Pulsed EPR Dipolar Spectroscopy under the Breakdown of the High-Field Approximation: The High-Spin Iron(III) Case

Pulsed EPR dipolar spectroscopy (PDS) offers several methods for measuring dipolar coupling and thus the distance between electron-spin centers. To date, PDS measurements to metal centers were limited to ions that adhere to the high-field approximation. Here, the PDS methodology is extended to cases where the high-field approximation breaks down on the example of the high-spin Fe³⁺/nitroxide spin-pair. First, the theory developed by Maryasov et al. (Appl. Magn. Reson. 2006 , 30, 683–702) was adapted to derive equations for the dipolar coupling constant, which revealed that the dipolar spectrum does not only depend on the length and orientation of the interspin distance vector with respect to the applied magnetic field but also on its orientation to the effective g-tensor of the Fe³⁺ ion. Then, it is shown on a model system and a heme protein that a PDS method called relaxation-induced dipolar modulation enhancement (RIDME) is well-suited to measuring such spectra and that the experimentally obtained dipolar spectra are in full agreement with the derived equations. Finally, a RIDME data analysis procedure was developed, which facilitates the determination of distance and angular distributions from the RIDME data. Thus, this study enables the application of PDS to for example, the highly relevant class of high-spin Fe³⁺ heme proteins.     

Photocatalytic Removal of Soot: Unravelling of the Reaction Mechanism by EPR and in situ FTIR Spectroscopy

Photocatalytic soot oxidation is studied on P25 TiO₂ as an important model reaction for self‐cleaning processes by means of electron paramagnetic resonance (EPR) and Fourier transform infrared (FTIR) spectroscopy. Contacting of carbon black with P25 leads on the one hand to a reduction of the local dioxygen concentration in the powder. On the other hand, the weakly adsorbed radicals on the carbon particles are likely to act as alternative traps for the photogenerated conduction‐band electrons. We find furthermore that the presence of dioxygen and oxygen‐related radicals is vital for the photocatalytic soot degradation. The complete oxidation of soot to CO₂ is evidenced by in situ FTIR spectroscopy, no intermediate CO is detected during the photocatalytic process.     

High-spin chloro mononuclear MnIII complexes: a multifrequency high-field EPR study.

The isolation, structural characterization, and electronic properties of two six-coordinated chloromanganese (III) complexes, [Mn(terpy)(Cl)3] (1) and [Mn(Phterpy)(Cl)3] (2), are reported (terpy = 2,2':6'2"-terpyridine, Phterpy = 4'-phenyl-2,2':6',2"-terpyridine). These complexes complement a series of mononuclear azide and fluoride Mn(lll) complexes synthesized with neutral N-tridentate ligands, [Mn(L)(X)3] (X = F- or N3 and L = terpy or bpea [N,N-bis(2-pyridylmethyl)-ethylamine)], previously described. Similar to these previous complexes, 1 and 2 exhibit a Jahn-Teller distortion of the octahedron, characteristic of a high-spin Mn(III) complex (S = 2). The analysis of the crystallographic data shows that, in both cases, the manganese ion lies in the center of a distorted octahedron characterized by an elongation along the tetragonal axis. Their electronic properties were investigated by multifrequency EPR (190-475 GHz) performed in the solid state at different temperatures (5-15 K). This study confirms our previous results and further shows that: i) the sign of D is correlated with the nature of the tetragonal distortion; ii) the magnitude of D is not sensitive to the nature of the anions in our series of rhombic complexes, contrary to the porphyrinic systems; iii) the [E/D] values (0.124 for 1 and 0.085 for 2) are smaller compared to those found for the [Mn(L)(X)3] complexes (in the range of 0.146 to 0.234); and iv) the E term increases when the ligand-field strength of the equatorial ligands decreases.     

The reaction mechanism of p-toluenediamine anodic oxidation: an in situ ESR-UV/Vis/NIR spectroelectrochemical study.

In situ ESR-UV/Vis spectroelectrochemistry is applied to obtain new insights into the intermediates and reaction products of the anodic oxidation of p-toluenediamine in aqueous solution at different pH values. A strong pH dependence of the stability of the cation radical is found. While the absence of a stable radical was proved by ESR spectroscopy at pH 2 and 10, this radical is detected at medium pH values and assigned to the semiquinonediimine structure. The UV/Vis absorption of the radical is observed at these pH values as well. The p-toluenediimine intermediate and the trimeric reaction product were followed during the electrode reaction by UV/Vis spectroscopy at all pH values.     

Temperature dependence of X- and Q-band EPR spectra of the dinuclear manganese(II) complex [(NO2Bpmp)Mn2(mu-OAc)2]+: determination of the exchange constant and of the spin parameters for the S=1, 2, and 3 spin states

A new dinuclear manganese(II) complex was synthesised with the biscompartimental ligand 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-nitrophenol (NO(2)BpmpH) and characterised by X-ray crystallography. Magnetic susceptibility measurements revealed that the two high-spin Mn(II) ions are antiferromagnetically coupled with a singlet-to-triplet separation of 7.2 cm(-1). The powder EPR spectra were recorded for both X- and Q-bands between 1.8 K and 35 K. A detailed analysis of these spectra led to the determination of three out of five individual spin-state zero-field splitting parameters. From the proposed simulations, the exchange coupling constant J and the intermetallic distance have been computed.     

EPR study of nitroxides formed from the reaction of nitric oxide with photolyzed amides

Free radicals generated from UV irradiation of simple aliphatic amides in anaerobic and nitric oxide (NO)‐saturated liquid mixtures or solutions gave EPR spectra of nitroxides. The application of isotopic effects to EPR spectra and the generation of radicals by transient radical attack on substrate molecules or by photolysing amine or acetoin were used to help identify photochemically produced radicals from the amides. The aliphatic amides used were formamide, acetamide and their N‐methyl‐ or deuterium‐substituted derivatives. Transient radicals used to attack the amides via hydrogen‐atom abstraction were generated from the initiator AIBN or AAPH. The observation of various nitroxides indicates the reactivity of NO for trapping acyl, carbamoyl and other carbon‐centered radicals. Possibly mechanistic pathways diagnosed with this trap are proposed. Copyright © 2003 John Wiley & Sons, Ltd.     

Synthesis, structure, and characterisation of a new phenolato-bridged manganese complex [Mn2(mL)2]2+: chemical and electrochemical access to a new mono-mu-oxo dimanganese core unit

The dinuclear phenolato-bridged complex [(mL)Mn(II)Mn(II)(mL)](ClO(4))(2) (1(ClO(4))(2)) has been obtained with the new [N(4)O] pentadentate ligand mL(-) (mLH=N,N'-bis-(2-pyridylmethyl)-N-(2-hydroxybenzyl)-N'-methyl-ethane-1,2-diamine) and has been characterised by X-ray crystallography. X- and Q-band EPR spectra were recorded and their variation with temperature was examined. All spectra exhibit features extending over 0-800 mT at the X band and over 100-1450 mT at the Q band, features that are usually observed for dinuclear Mn(II) complexes. Cyclic voltammetry of 1 exhibits two irreversible oxidation waves at E(1)(p)=0.89 V and E(2)(p)=1.02 V, accompanied on the reverse scan by an ill-defined cathodic wave at E(1')(p)=0.56 V (all measured versus the saturated calomel electrode (SCE)). Upon chemical oxidation with tBuOOH (10 equiv) at 20 degrees C, 1 is transformed into the mono-mu-oxo species [(mL)Mn(III)-(mu-O)-Mn(III)(mL)](2+) (2), which eventually partially evolves into the di-mu-oxo species [(mL)Mn(III)-(mu-O)(2)-Mn(IV)(mL)](n+) (3) in which one of the aromatic rings of the ligand is decoordinated. The UV/Vis spectrum of 2 displays a large absorption band at 507 nm, which is attributed to a phenolate-->Mn(III) charge-transfer transition. The cyclovoltammogram of 2 exhibits two reversible oxidation waves, at 0.65 and 1.16 V versus the SCE, corresponding to the Mn(III)Mn(III)/Mn(III)Mn(IV) and Mn(III)Mn(IV)/Mn(IV)Mn(IV) oxidation processes, respectively. The one-electron electrochemical oxidation of 2 leads to the mono-mu-oxo mixed-valent species [(mL)Mn(III)-(mu-O)-Mn(IV)(mL)](3+) (2 ox). The UV/Vis spectrum of 2 ox exhibits one large band at 643 nm, which is attributed to the phenolate-->Mn(IV) charge-transfer transition. 2 ox can also be obtained by the direct electrochemical oxidation of 1 in the presence of an external base. The 2 ox and 3 species exhibit a 16-line EPR signal with first peak to last trough widths of 125 and 111 mT, respectively. Both spectra have been simulated by using colinear rhombic Mn-hyperfine tensors. Mechanisms for the chemical formation of 2 and the electrochemical oxidation of 1 into 2 ox are proposed.     

N,N-二甲基苯胺-对氨基二甲替苯胺氧化偶联反应催化分光光度法测定铬(Ⅵ)

在pH=3.2乙酸-乙酸钠缓冲介质中,研究了铬(Ⅵ)催化过氧化氢氧化对-氨基二甲替苯胺盐酸盐和N,N-二甲基苯胺的氧化偶联反应及其机理,建立了催化光度法测定痕量铬(Ⅵ)的新方法.该方法线性范围为0～0.12 mg/L;检出限为1.0×10-7 g/L;相对标准偏差为0.12%,直接用于水中铬(Ⅵ)的测定,结果满意.     

Comparative EPR study of copper(II) complexes with threonine derivatives

Summary In order to get better insight into the structural reasons for different properties of copper(II) complexes withL-threonine,L-allo-threonine,L-N,N-dimethyl-threonine, andL-N,N-dimethyl-allo-threonine, their EPR spectra were studied as a function ofpH and temperature. AtpD9.4, in all complexes a change in the copper(II) coordination sphere from the glycine to the hydroxy type was observed. Inbis(L-threoninato)copper(II), the hydroxy type formed atpD9.4 was found to be stablized by increasing the temperature of the solution from 280 to 320 K. In all other copper(II) complexes, the conformational change is accompanied by the disruption of the Cu-N bond of one chelate ring.

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Vergleichende EPR-Untersuchungen von Kupfer(II)-Threonin-Komplexen

Zuzammenfassung Um einen besseren Einblick in die Beziehungen zwischen Struktur und Eigenschaften von Kupfer(II)-Komplexen mitL-Threonin,L-allo-Threonin,L-N,N-Dimethyl-Threonin undL-N,N-Dimethyl-allo-Threonin zu gewinnen, wurden ihre EPR-Spektren in Abhängigkeit vompH-Wert und von der Temperatur untersucht. Bei einempH-Wert von 9.4 (in deuterierter Lösung) wurde eine Veränderung in der Kupfer (II)-Koordinationssphäre festgestellt, die von einer Glycin-ähnlichen Konformation in eine vermutlich Hydroxy-ähnliche Konformation übergeht. Derbis(L-Threoninato)-Kupfer (II)-Hydroxy-Komplex wird durch eine Temperaturerhöhung von 280 auf 320K stabilisiert. Die Veränderung der Koordination vom Glycin-Typ wird von einem Bruch der Cu-N-Bindung eines Chelatrings begleitet.

     

XANES/EPR Evidence of the Oxidation of Nickel(II) Quinolinylpropioamide and Its Application in Csp3−H Functionalization

An in situ generated oxidation species of nickel quinolinylpropioamide intermediate was produced. Characterization by X-ray absorption near edge structure (XANES) and EPR provides complementary insights into this oxidized nickel species. With aliphatic amides and isocyanides as substrates, a nickel-catalyzed facile synthesis of structurally diverse five-membered lactams could be achieved.     

O,N‐Chelated Vanadium(IV) Oxo Aminophenolate Complexes: the Effect of Steric Bulk on the Vanadium Coordination Geometry. Can this Influence be detected Spectroscopically?

In order to study the effect of steric bulk on the vanadium coordination geometry in O, N‐chelated vanadium oxo (bis)phenolates, six different ortho‐aminophenolate ligands have been used. The ortho‐aminophenolate system was changed at three different places, i.e. 1) the second ortho position (C⁶) of the arene ring (R), 2) the substituents at the amino nitrogen (R′ and R″), and 3) the benzylic carbon atom (R*). The phenols were used in the preparation of the vanadium oxo (bis)phenolate complexes. In order to study whether it is possible to predict geometrical features of these vanadium complexes, UV/Vis, solution and frozen state EPR and ¹⁴N ESEEM spectroscopic data was measured and compared to the structural features of four structurally characterized vanadium oxo (bis)phenolates. Unfortunately, it turned out that is was not possible to correlate the EPR parameters, the UV/Vis HOMO‐LUMO transitions or ¹⁴N hyperfine couplings to the structural parameters.     

Oxido-pincer complexes of copper(II) - An EXAFS and EPR study of mono- and binuclear [(pydotH2)CuCl2]n (n = 1 or 2)

The oxido-pincer ligand pydotH₂ (2,6-bis(1-hydroxy-1-o-tolyl-ethyl-η²O,O′)pyridine) forms two different Cu^II containing complexes when prepared from anhydrous CuCl₂. A combination of EPR spectroscopy and EXAFS allowed to structurally characterise the light-green dimer of the formula [(pydotH₂)CuCl(μ-Cl)₂ClCu(pydotH₂)] and the penta-coordinate olive-green monomer [(pydotH₂)CuCl₂]. The molecular entities imply that the ligand remains protonated upon coordination. When dissolved in DMF both compounds form monomeric species [(pydotH₂)CuCl₂(DMF)] which could be characterised in detail by EPR, UV-Vis/NIR spectroscopy and electrochemical measurements. The assignments were supported by comparison with Cu^II complexes of the related ligands 2,6-bis(hydroxymethyl)pyridine (pydimH₂) and 2,6-bis(1-hydroxy-1-methyl)pyridine (pydipH₂).     

Electrocatalytic Oxidation and Determination of Cysteamine by Poly‐N,N‐dimethylaniline/Ferrocyanide Film Modified Carbon Paste Electrode

Functionalized poly‐N,N‐dimethylaniline film was prepared by adsorption of ferrocyanide onto the polymer forming at the surface of carbon paste electrode (CPE) in aqueous solution by using potentiostatic method. The electrocatalytic ability of poly‐N,N‐dimethylaniline/ferrocyanide film modified carbon paste electrode (PDMA/FMCPE) was demonstrated by oxidation of cysteamine. Cyclic voltammetry and chronoamperometry techniques were used to investigate this ability. Results showed that pH 7.00 is the most suitable for this purpose. It is found that the catalytic reaction rate constant, (k_h), is equal to 2.142×10³ M^?1 s^?1 by the data of chronoamperometry. The catalytic reduction peak current was linearly dependent on the cysteamine concentration and the linearity range obtained was 8.00×10^?5 M–1.14×10^?2 M. Detection limit was determined 7.97×10^?5 M (2σ). This method has been successfully employed for quantification of cysteamine in real sample.     

Magnetic properties of poly(propylene imine)-copper dendromesogenic complexes: An EPR study.

Copper(II) complexes formed by coordination of the Cu(II) ion with liquid-crystalline poly(propylene imine) dendrimer ligand (L) of the first (complex 1) and second (complex 2) generations with various Cu(II) contents (x = Cu/L) have been studied by electron paramagnetic resonance (EPR) spectroscopy. The existence of a redox-active blue complex 1 (x = 1.9) and the copper(II) nitrate electron transfer associated with the valence tautomerism are revealed for the first time in copper-based dendrimers. It has been shown that the electronic structure of the blue complex 1 (x = 1.9) is adequately described as a mixed-valence dimer containing d9- and diamagnetic d10-configurated copper ions, and an antiferromagnetically coupled NO3* radical arising on the nitrate-bridged counter ligand. The activation energy value found for the electron transfer is about 0.35 meV, which indicates a low-energy charge dynamic. The ability of the blue and green complexes 1 (x = 1.9) dissolved in isotropic solvents to orient themselves in the magnetic field was revealed by EPR spectroscopy. The degree of orientation of the molecular z axis (S(z)) of these complexes in the magnetic field differs, depending on the type of copper(II)-complexing site in the dendrimer ligand, and can reach 0.76, which is close to S(z) = 1 (completely aligned system). A combination of magnetic and orientational parameters indicates an NO4 environment of the Cu(II) ion in green complex 1 (x = 1.9), and confirms the chain structure with intermolecular Cu(II)-NO3-Cu(II) bridges between Cu(II) centres in columns.     

Probing the Structural Dynamics of a Bacterial Chaperone in Its Native Environment by Nitroxide-Based EPR Spectroscopy

One of the greatest current challenges in structural biology is to study protein dynamics over a wide range of timescales in complex environments, such as the cell. Among magnetic resonances suitable for this approach, electron paramagnetic resonance spectroscopy coupled to site-directed spin labeling (SDSL-EPR) has emerged as a promising tool to study protein local dynamics and conformational ensembles. In this work, we exploit the sensitivity of nitroxide labels to report protein local dynamics at room temperature. We demonstrate that such studies can be performed while preserving both the integrity of the cells and the activity of the protein under investigation. Using this approach, we studied the structural dynamics of the chaperone NarJ in its natural host, Escherichia coli. We established that spin-labeled NarJ is active inside the cell. We showed that the cellular medium affects NarJ structural dynamics in a site-specific way, while the structural flexibility of the protein is maintained. Finally, we present and discuss data on the time-resolved dynamics of NarJ in cellular context.     

Semi-Rigid Nitroxide Spin Label for Long-Range EPR Distance Measurements of Lipid Bilayer Embedded β-Peptides

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.     

Evolution of Ate-Organoiron(II) Species towards Lower Oxidation States: Role of the Steric and Electronic Factors

Ate-iron(II) species such as [Ar₃Fe^II]⁻ (Ar=aryl) are key intermediates in Fe-catalyzed couplings between aryl nucleophiles and organic electrophiles. They can be active species in the catalytic cycle, or lead to Fe⁰ and Fe^I oxidation states, which can themselves be catalytically active or lead to unwished organic byproducts. Analysis of the reactivity of the intermediates obtained by step-by-step displacement of the mesityl groups in high-spin [Mes₃Fe^II]⁻ by less hindered phenyl ligands was performed, and uncovered the crucial role of both steric and electronic parameters in the formation of the Fe⁰ and Fe^I oxidation states. The formation of quaternized [Ar₄Fe^IIMgBr(THF)]⁻ intermediates allows the bielectronic reductive elimination energy required for the formation of Fe⁰ to be reduced. Similarly, the small steric pressure of the aryl groups in [Ar₃Fe^II]⁻ enables the formation of aryl-bridged [{Fe^II(Ar)₂}₂(μ-Ar)₂]²⁻ species, which afford the Fe^I oxidation state by bimetallic reductive elimination. These results are supported by ¹H NMR, EPR, and ⁵⁷Fe Mössbauer spectroscopies, as well as by DFT calculations.