentrations of nitroxides which are not relevant to the concentrations that could be attained in vivo. This paper aimed to examine the effects of low (≤10 μM) concentrations of three nitroxides, 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO), 4-hydroxy-TEMPO (TEMPOL) and 4-amino-TEMPO (TEMPAMINE), in pure chemical systems and on SH-SY5Y cells transfected with the human tau protein (TAU cells), a model of chronic cellular oxidative stress, and transfected with the empty plasmid (EP cells). All nitroxides were active in antioxidant-activity tests except for the 2,2'-azinobis-(3-ethylbenzthiazolin-6-sulfonate) radical (ABTS•) decolorization assay and reduced Fe3+, inhibited autoxidation of adrenalin and pyrogallol and oxidation of dihydrorhodamine123 by 3-morpholino-sydnonimine SIN-1. TEMPO protected against fluorescein bleaching from hypochlorite, but TEMPAMINE enhanced the bleaching. Nitroxides showed no cytotoxicity and were reduced by the cells to non-paramagnetic derivatives. They decreased the level of reactive oxygen species, depleted glutathione, and increased mitochondrial-membrane potential in both types of cells, and increased lipid peroxidation in TAU cells. These results demonstrate that even at low micromolar concentrations nitroxides can affect the cellular redox equilibrium and other biochemical parameters.

Preserving Retinal Structure and Function with the Novel Nitroxide Antioxidant, DCTEIO

Oxidative stress is a major contributor to progressive neurodegenerative disease and may be a key target for the development of novel preventative and therapeutic strategies. Nitroxides have been successfully utilised to study changes in redox status (biological probes) and modulate radical-induced oxidative stress. This study investigates the efficacy of DCTEIO (5,6-dicarboxy-1,1,3,3-tetraethyllisoindolin-2-yloxyl), a stable, kinetically-persistent, nitroxide-based antioxidant, as a retinal neuroprotectant. The preservation of retinal function following an acute ischaemic/reperfusion (I/R) insult in the presence of DCTEIO was quantified by electroretinography (ERG). Inflammatory responses in retinal glia were analysed by GFAP and IBA-1 immunohistochemistry, and retinal integrity assessed by histology. A nitroxide probe combined with flow cytometry provided a rapid technique to assess oxidative stress and the mitigation offered by antioxidant compounds in cultured 661W photoreceptor cells. DCTEIO protected the retina from I/R-induced damage, maintaining retinal function. Histological analysis showed preservation of retinal integrity with reduced disruption and disorganisation of the inner and outer nuclear layers. I/R injury upregulated GFAP expression, indicative of retinal stress, which was significantly blunted by DCTEIO. The number of 'activated' microglia, particularly in the outer retina, in response to cellular stress was also significantly reduced by DCTEIO, potentially suggesting reduced inflammasome activation and cell death. DCTEIO mitigated oxidative stress in 661W retinal cell cultures, in a dose-dependent fashion. Together these findings demonstrate the potential of DCTEIO as a neuroprotective therapeutic for degenerative diseases of the CNS that involve an ROS-mediated component, including those of the retina e.g. age-related macular degeneration and glaucoma.

Frustrated Radical Pairs in Organic Synthesis

Frustrated radical pairs (FRPs) describe the phenomenon that two distinct radicals─which would otherwise annihilate each other to form a closed-shell covalent adduct─can coexist in solution, owing to steric repulsion or weak bonding association. FRPs are typically formed via spontaneous single-electron transfer between two sterically encumbered precursors─an oxidant and a reductant─under ambient conditions. The two components of a FRP exhibit orthogonal chemical properties and can often act in cooperativity to achieve interesting radical reactivities. Initially observed in the study of traditional frustrated Lewis pairs, FRPs have recently been shown to be capable of homolytically activating various chemical bonds. In this Perspective, we will discuss the discovery of FRPs, their fundamental reactivity in chemical bond activation, and recent developments of their use in synthetic organic chemistry, including in C-H bond functionalization. We anticipate that FRPs will provide new reaction strategies for solving challenging problems in modern organic synthesis.

Marcus Cross-Relationship Probed by Time-Resolved CIDNP

The time-resolved CIDNP method can provide information about degenerate exchange reactions (DEEs) involving short-lived radicals. In the temperature range from 8 to 65 °C, the DEE reactions of the guanosine-5'-monophosphate anion GMP(-H)- with the neutral radical GMP(-H)•, of the N-acetyl tyrosine anion N-AcTyrO- with a neutral radical N-AcTyrO•, and of the tyrosine anion TyrO- with a neutral radical TyrO• were studied. In all the studied cases, the radicals were formed in the reaction of quenching triplet 2,2'-dipyridyl. The reorganization energies were obtained from Arrhenius plots. The rate constant of the reductive electron transfer reaction in the pair GMP(-H)•/TyrO- was determined at T = 25 °C. Rate constants of the GMP(-H)• radical reduction reactions with TyrO- and N-AcTyrO- anions calculated by the Marcus cross-relation differ from the experimental ones by two orders of magnitude. The rate constants of several other electron transfer reactions involving GMP(-H)-/GMP(-H)•, N-AcTyrO-/N-AcTyrO•, and TyrO-/TyrO• pairs calculated by cross-relation agree well with the experimental values. The rate of nuclear paramagnetic relaxation was found for the 3,5 and β-protons of TyrO• and N-AcTyrO•, the 8-proton of GMP(-H)•, and the 3,4-protons of DPH• at each temperature. In all cases, the dependences of the rate of nuclear paramagnetic relaxation on temperature are described by the Arrhenius dependence.

Organic Synthesis Using Nitroxides

Nitroxides, also known as nitroxyl radicals, are long-lived or stable radicals with the general structure R1R2N-O•. The spin distribution over the nitroxide N and O atoms contributes to the thermodynamic stability of these radicals. The presence of bulky N-substituents R1 and R2 prevents nitroxide radical dimerization, ensuring their kinetic stability. Despite their reactivity toward various transient C radicals, some nitroxides can be easily stored under air at room temperature. Furthermore, nitroxides can be oxidized to oxoammonium salts (R1R2N═O+) or reduced to anions (R1R2N-O-), enabling them to act as valuable oxidants or reductants depending on their oxidation state. Therefore, they exhibit interesting reactivity across all three oxidation states. Due to these fascinating properties, nitroxides find extensive applications in diverse fields such as biochemistry, medicinal chemistry, materials science, and organic synthesis. This review focuses on the versatile applications of nitroxides in organic synthesis. For their use in other important fields, we will refer to several review articles. The introductory part provides a brief overview of the history of nitroxide chemistry. Subsequently, the key methods for preparing nitroxides are discussed, followed by an examination of their structural diversity and physical properties. The main portion of this review is dedicated to oxidation reactions, wherein parent nitroxides or their corresponding oxoammonium salts serve as active species. It will be demonstrated that various functional groups (such as alcohols, amines, enolates, and alkanes among others) can be efficiently oxidized. These oxidations can be carried out using nitroxides as catalysts in combination with various stoichiometric terminal oxidants. By reducing nitroxides to their corresponding anions, they become effective reducing reagents with intriguing applications in organic synthesis. Nitroxides possess the ability to selectively react with transient radicals, making them useful for terminating radical cascade reactions by forming alkoxyamines. Depending on their structure, alkoxyamines exhibit weak C-O bonds, allowing for the thermal generation of C radicals through reversible C-O bond cleavage. Such thermally generated C radicals can participate in various radical transformations, as discussed toward the end of this review. Furthermore, the application of this strategy in natural product synthesis will be presented.

Spatial-Selective Volumetric 4D Printing and Single-Photon Grafting of Biomolecules within Centimeter-Scale Hydrogels via Tomographic Manufacturing

Conventional additive manufacturing and biofabrication techniques are unable to edit the chemicophysical properties of the printed object postprinting. Herein, a new approach is presented, leveraging light-based volumetric printing as a tool to spatially pattern any biomolecule of interest in custom-designed geometries even across large, centimeter-scale hydrogels. As biomaterial platform, a gelatin norbornene resin is developed with tunable mechanical properties suitable for tissue engineering applications. The resin can be volumetrically printed within seconds at high resolution (23.68 ± 10.75 μm). Thiol-ene click chemistry allows on-demand photografting of thiolated compounds postprinting, from small to large (bio)molecules (e.g., fluorescent dyes or growth factors). These molecules are covalently attached into printed structures using volumetric light projections, forming 3D geometries with high spatiotemporal control and ≈50 μm resolution. As a proof of concept, vascular endothelial growth factor is locally photografted into a bioprinted construct and demonstrated region-dependent enhanced adhesion and network formation of endothelial cells. This technology paves the way toward the precise spatiotemporal biofunctionalization and modification of the chemical composition of (bio)printed constructs to better guide cell behavior, build bioactive cue gradients. Moreover, it opens future possibilities for 4D printing to mimic the dynamic changes in morphogen presentation natively experienced in biological tissues.

Two faces of TEMPO (2,2,6,6-tetramethylpiperidinyl-1-oxyl) - An antioxidant or a toxin?

2,2,6,6-Tetramethylpiperidine-1-oxyl, commonly known as TEMPO, is one of the compounds called nitroxides that are used in the chemical industry for synthesis of many organic compounds as well as for electrodes in all-organic radical batteries. Additionally, TEMPO is a widely used antioxidant in scientific studies. Technological progress and simultaneous care for the environment leads to resorting to new industrial methods which require the use of compounds that have not been fully tested for their impact on living organisms. Therefore, TEMPO may be an environmental pollutant and its effect on living organisms is not fully understood. The aim of our study was to determine the influence of TEMPO on the physiology, chronological lifespan and wide transcription changes of a eukaryotic model organism, namely the Saccharomyces cerevisiae yeast. For this purpose, we used the BY4741 wild-type and isogenic mutants with a disorder in the response to oxidative stress (sod1Δ, sod2Δ, yap1Δ) and repair of DNA damage (rad52Δ). We showed that supplementation with TEMPO inhibited the cell growth rate of all analyzed strains while simultaneously slowing down the aging of post-mitotic cells in the yeast population. In addition, TEMPO-treated yeast cells manifested a significantly increased level of metabolism in the wild-type and sod2Δ strains. TEMPO also displayed genoprotective effect by reducing the number of DNA double-strand breaks in cells. Here, we are the first to show the widespread effect of TEMPO on yeast. In conclusion, we have shown that, contrary to the commonly accepted notion, TEMPO has also a toxic effect, especially on active mitotic cells. We hypothesize that translation impairment or ribosome biogenesis disorder is likely to be considered secondary effects of TEMPO toxicity related to cell cycle arrest. Therefore, despite the growing interest in the use of this compound in the chemical industry, its toxic effect on the environment, especially biosphere, should be taken into account.

Polyradical PROXYL/TEMPO Conjugates Connected by Ester/Amide Bridges: Synthesis, Physicochemical Studies, and DFT Calculations

A series of di-/trinitroxide esters and amides featuring PROXYL and/or TEMPO radicals connected with alicyclic bridges were prepared in 61-92 % yields and their properties were analysed by using multiple experimental techniques. The examination of EPR spectra of radicals in organic solvents augmented with DFT calculations brought valuable information on the conformational dynamics and spin exchange mechanisms. Cyclic voltammetry investigations revealed (quasi)reversible electrochemical behaviour of studied nitroxides with their half-wave potentials ranging from -51 to -17 mV. SQUID measurements of selected radicals revealed that the magnetism of di- and trinitroxides is significantly different, since antiferromagnetic coupling in biradicals is notably larger than in triradicals. The single-crystal X-ray analysis of selected biradicals revealed the existence of 3D supramolecular networks of molecules linked through hydrogen-bonding interactions. These polynitroxide radicals can serve as promising bridging or chelating ligands in the synthesis of transition-metal-based molecular magnets.

Unified Synthesis of Azepines by Visible-Light-Mediated Dearomative Ring Expansion of Aromatic N-Ylides

Herein, we report a unified approach to azepines by dearomative photochemical rearrangement of aromatic N-ylides. Deprotonation of quaternary aromatic salts with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or N,N,N',N'-tetramethylquanidine (TMG) under visible light irradiation provides mono- and polycyclic azepines in yields up to 98%. This ring-expansion presents a new mode of access to functionalized azepines from N-heteroarenes using two straightforward steps and simple starting materials.

Human cataractous lenses contain cross-links produced by crystallin-derived tryptophanyl and tyrosyl radicals

Protein insolubilization, cross-linking and aggregation are considered critical to the development of lens opacity in cataract. However, the information about the presence of cross-links other than disulfides in cataractous lenses is limited. A potential role for cross-links produced from tryptophanyl radicals in cataract development is suggested by the abundance of the UV light-sensitive Trp residues in crystallin proteins. Here we developed a LC-MS/MS approach to examine the presence of Trp-Trp, Trp-Tyr and Tyr-Tyr cross-links and of peptides containing Trp-2H (-2.0156 Da) in the lens of three patients diagnosed with advanced nuclear cataract. In the proteins of two of the lenses, we characterized intermolecular cross-links between βB2-Tyr¹⁵³-Tyr¹⁰⁴-βA3 and βB2-Trp¹⁵⁰-Tyr¹³⁹-βS. An additional intermolecular cross-link (βB2-Tyr⁶¹-Trp²⁰⁰-βB3) was present in the lens of the oldest patient. In the proteins of all three lenses, we characterized two intramolecular Trp-Trp cross-links (Trp¹²³-Trp¹²⁶ in βB1 and Trp⁸¹-Trp⁸⁴ in βB2) and six peptides containing Trp -2H residues, which indicate the presence of additional Trp-Trp cross-links. Relevantly, we showed that similar cross-links and peptides with modified Trp-2H residues are produced in a time-dependent manner in bovine β-crystallin irradiated with a solar simulator. Therefore, different crystallin proteins cross-linked by crystalline-derived tryptophanyl and tyrosyl radicals are present in advanced nuclear cataract lenses and similar protein modifications can be promoted by solar irradiation even in the absence of photosensitizers. Overall, the results indicate that a role for Trp-Tyr and Trp-Trp cross-links in the development of human cataract is possible and deserves further investigation.

Hydroxylamines inhibit tyrosine oxidation and nitration: The role of their respective nitroxide radicals

In vivo, nitroxide antioxidants distribute within minutes throughout all tissues, but are reduced to their respective hydroxylamines due to the cellular reducing environment, which apparently limits their application. To distinguish their antioxidative activity from that of their respective nitroxides, the kinetics and mechanism of their inhibitory effect on the enzymatic oxidation and nitration of tyrosine have been studied. The inhibitory effect of the hydroxylamines on the oxidation and nitration of tyrosine induced by HRP/H₂O₂ and HRP/H₂O₂/nitrite was investigated by following the kinetics of the formation of their respective nitroxides, H₂O₂ decomposition, release of O₂ and accumulation of tyrosine oxidation and nitration products. The distinction between the antioxidative activities of nitroxides and of their respective hydroxylamines is hindered due to oxidation of hydroxylamines to nitroxides, which catalytically inhibit tyrosine oxidation and nitration. The results demonstrate that (i) hydroxylamines inhibit tyrosine oxidation and nitration and their inhibitory effect increases as the reduction potential of their respective nitroxides decreases; (ii) the 6-membered ring hydroxylamines are more effective antioxidants than the 5-membered hydroxylamine derived from 3-carbamoyl proxyl and (iii) the 6-membered ring hydroxylamines are as effective antioxidants as their respective nitroxides, whereas the 3-carbamoyl proxyl is even a weaker antioxidant than its respective hydroxylamine. In general, cyclic hydroxylamines are more effective antioxidants than common antioxidants such as ascorbic and uric acids, which are depleted giving rise to secondary radicals that, might be toxic. In the case of hydroxylamines, the secondary radicals are their respective nitroxides, which are efficient catalytic antioxidants.

Tempol reduces inflammation and oxidative damage in cigarette smoke-exposed mice by decreasing neutrophil infiltration and activating the Nrf2 pathway

Cigarette smoke is a complex mixture capable of triggering inflammation and oxidative damage in animals at pulmonary and systemic levels. Tempol (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) reduces tissue injury associated with inflammation in vivo by mechanisms that are not completely understood. Here we evaluated the effect of tempol on inflammation and oxidative damage induced by acute exposure to cigarette smoke in vivo. Male C57BL/6 mice (n = 32) were divided into 4 groups (n = 8 each): 1) control group exposed to ambient air (GC), 2) animals exposed to cigarette smoke for 5 days (CSG), mice treated 3) prior or 4) concomitantly with tempol (50 mg/kg/day) and exposed to cigarette smoke for 5 days. The results showed that the total number of leukocytes and neutrophils increased in the respiratory tract and lung parenchyma of mice exposed to cigarette smoke. Likewise, MPO levels and activity as well as lipid peroxidation and lung protein nitration and carbonylation also increased. Administration of tempol before or during exposure to cigarette smoke inhibited all the above parameters. Tempol also reduced the pulmonary expression of the inflammatory cytokines Il-6, Il-1β and Il-17 to basal levels and of Tnf-α by approximately 50%. In contrast, tempol restored Il-10 and Tgf-β levels and enhanced the expression of Nrf2-associated genes, such as Ho-1 and Gpx2. Accordingly, total GPx activity increased in lung homogenates of tempol-treated animals. Taken together, our results show that tempol protects mouse lungs from inflammation and oxidative damage resulting from exposure to cigarette smoke, likely through reduction of leukocyte infiltration and increased transcription of some of the Nrf2-controlled genes.

Chemical Deprenylation of N6 -Isopentenyladenosine (i6 A) RNA

N⁶ -isopentenyladenosine (i⁶ A) is an RNA modification found in cytokinins, which regulate plant growth/differentiation, and a subset of tRNAs, where it improves the efficiency and accuracy of translation. The installation and removal of this modification is mediated by prenyltransferases and cytokinin oxidases, and a chemical approach to selective deprenylation of i⁶ A has not been developed. We show that a selected group of oxoammonium cations function as artificial deprenylases to promote highly selective deprenylation of i⁶ A in nucleosides, oligonucleotides, and live cells. Importantly, other epigenetic modifications, amino acid residues, and natural products were not affected. Moreover, a significant phenotype difference in the Arabidopsis thaliana shoot and root development was observed with incubation of the cation. These results establish these small organic molecules as direct chemical regulators/artificial deprenylases of i⁶ A.

The heterocyclic compound Tempol inhibits the growth of cancer cells by interfering with glutamine metabolism

Tempol (4-hydroxy-2,2,6,6-Tetramethylpiperidine-1-oxyl, TPL), a nitroxide compound, inhibits proliferation and increases the vulnerability of cancer cells to apoptosis induced by cytotoxic agents. However, the molecular mechanism of TPL inhibiting cancer cell proliferation has not been fully understood. In this study, we evaluated the metabolic effect of TPL on cancer cells and explored its cancer therapeutic potential. Extracellular flow assays showed that TPL inhibited cellular basal and maximal oxygen consumption rates of mitochondrial. ¹³C metabolic flux analysis showed that TPL treatment had minimal effect on glycolysis. However, we found that TPL inhibits glutamine metabolism by interfering with the oxidative tricarboxylic acid cycle (TCA) process and reductive glutamine process. We found that the inhibitory effect of TPL on metabolism occurs mainly on the step from citrate to α-ketoglutarate or vice versa. We also found that activity of isocitrate dehydrogenase IDH1 and IDH2, the key enzymes in TCA, were inhibited by TPL treatment. In xenograft mouse model, TPL treatment reduced tumor growth by inhibiting cellular proliferation of xenograft tumors. Thus, we provided a mechanism of TPL inhibiting cancer cell proliferation by interfering with glutamine utilization that is important for survival and proliferation of cancer cells. The study may help the development of a therapeutic strategy of TPL combined with other anticancer medicines.

It takes two for chronic wounds to heal: dispersing bacterial biofilm and modulating inflammation with dual action plasma coatings

Chronic wounds are affecting increasingly larger portions of the general population and their treatment has essentially remained unchanged for the past century. This lack of progress is due to the complex problem that chronic wounds are simultaneously infected and inflamed. Both aspects need to be addressed together to achieve a better healing outcome. Hence, we hereby demonstrate that the stable nitroxide radical (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) can be plasma polymerized into smooth coatings (TEMPOpp), as seen via atomic force microscopy, X-ray photoelectron spectroscopy and ellipsometry. Upon contact with water, these coatings leach nitroxides into aqueous supernatant, as measured via EPR. We then exploited the known cell-signalling qualities of TEMPO to change the cellular behaviour of bacteria and human cells that come into contact with the surfaces. Specifically, the TEMPOpp coatings not only suppressed biofilm formation of the opportunistic bacterium Staphylococcus epidermidis but also dispersed already formed biofilm in a dose-dependent manner; a crucial aspect in treating chronic wounds that contain bacterial biofilm. Thus the coatings' microbiological efficacy correlated with their thickness and the thickest coating was the most efficient. Furthermore, this dose-dependent effect was mirrored in significant cytokine reduction of activated THP-1 macrophages for the four cytokines TNF-α, IL-1β, IL-6 and IP-10. At the same time, the THP-1 cells retained their ability to adhere and colonize the surfaces, as verified via SEM imaging. Thus, summarily, we have exploited the unique qualities of plasma polymerized TEMPO coatings in targeting both infection and inflammation simultaneously; demonstrating a novel alternative to how chronic wounds could be treated in the future.

We plasma polymerized the stable nitroxide radical TEMPO into thin coatings and exploited the coatings' unique qualities in targeting both infection and inflammation simultaneously; demonstrating a novel alternative as to how chronic wounds could be treated in the future.

The Persistent Radical Effect in Organic Synthesis

Radical-radical couplings are mostly nearly diffusion-controlled processes. Therefore, the selective cross-coupling of two different radicals is challenging and not a synthetically valuable transformation. However, if the radicals have different lifetimes and if they are generated at equal rates, cross-coupling will become the dominant process. This high cross-selectivity is based on a kinetic phenomenon called the persistent radical effect (PRE). In this Review, an explanation of the PRE supported by simulations of simple model systems is provided. Radical stabilities are discussed within the context of their lifetimes, and various examples of PRE-mediated radical-radical couplings in synthesis are summarized. It is shown that the PRE is not restricted to the coupling of a persistent with a transient radical. If one coupling partner is longer-lived than the other transient radical, the PRE operates and high cross-selectivity is achieved. This important point expands the scope of PRE-mediated radical chemistry. The Review is divided into two parts, namely 1) the coupling of persistent or longer-lived organic radicals and 2) "radical-metal crossover reactions"; here, metal-centered radical species and more generally longer-lived transition-metal complexes that are able to react with radicals are discussed-a field that has flourished recently.

[Effect of the chemoprotectant tempol on anti-tumor activity of cisplatin]

OBJECTIVE

To investigate the effect of the chemoprotectant tempol on the anti-tumor activity of cisplatin (DDP).

METHODS

The cellular toxicity of tempol in human colon cancer SW480 cells and mouse colon cancer CT26 cells were evaluated using MTT and cell counting kit-8 assays. CalcuSyn software analysis was used to determine the interaction between tempol and DDP in inhibition of the cell viability. A subcutaneous homograft mouse model of colon cancer was established. The mice were randomly divided into control group, tempol group, cisplatin group and tempol + DDP treatment group with intraperitoneal injections of the indicated agents. The tumor size, body weight and lifespan of the mice were measured, and HE staining was used to analyze the cytotoxic effect of the agents on the kidney and liver. Immunohistochemistry and Western blotting were performed to detect the expression of Bax and Bcl2 in the tumor tissue, and TUNEL staining was used to analyze the tumor cell apoptosis. The level of reactive oxygen species (ROS) in the tumor tissue was determined using flow cytometry.

RESULTS

Tempol showed inhibitory effects on the viability of SW480 and CT26 cells. CalcuSyn software analysis showed that tempol had a synergistic anti-tumor effect with DDP (CI < 1). In the homograft mouse model, tempol treatment alone did not produce obvious anti-tumor effect. HE staining showed that the combined use of tempol and DDP alleviated DDP-induced fibrogenesis in the kidneys, but tempol also reduced the anti-tumor activity of DDP. Compared with the mice treated with DDP alone, the mice treated with both tempol and DDP had a significantly larger tumor size (P < 0.01) and a shorter lifespan (P < 0.05). Tempol significantly reversed DDP-induced expression of Bax and Bcl2 in the tumor tissue and tumor cell apoptosis (P < 0.001), and obviously reduced the elevation of ROS level in the tumor tissue induced by DDP treatment (P < 0.05).

CONCLUSIONS

Tempol can attenuate the anti-tumor effect of DDP while reducing the side effects of DDP. Caution must be taken and the risks and benefits should be carefully weighed when considering the use of tempol as an anti-oxidant to reduce the toxicities of DDP.

Mechanistic insight into the catalytic inhibition by nitroxides of tyrosine oxidation and nitration

BACKGROUND

Nitroxide antioxidants (RNO^•) protect from injuries associated with oxidative stress. Tyrosine residues in proteins are major targets for oxidizing species giving rise to irreversible cross-linking and protein nitration, but the mechanisms underlying the protective activity of RNO^• on these processes are not sufficiently clear.

METHODS

Tyrosine oxidation by the oxoammonium cation (RN⁺=O) was studied by following the kinetics of RNO^• formation using EPR spectroscopy. Tyrosine oxidation and nitration were investigated using the peroxidase/H₂O₂ system without and with nitrite. The inhibitory effect of RNO^• on these processes was studied by following the kinetics of the evolved O₂ and accumulation of tyrosine oxidation and nitration products.

RESULTS

Tyrosine ion is readily oxidized by RN⁺=O, and the equilibrium constant of this reaction depends on RNO^• structure and reduction potential. RNO^• catalytically inhibits tyrosine oxidation and nitration since it scavenges both tyrosyl and ^•NO₂ radicals while recycling through RN⁺=O reduction by H₂O₂, tyrosine and nitrite. The inhibitory effect of nitroxide on tyrosine oxidation and nitration increases as its reduction potential decreases where the 6-membered ring nitroxides are better catalysts than the 5-membered ones.

CONCLUSIONS

Nitroxides catalytically inhibit tyrosine oxidation and nitration. The proposed reaction mechanism adequately fits the results explaining the dependence of the nitroxide inhibitory effect on its reduction potential and on the concentrations of the reducing species present in the system.

GENERAL SIGNIFICANCE

Nitroxides protect against both oxidative and nitrative damage. The proposed reaction mechanism further emphasizes the role of the reducing environment to the efficacy of these catalysts.

Antioxidative biointerface: biocompatible materials scavenging reactive oxygen species

Oxidative stress caused by reactive oxygen species (ROS) occurs as events in which living tissues contact certain materials. These events include cell cultures and implantation of materials. Because of the high reactivity of ROS, they damage cells by oxidizing DNA, lipids, and proteins. Conversely, ROS also act as signaling molecules regulating cellular morphology. In particular, mitochondrial ROS are involved in the regulation of cellular physiology, including differentiation, autophagy, metabolic adaptation, apoptosis, and immunity. The balance between generation and elimination of ROS is essential for signaling pathways and proper cell function, and redox imbalance leads to cellular dysfunction and disturbs cellular homeostasis. To reduce oxidative stress, versatile antioxidants, including natural compounds, have been used; however, their poor bioavailability and pro-oxidant effects have limited the versatility of these antioxidants. Recent developments of antioxidative biointerfaces may represent a potent solution to this issue. Designed biointerfaces composed of polymer antioxidants eliminate excessive ROS at the interface between living tissues and materials, and do not disturb regulated redox balance inside cells, thus eliminating unexpected cell responses, such as inflammation and dysfunction.

Polyradical PROXYL/TEMPO-Derived Amides: Synthesis, Physicochemical Studies, DFT Calculations, and Antimicrobial Activity

A series of polynitroxide amides possessing 2,2,5,5-tetramethyl-1-pyrrolidinyloxy (PROXYL) and/or 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) units connected through various bridges were synthesized and their properties were analyzed. EPR spectroscopy provided detailed insight into their paramagnetic character and related properties. A thorough examination of the EPR spectra of dinitroxides in organic solvents provided valuable information on the intramolecular motions, thermodynamics, and spin-exchange mechanisms. Analysis of low-temperature X- and Q-band EPR spectra of the dissolved dinitroxides provided spin-spin distances that were comparable with the theoretical values obtained by DFT. Cyclic voltammetry investigations revealed (quasi)reversible electrochemical behavior for PROXYL-derived biradicals, whereas significant loss of the reversibility was found for TEMPO-containing bi- and polyradicals. The inhibitory activities of the nitroxides against model bacteria, yeasts, and filamentous fungi were assessed.

Mechanistic insights into the bleaching of melanin by alkaline hydrogen peroxide

This work aims to determine the roles of reactive oxygen species HO∙ and HO₂^- in the bleaching of melanins by alkaline hydrogen peroxide. Experiments using melanosomes isolated from human hair indicated that the HO∙ radical generated in the outside solution does not contribute significantly to bleaching. However, studies using soluble Sepia melanin demonstrated that both HO₂^- and HO∙ will individually bleach melanin. Additionally, when both oxidants are present, bleaching is increased dramatically in both rate and extent. Careful experimental design enabled the separation of the roles and effects of these key reactive species, HO∙ and HO₂^-. Rationalisation of the results presented, and review of previous literature, allowed the postulation of a simplified general scheme whereby the strong oxidant HO∙ is able to pre-oxidise melanin units to o-quinones enabling more facile ring opening by the more nucleophilic HO₂^-. In this manner the efficiency of the roles of both species is maximised.

Indole-TEMPO conjugates alleviate ischemia-reperfusion injury via attenuation of oxidative stress and preservation of mitochondrial function

Mitochondrial oxidative damage contributes to a wide range of pathologies including ischemia/reperfusion injury. Accordingly, protecting mitochondria from oxidative damage should possess therapeutic relevance. In the present study, we have designed and synthesized a series of novel indole-TEMPO conjugates that manifested good anti-inflammatory properties in a murine model of xylene-induced ear edema. We have demonstrated that these compounds can protect cells from simulated ischemia/reperfusion (s-I/R)-induced reactive oxygen species (ROS) overproduction and mitochondrial dysfunction. Furthermore, we have demonstrated that indole-TEMPO conjugates can attenuate organ damage induced in rodents via intestinal I/R injury. We therefore propose that the pharmacological profile and mechanism of action of these indole-TEMPO conjugates involve convergent roles, including the ability to decrease free radical production via lipid peroxidation which couples to an associated decrease in ROS-mediated activation of the inflammatory process. We further hypothesize that the protective effects of indole-TEMPO conjugates partially reside in maintaining optimal mitochondrial function.

The nitroxide Tempo inhibits hydroxyl radical production from the Fenton-like reaction of iron(II)-citrate with hydrogen peroxide

In vivo physiological ligand citrate can bind iron(II) ions to form the iron(II)-citrate complex. Inhibition of hydroxyl radical (OH) production from the Fenton-like reaction of iron(II)-citrate with H₂O₂ is biologically important, as this reaction may account for one of the mechanisms of the labile iron pool in vivo to induce oxidative stress and pathological conditions. Nitroxides have promising potentials as therapeutic antioxidants. However, there are controversial findings indicating that they not only act as antioxidants but also as pro-oxidants when engaged in Fenton reactions. Although the underlying mechanisms are proposed to be the inhibition or enhancement of the OH production by nitroxides, the proposed elucidations are only based on assessing biological damages and not demonstrated directly by measuring the OH production in the presence of nitroxides. In this study, therefore, we employed EPR and fluorescence spectroscopies to show direct evidence that nitroxide 2,2,6,6-tetramethyl-piperidine-1-oxyl (Tempo) inhibited OH production from the Fenton-like reaction of iron(II)-citrate with H₂O₂ by up to 90%. We also demonstrated spectrophotometrically, for the first time, that this inhibition was due to oxidation of the iron(II)-citrate by Tempo with a stoichiometry of Tempo:Iron(III)-citrate = 1.1:1.0. A scheme was proposed to illustrate the roles of nitroxides engaged in Fenton/Fenton-like reactions.

Nitroxides catalytically inhibit nitrite oxidation and heme inactivation induced by H2O2, nitrite and metmyoglobin or methemoglobin

Stable nitroxide radicals have multiple biological effects, although the mechanisms underlying them are not fully understood. Their protective effect against oxidative damage has been mainly attributed to scavenging deleterious radicals, oxidizing reduced metal ions and reducing oxyferryl centers of heme proteins. Yet, the potential of nitroxides to protect heme proteins against inactivation while suppressing or enhancing their catalytic activities has been largely overlooked. We have studied the effect of nitroxides, including TPO (2,2,6,6-tetramethylpiperidin-N-oxyl), 4-OH-TPO, 4-oxo-TPO and 3-carbamoyl proxyl, on the peroxidase-like activity of metmyoglobin (MbFe^III) and methemoglobin (HbFe^III) using nitrite as an electron donor by following heme absorption, H₂O₂ consumption, O₂ evolution and nitrite oxidation. The results demonstrate that the peroxidase-like activity is accompanied by a progressive heme inactivation where MbFe^III is far more resistant than HbFe^III. Nitroxides convert the peroxidase-like activity into catalase-like activity while inhibiting heme inactivation and nitrite oxidation in a dose-dependent manner. The nitroxide facilitates H₂O₂ dismutation, yet none of its reactions with any of the intermediates formed in these systems is rate-determining, and therefore its effect on the rate of the catalysis is hardly dependent on the kind of the nitroxide derivative and its concentration. The nitroxide at µM concentrations range catalytically inhibits nitrite oxidation, and consequently prevents tyrosine nitration induced by heme protein/H₂O₂/nitrite due to its fast oxidation by ^•NO₂ forming the respective oxoammonium cation, which is reduced back to the nitroxide by H₂O₂ and by superoxide radical. The nitroxides are superior over common antioxidants, which their reaction with ^•NO₂ always yields secondary radicals leading eventually to consumption of the antioxidant. A mechanism is proposed, and the kinetic simulations fit very well the experimental data in the case of MbFe^III where most of the rate constants of the reactions involved are independently known.

The Effect of Covalently-Attached ATRP-Synthesized Polymers on Membrane Stability and Cytoprotection in Human Erythrocytes

Erythrocytes have been described as advantageous drug delivery vehicles. In order to ensure an adequate circulation half-life, erythrocytes may benefit from protective enhancements that maintain membrane integrity and neutralize oxidative damage of membrane proteins that otherwise facilitate their premature clearance from circulation. Surface modification of erythrocytes using rationally designed polymers, synthesized via atom-transfer radical polymerization (ATRP), may further expand the field of membrane-engineered red blood cells. This study describes the fate of ATRP-synthesized polymers that were covalently attached to human erythrocytes as well as the effect of membrane engineering on cell stability under physiological and oxidative conditions in vitro. The biocompatible, membrane-reactive polymers were homogenously retained on the periphery of modified erythrocytes for at least 24 hours. Membrane engineering stabilized the erythrocyte membrane and effectively neutralized oxidative species, even in the absence of free-radical scavenger-containing polymers. The targeted functionalization of Band 3 protein by NHS-pDMAA-Cy3 polymers stabilized its monomeric form preventing aggregation in the presence of the crosslinking reagent, bis(sulfosuccinimidyl)suberate (BS3). A free radical scavenging polymer, NHS-pDMAA-TEMPO˙, provided additional protection of surface modified erythrocytes in an in vitro model of oxidative stress. Preserving or augmenting cytoprotective mechanisms that extend circulation half-life is an important consideration for the use of red blood cells for drug delivery in various pathologies, as they are likely to encounter areas of imbalanced oxidative stress as they circuit the vascular system.

Nitroxide radicals as research tools: Elucidating the kinetics and mechanisms of catalase-like and "suicide inactivation" of metmyoglobin

BACKGROUND

Metmyoglobin (MbFe(III)) reaction with H(2)O(2) has been a subject of study over many years. H(2)O(2) alone promotes heme destruction frequently denoted "suicide inactivation," yet the mechanism underlying H(2)O(2) dismutation associated with MbFe(III) inactivation remains obscure.

METHODS

MbFe(III) reaction with excess H(2)O(2) in the absence and presence of the nitroxide was studied at pH 5.3-8.1 and 25°C by direct determination of reaction rate constants using rapid-mixing stopped-flow technique, by following H(2)O(2) depletion, O(2) evolution, spectral changes of the heme protein, and the fate of the nitroxide by EPR spectroscopy.

RESULTS

The rates of both H(2)O(2) dismutation and heme inactivation processes depend on [MbFe(III)], [H(2)O(2)] and pH. Yet the inactivation stoichiometry is independent of these variables and each MbFe(III) molecule catalyzes the dismutation of 50±10 H(2)O(2) molecules until it is inactivated. The nitroxide catalytically enhances the catalase-like activity of MbFe(III) while protecting the heme against inactivation. The rate-determining step in the absence and presence of the nitroxide is the reduction of MbFe(IV)O by H(2)O(2) and by nitroxide, respectively.

CONCLUSIONS

The nitroxide effects on H(2)O(2) dismutation catalyzed by MbFe(III) demonstrate that MbFe(IV)O reduction by H(2)O(2) is the rate-determining step of this process. The proposed mechanism, which adequately fits the pro-catalytic and protective effects of the nitroxide, implies the intermediacy of a compound I-H(2)O(2) adduct, which decomposes to a MbFe(IV)O and an inactivated heme at a ratio of 25:1.

GENERAL SIGNIFICANCE

The effects of nitroxides are instrumental in elucidating the mechanism underlying the catalysis and inactivation routes of heme proteins.

Photoinduced Copolymerization of APMP-MMA: The Role of Reactive Hindered Amine APMP

4-Acryloyl-1,2,2,6,6-pentamethyl-piperidinol (APMP) is a reactive hindered amine that prolongs the service life of the polymeric materials and exhibits high stability, good resistance to extraction, and low toxicity. In this paper, a photoinduced free radical copolymerization of APMP and methyl methacrylate (MMA) is accomplished at ambient temperature in solution. APMP plays a key role in the copolymerization, owing to the nitroxides generated in situ from the moiety of 1,2,2,6,6-pentamethyl-piperidine under UV irradiation, and mediates the copolymerization, which is confirmed by the linear kinetics. With the increment of initial monomer feed ratios of APMP/MMA, both the copolymerization rate and the average molecular weight increase. According to the reactivity ratios from the extended Kelen-Tüdos (KT) method at high conversion by1H NMR spectroscopy, a nonlinear model is established and the sequential distribution in the copolymers is also investigated. The dispersion of APMP units is regulated by the feed ratios and reactivity ratios.

Nitroxides protect against peroxynitrite-induced nitration and oxidation

Nitroxides are promising compounds for prevention of undesired protein modifications. The aim of this study was to compare the efficiency of 11 nitroxides, derivatives of 2,2,6,6-tetramethylpiperidine-1-oxide (TEMPO) and 2,2,5,5-tetramethylpirrolidine-1-oxyl (PROXYL) in prevention of nitration and oxidation of model compounds and human serum albumin (HSA). Most nitroxides were very efficient in preventing loss of fluorescein fluorescence induced by peroxynitrite (PN) (IC50 in the nanomolar range) and preventing HSA nitration. The loss of fluorescein fluorescence was demonstrated to be due to nitration. Nitroxides were more effective in prevention nitration than oxidation reactions. They showed a concentration window for preventing dihydrorhodamine (DHR) 123 oxidation but exerted a prooxidant effect at both high and low concentrations. No prooxidant effect of nitroxides was seen in prevention of DHR123 oxidation induced by SIN-1. In all essays hydrophobic nitroxides (especially 4-nonylamido-TEMPO and 3-carbamolyl-dehydroPROXYL) showed the lowest efficiency. An exception was the prevention of thiol group oxidation by PN and SIN-1 where hydrophobic nitroxides were the most effective, apparently due to binding to the protein. Nitroxides showed low toxicity to MCF-7 cells. Most nitroxides, except for the most hydrophobic ones, protected cells from the cytotoxic action of SIN-1 and SIN-1-induced protein nitration. These results point to potential usefulness of nitroxides for prevention of PN-induced oxidation and, especially, nitration.

Structure and packing of aminoxyl and piperidinyl acrylamide monomers

The closely related title compounds, 4-acrylamido-2,2,6,6-tetramethylpiperidine-1-oxyl, C12H21N2O2, (I), and N-(2,2,6,6-tetramethylpiperidin-4-yl)acrylamide monohydrate, C12H22N2O·H2O, (II), are important monomers in the preparation of redox-active polymers. They comprise an acrylamide group of the usual s-cis configuration appended to a 2,2,6,6-tetramethyl-substituted piperidine-1-oxyl radical or a piperidinyl chair, respectively. The adjacent amide and piperidinyl H atoms are approximately trans across the C-N bond. The packing in (I) is dominated by N-H...O hydrogen bonds; these are supported by C-H...O contacts to form an R2(1)(6) ring repeat, a motif which has been observed in other acrylamide structures. In (II), hydrogen bonds are again key to the packing arrangements. In this case, the incorporated solvent water molecule acts as an acceptor through its O atom and as a donor through both H atoms, binding three adjacent piperidinylacrylamide molecules into layers. In both structures, weak C-H...O contacts involving the piperidinyl methyl H atoms and a proximal acrylamide carbonyl O atom extend the structure in the third dimension.

Reactive oxygen species and c-Jun N-terminal kinases contribute to TEMPO-induced apoptosis in L5178Y cells

The biological consequences of exposure to piperidine nitroxides is a concern, given their widespread use in manufacturing processes and their potential use in clinical applications. Our previous study reported that TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl), a low molecular weight free radical, possesses pro-oxidative activity in L5178Y cells. In this study, we investigated and characterized the role of reactive oxygen species (ROS) in TEMPO-induced toxicity in L5178Y cells. We found that TEMPO induced time- and concentration-dependent intracellular ROS production and glutathione depletion. TEMPO also induced apoptosis as demonstrated by increased caspase-3/7 activity, an increased proportion of annexin V stained cells, and decreased expression of anti-apoptotic proteins including Bcl-2, Bcl-xL and Mcl-1. N-acetylcysteine, a ROS scavenger, attenuated the ROS production and apoptosis induced by TEMPO. Moreover, Western blot analyses revealed that TEMPO activated γ-H2A.X, a hallmark of DNA damage, and c-Jun N-terminal kinases (JNK), a key member in the mitogen-activated protein kinase (MAPK) signaling pathway. Addition of SP600125, a JNK-specific inhibitor, blocked TEMPO-mediated JNK phosphorylation and also attenuated TEMPO-induced apoptosis. These findings indicate that both ROS production and JNK activation are involved in TEMPO-induced apoptosis, and may contribute to the toxicity of TEMPO in L5178Y cells.

The nitroxide radical TEMPOL prevents obesity, hyperlipidaemia, elevation of inflammatory cytokines, and modulates atherosclerotic plaque composition in apoE-/- mice

OBJECTIVE

The nitroxide compound TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl radical) has been shown to prevent obesity-induced changes in adipokines in cell and animal systems. In this study we investigated whether supplementation with TEMPOL inhibits inflammation and atherosclerosis in apoE-/- mice fed a high fat diet (HFD).

METHODS

ApoE-/- mice were fed for 12 weeks on standard chow diet or a high-fat diet. Half the mice were supplemented with 10 mg/g TEMPOL in their food. Plasma samples were analysed for triglycerides, cholesterol, low- and high-density lipoprotein cholesterol, inflammatory cytokines and markers (interleukin-6, IL-6; monocyte-chemotactic protein, MCP-1; myeloperoxidase, MPO; serum amyloid A, SAA; adiponectin; leptin). Plaques in the aortic sinus were analysed for area, and content of collagen, lipid, macrophages and smooth muscle cells.

RESULTS

High fat feeding resulted in marked increases in body mass and plasma lipid levels. Dietary TEMPOL decreased both parameters. In the high-fat-fed mice significant elevations in plasma lipid levels and the inflammatory markers IL-6, MCP-1, MPO, SAA were detected, along with an increase in leptin and a decrease in adiponectin. TEMPOL supplementation reversed these effects. When compared to HFD-fed mice, TEMPOL supplementation increased plaque collagen content, decreased lipid content and increased macrophage numbers.

CONCLUSIONS

These data indicate that in a well-established model of obesity-associated hyperlipidaemia and atherosclerosis, TEMPOL had a significant impact on body mass, atherosclerosis, hyperlipidaemia and inflammation. TEMPOL may therefore be of value in suppressing obesity, metabolic disorders and increasing atherosclerotic plaque stability.

The antioxidant mechanism of nitroxide TEMPO: scavenging with glutathionyl radicals

A rhodamine-nitroxide probe (R-NO˙) was introduced to probe glutathionyl radicals (GS˙) with high sensitivity and selectivity.

Inhibition of myeloperoxidase- and neutrophil-mediated oxidant production by tetraethyl and tetramethyl nitroxides

The powerful oxidant HOCl (hypochlorous acid and its corresponding anion, (-)OCl) generated by the myeloperoxidase (MPO)-H2O2-Cl(-) system of activated leukocytes is strongly associated with multiple human inflammatory diseases; consequently there is considerable interest in inhibition of this enzyme. Nitroxides are established antioxidants of low toxicity that can attenuate oxidation in animal models, with this ascribed to superoxide dismutase or radical-scavenging activities. We have shown (M.D. Rees et al., Biochem. J. 421, 79-86, 2009) that nitroxides, including 4-amino-TEMPO (4-amino-2,2,6,6-tetramethylpiperidin-1-yloxyl radical), are potent inhibitors of HOCl formation by isolated MPO and activated neutrophils, with IC50 values of ~1 and ~6 µM respectively. The utility of tetramethyl-substituted nitroxides is, however, limited by their rapid reduction by biological reductants. The corresponding tetraethyl-substituted nitroxides have, however, been reported to be less susceptible to reduction. In this study we show that the tetraethyl species were reduced less rapidly than the tetramethyl species by both human plasma (89-99% decreased rate of reduction) and activated human neutrophils (62-75% decreased rate). The tetraethyl-substituted nitroxides retained their ability to inhibit HOCl production by MPO and activated neutrophils with IC50 values in the low-micromolar range; in some cases inhibition was enhanced compared to tetramethyl substitution. Nitroxides with rigid structures (fused oxaspiro rings) were, however, inactive. Overall, these data indicate that tetraethyl-substituted nitroxides are potent inhibitors of oxidant formation by MPO, with longer plasma and cellular half-lives compared to the tetramethyl species, potentially allowing lower doses to be employed.

A novel protecting group methodology for syntheses using nitroxides

The methoxyamine group represents an ideal protecting group for the nitroxide moiety. It can be easily and selectively introduced in high yield (typically >90%) to a range of functionalised nitroxides using FeSO4·7H2O and H2O2 in DMSO. Its removal is readily achieved under mild conditions in high yield (70-90%) using mCPBA in a Cope-type elimination process.

The carbonylation and covalent dimerization of human superoxide dismutase 1 caused by its bicarbonate-dependent peroxidase activity is inhibited by the radical scavenger tempol

The nitroxide tempol inhibited the carbonylation and covalent dimerization of human superoxide dismutase 1 caused by its bicarbonate-dependent peroxidase activity. Tempol acted by scavenging the produced carbonate radical and by recombining with hSOD1-Trp32• radicals as indicated by MS/MS evidence.