Reversible reduction of nitroxides to hydroxylamines: roles for ascorbate and glutathione

Evaluation of Copper(II) Transfer between Amyloid-beta Peptides by Relaxation-Induced Dipolar Modulation Enhancement (RIDME)

In the brains of Alzheimer's disease patients, fibrillar aggregates containing amyloid-beta (Aβ) peptides are found, along with elevated concentrations of Cu(II) ions. The aggregation pathways of Aβ peptides can be modulated by Cu(II) ions and is determined by the formation and nature of the Cu(II)-Aβ complex. If spin-labeled, the Cu(II)-Aβ complex contains two dipolar coupled paramagnetic centers, the spin label and the Cu(II) ion. Measurement of the dipolar coupling between these paramagnetic centers by relaxation-induced dipolar modulation enhancement (RIDME) allows to monitor the complex formation and thus opens a way to follow the Cu(II) transfer between peptides if a mixture of wild-type and spin-labeled ones is used. We evaluate this approach for a specific Cu(II)-Aβ complex, the aggregation-inert Component II. The kinetics of the Cu(II) transfer can be resolved by performing RIDME in a time-dependent manner. A temporal resolution of seconds has been achieved, with the potential to reach milliseconds, using a rapid-freeze quench device to stop the Cu(II) transfer in solution after defined incubation times.

Contrast Agents Based on Human Serum Albumin and Nitroxides for 1H-MRI and Overhauser-Enhanced MRI

In cancer diagnostics, magnetic resonance imaging (MRI) uses contrast agents to enhance the distinction between the target tissue and background. Several promising approaches have been developed to increase MRI sensitivity, one of which is Overhauser dynamic nuclear polarization (ODNP)-enhanced MRI (OMRI). In this study, a macromolecular construct based on human serum albumin and nitroxyl radicals (HSA-NIT) was developed using a new synthesis method that significantly increased the modification to 21 nitroxide residues per protein. This was confirmed by electron paramagnetic resonance (EPR) spectroscopy and matrix-assisted laser desorption/ionization time-of-flight (MALDI ToF) mass spectrometry. Gel electrophoresis and circular dichroism showed no significant changes in the structure of HSA-NITs, and no oligomers were formed during modification. The cytotoxicity of HSA-NITs was comparable to that of native albumin. HSA-NITs were evaluated as potential "metal-free" organic radical relaxation-based contrast agents for 1H-MRI and as hyperpolarizing contrast agents for OMRI. Relaxivities (longitudinal and transversal relaxation rates r1 and r2) for HSA-NITs were measured at different magnetic field strengths (1.88, 3, 7, and 14 T). Phantoms were used to demonstrate the potential use of HSA-NIT as a T1- and T2-weighted relaxation-based contrast agent at 3 T and 14 T. The efficacy of 1H Overhauser dynamic nuclear polarization (ODNP) in liquids at an ultralow magnetic field (ULF, B0 = 92 ± 0.8 μT) was investigated for HSA-NIT conjugates. The HSA-NITs themselves did not show ODNP enhancement; however, under the proteolysis conditions simulating cancer tissue, HSA-NIT conjugates were cleaved into lower-molecular-weight (MW) protein fragments that activate ODNP capabilities, resulting in a maximum achievable enhancement |Emax| of 40-50 and a radiofrequency power required to achieve half of Emax, P1/2, of 21-27 W. The HSA-NIT with a higher degree of modification released increased the number of spin probes upon biodegradation, which significantly enhanced the Overhauser effect. Thus, HSA-NITs may represent a new class of MRI relaxation-based contrast agents as well as novel cleavable conjugates for use as hyperpolarizing contrast agents (HCAs) in OMRI.

Efficient Scavenging of TEMPOL Radical by Ascorbic Acid in Solution and Related Prolongation of 13C and 1H Nuclear Spin Relaxation Times of the Solute

Dynamic nuclear polarization for nuclear magnetic resonance (NMR) spectroscopy and imaging uses free radicals to strongly enhance the NMR signal of a compound under investigation. At the same time, the radicals shorten significantly its nuclear spin relaxation times which reduces the time window available for the experiments. Radical scavenging can overcome this drawback. Our work presents a detailed study of the reduction of the TEMPOL radical by ascorbic acid in solution by high-resolution NMR. Carbon-13 and hydrogen-1 nuclear spin relaxations are confirmed to be restored to their values without TEMPOL. Reaction mechanism, kinetics, and the influence of pD and viscosity are thoroughly discussed. The detailed investigation conducted in this work should help with choosing suitable concentrations in the samples for dynamic nuclear polarization and optimizing the measurement protocols.

The Cellular and Organismal Effects of Nitroxides and Nitroxide-Containing Nanoparticles

Nitroxides are stable free radicals that have antioxidant properties. They react with many types of radicals, including alkyl and peroxyl radicals. They act as mimics of superoxide dismutase and stimulate the catalase activity of hemoproteins. In some situations, they may exhibit pro-oxidant activity, mainly due to the formation of oxoammonium cations as products of their oxidation. In this review, the cellular effects of nitroxides and their effects in animal experiments and clinical trials are discussed, including the beneficial effects in various pathological situations involving oxidative stress, protective effects against UV and ionizing radiation, and prolongation of the life span of cancer-prone mice. Nitroxides were used as active components of various types of nanoparticles. The application of these nanoparticles in cellular and animal experiments is also discussed.

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.

Synthesis and Properties of (1R(S),5R(S),7R(S),8R(S))-1,8-Bis(hydroxymethyl)-6-azadispiro[4.1.4.2]tridecane-6-oxyl: Reduction-Resistant Spin Labels with High Spin Relaxation Times

Site-directed spin labeling followed by investigation using Electron Paramagnetic Resonance spectroscopy is a rapidly expanding powerful biophysical technique to study structure, local dynamics and functions of biomolecules using pulsed EPR techniques and nitroxides are the most widely used spin labels. Modern trends of this method include measurements directly inside a living cell, as well as measurements without deep freezing (below 70 K), which provide information that is more consistent with the behavior of the molecules under study in natural conditions. Such studies require nitroxides, which are resistant to the action of biogenic reductants and have high spin relaxation (dephasing) times, T_m. (1R(S),5R(S),7R(S),8R(S))-1,8-bis(hydroxymethyl)-6-azadispiro[4.1.4.2]tridecane-6-oxyl is a unique nitroxide that combines these features. We have developed a convenient method for the synthesis of this radical and studied the ways of its functionalization. Promising spin labels have been obtained, the parameters of their spin relaxation T₁ and T_m have been measured, and the kinetics of reduction with ascorbate have been studied.

Ultrafast Bioorthogonal Spin-Labeling and Distance Measurements in Mammalian Cells Using Small, Genetically Encoded Tetrazine Amino Acids

Site-directed spin-labeling (SDSL)─in combination with double electron-electron resonance (DEER) spectroscopy─has emerged as a powerful technique for determining both the structural states and the conformational equilibria of biomacromolecules. DEER combined with in situ SDSL in live cells is challenging since current bioorthogonal labeling approaches are too slow to allow for complete labeling with low concentrations of spin label prior to loss of signal from cellular reduction. Here, we overcome this limitation by genetically encoding a novel family of small, tetrazine-bearing noncanonical amino acids (Tet-v4.0) at multiple sites in proteins expressed in Escherichia coli and in human HEK293T cells. We achieved specific and quantitative spin-labeling of Tet-v4.0-containing proteins by developing a series of strained trans-cyclooctene (sTCO)-functionalized nitroxides─including a gem-diethyl-substituted nitroxide with enhanced stability in cells─with rate constants that can exceed 106 M-1 s-1. The remarkable speed of the Tet-v4.0/sTCO reaction allowed efficient spin-labeling of proteins in live cells within minutes, requiring only sub-micromolar concentrations of sTCO-nitroxide. DEER recorded from intact cells revealed distance distributions in good agreement with those measured from proteins purified and labeled in vitro. Furthermore, DEER was able to resolve the maltose-dependent conformational change of Tet-v4.0-incorporated and spin-labeled MBP in vitro and support assignment of the conformational state of an MBP mutant within HEK293T cells. We anticipate the exceptional reaction rates of this system, combined with the relatively short and rigid side chains of the resulting spin labels, will enable structure/function studies of proteins directly in cells, without any requirements for protein purification.

Conformational tuning improves the stability of spirocyclic nitroxides with long paramagnetic relaxation times

Nitroxides are widely used as probes and polarization transfer agents in spectroscopy and imaging. These applications require high stability towards reducing biological environments, as well as beneficial relaxation properties. While the latter is provided by spirocyclic groups on the nitroxide scaffold, such systems are not in themselves robust under reducing conditions. In this work, we introduce a strategy for stability enhancement through conformational tuning, where incorporating additional substituents on the nitroxide ring effects a shift towards highly stable closed spirocyclic conformations, as indicated by X-ray crystallography and density functional theory (DFT) calculations. Closed spirocyclohexyl nitroxides exhibit dramatically improved stability towards reduction by ascorbate, while maintaining long relaxation times in electron paramagnetic resonance (EPR) spectroscopy. These findings have important implications for the future design of new nitroxide-based spin labels and imaging agents.

An EPR Study on Highly Stable Nitroxyl-Nitroxyl Biradicals for Dynamic Nuclear Polarization Applications at High Magnetic Fields

Nitroxide biradicals are efficient polarizing agents in dynamic nuclear polarization (DNP) solid-state nuclear magnetic resonance. Many recently reported radicals possess substantial DNP efficiency in organic solvents but have poor solubility in water media which is unfavorable for biological applications. In this paper, we report DNP efficiency at a high magnetic field for two water-soluble biradicals resistant to reducing media. Water solubility was achieved by obtaining the radicals in the form of quaternary ammonium salts. Parameters of hyperfine interaction and exchange interaction were quantified by EPR spectroscopy, and their influence on the DNP effect was determined. The resistance of the biradicals to strongly reducing media was characterized. High stability was achieved using tetraethyl substituents and pyrrolidine moieties.

Structural Features Governing the Metabolic Stability of Tetraethyl-Substituted Nitroxides in Rat Liver Microsomes

Nitroxides are potent tools for studying biological systems by electron paramagnetic resonance (EPR). Whatever the application, a certain stability is necessary for successful detection. Since conventional tetramethyl-substituted cyclic nitroxides have insufficient in vivo stability, efforts have recently been made to synthesize more stable, tetraethyl-substituted nitroxides. In our previous study on piperidine nitroxides, the introduction of steric hindrance around the nitroxide moiety successfully increased the resistance to reduction into hydroxylamine. However, it also rendered the carbon backbone susceptible to modifications by xenobiotic metabolism due to increased lipophilicity. Here, we focus on a new series of three nitroxide candidates with tetraethyl substitution, namely with pyrrolidine, pyrroline, and isoindoline cores, to identify which structural features afford increased stability for future probe design and application in in vivo EPR imaging. In the presence of rat liver microsomes, pyrrolidine and pyrroline tetraethyl nitroxides exhibited a higher stability than isoindoline nitroxide, which was studied in detail by HPLC-HRMS. Multiple metabolites suggest that the aerobic transformation of tetraethyl isoindoline nitroxide is initiated by hydrogen abstraction by P450-Fe^V = O from one of the ethyl groups, followed by rearrangement and further modifications by cytochrome P450, as supported by DFT calculations. Under anaerobic conditions, only reduction by rat liver microsomes was observed with involvement of P450-Fe^II.

Ultra-Fast Bioorthogonal Spin-Labeling and Distance Measurements in Mammalian Cells Using Small, Genetically Encoded Tetrazine Amino Acids

Studying protein structures and dynamics directly in the cellular environments in which they function is essential to fully understand the molecular mechanisms underlying cellular processes. Site-directed spin-labeling (SDSL)-in combination with double electron-electron resonance (DEER) spectroscopy-has emerged as a powerful technique for determining both the structural states and the conformational equilibria of biomacromolecules. In-cell DEER spectroscopy on proteins in mammalian cells has thus far not been possible due to the notable challenges of spin-labeling in live cells. In-cell SDSL requires exquisite biorthogonality, high labeling reaction rates and low background signal from unreacted residual spin label. While the bioorthogonal reaction must be highly specific and proceed under physiological conditions, many spin labels display time-dependent instability in the reducing cellular environment. Additionally, high concentrations of spin label can be toxic. Thus, an exceptionally fast bioorthogonal reaction is required that can allow for complete labeling with low concentrations of spin-label prior to loss of signal. Here we utilized genetic code expansion to site-specifically encode a novel family of small, tetrazine-bearing non-canonical amino acids (Tet-v4.0) at multiple sites in green fluorescent protein (GFP) and maltose binding protein (MBP) expressed both in E. coli and in human HEK293T cells. We achieved specific and quantitative spin-labeling of Tet-v4.0-containing proteins by developing a series of strained trans -cyclooctene (sTCO)-functionalized nitroxides-including a gem -diethyl-substituted nitroxide with enhanced stability in cells-with rate constants that can exceed 10 6 M -1 s -1 . The remarkable speed of the Tet-v4.0/sTCO reaction allowed efficient spin-labeling of proteins in live HEK293T cells within minutes, requiring only sub-micromolar concentrations of sTCO-nitroxide added directly to the culture medium. DEER recorded from intact cells revealed distance distributions in good agreement with those measured from proteins purified and labeled in vitro . Furthermore, DEER was able to resolve the maltose-dependent conformational change of Tet-v4.0-incorporated and spin-labeled MBP in vitro and successfully discerned the conformational state of MBP within HEK293T cells. We anticipate the exceptional reaction rates of this system, combined with the relatively short and rigid side chains of the resulting spin labels, will enable structure/function studies of proteins directly in cells, without any requirements for protein purification.

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Site directed spin labeling to elucidating the mechanism of the cyanobacterial circadian clock

Electron Paramagnetic Resonance (EPR) is a spectroscopic technique that provides structural and dynamic information on unpaired spins and their surrounding environments. Introduction of exogenous spin labels via site directed spin labeling (SDSL) enables characterization of systems of interests lacking intrinsic unpaired spins. This chapter describes the use of SDSL in quantifying KaiB-KaiC binding in the cyanobacterial circadian clock (Kai Clock), exploiting the changes in mobility of the local environment around the spin label on KaiB-KaiC interactions. While the Kai system serves as our model system to demonstrate SDSL-EPR utility in quantifying protein-protein interactions, this technique is readily amenable to other systems of interest whenever specific protein-protein interactions need to be isolated. We first present a protocol for spin labeling KaiB. Then, we detail the sample preparation and acquisition processes to maximize signal-to-noise for downstream analysis. We close this chapter by highlighting recent advances in SDSL technology to incorporate spin labels into proteins of interest and in EPR technology to improve detection sensitivity that may allow greater flexibilities to the types of experiments possible.

Stability of the nitroxide biradical AMUPol in intact and lysed mammalian cells

Dynamic Nuclear Polarization (DNP) enhanced solid state NMR increases experimental sensitivity, potentially enabling detection of biomolecules at their physiological concentrations. The sensitivity of DNP experiments is due to the transfer of polarization from electron spins of free radicals to the nuclear spins of interest. Here, we investigate the reduction of AMUPol in both lysed and intact HEK293 cells. We find that nitroxide radicals are reduced with first order reduction kinetics by cell lysates at a rate of ∼ 12% of the added nitroxide radical concentration per hour. We also found that electroporation delivered a consistent amount of AMUPol to intact cells and that nitroxide radicals are reduced just slightly more rapidly (∼15% per hour) by intact cells than by cell lysates. The two nitroxide radicals of AMUPol are reduced independently and this leads to considerable accumulation of the DNP-silent monoradical form of AMUPol, particularly in preparations of intact cells where nearly half of the AMUPol is already reduced to the DNP silent monoradical form at the earliest experimental time points. This confirms that the loss of the DNP-active biradical form of AMUPol is faster than the nitroxide reduction rate. Finally, we investigate the effect of adding N-ethyl maleimide, a well-known inhibitor of thiol (-SH) group-based reduction of nitroxide biradicals in cells, on AMUPol reduction, cellular viability, and DNP performance. Although pre-treatment of cells with NEM effectively inhibited the reduction of AMUPol, exposure to NEM compromised cellular viability and, surprisingly, did not improve DNP performance. Collectively, these results indicate that, currently, the most effective strategy to obtain high DNP enhancements for DNP-assisted in-cell NMR is to minimize room temperature contact times with cellular constituents and suggest that the development of bio-resistant polarization agents for DNP could considerably increase the sensitivity of DNP-assisted in-cell NMR experiments.

Nitroxyl Radical as a Theranostic Contrast Agent in Magnetic Resonance Redox Imaging

Significance:In vivo assessment of paramagnetic and diamagnetic conversions of nitroxyl radicals based on cyclic redox mechanism can be an index of tissue redox status. The redox mechanism of nitroxyl radicals, which enables their use as a normal tissue-selective radioprotector, is seen as being attractive on planning radiation therapy. Recent Advances:In vivo redox imaging using nitroxyl radicals as redox-sensitive contrast agents has been developed to assess tissue redox status. Chemical and biological behaviors depending on chemical structures of nitroxyl radical compounds have been understood in detail. Polymer types of nitroxyl radical contrast agents and/or nitroxyl radical-labeled drugs were designed for approaching theranostics. Critical Issues: Nitroxyl radicals as magnetic resonance imaging (MRI) contrast agents have several advantages compared with those used in electron paramagnetic resonance (EPR) imaging, while support by EPR spectroscopy is important to understand information from MRI. Redox-sensitive paramagnetic contrast agents having a medicinal benefit, that is, nitroxyl-labeled drug, have been developed and proposed. Future Directions: A development of suitable nitroxyl contrast agent for translational theranostic applications with high reaction specificity and low normal tissue toxicity is under progress. Nitroxyl radicals as redox-sensitive magnetic resonance contrast agents can be a useful tool to detect an abnormal tissue redox status such as disordered oxidative stress. Antioxid. Redox Signal. 36, 95-121.

Interaction kinetics and accessibility of sulfadiazine in model clay-humic acid suspension: Electron spin resonance investigations with nitroxide spin label

The characterization of the interaction of sulfonamides with soil is of particular interest in environmental risk and persistence assessment. In the present work electron spin resonance spectroscopy (ESR) was used to investigate the interaction kinetics of spin labelled sulfadiazine (SL-SDZ) with model clay-humic acid suspensions. The ESR spectra showed that SL-SDZ incubated with Leonardite humic acid (LHA) and Ca-hectorite as model clay was immobilized due to covalent binding of its aniline moiety to LHA. From the immobilization kinetics measured over a period of 1200 h a pseudo-first order reaction with a time constant of 82.6 ± 25.0 h of covalent binding was determined. Additionally, SL-SDZ was strongly sorbed by LHA immediately after incubation but not durably sequestered. Compared to incubation without Ca-hectorite the covalent binding kinetics of SL-SDZ as well as its strong sorption were retarded.

Cleavage-Resistant Protein Labeling With Hydrophilic Trityl Enables Distance Measurements In-Cell

Sensitive in-cell distance measurements in proteins using pulsed-electron spin resonance (ESR) require reduction-resistant and cleavage-resistant spin labels. Among the reduction-resistant moieties, the hydrophilic trityl core known as OX063 is promising due to its long phase-memory relaxation time (T_m). This property leads to a sufficiently intense ESR signal for reliable distance measurements. Furthermore, the T_m of OX063 remains sufficiently long at higher temperatures, opening the possibility for measurements at temperatures above 50 K. In this work, we synthesized deuterated OX063 with a maleimide linker (mOX063-d₂₄). We show that the combination of the hydrophilicity of the label and the maleimide linker enables high protein labeling that is cleavage-resistant in-cells. Distance measurements performed at 150 K using this label are more sensitive than the measurements at 80 K. The sensitivity gain is due to the significantly short longitudinal relaxation time (T₁) at higher temperatures, which enables more data collection per unit of time. In addition to in vitro experiments, we perform distance measurements in Xenopus laevis oocytes. Interestingly, the T_m of mOX063-d₂₄ is sufficiently long even in the crowded environment of the cell, leading to signals of appreciable intensity. Overall, mOX063-d₂₄ provides highly sensitive distance measurements both in vitro and in-cells.

Bioorthogonal Retro-Cope Elimination Reaction of N,N-Dialkylhydroxylamines and Strained Alkynes

A bioorthogonal reaction between N,N-dialkylhydroxylamines and cyclooctynes is described. This reaction features a highly regioselective transformation between small, easily functionalizable reaction components with second-order rate constants reaching 84 M^-1 s^-1. The reaction is orthogonal to the inverse-electron demand Diels-Alder reactions between tetrazine and strained alkenes, and its components exhibit exquisite stability and chemoselectivity in cell lysate. This retro-Cope elimination reaction introduces a new member to the bioorthogonal reaction compendium outside the prolific class of cycloaddition reactions.

Nitroxide spin labels and EPR spectroscopy: A powerful association for protein dynamics studies

Site-Directed Spin Labelling (SDSL) technique is based on the attachment of a paramagnetic label onto a specific position of a protein (or other bio-molecules) and the subsequent study by Electron Paramagnetic Resonance (EPR) spectroscopy. In particular, continuous-wave EPR (cw-EPR) spectra can detect the local conformational dynamics for proteins under various conditions. Moreover, pulse-EPR experiments on doubly spin-labelled proteins allow measuring distances between spin centres in the 1.5-8 nm range, providing information about structures and functions. This review focuses on SDSL-EPR spectroscopy as a structural biology tool to investigate proteins using nitroxide labels. The versatility of this spectroscopic approach for protein structural characterization has been demonstrated through the choice of recent studies. The main aim is to provide a general overview of the technique, particularly for non-experts, to spread the applicability of this technique in various fields of structural biology.

A radioiodinated nitroxide probe with improved stability against bioreduction for in vivo detection of lipid radicals

It is well known that lipid carbon radicals (lipid radicals) are the origin of lipid peroxidation and are involved in various diseases such as cancer. Therefore, the in vivo detection of lipid radicals would be expected to lead to early diagnosis of these diseases. However, there are no methods for measuring lipid radicals in vivo. Nitroxides are known to be highly reactive with lipid radicals, but they tend to be reduced in vivo. Focusing on the excellent detection sensitivity of nuclear medical imaging, we have developed a radioiodinated nitroxide derivative with resistance to bioreduction for the in vivo detection of lipid radicals. The desired compound was obtained successfully and was highly stable against bioreduction while maintaining high reactivity toward lipid radicals. The I-125 labeling was efficacious with radiochemical yields of 84-87% and radiochemical purities of >99%. A cellular uptake assay showed that the radioiodinated compound was significantly taken up by cells under lipid radical-producing conditions compared to that in the absence of lipid radical production. A biodistribution study indicated that the radioiodinated compound accumulated more in organs where lipid peroxidation was promoted than the methoxyamine derivative, which lost reactivity to lipid radicals. These results indicated that the developed probe became trapped in cells or organs by reacting with lipid radicals. Thus, the radioiodinated nitroxide is a candidate probe for in vivo detection of lipid radicals.

New Insight into the Mechanism of Drug Release from Poly(d,l-lactide) Film by Electron Paramagnetic Resonance

A novel approach based on convolution of the electron paramagnetic resonance (EPR) spectra was used for quantitative study of the release kinetics of paramagnetic dopants from poly(d,l-lactide) films. A non-monotonic dependence of the release rate on time was reliably recorded. The release regularities were compared with the dynamics of polymer structure changes determined by EPR, SEM, and optic microscopy. The data obtained allow for the conclusion that the main factor governing dopant release is the formation of pores connected with the surface. In contrast, the contribution of the dopant diffusion through the polymer matrix is negligible. The dopant release can be divided into two phases: release through surface pores, which are partially closed with time, and release through pores initially formed inside the polymer matrix due to autocatalytic hydrolysis of the polymer and gradually connected to the surface of the sample. For some time, these processes co-occur. The mathematical model of the release kinetics based on pore formation is presented, describing the kinetics of release of various dopants from the polymer films of different thicknesses.

Myeloperoxidase: A versatile mediator of endothelial dysfunction and therapeutic target during cardiovascular disease

Myeloperoxidase (MPO) is a prominent mammalian heme peroxidase and a fundamental component of the innate immune response against microbial pathogens. In recent times, MPO has received considerable attention as a key oxidative enzyme capable of impairing the bioactivity of nitric oxide (NO) and promoting endothelial dysfunction; a clinically relevant event that manifests throughout the development of inflammatory cardiovascular disease. Increasing evidence indicates that during cardiovascular disease, MPO is released intravascularly by activated leukocytes resulting in its transport and sequestration within the vascular endothelium. At this site, MPO catalyzes various oxidative reactions that are capable of promoting vascular inflammation and impairing NO bioactivity and endothelial function. In particular, MPO catalyzes the production of the potent oxidant hypochlorous acid (HOCl) and the catalytic consumption of NO via the enzyme's NO oxidase activity. An emerging paradigm is the ability of MPO to also influence endothelial function via non-catalytic, cytokine-like activities. In this review article we discuss the implications of our increasing knowledge of the versatility of MPO's actions as a mediator of cardiovascular disease and endothelial dysfunction for the development of new pharmacological agents capable of effectively combating MPO's pathogenic activities. More specifically, we will (i) discuss the various transport mechanisms by which MPO accumulates into the endothelium of inflamed or diseased arteries, (ii) detail the clinical and basic scientific evidence identifying MPO as a significant cause of endothelial dysfunction and cardiovascular disease, (iii) provide an up-to-date coverage on the different oxidative mechanisms by which MPO can impair endothelial function during cardiovascular disease including an evaluation of the contributions of MPO-catalyzed HOCl production and NO oxidation, and (iv) outline the novel non-enzymatic mechanisms of MPO and their potential contribution to endothelial dysfunction. Finally, we deliver a detailed appraisal of the different pharmacological strategies available for targeting the catalytic and non-catalytic modes-of-action of MPO in order to protect against endothelial dysfunction in cardiovascular disease.

Synthesis, Characterization, and Application of a Highly Hydrophilic Triarylmethyl Radical for Biomedical EPR

Stable tetrathiatriarylmethyl radicals have significantly contributed to the recent progress in biomedical electron paramagnetic resonance (EPR) due to their unmatched stability in biological media and long relaxation times. However, the lipophilic core of the most commonly used structure (Finland trityl) is responsible for its interaction with plasma biomacromolecules, such as albumin, and self-aggregation at high concentrations and/or low pH. While Finland trityl is generally considered inert toward many reactive radical species, we report that sulfite anion radical efficiently substitutes the three carboxyl moieties of Finland trityl with a high rate constant of 3.53 × 10⁸ M^-1 s^-1, leading to a trisulfonated Finland trityl radical. This newly synthesized highly hydrophilic trityl radical shows an ultranarrow linewidth (ΔB_pp = 24 mG), a lower affinity for albumin than Finland trityl, and a high aqueous solubility even at acidic pH. Therefore, this new tetrathiatriarylmethyl radical can be considered as a superior spin probe in comparison to the widely used Finland trityl. One of its potential applications was demonstrated by in vivo mapping oxygen in a mouse model of breast cancer. Moreover, we showed that one of the three sulfo groups can be easily substituted with S-, N-, and P-nucleophiles, opening access to various monofunctionalized sulfonated trityl radicals.

Reduction Resistant and Rigid Nitroxide Spin-Labels for DNA and RNA

Electron paramagnetic resonance (EPR) spectroscopy, coupled with site-directed spin labeling (SDSL), is a useful method for studying conformational changes of biomolecules in cells. To employ in-cell EPR using nitroxide-based spin labels, the structure of the nitroxides must confer reduction resistance to withstand the reductive environment within cells. Here, we report the synthesis of two new spin labels, EÇ and EÇm, both of which possess the rigidity and the reduction resistance needed for extracting detailed structural information by EPR spectroscopy. EÇ and EÇm were incorporated into DNA and RNA, respectively, by oligonucleotide synthesis. Both labels were shown to be nonperturbing of the duplex structure. The partial reduction of EÇm during RNA synthesis was circumvented by the protection of the nitroxide as a benzoylated hydroxylamine.

2-Butyl-2-tert-butyl-5,5-diethylpyrrolidine-1-oxyls: Synthesis and Properties

Nitroxides are broadly used as molecular probes and labels in biophysics, structural biology, and biomedical research. Resistance of a nitroxide group bearing an unpaired electron to chemical reduction with low-molecular-weight antioxidants and enzymatic systems is of critical importance for these applications. The redox properties of nitroxides are known to depend on the ring size (for cyclic nitroxides) and electronic and steric effects of the substituents. Here, two highly strained nitroxides, 5-(tert-butyl)-5-butyl-2,2-diethyl-3-hydroxypyrrolidin-1-oxyl (4) and 2-(tert-butyl)-2-butyl-5,5-diethyl-3,4-bis(hydroxymethyl)pyrrolidin-1-oxyl (5), were prepared via a reaction of the corresponding 2-tert-butyl-1-pyrroline 1-oxides with butyllithium. Thermal stability and kinetics of reduction of the new nitroxides by ascorbic acid were studied. Nitroxide 5 showed the highest resistance to reduction.

DNA complexes with human apurinic/apyrimidinic endonuclease 1: structural insights revealed by pulsed dipolar EPR with orthogonal spin labeling

A DNA molecule is under continuous influence of endogenous and exogenous damaging factors, which produce a variety of DNA lesions. Apurinic/apyrimidinic sites (abasic or AP sites) are among the most common DNA lesions. In this work, we applied pulse dipolar electron paramagnetic resonance (EPR) spectroscopy in combination with molecular dynamics (MD) simulations to investigate in-depth conformational changes in DNA containing an AP site and in a complex of this DNA with AP endonuclease 1 (APE1). For this purpose, triarylmethyl (TAM)-based spin labels were attached to the 5' ends of an oligonucleotide duplex, and nitroxide spin labels were introduced into APE1. In this way, we created a system that enabled monitoring the conformational changes of the main APE1 substrate by EPR. In addition, we were able to trace substrate-to-product transformation in this system. The use of different (orthogonal) spin labels in the enzyme and in the DNA substrate has a crucial advantage allowing for detailed investigation of local damage and conformational changes in AP-DNA alone and in its complex with APE1.

Synthesis and Evaluation as a Blood-Brain Barrier-Permeable Probe of 7-N-(PROXYL-3-yl-methyl)theophylline

The drug-nitroxide radical hybrid-compound 7-N-((2,2,5,5-tetramethylpyrrolidine-1-yloxy(PROXYL))-3-yl-methyl)theophylline (3) was synthesized by coupling 7-N-tosyltheophylline with 3-hydroxymethyl-PROXYL, HMP). The stability of 3 relative to that of HMP was examined in the presence of the anti-oxidant, ascorbic acid (AsA). The initial reduction rate constants of 3 and HMP were 11.9±5.3 and 6.1±5.2 M^-1 min^-1, respectively. In the presence of glutathione (GSH), these constants increased slightly to 22.3±6.8 and 9.1±2.4 M^-1 min^-1, respectively. Two-dimensional cranial electron paramagnetic resonance imaging of mice intravenously injected with 3 via the tail vein revealed that probe 3 enters the mouse brain by passing through the blood-brain barrier (BBB).

Stability of nitroxide biradical TOTAPOL in biological samples

We characterize chemical reduction of a nitroxide biradical, TOTAPOL, used in dynamic nuclear polarization (DNP) experiments, specifically probing the stability in whole-cell pellets and lysates, and present a few strategies to stabilize the biradicals for DNP studies. DNP solid-state NMR experiments use paramagnetic species such as nitroxide biradicals to dramatically increase NMR signals. Although there is considerable excitement about using nitroxide-based DNP for detecting the NMR spectra of proteins in whole cells, nitroxide radicals are reduced in minutes in bacterial cell pellets, which we confirm and quantify here. We show that addition of the covalent cysteine blocker N-ethylmaleimide to whole cells significantly slows the rate of reduction, suggesting that cysteine thiol radicals are important to in vivo radical reduction. The use of cell lysates rather than whole cells also slows TOTAPOL reduction, which suggests a possible role for the periplasm and oxidative phosphorylation metabolites in radical degradation. Reduced TOTAPOL in lysates can also be efficiently reoxidized with potassium ferricyanide. These results point to a practical and robust set of strategies for DNP of cellular preparations.

"Redox Imaging" to Distinguish Cells with Different Proliferative Indexes: Superoxide, Hydroperoxides, and Their Ratio as Potential Biomarkers

The present study was directed to the development of EPR methodology for distinguishing cells with different proliferative activities, using “redox imaging.” Three nitroxide radicals were used as redox sensors: (a) mito-TEMPO—cell-penetrating and localized mainly in the mitochondria; (b) methoxy-TEMPO—cell-penetrating and randomly distributed between the cytoplasm and the intracellular organelles; and (c) carboxy-PROXYL—nonpenetrating in living cells and evenly distributed in the extracellular environment. The experiments were conducted on eleven cell lines with different proliferative activities and oxidative capacities, confirmed by conventional analytical tests. The data suggest that cancer cells and noncancer cells are characterized by a completely different redox status. This can be analyzed by EPR spectroscopy using mito-TEMPO and methoxy-TEMPO, but not carboxy-PROXYL. The correlation analysis shows that the EPR signal intensity of mito-TEMPO in cell suspensions is closely related to the superoxide level. The described methodology allows the detection of overproduction of superoxide in living cells and their identification based on the intracellular redox status. The experimental data provide evidences about the role of superoxide and hydroperoxides in cell proliferation and malignancy.

Molecular Probes for Evaluation of Oxidative Stress by In Vivo EPR Spectroscopy and Imaging: State-of-the-Art and Limitations

Oxidative stress, defined as a misbalance between the production of reactive oxygen species and the antioxidant defenses of the cell, appears as a critical factor either in the onset or in the etiology of many pathological conditions. Several methods of detection exist. However, they usually rely on ex vivo evaluation or reports on the status of living tissues only up to a few millimeters in depth, while a whole-body, real-time, non-invasive monitoring technique is required for early diagnosis or as an aid to therapy (to monitor the action of a drug). Methods based on electron paramagnetic resonance (EPR), in association with molecular probes based on aminoxyl radicals (nitroxides) or hydroxylamines especially, have emerged as very promising to meet these standards. The principles involve monitoring the rate of decrease or increase of the EPR signal in vivo after injection of the nitroxide or the hydroxylamine probe, respectively, in a pathological versus a control situation. There have been many successful applications in various rodent models. However, current limitations lie in both the field of the technical development of the spectrometers and the molecular probes. The scope of this review will mainly focus on the latter.

Increasing nitroxide lifetime in cells to enable in-cell protein structure and dynamics measurements by electron spin resonance spectroscopy

There is increasing evidence that the stability, structure, dynamics, and function of many proteins differ in cells versus in vitro. The determination of protein structure and dynamics within the native cellular environment may lead to better understanding of protein behavior. Electron spin resonance (ESR) has emerged as a technique that can report on protein structure and dynamics within cells. Nitroxide based spin labels are capable of reporting on protein dynamics, structure, and backbone flexibility but are limited due to nitroxide reduction occurring in cells. In order to overcome this limitation, we used the oxidizing agent potassium ferricyanide (K₃Fe(CN)₆) as well as the cleavage resistant spin label 3-malemido-PROXYL (5-MSL). Furthermore, we hypothesized that injection concentration is an important parameter regarding nitroxide reduction kinetics. By increasing the injection concentration of doubly 5-MSL labeled protein into Xenopus laevis oocytes, we found an increased nitroxide lifetime. Our work demonstrates unprecedented incubation times of 3-h in-cell and 5-h in-cytosol for double electron-electron resonance (DEER) experiments using nitroxide spin labels. This allows for more meaningful measurements of larger protein systems which may require longer incubation times for equilibration in the cellular milieu. Even longer incubation times are possible by combining our approach with more shielded nitroxides and Q-band.

Non-invasive mapping of glutathione levels in mouse brains by in vivo electron paramagnetic resonance (EPR) imaging: Applied to a kindling mouse model

Glutathione (GSH) is an important antioxidant that can protect cells under oxidative stress. Thus, a non-invasive method to measure and map the distribution of GSH in live animals is needed. To image the distribution of GSH levels in specific brain regions, a new method using electron paramagnetic resonance (EPR) imaging with a nitroxide imaging probe was developed. Pixel-based mapping of brain GSH levels was successfully obtained by using the linear relationship between reduction rates for nitroxides in brains, measured by an in vivo EPR imager, and brain GSH levels, measured by an in vitro biochemical assay. The newly developed method was applied to a kindling mouse model induced with pentylenetetrazole (PTZ) to visualize changes in GSH levels in specific brain regions after seizure. The obtained map of brain GSH levels clearly indicated decreased GSH levels around the hippocampal region compared to control mice.

Nitroxyl Modified Tobacco Mosaic Virus as a Metal-Free High-Relaxivity MRI and EPR Active Superoxide Sensor

Superoxide overproduction is known to occur in multiple disease states requiring critical care; yet, noninvasive detection of superoxide in deep tissue remains a challenge. Herein, we report a metal-free magnetic resonance imaging (MRI) and electron paramagnetic resonance (EPR) active contrast agent prepared by "click conjugating" paramagnetic organic radical contrast agents (ORCAs) to the surface of tobacco mosaic virus (TMV). While ORCAs are known to be reduced in vivo to an MRI/EPR silent state, their oxidation is facilitated specifically by reactive oxygen species-in particular, superoxide-and are largely unaffected by peroxides and molecular oxygen. Unfortunately, single molecule ORCAs typically offer weak MRI contrast. In contrast, our data confirm that the macromolecular ORCA-TMV conjugates show marked enhancement for T1 contrast at low field (<3.0 T) and T2 contrast at high field (9.4 T). Additionally, we demonstrated that the unique topology of TMV allows for a "quenchless fluorescent" bimodal probe for concurrent fluorescence and MRI/EPR imaging, which was made possible by exploiting the unique inner and outer surface of the TMV nanoparticle. Finally, we show TMV-ORCAs do not respond to normal cellular respiration, minimizing the likelihood for background, yet still respond to enzymatically produced superoxide in complicated biological fluids like serum.

Nitroxides as Antioxidants and Anticancer Drugs

Nitroxides are stable free radicals that contain a nitroxyl group with an unpaired electron. In this paper, we present the properties and application of nitroxides as antioxidants and anticancer drugs. The mostly used nitroxides in biology and medicine are a group of heterocyclic nitroxide derivatives of piperidine, pyrroline and pyrrolidine. The antioxidant action of nitroxides is associated with their redox cycle. Nitroxides, unlike other antioxidants, are characterized by a catalytic mechanism of action associated with a single electron oxidation and reduction reaction. In biological conditions, they mimic superoxide dismutase (SOD), modulate hemoprotein’s catalase-like activity, scavenge reactive free radicals, inhibit the Fenton and Haber-Weiss reactions and suppress the oxidation of biological materials (peptides, proteins, lipids, etc.). The use of nitroxides as antioxidants against oxidative stress induced by anticancer drugs has also been investigated. The application of nitroxides and their derivatives as anticancer drugs is discussed in the contexts of breast, hepatic, lung, ovarian, lymphatic and thyroid cancers under in vivo and in vitro experiments. In this article, we focus on new natural spin-labelled derivatives such as camptothecin, rotenone, combretastatin, podophyllotoxin and others. The applications of nitroxides in the aging process, cardiovascular disease and pathological conditions were also discussed.

Nitroxide-Based Macromolecular Contrast Agents with Unprecedented Transverse Relaxivity and Stability for Magnetic Resonance Imaging of Tumors

Metal-free magnetic resonance imaging (MRI) agents could overcome the established toxicity associated with metal-based agents in some patient populations and enable new modes of functional MRI in vivo. Herein, we report nitroxide-functionalized brush-arm star polymer organic radical contrast agents (BASP-ORCAs) that overcome the low contrast and poor in vivo stability associated with nitroxide-based MRI contrast agents. As a consequence of their unique nanoarchitectures, BASP-ORCAs possess per-nitroxide transverse relaxivities up to ∼44-fold greater than common nitroxides, exceptional stability in highly reducing environments, and low toxicity. These features combine to provide for accumulation of a sufficient concentration of BASP-ORCA in murine subcutaneous tumors up to 20 h following systemic administration such that MRI contrast on par with metal-based agents is observed. BASP-ORCAs are, to our knowledge, the first nitroxide MRI contrast agents capable of tumor imaging over long time periods using clinical high-field 1H MRI techniques.

Regulation of the nitric oxide oxidase activity of myeloperoxidase by pharmacological agents

The leukocyte-derived heme enzyme myeloperoxidase (MPO) is released extracellularly during inflammation and impairs nitric oxide (NO) bioavailability by directly oxidizing NO or producing NO-consuming substrate radicals. Here, structurally diverse pharmacological agents with activities as MPO substrates/inhibitors or antioxidants were screened for their effects on MPO NO oxidase activity in human plasma and physiological model systems containing endogenous MPO substrates/antioxidants (tyrosine, urate, ascorbate). Hydrazide-based irreversible/reversible MPO inhibitors (4-ABAH, isoniazid) or the sickle cell anaemia drug, hydroxyurea, all promoted MPO NO oxidase activity. This involved the capacity of NO to antagonize MPO inhibition by hydrazide-derived radicals and/or the ability of drug-derived radicals to stimulate MPO turnover thereby increasing NO consumption by MPO redox intermediates or NO-consuming radicals. In contrast, the mechanism-based irreversible MPO inhibitor 2-thioxanthine, potently inhibited MPO turnover and NO consumption. Although the phenolics acetaminophen and resveratrol initially increased MPO turnover and NO consumption, they limited the overall extent of NO loss by rapidly depleting H₂O₂ and promoting the formation of ascorbyl radicals, which inefficiently consume NO. The vitamin E analogue trolox inhibited MPO NO oxidase activity in ascorbate-depleted fluids by scavenging NO-consuming tyrosyl and urate radicals. Tempol and related nitroxides decreased NO consumption in ascorbate-replete fluids by scavenging MPO-derived ascorbyl radicals. Indoles or apocynin yielded marginal effects. Kinetic analyses rationalized differences in drug activities and identified criteria for the improved inhibition of MPO NO oxidase activity. This study reveals that widely used agents have important implications for MPO NO oxidase activity under physiological conditions, highlighting new pharmacological strategies for preserving NO bioavailability during inflammation.

Cu(II)-Based Paramagnetic Probe to Study RNA-Protein Interactions by NMR

Paramagnetic NMR techniques allow for studying three-dimensional structures of RNA-protein complexes. In particular, paramagnetic relaxation enhancement (PRE) data can provide valuable information about long-range distances between different structural components. For PRE NMR experiments, oligonucleotides are typically spin-labeled using nitroxide reagents. The current work describes an alternative approach involving a Cu(II) cyclen-based probe that can be covalently attached to an RNA strand in the vicinity of the protein's binding site using "click" chemistry. The approach has been applied to study binding of HIV-1 nucleocapsid protein 7 (NCp7) to a model RNA pentanucleotide, 5'-ACGCU-3'. Coordination of the paramagnetic metal to glutamic acid residue of NCp7 reduced flexibility of the probe, thus simplifying interpretation of the PRE data. NMR experiments showed attenuation of signal intensities from protein residues localized in proximity to the paramagnetic probe as the result of RNA-protein interactions. The extent of the attenuation was related to the probe's proximity allowing us to construct the protein's contact surface map.

Intermolecular Paramagnetic Relaxation Enhancement (PRE) Studies of Transient Complexes in Intrinsically Disordered Proteins

NMR interchain paramagnetic relaxation enhancement (PRE) techniques are a very powerful approach for detecting transient interchain interactions between intrinsically disordered proteins. These experiments, requiring a mixed sample containing a 1:1 ratio of isotope-labeled (15)N protein and natural abundance (14)N protein with a paramagnetic spin label, provide data that is limited to interchain interactions only. Application of these experiments to weakly associated transient species such as those that are present in the very early stages of self-assembly processes will aid our understanding of protein aggregation or fibril formation processes.

Room-temperature electron spin relaxation of nitroxides immobilized in trehalose: Effect of substituents adjacent to NO-group

Trehalose has been recently promoted as efficient immobilizer of biomolecules for room-temperature EPR studies, including distance measurements between attached nitroxide spin labels. Generally, the structure of nitroxide influences the electron spin relaxation times, being crucial parameters for room-temperature pulse EPR measurements. Therefore, in this work we investigated a series of nitroxides with different substituents adjacent to NO-moiety including spirocyclohexane, spirocyclopentane, tetraethyl and tetramethyl groups. Electron spin relaxation times (T1, Tm) of these radicals immobilized in trehalose were measured at room temperature at X- and Q-bands (9/34GHz). In addition, a comparison was made with the corresponding relaxation times in nitroxide-labeled DNA immobilized in trehalose. In all cases phase memory times Tm were close to 700ns and did not essentially depend on structure of substituents. Comparison of temperature dependences of Tm at T=80-300K shows that the benefit of spirocyclohexane substituents well-known at medium temperatures (∼100-180K) becomes negligible at 300K. Therefore, unless there are specific interactions between spin labels and biomolecules, the room-temperature value of Tm in trehalose is weakly dependent on the structure of substituents adjacent to NO-moiety of nitroxide. The issues of specific interactions and stability of nitroxide labels in biological media might be more important for room temperature pulsed dipolar EPR than differences in intrinsic spin relaxation of radicals.

Site-directed spin labeling of proteins for distance measurements in vitro and in cells

We here review strategies for site-directed spin labeling (SDSL) of proteins and discuss their potential for EPR distance measurements to study protein function in vitro and in cells.

In vivo imaging of free radicals produced by multivitamin-mineral supplements

Background

Redox active minerals in dietary supplements can catalyze unwanted and potentially harmful oxidations.

Methods

To determine if this occurs in vivo we employed electron paramagnetic (EPR) imaging. We used 1-hydroxy-3-carboxy-2,2,5,5-tetramethylpyrrolidine (CPH) as a reporter for one-electron oxidations, e.g. free radical-mediated oxidations; the one-electron oxidation product of CPH, 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy (CP^•), is a nitroxide free radical that is relatively persistent in vivo and detectable by EPR. As model systems, we used research formulations of vitamin mineral supplements (RVM) that are typical of commercial products.

Results

In in vitro experiments, upon suspension of RVM in aqueous solution, we observed: (1) the uptake of oxygen in the solution, consistent with oxidation of the components in the RVM; (2) the ascorbate free radical, a real-time indicator of ongoing oxidations; and (3) when amino acid/oligosaccharide (AAOS; glycinate or aspartate with non-digestible oligofructose) served as the matrix in the RVM, the rate of oxidation was significantly slowed. In a murine model, EPR imaging showed that the ingestion of RVM along with CPH results in the one-electron oxidation of CPH by RVM in the digestive system. The resulting CP^• distributes throughout the body. Inclusion of AAOS in the RVM formulation diminished the oxidation of CPH to CP^• in vivo.

Conclusions

These data demonstrate that typical formulations of multivitamin/multimineral dietary supplements can initiate the oxidation of bystander substances and that AAOS-complexes of essential redox active metals, e.g. copper and iron, have reduced ability to catalyze free radical formation and associated detrimental oxidations when a part of a multivitamin/multimineral formulation.

Magnetic Resonance Imaging of Mitochondrial Dysfunction and Metabolic Activity, Accompanied by Overproduction of Superoxide

This study shows that a mitochondria-penetrating nitroxide probe (mito-TEMPO) allows detection of superoxide and visualization of mitochondrial dysfunction in living cells due to the effect of T1 shortening in MRI. Mitochondrial dysfunction was induced by treatment of cells with rotenone and 2-methoxyestradiol (2-ME/Rot). The MRI measurements were performed on 7T MRI. The 2-ME/Rot-treated cells were characterized by overproduction of superoxide, which was confirmed by a conventional dihydroethidium test. In the presence of mito-TEMPO, the intensity of MRI signal in 2-ME/Rot-treated cells was ∼30-40% higher, in comparison with that in untreated cells or culture media. In model (cell-free) systems, we observed that superoxide, but not hydrogen peroxide, increased the intensity of T1-weighted MRI signal of mito-TEMPO. Moreover, the superoxide restores the T1-weighted MRI contrast of mito-TEMPOH, a noncontrast (diamagnetic) analogue of mito-TEMPO. This was also confirmed by using EPR spectroscopy. The results demonstrate that superoxide radical is involved in the enhancement of T1-weighted MRI contrast in living cells, in the absence and presence of mito-TEMPO. This report gives a direction for discovering new opportunities for functional MRI, for detection of metabolic activity, accompanied by overproduction of superoxide, as well as by disturbance of the balance between superoxide and hydrogen peroxide, a very important approach to clarify the fine molecular mechanisms in the regulation of many pathologies. The visualization of mitochondrial activity in real-time can be crucial to clarify the molecular mechanism of the functional MRI in its commonly accepted definition, as a method for detection of neurovascular coupling.