Integrated experimental and computational spectroscopy study on the protonation of the α-nitronyl nitroxide radical unit

The stability of the α-nitronyl nitroxide radical unit under acidic conditions is investigated by an integrated experimental UV/Vis spectroscopy and TDDFT study. In the field of molecular magnetism, α-nitronyl nitroxide radicals are important as purely organic spin carriers due to their synthetic versatility. Here, the existence of an intermediate of the protonated α-nitronyl nitroxide radical unit is demonstrated for the first time and a proposed disproportionation reaction is confirmed.     

Perylene-based profluorescent nitroxides for the rapid monitoring of polyester degradation upon weathering: An assessment

A profluorescent nitroxide possessing an isoindoline nitroxide moiety linked to a perylene fluorophore was developed to monitor radical mediated degradation of melamine-formaldehyde crosslinked polyester coil coatings in an industry standard accelerated weathering tester. Trapping of polyester-derived radicals (most likely C-radicals) that are generated during polymer degradation leads to fluorescent closed-shell alkoxy amines, which was used to obtain time-dependent degradation profiles to assess the relative stability of different polyesters towards weathering. The nitroxide probe couples excellent thermal stability and satisfactory photostability with high sensitivity and enables detection of free radical damage in polyesters under conditions that mimic exposure to the environment on a time scale of hours rather than months or years required by other testing methods. There are indications that the profluorescent nitroxide undergoes partial photo-degradation in the absence of polymer-derived radicals. Unexpectedly, it was also found that UV-induced fragmentation of the NO–C bond in closed-shell alkoxy amines leads to regeneration of the profluorescent nitroxide and the respective C-radical. The maximum fluorescence intensity that could be achieved with a given probe concentration is therefore not only determined by the amount of polyester radicals formed during accelerated weathering, but also by the light-driven side reactions of the profluorescent nitroxide and the corresponding alkoxy amine radical trapping products. Studies to determine the optimum probe concentration in the polymer matrix revealed that aggregation and re-absorption effects lowered the fluorescence intensity at higher concentrations of the profluorescent nitroxide, but too low probe concentrations, where these effects would be avoided, were not sufficient to trap the amount of polyester radicals formed upon weathering. The optimized experimental conditions were used to assess the impact of temperature and UV irradiance on polymer degradation during accelerated weathering.     

A long-lived luminescence and EPR bimodal lanthanide-based probe for free radicals

We developed a novel spin-labeled terbium complex Tb(3+)/cs124-DTPA-TEMPO (1) by covalently labeling a nitroxide radical on the terbium complex for monitoring free radicals of various areas. This lanthanide complex probe shows a high EPR signal which resulted from the nitroxide radical moiety, and is weakly luminescent which resulted from the intramolecular quenching effect of the nitroxide radical on sensitised terbium luminescence. The intensity of both the EPR and luminescence can be modulated by eliminating the paramagnetism of the nitroxide radical through recognition of a carbon-centered radical analyte and thus gives a quantification of the analyte. We have preliminarily applied this probe in the luminescent detection of model carbon-centered radicals and hydroxyl radicals (·OH). This probe is water-soluble and contains lanthanide-luminescence properties, favorable for the time-resolved luminescence technique. The investigation of the intramolecular quenching process has showed that the labeled nitroxide radical quenches multiple excited states of the terbium complex, resulting in highly efficient quenching of terbium luminescence. This probe is the first example of intramolecular modulation of lanthanide luminescence by a nitroxide radical.     

Gas‐phase radical–radical recombination reactions of nitroxides with substituted phenyl radicals

Fourier‐transform ion cyclotron resonance mass spectrometry has been used to examine gas‐phase reactions of four different nitroxide free radicals with eight positively charged pyridyl and phenyl radicals (some containing a Cl, F, or CF₃ substituent). All the radicals reacted rapidly (near collision rate) with nitroxides by radical–radical recombination. However, some of the radicals were also able to abstract a hydrogen atom from the nitroxide. The results establish that the efficiency (k_reaction/k_collision) of hydrogen atom abstraction varies with the electrophilicity of the radical, and hence is attributable to polar effects (a lowering of the transition‐state energy by an increase in its polar character). The efficiency of the recombination reaction is not sensitive to substituents, presumably due to a very low reaction barrier. Even so, after radical–radical recombination has occurred, the nitroxide adduct was found to fragment in different ways depending on the structure of the radical. For example, a cationic fragment was eliminated from the adducts of the more electrophilic radicals via oxygen anion abstraction by the radical (i.e., the nitroxide adduct cleaves heterolytically), whereas adducts of the less electrophilic radicals predominantly fragmented via homolytic cleavage (oxygen atom abstraction). Therefore, differences in the product branching ratios were found to be attributable to polar factors. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 216–229 2004     

Effect of nitroxide radicals on chemically induced dynamic electron polarization of spin-correlated radical pairs in aqueous micellar solutions of sodium dodecyl sulfate

The multispin systems consisting of spin-correlated radical pairs (SCRPs) and stable nitroxide radicals, localized in micelles of sodium dodecyl sulfate (SDS), were studied by ESR and pulse laser photolysis techniques. In all the systems studied, the stable nitroxide radicals exert no effect on the shape of the ESR spectra of the SCRPs (in particular, on the shape of their antiphase structure) and on the decay kinetics of the ESR signal of the SCRPs. In the SDS micelles, the electron spin polarization transfer from the nonequilibrium electron spin states of the molecular triplets (SCRP precursors) is the most efficient mechanism of generation of the electron spin polarization in nitroxide radicals. The experimental data also show that the nitroxide radicals and SCRP radicals are most probably distributed uniformly in the micellar phase. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1390–1401, July, 2008.     

A new model for Overhauser enhanced nuclear magnetic resonance using nitroxide radicals

Nitroxide free radicals are the most commonly used source for dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) experiments and are also exclusively employed as spin labels for electron spin resonance (ESR) spectroscopy of diamagnetic molecules and materials. Nitroxide free radicals have been shown to have strong dipolar coupling to (1)H in water, and thus result in large DNP enhancement of (1)H NMR signal via the well known Overhauser effect. The fundamental parameter in a DNP experiment is the coupling factor, since it ultimately determines the maximum NMR signal enhancements which can be achieved. Despite their widespread use, measurements of the coupling factor of nitroxide free radicals have been inconsistent, and current models have failed to successfully explain our experimental data. We found that the inconsistency in determining the coupling factor arises from not taking into account the characteristics of the ESR transitions, which are split into three (or two) lines due to the hyperfine coupling of the electron to the (14)N nuclei (or (15)N) of the nitric oxide radical. Both intermolecular Heisenberg spin exchange interactions as well as intramolecular nitrogen nuclear spin relaxation mix the three (or two) ESR transitions. However, neither effect has been taken into account in any experimental studies on utilizing or quantifying the Overhauser driven DNP effects. The expected effect of Heisenberg spin exchange on Overhauser enhancements has already been theoretically predicted and observed by Bates and Drozdoski [J. Chem. Phys. 67, 4038 (1977)]. Here, we present a new model for quantifying Overhauser enhancements through nitroxide free radicals that includes both effects on mixing the ESR hyperfine states. This model predicts the maximum saturation factor to be considerably higher by the effect of nitrogen nuclear spin relaxation. Because intramolecular nitrogen spin relaxation is independent of the nitroxide concentration, this effect is still significant at low radical concentrations where electron spin exchange is negligible. This implies that the only correct way to determine the coupling factor of nitroxide free radicals is to measure the maximum enhancement at different concentrations and extrapolate the results to infinite concentration. We verify our model with a series of DNP experimental studies on (1)H NMR signal enhancement of water by means of (14)N as well as (15)N isotope enriched nitroxide radicals.     

Nitroxides scavenge myeloperoxidase-catalyzed thiyl radicals in model systems and in cells

Nitroxide radicals possess important antioxidant activity in live tissues because of their ability to scavenge reactive radicals. Despite the fact that, in cells, damaging free radicals are primarily quenched by glutathione (GSH) with subsequent formation of harmful glutathionyl radical (GS(*)), interactions of nitroxide radicals with GS(*) and thiols have not been studied in detail. In addition, intracellular metabolic pathways leading to the formation of secondary amines from nitroxides are unknown. Here we report that GS(*) radicals react efficiently and irreversibly with nitroxides to produce secondary amines. We developed a sensitive method for the detection of GS(*) based on their specific interaction with Ac-Tempo, a nonfluorescent conjugate of fluorogenic acridine with paramagnetic nitroxide Tempo, and used it to characterize interactions between nitroxide and thiyl radicals generated through phenoxyl radical recycling by peroxidase. During reaction of Ac-Tempo with GS(*), Tempo EPR signals decayed and acridine fluorescence concurrently increased. DMPO and PBN, spin traps for GS(*), inhibited this interaction. Using combined HPLC and mass spectrometry, we determined that 90% of the Ac-Tempo was converted into fluorescent acridine (Ac)-piperidine; GSH was primarily oxidized into sulfonic acid. In myeloperoxidase-rich HL-60 cells, Ac-piperidine fluorescence was observed upon stimulation of GS(*) generation by H(2)O(2) and phenol. Development of fluorescence was prevented by preincubation of cells with the thiol-blocking reagent N-ethylmaleimide as well as with peroxidase inhibitiors. Furthermore, Ac-Tempo preserved intracellular GSH and protected cells from phenol/GS(*) toxicity, suggesting a new mechanism for the free-radical scavenging activity of nitroxides in live cells.     

Redox polymers incorporating pendant 6-oxoverdazyl and nitronyl nitroxide radicals

Polymers comprised of redox-active organic radicals have emerged as promising materials for use in a variety of organic electronics, including fast-charging batteries. Despite these advances, relatively little attention has been focused on the diversification of the families of radicals that are commonly incorporated into polymer frameworks, with most radical polymers being comprised of nitroxide radicals. Here, we report two new examples prepared via ring-opening methathesis polymerization containing 6-oxoverdazyl and nitronyl nitroxide radicals appended to their backbones. The polymerization reaction and optoelectronic properties were explored in detail, revealing high radical content and redox activity that may be advantageous for their use as semiconducting thin films. Initial studies revealed that current–voltage curves obtained from thin films of the title polymers exhibited memory effects making them excellent candidates for use in resistive memory applications. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 309–319     

Scavenging with TEMPO* to identify peptide- and protein-based radicals by mass spectrometry: advantages of spin scavenging over spin trapping

The detection and characterization of radicals in biomolecules are challenging due to their high reactivity and low concentration. Mass spectrometry (MS) provides a tool for the unambiguous identification of protein-based radicals by exploiting their reactivity with suitable reagents. To date, protein-radical detection by MS has been modeled after electron paramagnetic resonance experiments, in which diamagnetic spin traps, such as 3,5-dibromo-4-nitrosobenzene sulfonic acid, convert unstable radicals to more stable spin adducts. Since MS detects mass changes, and not unpaired spins, conversion of radicals to stable diamagnetic adducts is more desirable. The use of 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO(*)) in the MS identification of protein-based radicals was explored here to establish whether scavenging via radical combination would give rise to TEMPO adducts that were stable to MS analysis. The horseradish peroxidase/H(2)O(2) reaction was used to generate radicals in derivatives of tyrosine, tryptophan, and phenylalanine as models of protein-based radicals. TEMPO(*) was added as a radical scavenger, and the products were analyzed by electrospray ionization (ESI) MS. Dramatically higher mass-adduct yields were obtained using radical scavenging vs radical trapping, which greatly enhanced the sensitivity of radical detection. The efficiency of TEMPO(*) in protein radical scavenging was examined in horse heart myoglobin and cytochrome c peroxidase (CCP) from Saccharomyces cerevisiae. On H(2)O(2) binding to their ferric hemes, two oxidizing equivalents are transferred to the proteins as an Fe(IV)=O species and a polypeptide-based radical. In addition, CCP has been shown to reduce up to 10 equiv of H(2)O(2) using endogenous donors, thereby generating as many as 20 radicals on its polypeptide. Following myoglobin and CCP incubation with a 10-fold molar excess of H(2)O(2) and TEMPO(*), matrix-assisted laser desorption ionization (MALDI) time-of-flight analysis of the tryptic peptides derived from the proteins revealed 1 and 9 TEMPO adducts of myoglobin and CCP, respectively. Given the high scavenging efficiency of TEMPO(*) and the stability of TEMPO-labeled peptides in ESI and MALDI sources, scavenging by stable nitroxide radicals coupled with MS analysis should provide sensitive and powerful technology for the characterization of protein-based radicals.     

An intrinsic radical stability scale from the perspective of bond dissociation enthalpies: a companion to radical electrophilicities

Bond dissociation enthalpies (BDEs) of a large series of molecules of the type A-B, where a series of radicals A ranging from strongly electrophilic to strongly nucleophilic are coupled with a series of 8 radicals (CH2OH, CH3, NF2, H, OCH3, OH, SH, and F) also ranging from electrophilic to nucleophilic, are computed and analyzed using chemical concepts emerging from density functional theory, more specifically the electrophilicities of the individual radical fragments A and B. It is shown that, when introducing the concept of relative radical electrophilicity, an (approximately) intrinsic radical stability scale can be developed, which is in good agreement with previously proposed stability scales. For 47 radicals, the intrinsic stability was estimated from computed BDEs of their combinations with the strongly nucleophilic hydroxymethyl radical, the neutral hydrogen atom, and the strongly electrophilic fluorine atom. Finally, the introduction of an extra term containing enhanced Pauling electronegativities in the model improves the agreement between the computed BDEs and the ones estimated from the model, resulting in a mean absolute deviation of 16.4 kJ mol(-1). This final model was also tested against 82 experimental values. In this case, a mean absolute deviation of 15.3 kJ mol(-1) was found. The obtained sequences for the radical stabilities are rationalized using computed spin densities for the radical systems.     

Photoelectron spectra studies of organic free radicals. The nitroxide radical

The photoelectron spectra of the nitroxide radicals, di-tert-butylnitroxide (DTBN) and 2,2,6,6-tetramethyl-piperidine-N-oxyl, have been studied and molecular orbital calculations made. The adiabatic first ionization potentials were found to be 6.77 and 6.73 eV for these two nitroxide radicals respectively. Four vertical ionization potentials which are common to each nitroxide radical were attributed to ionization of the odd electron in the NO anti-bonding π orbital, oxygen lone pair electrons and NO bonding π electrons. Doublet splitting of the lone pair electron peak with different peak intensities can be quantitatively understood in terms of triplet and singlet states of the photoionized nitroxide cation.     

Photoelectron spectroscopy of free radicals: 4-methyl-sulfonyloxy-TEMPO

HeI and HeII photoelectron spectra of persistent free radical: 2,2,6,6-tetramethyl-4-(methylsulfonyloxy)piperidinyl-1-oxide (CH3SO3-TEMPO) have been measured. The analysis of the electronic structure is based on comparison with the spectra of related radicals and with DFT calculations. The electronic structure of the nitroxide group in the title radical seems unaffected by the presence of substituent group. This is consistent with results observed for other nitroxide radicals and with the electrochemical oxidation potentials.     

Evaluation method of steric shielding effect around nitroxide radical reaction center based on molecular volume within a virtual ball

Steric shielding affects the stability of various molecules such as nitroxide radicals, which have many applications in a variety of fields. Thus, the mechanisms that maintain molecular stability are of particular interest. A new method for nitroxide radicals to quantify the steric shielding effect around the reaction center in a molecule was developed. The steric hindrance in this method is defined as the volume of the molecules contained within a virtual ball centered on the radical atom. With the proposed method, it is possible to evaluate the influences of the β-substituent, the basic molecular skeleton, and other parameters on steric hindrance, which cannot be calculated with E_s^c, a known parameter that indicates bulkiness. This method can acquire more accurate data that more closely resembles the behavior of the actual molecule. Because this method is very simple in that it requires only the optimal stable structure of the molecule, it can be used to estimate the steric shielding of various nitroxide radicals as well as other molecules.

     

Synthesis,structure and magnetic properties of a Pr(III) complex with chelating nitronyl nitroxide radicals

The synthesis and structure of Pr(III) complex with chelating nitronyl nitroxide radicals of formula [Pr(III)(NIT2Py)₂(NO₃)₃] (NIT2Py?=?2-(2′-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide) is reported. Pr(III) is ten-coordinate with three bidentate nitrate anions and two radicals. The radical behaves as a bidentate chelating ligand through one oxygen atom of the nitronyl nitroxide group and one nitrogen atom of a pyridine ring. The electronic spectrum for the complex in THF and magnetic susceptibilities from 77–300?K are reported.     

Investigation of the anthracene-nitroxide hybrid molecule as a probe for hydroxyl radicals.

A new method for the determination of hydroxyl radicals is proposed. The method is based on the use of a hybrid molecule consisting of a fluorescent chromophore, anthracene, and a nitroxide radical. In the hybrid molecule, the nitroxide quenches the fluorescence of anthracene strongly. The reaction of hydroxyl radicals with dimethyl sulfoxide generates quantitatively methyl radicals, which then combine with the nitroxide moiety of the hybrid molecules to result in an increase in the fluorescence intensity. The fluorescence increase is proportional to the concentration of hydroxyl radicals. The proposed method is capable of detecting hydroxyl radicals generated in the Fenton system. It is a simple and sensitive technique for the determination of hydroxyl radicals.     

Tunable delocalization of unpaired electrons of nitroxide radicals for sickle-cell disease drug improvements

Hydroxyurea is a drug recently approved to treat sickle cell diseases. Hydroxyurea benefits the patients by increasing the level of fetal hemoglobin via a nitroxide radical pathway. Here, we report an unpaired-electron-delocalization approach to tune the stability of nitroxide radicals. In this approach, the substitution by an unsaturated alkyl group containing conjugated C=C double bonds for the hydrogen on the nitrogen atom attached to the hydroxyl of hydroxyurea can significantly increase its ability to generate nitroxide radical. Furthermore, the increase can be remarkably enhanced by increasing the number of conjugated C=C double bonds. For a hydroxyurea derivative that contains two conjugated C=C double bonds, the reaction rate to generate its radical is 118 times faster than that of hydroxyurea, and for a hydroxyurea derivative containing 20 conjugated C=C double bonds, the reaction rate to form its radical is 238 times faster than that of hydroxyurea. For this reason, hydroxyurea derivatives with conjugated C=C double bonds may constitute new potential drugs for the treatment of sickle-cell diseases.     

Synthesis,structures, and magnetic and optical properties of a series of europium(III) and gadolinium(III) complexes with chelating nitronyl and imino nitroxide free radicals

This paper reports the synthesis, structures, and magnetic and optical properties of a series of gadolinium(III) (1a-4a) and europium(III) (1b-4b) complexes with nitronyl or imino nitroxide radicals. The crystal structures of compounds 1a and 1b consist of [Ln(III)(radical)(2)(NO(3))(3)] entities in which the gadolinium(III) (1a) or europium(III) ion (1b) is 10-coordinated to two nitronyl nitroxide radicals and three nitrato ligands. The crystal structures of compounds 2a-4a and 2b-4b consist of [Ln(III)(hfac)(3)(radical)] entities in which the gadolinium(III) (2a-4a) or europium(III) ion (2b-4b) is 8-coordinated to one nitronyl (2a and 2b) or one imino (3a, 4a and 3b, 4b) nitroxide radical and three hexafluoroacetylacetonato ligands. The gadolinium(III) complexes (1a-4a) are isostructural with their europium(III) analogues (1b-4b). The magnetic properties of the gadolinium complexes were studied. Along the series 1a-4a only compound 2a exhibits a ferromagnetic Gd(III)-radical coupling (J(Gd-rad) = +1.7 cm(-1)), while for the others this coupling is antiferromagnetic (1a: J(Gd-rad1) = -4.05 cm(-1) and J(Gd-rad2) = -0.80 cm(-1); 3a: J(Gd-rad) = -2.6 cm(-1); 4a: J(Gd-rad) = -1.9 cm(-1)). The first full luminescence spectra of lanthanide complexes with free radical ligands are reported between 650 and 1200 nm. The rich vibronic structure in luminescence and absorption spectra indicates that several excited states define the absorption spectra between 400 and 800 nm. Qualitative trends can be established between magnetic ground state properties and the energies and fine structure of the title compounds.     

The magnetochemistry of verdazyl radical-based materials

Stable organic radicals have received much attention as building blocks for the construction of molecular magnetic materials because they are readily functionalized using modern synthetic techniques. In this context, the nitroxide radical family has been the dominant class of radicals in molecular magnetochemistry. However, other stable radical systems have also been explored. One such example is the verdazyl family of radicals. Their high chemical stability and synthetic versatility make verdazyls one of the more attractive alternatives to nitroxides in molecular magnet design. This article reviews the magnetism of verdazyl-based systems, including through-bond coupling in polyradicals, coordination complexes and intermolecular interactions in the solid state.     

Synthesis and spectral properties of polymethine-cyanine dye-nitroxide radical hybrid compounds for use as fluorescence probes to monitor reducing species and radicals

Various hybrid compounds comprised of two types of nitroxide radicals and either a pentamethine (Cy5) or trimethine cyanine (Cy3) were synthesized. The nitroxide radicals were linked either via an ester-bond to one or two N-alkyl carboxyl-terminated groups of Cy5, or via two amido-bonds (aminocarbonyl or carbonylamino group) to the 5-position of the indolenine moieties of Cy5 and Cy3. Changes in fluorescence and ESR intensities of the hybrid compounds were measured before and after addition of Na ascorbate in PBS (pH 7.0) to reduce the radicals. Among the hybrid compounds synthesized, those that linked the nitroxide radicals via an aminocarbonyl residue at the 5-position of the indolenine moieties on Cy5 and Cy3 exhibited a 1.8- and 5.1-fold increase in fluorescence intensity with the reduction of the nitroxide segment by the addition of Na ascorbate, respectively. In contrast, fluorescence intensity was not enhanced in the other hybrid compounds. Thus, the hybrid compounds which exhibited an increase in fluorescent intensity with radical reduction can be used in the quantitative measurement of reducing species such as Fe(2+) and ascorbic acid, and hydroxyl radicals. Because these hybrid compounds have the advantage of fluorescing at longer wavelengths-661 (Cy5) or 568 (Cy3)nm, respectively, they can be used to measure radical-reducing species or radicals either in solution or in vivo.     

Saturation factor of nitroxide radicals in liquid DNP by pulsed ELDOR experiments

We propose the use of the pulse electron double resonance (ELDOR) method to determine the effective saturation factor of nitroxide radicals for dynamic nuclear polarization (DNP) experiments in liquids. The obtained values for the nitroxide radical TEMPONE-D,(15)N at different concentrations are rationalized in terms of spin relaxation and are shown to fulfil the Overhauser theory.