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Nitroxides find application in various areas of chemistry, and a more in‐depth understanding of factors controlling their reactivity with metal complexes is warranted to promote further developments. Here, we report on the effect of the metal centre Lewis acidity on both the distribution of the O‐ and N‐centered spin density in 2,2,6,6‐tetramethylpiperidine‐N‐oxyl (TEMPO) and turning TEMPO from the O‐ to N‐radical mode scavenger in metal‐TEMPO systems. We use Et(Cl)Zn/TEMPO model reaction system with tuneable reactivity in the solid state and solution. Among various products, a unique Lewis acid‐base adduct of Cl2Zn with the N‐ethylated TEMPO was isolated and structurally characterised, and the so‐called solid‐state ′slow chemistry′ reaction led to a higher yield of the N‐alkylated product. The revealed structure‐activity/selectivity correlations are exceptional yet are entirely rationalised by the mechanistic underpinning supported by theoretical calculations of studied model systems. This work lays a foundation and mechanistic blueprint for future metal/nitroxide systems exploration. [ABSTRACT FROM AUTHOR]

A series of pyridyl‐substituted nitronyl nitroxides was synthesized and structurally characterized. A comprehensive magnetochemical and quantum chemical study of extended raw of the nitroxides with different substituents R in the pyridine fragment was performed. It was shown, that temperature‐dependent magnetic properties are determined by the short contacts between nitroxide groups of adjacent molecules as well as between nitroxide group and methyl substituents in the pseudo axial positions of imidazoline fragments. Quantum chemistry allows to select the appropriate model of exchange cluster for analysis of experimental magnetic data and evaluation of the exchange interaction parameters. For NN–PyCl the "order‐disorder" transition was detected by means of low temperature XRD. The difference in the experimental and calculated exchange interaction energies may serve as an indicator of temperature‐induced structural rearrangements. For instance, for methyl substituted nitronyl nitroxide NN‐PyMe structural transformations and significant changes in exchange interaction energies were observed. [ABSTRACT FROM AUTHOR]

A diradical with engineered g‐asymmetry was synthesized by grafting a nitroxide radical onto the [Y(Pc)2]⋅ radical platform. Various spectroscopic techniques and computational studies revealed that the electronic structures of the two spin systems remained minimally affected within the diradical system. Fluid‐solution Electron Paramagnetic Resonance (EPR) experiments revealed a weak exchange coupling with |J| ~ 0.014 cm−1, subsequently rationalized by CAS‐SCF calculations. Frozen solution continuous‐wave (CW) EPR experiments showed a complicated and power‐dependent spectrum that eluded analysis using the point‐dipole model. Pulse EPR manipulations with varying microwave powers, or under varying magnetic fields, demonstrated that different resonances could be selectively enhanced or suppressed, based on their different tipping angles. In particular, Field‐Swept Echo‐Detected (FSED) spectra revealed absorptions of MW power‐dependent intensities, while Field‐Swept Spin Nutation (FSSN) experiments revealed two distinct Rabi frequencies. This study introduces a methodology to synthesize and characterize g‐asymmetric two‐spin systems, of interest in the implementation of spin‐based CNOT gates. [ABSTRACT FROM AUTHOR]

Dinitrogen trioxide (N2O3) is a potent nitrosating agent featured with high reactivity and appealing atom economy. Because of its instability and the entanglement of chemical and phase equilibria, N2O3 has rarely been utilized in organic synthesis as a stock reagent with well-defined composition. In this review, the preparations of pure N2O3 and its concentrated solution (>0.1 M) are discussed from the aspect of phase equilibrium. Understanding the physical and chemical characteristics of N2O3, along with how reaction parameters (temperature, pressure, molar ratio) interact, plays a crucial role in managing the concentration of N2O3 in the liquid phase. This control holds practical significance in achieving quantitative reactions. [ABSTRACT FROM AUTHOR]

The flavoprotein Cytochrome P450 reductase (CPR) is the unique electron pathway from NADPH to Cytochrome P450 (CYPs). The conformational dynamics of human CPR in solution, which involves transitions from a "locked/closed" to an "unlocked/open" state, is crucial for electron transfer. To date, however, the factors guiding these changes remain unknown. By Site‐Directed Spin Labelling coupled to Electron Paramagnetic Resonance spectroscopy, we have incorporated a non‐canonical amino acid onto the flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) domains of soluble human CPR, and labelled it with a specific nitroxide spin probe. Taking advantage of the endogenous FMN cofactor, we successfully measured for the first time, the distance distribution by DEER between the semiquinone state FMNH• and the nitroxide. The DEER data revealed a salt concentration‐dependent distance distribution, evidence of an "open" CPR conformation at high salt concentrations exceeding previous reports. We also conducted molecular dynamics simulations which unveiled a diverse ensemble of conformations for the "open" semiquinone state of the CPR at high salt concentration. This study unravels the conformational landscape of the one electron reduced state of CPR, which had never been studied before. [ABSTRACT FROM AUTHOR]

NMR spectroscopy techniques can provide important information about protein‐ligand interactions. Here we tested an NMR approach which relies on the measurement of paramagnetic relaxation enhancements (PREs) arising from analogous cationic, anionic or neutral soluble nitroxide molecules, which distribute around the protein‐ligand complex depending on near‐surface electrostatic potentials. We applied this approach to two protein‐ligand systems, interleukin‐8 interacting with highly charged glycosaminoglycans and the SH2 domain of Grb2 interacting with less charged phospho‐tyrosine tripeptides. The electrostatic potential around interleukin‐8 and its changes upon binding of glycosaminoglycans could be derived from the PRE data and confirmed by theoretical predictions from Poisson‐Boltzmann calculations. The ligand influence on the PREs and NMR‐derived electrostatic potentials of Grb2 SH2 was localized to a narrow protein region which allowed the localization of the peptide binding pocket. Our analysis suggests that experiments with nitroxide cosolutes can be useful for investigating protein‐ligand electrostatic interactions and mapping ligand binding sites. [ABSTRACT FROM AUTHOR]

A novel synthetic approach has been employed to synthesize a series of new nitronyl nitroxides: 2‐(1‐propyl‐1H‐imidazol‐5‐yl)‐ (Ln−Pr), 2‐(1‐isopropyl‐1H‐imidazol‐5‐yl)‐ (Li−Pr) and 2‐(1‐butyl‐1H‐imidazol‐5‐yl)‐4,4,5,5‐tetramethyl‐4,5‐dihydro‐1H‐imidazole‐3‐oxide‐1‐oxyl (Ln−Bu). The reaction of Cu(hfac)2 with LR in a 1 : 2 ratio yields mononuclear heterospin complexes [Cu(hfac)2(LR)2] (LR=Ln‐Pr, Li‐Pr, Ln‐Bu), which have a similar crystal structure to the "jumping" crystals [Cu(hfac)2(LMe)2] that exhibit chemomechanical activity. It was shown that an increase in the alkyl substituent R leads to changes in the crystal packing of the molecules and the absence of chemomechanical activity. Furthermore, it was found that two polymorph modifications of the heterospin complex [Cu(hfac)2(Ln−Pr)2] can be obtained, and magnetic properties of [Cu(hfac)2(Ln−Pr)2] strongly depend on the angle between the planes of the paramagnetic fragment O•–N‐C=N→O and the imidazole ring in Ln−Pr. [ABSTRACT FROM AUTHOR]

High‐spin organic tetraradicals with significant intramolecular exchange interactions have high potential for advanced technological applications and fundamental research, but those synthesized to date possess limited stability and processability. In this work, we have designed a tetraradical based on the Blatter's radical and nitronyl nitroxide radical moieties and successfully synthesized it by using the palladium‐catalyzed cross‐coupling reaction of a triiodo‐derivative of the 1,2,4‐benzotriazinyl radical with gold(I) nitronyl nitroxide‐2‐ide complex in the presence of a newly developed efficient catalytic system. The molecular and crystal structure of the tetraradical was confirmed by X‐ray diffraction analysis. The tetraradical possesses good thermal stability with decomposition onset at ∼150 °C under an inert atmosphere and exhibits reversible redox waves at −0.54 and 0.45 V versus Ag/AgCl. The magnetic properties of the tetraradical were characterized by SQUID magnetometry of polycrystalline powders and EPR spectroscopy in various matrices. The collected data, analyzed by using high‐level quantum chemical calculations, confirmed that the tetraradical has a triplet ground state and a nearby excited quintet state. The unique high stability of the prepared triazinyl‐nitronylnitroxide tetraradical is a new milestone in the field of creating high‐spin systems. [ABSTRACT FROM AUTHOR]

New nitroxides based on aza‐crown ethers were prepared and employed as selective sensors for the detection of inorganic and organic cations by EPR analysis of the corresponding host–guest complexes. The nitroxide unit behaves as a sensitive probe for a number of alkali and alkaline earth metal cations affording EPR spectra differing in the value of nitrogen hyperfine constants and in the appearance of splitted signals due to the non‐zero nuclear spin of some metal cation upon complexation. Owing to the remarkable EPR spectral differences between the host and the corresponding cation complex the new macrocycles are likely to act as multitasking tools to recognize several cationic species. EPR behaviour of the larger nitroxide azacrown 1⋅ when acting as a wheel in a radical synthetic bistable [2]rotaxane containing both secondary dialkylammonium and 1,2‐bis(pyridinium) molecular stations, was also investigated. Reversible movements of the macrocycle between the two recognition sites in the rotaxane were promptly revealed by EPR, which shows significant changes either in nitrogen coupling constant values (aN) or in the spectral shape in the two rotaxane co‐conformations. [ABSTRACT FROM AUTHOR]

Nitroxide (NO) spin radicals are effective in characterizing structures, interactions and dynamics of biomolecules. The EPR applications in cell lysates or intracellular milieu require stable spin labels, but NO radicals are unstable in such conditions. We showed that the destabilization of NO radicals in cell lysates or even in cells is caused by NADPH/NADH related enzymes, but not by the commonly believed reducing reagents such as GSH. Maleimide stabilizes the NO radicals in the cell lysates by consumption of the NADPH/NADH that are essential for the enzymes involved in destabilizing NO radicals, instead of serving as the solo thiol scavenger. The maleimide treatment retains the crowding properties of the intracellular components and allows to perform long‐time EPR measurements of NO labeled biomolecules close to the intracellular conditions. The strategy of maleimide treatment on cell lysates for the EPR applications has been demonstrated on double electron‐electron resonance (DEER) measurements on a number of NO labeled protein samples. The method opens a broad application range for the NO labeled biomolecules by EPR in conditions that resemble the intracellular milieu. [ABSTRACT FROM AUTHOR]

The reaction of nitronyl nitroxide biradical NITPhMeImbis [5-(2-methylimidazole)-1,3-bis(1-oxyl-3'-oxido-4',4',5',5'-tetramethyl-4,5-hydro-1H-imidazol-2-yl)-benzene] with Ln(hfac)3·2H2O and Cu(hfac)2 (hfac=hexafluoroacetylacetonate), led to two series of 2p-3d-4f complexes, namely, nona-spin clusters, [Ln2 Cu3 (hfac)12 (NITPhMeImbis)2] (Ln=Gd 1, Dy 2), or one-dimensional chains [LnCu2 (hfac)7 (NITPhMeImbis)] (Ln=Y 3, Dy 4, Tb 5) depending on the temperature of the reaction. All five complexes contain a biradical-Ln unit in which the biradical chelates the LnIII ion by the means of one aminoxyl (i. e. NO) group of each NIT unit. For the discrete complexes, a Cu(hfac)2 links two biradical-Ln units via one of the remaining NO groups, while for the chain compounds, the two remaining NO groups of the biradical-Ln moiety are each coordinated to a Cu(hfac)2 unit to form a 1D coordination polymer. Moreover, a terminal Cu(hfac)2 unit is coordinated to the imidazole-N atom of the NITPhMeImbis ligand. Spin dynamics investigations evidenced the onset of slow relaxation of the magnetization for 2, whereas 4 and 5 exhibit a typical single-chain magnet behavior. This highlights the vital role of the 1D spin correlation in the blocking of the magnetization. These results illustrate that from the same basic building blocks, magnetic relaxation can be carefully modulated by structural adjustments. [ABSTRACT FROM AUTHOR]

Nitronyl nitroxides are functional building blocks in cutting‐edge research fields, such as the design of molecular magnets, the development of redox and photoswitchable molecular systems and the creation of redox‐active components for organic and hybrid batteries. The key importance of the nitronyl nitroxide function is to translate molecular‐level‐optimized structures into nano‐scale devices and new technologies. In spite of great importance, efficient and versatile synthetic approaches to these compounds still represent a challenge. Particularly, methods for the direct introduction of a nitronyl nitroxide moiety into aromatic systems possess many limitations. Here, we report gold derivatives of nitronyl nitroxide that can enter Pd(0)‐catalysed cross‐coupling reactions with various aryl bromides, affording the corresponding functionalized nitronyl nitroxides. Based on the high thermal stability and enhanced reactivity in catalytic transformation, a new reagent is suggested for the synthesis of radical systems via a universal cross‐coupling approach. [ABSTRACT FROM AUTHOR]

In spite of achievement of a lot of Ln‐radical SMMs, how to improve magnetic behavior of Ln‐radical system remains challenging. Here, two series of Ln‐radical complexes have successfully been built using an imino nitroxide biradical, namely, [Ln2(hfac)6(ImPhPyobis)2] (LnIII=Gd 1, Tb 2, Dy 3) and [Ln2Cu2(hfac)10(ImPhPyobis)2] (LnIII=Gd 4, Dy 5; hfac=hexafluoroacetylacetonate and ImPhPyobis=5‐(4‐oxypyridinium‐1‐yl)‐1,3‐bis(1'‐oxyl‐4',4',5',5'‐tetramethyl‐4,5‐hydro‐1H‐imidazol‐2‐yl)benzene). For these biradical‐metal complexes, two imino nitroxide biradicals bind two Ln(III) ions via their oxygen atoms coming from 4‐oxypyridinium units to produce a binuclear {Ln2O2} unit. Those imino nitroxide groups are free for complexes 1–3, however one of imino nitroxide groups of the biradical is ligated to the copper(II) ion for complexes 4 and 5. The distinct magnetic relaxation behaviors are observed for two Dy derivatives, as revealed by ac magnetic studies: complex 3 presents one magnetic process with the effective energy barrier(Ueff) of 74.0 K while complex 5 exhibits dual relaxation processes with Ueff values for the fast‐ and slow‐relaxation being 20.2 K and 30.9 K, respectively, which implies that the second coordination sphere of Dy ion plays a critical role for magnetic relaxation. [ABSTRACT FROM AUTHOR]

The nitroxide TPA (2,2,5,5‐tetramethyl‐pyrrolin‐1‐oxyl‐3‐acetylene) is an excellent spin label for EPR studies of RNA. Previous synthetic methods, however, are complicated and require special equipment. Herein, we describe a uridine derived phosphoramidite with a photocaged TPA unit attached. The light sensitive 2‐nitrobenzyloxymethyl group can be removed in high yield by short irradiation at 365 nm. Based on this approach, a doubly spin‐labeled 27mer neomycin sensing riboswitch was synthesized and studied by PELDOR. The overall thermal stability of the fold is not much reduced by TPA. In‐line probing nevertheless detected changes in local mobility. [ABSTRACT FROM AUTHOR]

This article, titled "Selective Transition Enhancement in a g-Engineered Diradical," discusses the synthesis of an asymmetric diradical by grafting a nitroxide radical onto the [Y(Pc)2]⋅ radical system. The molecule exhibits a weak exchange interaction between its two radical units, resulting in electron paramagnetic resonance transitions of different tipping angles. These transitions can be selectively enhanced by choosing the appropriate magnetic field and microwave power. The research article provides more detailed information on this topic. [Extracted from the article]

The single‐molecular conductance between two π‐conjugated wires with and without a radical substituent has been compared. Specifically, methyl‐ and iminonitroxide‐substituted 4‐(biphenyl‐4‐yl)pyridine wires bound onto a porphyrin template were subjected to scanning tunneling microscopy (STM) apparent‐height measurement at the interface between highly oriented pyrolytic graphite (HOPG) and octan‐1‐oic acid. Statistical analysis of the STM images revealed that the radical‐substituted wire has 3.2±1.7‐fold higher conductance than the methyl‐substituted reference. Although density functional theory (DFT) calculation suggests that only 17 % of the SOMO is distributed on the wire moiety, the effect was significant. This study presents the potential of radical substituents to achieve high conductivity in molecular wires. [ABSTRACT FROM AUTHOR]

The assessment of unregulated level of enzyme activity is a crucial parameter for early diagnoses in a wide range of pathologies. In this study, we propose the use of electron paramagnetic resonance (EPR) as an easy method to probe carboxylesterase (CE) enzymatic activity in vitro. For this application, were synthesized two amphiphilic, nitroxide containing esters, namely Tempo‐C12 (T‐C12) and Tempo‐2‐C12 (T‐2‐C12). They exhibit low solubility in water and form stable micelles in which the radicals are EPR almost silent, but the hydrolysis of the ester bond yields narrows and intense EPR signals. The intensity of the EPR signals is proportional to the enzymatic activity. CEs1, CEs2 and esterase from porcine liver (PLE) were investigated. The obtained results show that T‐C12 and T‐2‐C12‐containing systems display a much higher selectivity toward the CEs2, with a Limit of Detection of the same order of those ones obtained with optical methods. [ABSTRACT FROM AUTHOR]

Ligand reorganization has been shown to have a profound effect on the outcome of cerium redox chemistry. Through the use of a tethered, tripodal, trianionic nitroxide ligand, [((2-tBuNOH)C6 H4 CH2 )3 N](3-) (TriNOx (3-) ), controlled redox chemistry at cerium was accomplished, and typically reactive complexes of tetravalent cerium were isolated. These included rare cationic complexes [Ce(TriNOx )thf][BAr(F) 4 ], in which Ar(F) =3,5-(CF3 )2 -C6 H3 , and [Ce(TriNOx )py][OTf]. A rare complete Ce-halide series, Ce(TriNOx )X, in which X=F(-) , Cl(-) , Br(-) , I(-) , was also synthesized. The solution chemistry of these complexes was explored through detailed solution-phase electrochemistry and (1) H NMR experiments and showed a unique shift in the ratio of species with inner- and outer-sphere anions with size of the anionic X(-) group. DFT calculations on the series of calculations corroborated the experimental findings.

Multinuclear AuI complexes with two or three nitronyl nitroxide‐2‐ide radical anion and phosphine‐ligand scaffolds, (NN‐Au)2‐1 o, (NN‐Au)2‐1 m, and (NN‐Au)2‐1 p, have been synthesized to investigate the influence of AuI−AuI (aurophilic) interactions on the properties of multispin molecular systems. The desired complexes were successfully prepared in moderate yields in a one‐pot synthesis from the corresponding phosphine ligand, AuI source, parent NN, and sodium hydroxide. Among the prepared complexes, (NN‐Au)2‐1 o, in which an aurophilic interaction was clearly observed by crystal structure analysis, showed characteristic spin−spin interactions, electrochemical properties, and solvatochromic behavior. The results from theoretical calculations also suggested that the differences in properties between complex (NN‐Au)2‐1 o and the other complexes are due to intramolecular aurophilic interactions. [ABSTRACT FROM AUTHOR]

1,3‐Bis(diphenylene)‐2‐phenylallyl (BDPA)‐based radicals are of interest as polarizing agents for dynamic nuclear polarization (DNP). For this purpose, a BDPA‐nitroxide biradical, employing a phosphodiester linkage, was synthesized. Contrary to what is commonly assumed, BDPA‐derived radicals were observed to have limited stability. Hence, the effects of various factors on the stability of BDPA radicals were investigated. Solvent polarity was found to play a significant role on degradation; a polar BDPA radical was observed to degrade faster in a non‐polar solvent, whereas non‐polar radicals were more unstable in polar solvents. The rate of decomposition was found to increase non‐linearly with increasing radical concentration; a 2‐fold increase in concentration led to a 3‐fold increase in the rate of degradation. Collectively, these results indicate that the dimerization is a significant degradation pathway for BDPA radicals and indeed, a dimer of one BDPA radical was detected by mass spectrometry. [ABSTRACT FROM AUTHOR]

The α‐methylcyclopropyl (MCP) group with conformationally dependent electronic properties is suggested as an additional structural "instrument" for stabilization of both open‐shell and ionic states of diarylnitroxides, to be used as "ambipolar" redox active materials. New MCP‐substituted diphenylnitroxides (fully characterized by electrochemical, spectral, and X‐ray data) are the most stable nitroxides of the diaryl type known to date [τ1/2 in benzene exceeds three months (2310 h)]. The radicals are capable to reversible oxidation and reduction, yielding stable oxoammonium cations and aminoxyl anions. DFT investigation of the electronic structure and geometry of the compounds confirmed the conformational switching of the cyclopropyl orientation relative to the adjacent aromatic π system is dependent on the nitroxide's redox state. Additional through‐space stabilizing interaction between the π‐acceptor orbital of the NO+ moiety and the cyclopropyl "banana" bond orbital was also detected, highlighting its good hyperconjugative ability. The estimated σ(para)MCP value (−0.32) confirms its strong electron‐donating properties. [ABSTRACT FROM AUTHOR]

Three 1 N‐phenyl nitronyl nitroxide (NN) 4‐substituted dithieno[3,2‐b:2′,3′‐d]pyrrole (DTP) derivatives with R1=4‐phenyl‐, 4H‐, and 4‐methylthiothiophenyl‐ (R12DTP‐Ph‐NN, R1=H, Ph and MeSTh) were designed, synthesized and characterized. The electrochemical properties were studied by cyclic voltammetry (CV). All the molecules exhibited two main oxidation peaks, first for radical cation and next for dication formation. The cation and dication formation were also confirmed by UV/Vis absorption spectroscopy for Ph2DTP‐Ph‐NN and MeSTh2DTP‐Ph‐NN titrated with tris(4‐bromophenyl)aminiumhexachloroantimonate (magic blue). In addition, the cation and dication formation were verified by EPR spectroscopy. Finally, the exchange interactions (J/kB) of NN and radical cation were calculated by DFT studies. [ABSTRACT FROM AUTHOR]

A triplet ground‐state diradical molecule, bis(nitronyl nitroxide)‐substituted diphenyldihydrophenazine (1..), that can be converted into a one‐electron oxidized species, 1...+, in the quartet ground state has been developed. Surprisingly, these species, 1.. and 1...+, can be used under ambient conditions because they are reasonably stable under aerobic conditions, even in solution. The temperature‐dependent magnetic susceptibilities reveal that 1.. and 1...+ are in the triplet state, with a weak exchange interaction (J1/kB = +3.1 K) and quartet ground state with a strong exchange interaction (J2/kB = +160 K), respectively. The interconversion between the neutral and one‐electron oxidized species can be realized through electrochemical reactions. Significantly different absorption bands in the near‐IR region newly appeared in the electronic spectra acquired during electrochemical oxidation/reduction. [ABSTRACT FROM AUTHOR]

1‐Aminocyclopropane‐1‐carboxylic oxidase (ACCO) is a non‐heme iron(II)‐containing enzyme involved in the biosynthesis of the phytohormone ethylene, which regulates fruit ripening and flowering in plants. The active conformation of ACCO, and in particular that of the C‐terminal part, remains unclear and open and closed conformations have been proposed. In this work, a combined experimental and computational study to understand the conformation and dynamics of the C‐terminal part is reported. Site‐directed spin‐labeling coupled to electron paramagnetic resonance (SDSL‐EPR) spectroscopy was used. Mutagenesis experiments were performed to generate active enzymes bearing two paramagnetic labels (nitroxide radicals) anchored on cysteine residues, one in the main core and one in the C‐terminal part. Inter‐spin distance distributions were measured by pulsed EPR spectroscopy and compared with the results of molecular dynamics simulations. The results reveal the existence of a flexibility of the C‐terminal part. This flexibility generates several conformations of the C‐terminal part of ACCO that correspond neither to the existing crystal structures nor to the modelled structures. This highly dynamic region of ACCO raises questions on its exact function during enzymatic activity. [ABSTRACT FROM AUTHOR]

The spin–spin and magnetic properties of two (nitronyl nitroxide)‐(di‐p‐anisylamine‐phenothiazine) diradical cation salts, (DAA‐PTZ)+‐NN⋅MBr4− (M=Ga, Fe), have been investigated. These diradical‐cation species were prepared by the cross‐coupling of iodophenothiazine DAA‐PTZ‐I with NN‐AuPPh3 followed by oxidation with the thianthrenium radical cation (TA+⋅MBr4−). These salts were found to be highly stable under aerobic conditions. For the GaBr4 salt, large ferromagnetic intramolecular and small antiferromagnetic intermolecular interactions (J1/kB=+320 K and J2/kB=−2 K, respectively) were observed. The magnetic property of the Fe3+ salt was analyzed by using a six‐spin model assuming identical intramolecular exchange interaction (J3/kB=+320 K) and the other exchange interactions (J4/kB=−7 K and J5/kB=−4 K). A significant color change was observed in the UV/Vis/NIR absorption spectra upon electrochemical oxidation of the doublet DAA‐PTZ‐NN to the triplet (DAA‐PTZ)+‐NN. [ABSTRACT FROM AUTHOR]

Iron(III) porphyrins have the propensity to form μ2‐oxo‐dimers, the structures of which resemble two wheels on an axle. Whereas their crystal structure is known, their solution structure and internal dynamics is not. In the present work, the structure and dynamics of such dimers were studied by means of electron paramagnetic resonance (EPR) spectroscopy and quantum chemistry based molecular dynamics (MD) simulations by using the semiempirical tight‐binding method (GFN‐xTB). To enable EPR investigation of the dimers, a nitroxide was attached to each of the tetraphenylporphyrin cores through a linear and a bent linker. The inter‐nitroxide distance distributions within the dimers were determined by continuous‐wave (cw)‐EPR and pulsed electron–electron double resonance (PELDOR or DEER) experiments and, with the help of MD, interpreted in terms of the rotation of the porphyrin planes with respect to each other around the Fe–O–Fe axis. It was found that such rotation is restricted to the four registers defined by the phenyl substituents. Within the registers, the rotation angle swings between 30° and 60° in the proximal and between 125° and 145° in the distal register. With EPR, all four angles were found to be equally populated, whereas the 30° and 145° angles are strongly favored to the expense of the 60° and 125° angles in the MD simulation. In either case, the internal dynamics of these dimers thus resemble the motion of a step motor. Like wheels on an axle: Two iron(III) tetraphenylporphyrin–spacer–nitroxide complexes were synthesized and their monomers and μ2‐oxo‐bridged dimers characterized. Continuous‐wave and pulsed EPR spectroscopy in combination with molecular dynamics calculations show that the porphyrin moieties in the dimers rotate like a step motor around the Fe−O−Fe axis (see figure). [ABSTRACT FROM AUTHOR]

β‐Peptides are an interesting new class of transmembrane model peptides based on their conformationally stable and well‐defined secondary structures. Herein, we present the synthesis of the paramagnetic β‐amino acid β3‐hTOPP (4‐(3,3,5,5‐tetramethyl‐2,6‐dioxo‐4‐oxylpiperazin‐1‐yl)‐d‐β3‐homophenylglycine) that enables investigations of β‐peptides by EPR spectroscopy. This amino acid adds to the, to date, sparse number of β‐peptide spin labels. Its performance was evaluated by investigating the helical turn of a 314‐helical transmembrane model β‐peptide. Nanometer distances between two incorporated β3‐hTOPP labels in different environments were measured by using pulsed electron/electron double resonance (PELDOR/DEER) spectroscopy. Due to the semi‐rigid conformational design, the label delivers reliable distances and sharp (one‐peak) distance distributions even in the lipid bilayer. The results indicate that the investigated β‐peptide folds into a 3.2514 helix and maintains this conformation in the lipid bilayer. Nitroxide radicals: New semi‐rigid spin label β3‐hTOPP has allowed the investigation of a β‐peptide structure by EPR spectroscopy. Well‐defined inter‐spin distances between two β3‐hTOPP labels in a model β‐peptide have been measured in lipid bilayers using pulsed electron/electron double resonance (PELDOR) spectroscopy. The labelling strategy has permitted to determine the peptide helical structure with resolution at molecular‐length scale (see figure). [ABSTRACT FROM AUTHOR]

A substrate‐independent and versatile coating platform for (spatially resolved) surface functionalization, based on nitroxide radical coupling (NRC) reactions and the formation of thermo‐labile alkoxyamine functional groups, was introduced. Nitroxide‐decorated poly(glycidyl methacrylate) (PGMA) microspheres, obtained through bioinspired copolymer surface deposition using dopamine and a nitroxide functional dopamine derivative as monomers, were conjugated with small functional groups in a rewritable process. Reversible coding of the nitroxide functional microspheres by NRC and decoding through thermal alkoxyamine fission were monitored and characterized by electron paramagnetic resonance (EPR) spectroscopy and X‐ray photoelectron spectroscopy (XPS). In addition, this nitroxide coating system was exploited in "grafting‐to" polymer surface ligations of poly(methyl methacrylate) (PMMA) and poly(2,2,2‐trifluoroethyl methacrylate) (PTFEMA) in spatially confined areas. Polymer strands terminated with an Irgacure 2959 (2‐hydroxy‐4′‐(2‐hydroxyethoxy)‐2‐methylpropiophenone) photoinitiator were obtained through chain‐transfer polymerization, and subsequently coupled to nitroxide‐immobilized poly(dopamine) (PDA)‐coated silicon substrates by using rapid photoclick NRC reactions. Light‐driven polymer surface coding was visualized by time‐of‐flight secondary ion mass spectrometry (ToF‐SIMS) and XPS imaging. Coating with nitroxide: A versatile, bioinspired nitroxide functional coating platform is presented for precise programming of various materials. Nitroxide‐coated polymeric microspheres with rewritable properties were manufactured, exemplified in consecutive nitroxide radical coupling (coding) and thermal alkoxyamine fission (decoding) reaction steps. Photoinduced dual‐polymer‐patterned surfaces were further generated highlighting the broad applicability of our nitroxide coating system. [ABSTRACT FROM AUTHOR]

The electronic structure and magnetic interactions of three members of the breathing crystal Cu(hfac)2LR family (hfac=hexafluoroacetylacetonato, LR=pyrazole‐substituted nitronyl nitroxides with R=Me, Et, Pr, iPr, Bu), mainly Cu(hfac)2LPr (1), Cu(hfac)2LBu⋅0.5 C8H18 (2) and Cu(hfac)2LBu⋅0.5 C8H10 (3), have been analyzed by means of periodic plane‐wave based DFT+U calculations. These CuII‐nitroxide based molecular magnets display thermally and optically induced switchable behavior and light‐induced excited spin state trapping phenomena. The calculations confirm the presence of temperature‐dependent exchange interaction within the spin triads formed by the nitroxide‐copper(II)‐nitroxide units, in line with the changes observed in the effective magnetic moment. Moreover, they quantify the interchain interaction mediated by the terminal nitroxide group of two spin triads in neighboring polymer chains. This interaction competes with the exchange interaction within the spin triads at high temperature, and introduces 1D exchange channels that do not coincide with the polymeric chains. The density of states reveal that the low‐lying conduction states potentially involved in the UV/Vis transitions are located on the nitroxide radicals, the hfac groups and the Cu atoms. Then, the density of states is almost independent of the solvent and the R group. This suggests the possibility of light‐induced spin switching for other members of this family. The 500 nm band of the low‐temperature phase can be ascribed to a ligand‐to‐metal charge transfer transition between the nitroxide and Cu bands. Spinning around: The density of states (DOS) of Cu(hfac)2LPr gives insights on the states potentially accessible by electronic transitions in the UV/Vis range that could be involved in the photoswitching process, mainly, LMCT processes between the nitronyl‐nitroxide and the Cu 3d bands. The similarities found in the DOS for the three considered compounds suggest the possibility of light‐induced spin switching for other members of the breathing crystal family. [ABSTRACT FROM AUTHOR]

This Correspondence reports on a structural and mechanistic misinterpretation of an observed EPR spectrum, a misleading presentation of DNA cleavage data, and the physicochemically unsupported assignment of nitric oxide as a major DNA cleaving agent in the article cited in the title. [ABSTRACT FROM AUTHOR]

A unique superparamagnetic‐like behavior and a large "positive magneto‐LC effect" were observed in the solid phases and the hexagonal columnar (Colh) liquid crystalline (LC) phase, respectively, of novel achiral non‐π‐delocalized nitroxide diradical compounds (R,S)‐1, which showed polymorphism in the solid phases (solids I and II). The SQUID magnetization measurement revealed that (1) (R,S)‐1 containing a small amount of racemic diastereomers (R*,R*)‐1 possessed an unusual and large temperature‐independent magnetic susceptibility (χTIM>0) component in the original nanocrystalline solid I that was responsible for the observed superparamagnetic‐like behavior under low magnetic fields and did not arise from the contamination by extrinsic magnetic metal or metal ion impurities, besides ordinary temperature‐dependent paramagnetic susceptibility (χpara>0) and temperature‐independent diamagnetic susceptibility (χdia<0) components, (2) a large increase in molar magnetic susceptibility (χM) (positive magneto‐LC effect) that occurred at the solid I‐to‐liquid crystal transition upon heating was preserved as an additional χTIM increase in the resulting polymorphic nanocrystalline solid II by cooling, and (3) such unique magnetic phenomena were induced by thermal processing for (R,S)‐1 or by adding a small amount of (R*,R*)‐1 to (R,S)‐1 as the impurity. Magnetic and mesomorphic: Novel achiral non‐π‐delocalized nitroxide diradical compounds that showed a unique superparamagnetic‐like behavior in the nanocrystalline and columnar hexagonal liquid crystalline phases were prepared and characterized. [ABSTRACT FROM AUTHOR]

Abstract: Nitroxide biradicals are very efficient polarizing agents in magic angle spinning (MAS) cross effect (CE) dynamic nuclear polarization (DNP) nuclear magnetic resonance (NMR). Many recently synthesized, new radicals show superior DNP‐efficiency in organic solvents but suffer from insufficient solubility in water or glycerol/water for biological applications. We report DNP efficiencies for two new radicals, the water‐soluble bcTol‐M and cyolyl‐TOTAPOL, and include a comparison with three known biradicals, TOTAPOL, bcTol, and AMUPol. They differ by linker groups, featuring either a 3‐aminopropane‐1,2‐diol or a urea tether, or by the structure of the alkyl substituents that flank the nitroxide groups. For evaluating their performances, we measured both signal enhancements ϵ and DNP‐enhanced sensitivity κ, and compared the results to electron spin relaxation data recorded at the same magnetic field strength (9.4 T). In our study, differences in DNP efficiency correlate with changes in the nuclear polarization dynamics rather than electron relaxation. The ratios of their individual ϵ and κ differ by up to 20 %, which is explained by starkly different nuclear polarization build‐up rates. For the radicals compared here empirically, using proline standard solutions, the new radical bcTol‐M performs best while being most soluble in water/glycerol mixtures. [ABSTRACT FROM AUTHOR]

Abstract: Nitronyl nitroxide (NN)‐substituted conjugated oligomers, which were expected to self‐associate in biradical cation states, were designed to analyze the capability of π‐dimers as molecular wires. The oligomer moieties were composed of dithienyl‐N‐methylpyrrole with methoxy substituents at the inner β‐position of thiophene rings (DTP‐NN.) and its propylenedioxythiophene (ProDOT) inserted derivative (DTP‐P‐NN.), or two ethylenedioxythiophene (EDOT) and two ProDOT units (E2P2‐NN.). Among them, chemical one‐electron oxidation gave biradical cations (DTP‐P).+‐NN. and (E2P2).+‐NN. that formed π‐dimers (DTP‐P‐NN.)22+ and (E2P2‐NN.)22+ in dichloromethane at low temperatures. ESR studies of (DTP‐P‐NN.)22+ and (E2P2‐NN.)22+ showed the presence of a relatively strong exchange interaction between two NN radicals through the radical cation π‐dimer moieties. DFT calculations supported these experimental results and predicted that exchange interactions between two NN radicals were comparable or stronger than those through covalently linked quaterthiophene. Thus, the conjugated oligomer radical cation π‐dimers acted as efficient molecular wires for electronic communication. [ABSTRACT FROM AUTHOR]

Abstract: A nitroxide carrying a peptide specific to the binding pocket of the serine proteases chymotrypsin and cathepsin G is prepared. This peptide is attached as an enol ester to the nitroxide. Upon enzymatic hydrolysis of the peptide, the enol ester moiety is transformed into a ketone moiety. This transformation affords a difference of 5 G in phosphorus hyperfine coupling constant between the electronic paramagnetic resonance (EPR) signals of each nitroxide. This property is used to monitor the enzymatic activity of chymotrypsin and cathepsin G by EPR. Michaelis constants were determined and match those reported for conventional optical probes. [ABSTRACT FROM AUTHOR]

Abstract: Pulsed electron–electron double resonance spectroscopy (known as PELDOR or DEER) has recently become a very popular tool in structural biology. The technique can be used to accurately measure distance distributions within macromolecules or macromolecular complexes, and has become a standard method to validate structural models and to study the conformational flexibility of macromolecules. It can be applied in solution, in lipid environments or even in cells. Because most biological macromolecules are diamagnetic, they are normally invisible for PELDOR spectroscopy. To render a particular target molecule accessible for PELDOR, it can be engineered to contain only one or two surface‐exposed cysteine residues, which can be efficiently spin‐labelled using thiol‐reactive nitroxide compounds. This method has been coined “site‐directed spin labelling” (SDSL) and is normally straight‐forward. But, SDSL can be very challenging for proteins with many native cysteines, or even a single functionally or structurally important cysteine residue. For such cases, alternative spin labelling techniques are needed. Here we describe the concept of “inhibitor‐directed spin labelling” (IDSL) as an approach to spin label suitable cysteine‐rich proteins in a site‐directed and highly specific manner by employing bespoke spin‐labelled inhibitors. Advantages and disadvantages of IDSL are discussed. [ABSTRACT FROM AUTHOR]

Abstract: EPR studies on RNA are complicated by three major obstacles related to the chemical nature of nitroxide spin labels: Decomposition while oligonucleotides are chemically synthesized, further decay during enzymatic strand ligation, and undetected changes in conformational equilibria due to the steric demand of the label. Herein possible solutions for all three problems are presented: A 2‐nitrobenzyloxymethyl protective group for nitroxides that is stable under all conditions of chemical RNA synthesis and can be removed photochemically. By careful selection of ligation sites and splint oligonucleotides, high yields were achieved in the assembly of a full‐length HIV‐1 TAR RNA labeled with two protected nitroxide groups. PELDOR measurements on spin‐labeled TAR in the absence and presence of arginine amide indicated arrest of interhelical motions on ligand binding. Finally, even minor changes in conformation due to the presence of spin labels are detected with high sensitivity by in‐line probing. [ABSTRACT FROM AUTHOR]

Abstract: The synthesis of the new nitroxide crown ether 8 and its use as the wheel in a bistable [2]rotaxane, containing dialkylammonium and 4,4′‐bipyridinium recognition sites, is reported. The synthesis of 8 was achieved by the sequential addition of substituted phenyl groups to a nitrone derivatives leading to the preferential formation of the cis stereoisomer. Due to charge–dipole interactions between the nitroxide unit and the bipyridinium moiety, it was possible to probe the movement of the macrocycle between the two molecular stations of the [2]rotaxane after addition of a base by measuring the nitrogen hyperfine splitting in the corresponding EPR spectra. The equilibrium constant for the complexation of dibenzyl viologen by the macrocycle 8 was also determined by EPR titration. [ABSTRACT FROM AUTHOR]

With a view to developing a theranostic nanomedicine for targeted drug delivery systems visible by magnetic resonance (MR) imaging, robust metal-free magnetic nanoemulsions (mean particle size less than 20 nm) consisting of a biocompatible surfactant and hydrophobic, low molecular weight 2,2,5-trimethyl-5-(4-alkoxy)phenylpyrrolidine- N-oxyl radicals were prepared in pH 7.4 phosphate-buffered saline (PBS). The structure of the nanoemulsions was characterized by electron paramagnetic resonance spectroscopy, and dynamic light scattering and small-angle neutron-scattering measurements. The nanoemulsions showed high colloidal stability, low cytotoxicity, enough reduction resistance to excess ascorbic acid, and sufficient contrast enhancement in the proton longitudinal relaxation time ( T1) weighted MR images in PBS in vitro (and preliminarily in vivo). Furthermore, the hydrophobic anticancer drug paclitaxel could be encapsulated inside the nanoparticles, and the resulting paclitaxel-loaded nanoemulsions were efficiently incorporated into HeLa cells to suppress cell growth. [ABSTRACT FROM AUTHOR]

Precisely incorporating a wide range of structural and functional multiblocks along a polymer backbone is a significant challenge in polymer chemistry and offers promising opportunities to design highly ordered materials, including controlled polymer folding. Herein, a facile and versatile strategy for preparing functional multiblock copolymers composed of sequential peptides and well-defined vinyl polymers with a narrow polydispersity is reported. Cyclic oligopeptides have been developed that contain an alkoxyamine bond in the framework. By using this type of cyclic initiator, peptide-containing multiblock copolymers are successfully synthesized by nitroxide-mediated polymerization of styrene. To demonstrate the versatility of this method, radical (co)polymerizations were carried out for different monomers ( p-chlorostyrene, 4-vinylpyridine, and styrene/acrylonitrile) and by three different cyclic peptide initiators with specific amino acid sequences. The resultant multiblock copolymer is foldable through intramolecular interactions between peptide blocks. It is believed that this approach will significantly advance the field of controlled polymer synthesis for complex structures and single-chain folding. [ABSTRACT FROM AUTHOR]

Electron and nuclear spins are extremely sensitive probes of their local structural and dynamic surroundings. Their Zeeman energy levels are modified by different types of local magnetic and electric fields created by their structural environment, which influence their magnetic resonance condition. For this reason, electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopies have become extremely powerful tools of structural analysis, which are being widely used for the structural characterization of complex solids. Following a brief introduction into the basic theoretical foundations the most commonly used techniques and their application towards the structural characterization of paramagnetic solids based on aminoxyl radicals and their inorganic-organic nanocomposites will be described. Both ESR and NMR observables are useful for monitoring intermolecular interactions between unpaired electron spins, which are particularly important for the design of organically based ferromagnetic systems. ESR and NMR methods based on this effect can be used for monitoring the synthesis of polynitroxides and for evaluating the catalytic function of aminoxyl intercalation compounds. Finally, the sensitivity of ESR signals to motional dynamics can be exploited for characterizing molecule-surface interactions in nanocomposite systems. In the context of the latter work recently developed signal enhancement strategies are described, using polarization transfer from electron spins to nuclear spins for NMR spectroscopic detection. [ABSTRACT FROM AUTHOR]

Nitroxide-mediated polymerization (NMP) was applied to prepare alternating copolymers using 1,1,1,3,3,3-hexafluoroisopropyl acrylate (HFIPA) 1 and ((6-trimethylsilyl)-hex-5-yn-1-yl)vinyl ether (THVE) 2 as monomers. The alternating sequence of the resulting poly(HFIPA- alt-THVE) 3 was proved by using tandem mass spectrometry (MS/MS) and further supported by 1H NMR spectroscopy. Alternating alkyne and activated ester moieties of copolymer 3 were further functionalized with organic azides and amines using sequential Cu-catalyzed azide-alkyne click (CuAAC) and amidation reactions, providing dually functionalized poly(acrylamide- alt-triazole) polymers 5. Water-soluble alternating poly(acrylic acid- alt-triazole) copolymers 9 were obtained by saponification of HFIP esters. 1H and 19F NMR spectroscopy, IR spectroscopy as well as gel permeation chromatography (GPC) were used to characterize the polymers before and after functionalization. [ABSTRACT FROM AUTHOR]

α-Aminoxy peptides are peptidomimetic foldamers with high proteolytic and conformational stability. To gain an improved synthetic access to α-aminoxy oligopeptides we used a straightforward combination of solution- and solid-phase-supported methods and obtained oligomers that showed a remarkable anticancer activity against a panel of cancer cell lines. We solved the first X-ray crystal structure of an α-aminoxy peptide with multiple turns around the helical axis. The crystal structure revealed a right-handed 28-helical conformation with precisely two residues per turn and a helical pitch of 5.8 Å. By 2D ROESY experiments, molecular dynamics simulations, and CD spectroscopy we were able to identify the 28-helix as the predominant conformation in organic solvents. In aqueous solution, the α-aminoxy peptides exist in the 28-helical conformation at acidic pH, but exhibit remarkable changes in the secondary structure with increasing pH. The most cytotoxic α-aminoxy peptides have an increased propensity to take up a 28-helical conformation in the presence of a model membrane. This indicates a correlation between the 28-helical conformation and the membranolytic activity observed in mode of action studies, thereby providing novel insights in the folding properties and the biological activity of α-aminoxy peptides. [ABSTRACT FROM AUTHOR]

Hyperpolarization is generated by dissolution dynamic nuclear polarization (d-DNP) using a polymer-based polarizing agent dubbed FLAP (filterable labeled agents for polarization). It consists of a thermo-responsive poly( N-isopropylacrylamide), also known as pNiPAM-COOH, labeled with nitroxide radicals. The polymer powder is impregnated with an arbitrary solution of interest and frozen as is. Dissolution is followed by a simple filtration, leading to hyperpolarized solutions free from any contaminants. We demonstrated the use of FLAP to hyperpolarize partially deuterated water up to P(1H)=6 % with a long relaxation T1 >36 s characteristic of high purity. Water hyperpolarization can be transferred to drugs, metabolites, or proteins that are waiting in an NMR spectrometer, either by exchange of labile protons or through intermolecular Overhauser effects. We also show that FLAPs are suitable polarizing agents for 13C-labeled metabolites such as pyruvate, acetate, and alanine. [ABSTRACT FROM AUTHOR]

Site-directed spin labeling of RNA based on click chemistry is used in combination with pulsed electron-electron double resonance (PELDOR) to benchmark a nitroxide spin label, called here dŲ. We compare this approach with another established method that employs the rigid spin label Çm for RNA labeling. By using CD spectroscopy, thermal denaturation measurements, CW-EPR as well as PELDOR we analyzed and compared the influence of dŲ and Çm on a self-complementary RNA duplex. Our results demonstrate that the conformational diversity of dŲ is significantly reduced near the freezing temperature of a phosphate buffer, resulting in strongly orientation-selective PELDOR time traces of the dŲ-labeled RNA duplex. [ABSTRACT FROM AUTHOR]

Persistent radicals undergo hydrogen atom abstraction reactions with a great variety of substrates, but not with dihydrogen. It has now been found that the TEMPO radical splits dihydrogen under mild conditions in the presence of the strong bulky B(C6F5)3 boron Lewis acid. The reaction is thought to proceed by a typical frustrated Lewis pair mechanism with the TEMPO radical acting as the active Lewis base. The reaction was analyzed by DFT, which indicates that no significant spin density on the hydrogen atoms is accumulated along the H2 splitting reaction path. [ABSTRACT FROM AUTHOR]

A series of 18 nitroxide biradicals derived from bTurea has been prepared, and their enhancement factors ɛ (1H) in cross-effect dynamic nuclear polarization (CE DNP) NMR experiments at 9.4 and 14.1 T and 100 K in a DNP-optimized glycerol/water matrix ('DNP juice') have been studied. We observe that ɛ (1H) is strongly correlated with the substituents on the polarizing agents, and its trend is discussed in terms of different molecular parameters: solubility, average e-e distance, relative orientation of the nitroxide moieties, and electron spin relaxation times. We show that too short an e-e distance or too long a T1e can dramatically limit ɛ (1H). Our study also shows that the molecular structure of AMUPol is not optimal and its ɛ (1H) could be further improved through stronger interaction with the glassy matrix and a better orientation of the TEMPO moieties. A new AMUPol derivative introduced here provides a better ɛ (1H) than AMUPol itself (by a factor of ca. 1.2). [ABSTRACT FROM AUTHOR]

We reported a novel strategy for investigating small molecule binding to G-quadruplexes (GQs). A newly synthesized dinuclear platinum(II) complex (Pt2L) containing a nitroxide radical was shown to selectively bind a GQ-forming sequence derived from human telomere (hTel). Using the nitroxide moiety as a spin label, electron paramagnetic resonance (EPR) spectroscopy was carried out to investigate binding between Pt2L and hTel GQ. Measurements indicated that two molecules of Pt2L bind with one molecule of hTel GQ. The inter-spin distance measured between the two bound Pt2L, together with molecular docking analyses, revealed that Pt2L predominately binds to the neighboring narrow and wide grooves of the G-tetrads as hTel adopts the antiparallel conformation. The design and synthesis of nitroxide tagged GQ binders, and the use of spin-labeling/EPR to investigate their interactions with GQs, will aid the development of small molecules for manipulating GQs involved in crucial biological processes. [ABSTRACT FROM AUTHOR]

Controlled radical polymerization (CRP) and dyesensitized solar cells (DSSCs) are two fields of research that at an initial glance appear to have little in common. However, despite their obvious differences, both in application and in scientific nature, a closer look reveals a striking similarity between many of the compounds widely used as control agents in radical polymerization and as redox couples in dye-sensitized solar cells. Herein, we review the various redox active compounds used and examine the characteristics that give them the ability to perform this dual function. In addition we explore the advances in the understanding of the structural features that enhance their activity in both CRP and DSSCs. It is hoped that such a comparison will be conducive to improving process performance in both fields. [ABSTRACT FROM AUTHOR]

Novel profluorescent mono- and bis-isoindoline nitroxides linked to napthalimide and perylene diimide structural cores are described. These nitroxide-fluorophore probes display strongly suppressed fluorescence in comparison to their corresponding non-radical diamagnetic methoxyamine derivatives. The perylene-based probe possessing two isoindoline systems tethered through ethynyl linkages was shown to be the most photostable in solution, demonstrating significantly enhanced longevity over the 9,10-bis(phenylethynyl)anthracene fluorophore used in previous profluorescent nitroxide probes. [ABSTRACT FROM AUTHOR]

Nitroxide free radicals have been used to study the inner space of one of Rebek's water-soluble capsules. EPR and 1H NMR spectroscopy, ESI-MS, and DFT calculations showed a preference for the formation of 1:2 complexes. EPR titrations allowed us to determine binding constants ( Ka) in the order of 107 M−2. EPR spectral-shape analysis provided information on the guest rotational dynamics within the capsule. The interplay between optimum hydrogen bonding upon capsule formation and steric strain for guest accommodation highlights some degree of flexibility for guest inclusion, particularly at the center of the capsule where the hydrogen bond seam can be barely distorted or slightly disturbed. [ABSTRACT FROM AUTHOR]