Applications of pi-photon-induced transparency in two-frequency pulse electron paramagnetic resonance experiments

An approach to pulse electron paramagnetic resonance (EPR) experiments which are based on two different resonance fields is introduced. Instead of using two microwave (mw) sources or a magnetic field jump, bichromatic pulses consisting of a transverse microwave field with frequency omega(mw) and a longitudinal radio frequency field with frequency omega(rf) are employed. Such bichromatic pulses excite a number of multiple photon transitions at frequencies omega(mw)+komega(rf) (k in Z). The pi-photon-induced transparency phenomenon is used to select the required transitions. This approach is used in the stimulated soft electron spin echo envelope modulation and the four-pulse double electron-electron resonance experiments. The results obtained using the bichromatic pulse approach are in agreement with those obtained with the standard pulse EPR techniques. It is shown that applying bichromatic pulses is straightforward and advantageous in several respects.     

Structural and dynamic properties of oxygen vacancies in perovskite oxides--analysis of defect chemistry by modern multi-frequency and pulsed EPR techniques

Multi-frequency and pulsed electron paramagnetic resonance (EPR) provides a sensitive spectroscopic tool to elucidate the defect structure of transition-metal doped perovskite oxides, as well as to monitor dynamic processes of oxygen vacancies in these materials. In this regard, high-frequency EPR spectrometers and pulsed EPR techniques such as the hyperfine sublevel correlation experiment (HYSCORE) may now routinely be used for dedicated investigations, providing considerably more insight than the application of standard continuous-wave EPR. Recent results include the formation of defect complexes between acceptor-type transition-metal centers with either one or two oxygen vacancies for the reason of charge compensation. Furthermore, such defect complexes follow the domain switching upon poling ferroelectric compounds with correspondingly high electric fields. On the other hand, multi-valent manganese functional centers provide trapping centers for electronic and ionic charge carriers (e', ) such that valency altered acceptor states or defect complexes are formed. Additionally, the trapping of charge carriers at the intrinsic 'reduced' B-site ions, and , can be observed by means of EPR spectroscopy.     

Uses of electron paramagnetic resonance in studying fracture

The uses of electron paramagnetic resonance (EPR) in studying aspects of polymer fracture are discussed. The sensitivity of EPR is such that all phases of fracture are not amenable to investigation by these means. This paper attempts to define those areas where the authors' experience would indicate that success might or might not be expected. A discussion of the difference between the tensile fracture of drawn polymer fibers, in which strong signals are obtained, and cast and molded materials is given.     

Synthesis of μ2-Oxo-Bridged Iron(III) Tetraphenylporphyrin–Spacer–Nitroxide Dimers and their Structural and Dynamics Characterization by using EPR and MD Simulations

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.     

抗氧化剂活性的电子顺磁共振（EPR）研究

抗氧化剂在化工、食品以及生命科学等领域具有重要的作用,近年来更是由于其在解决与人类健康相关问题方面所起到的重要作用而受到广泛重视。本综述较为详细地介绍了利用电子顺磁共振（EPR）技术测定抗氧化剂活性的原理和方法,系统总结了近年来在天然抗氧化剂以及合成抗氧化剂化学活性方面的EPR研究最新进展,特别是定量测定表征抗氧化活性...     

Calculating slow-motional electron paramagnetic resonance spectra from molecular dynamics using a diffusion operator approach

A number of groups have utilized molecular dynamics (MD) to calculate slow-motional electron paramagnetic resonance (EPR) spectra of spin labels attached to biomolecules. Nearly all such calculations have been based on some variant of the trajectory method introduced by Robinson, Slutsky and Auteri (J. Chem. Phys. 1992,96, 2609-2616). Here we present an alternative approach that is specifically adapted to the diffusion operator-based stochastic Liouville equation (SLE) formalism that is also widely used to calculate slow-motional EPR line shapes. Specifically, the method utilizes MD trajectories to derive diffusion parameters such as the rotational diffusion tensor, diffusion tilt angles, and expansion coefficients of the orienting potential, which are then used as direct inputs to the SLE line shape program. This approach leads to a considerable improvement in computational efficiency over trajectory-based methods, particularly for high frequency, high field EPR. It also provides a basis for deconvoluting the effects of local spin label motion and overall motion of the labeled molecule or domain: once the local motion has been characterized by this approach, the label diffusion parameters may be used in conjunction with line shape analysis at lower EPR frequencies to characterize global motions. The method is validated by comparison of the MD predicted line shapes to experimental high frequency (250 GHz) EPR spectra.     

Study of relaxation rates of stable paramagnetic centers in gamma-irradiated alanine

The stable L-alanine radical induced by gamma-irradiation was examined by electron paramagnetic resonance (EPR), transfer saturation EPR and electron nuclear double resonance (ENDOR) in the temperature region of fast motion of the methyl group (180-320 K). From the obtained spectral line broadening and spectral intensity the correlation time for the methyl rotation was estimated. The complex processes determining the relaxation rate were examined in the same temperature interval. It was shown that important contributions to the relaxation rate arise from non-secular and pseudo-secular types of contributions. The non-secular contribution involves intramolecular dynamics while the pseudo-secular contribution originates from intermolecular motions. The obtained values for the dynamical parameters have been compared with those obtained by pulse EPR methods and by proton nuclear magnetic resonance (NMR) on undamaged crystals.     

Pulse EPR Methods for Studying Chemical and Biological Samples Containing Transition Metals

This review discusses the application of pulse EPR to the characterization of disordered systems, with an emphasis on samples containing transition metals. Electron nuclear double‐resonance (ENDOR), electron‐spin‐echo envelope‐modulation (ESEEM), and double electron–electron resonance (DEER) methodologies are outlined. The theory of field modulation is outlined, and its application is illustrated with DEER experiments. The simulation of powder spectra in EPR is discussed, and strategies for optimization are given. The implementation of this armory of techniques is demonstrated on a rich variety of chemical systems: several porphyrin derivatives that are found in proteins and used as model systems, otherwise highly reactive aminyl radicals stabilized with electron‐rich transition metals, and nitroxide–copper–nitroxide clusters. These examples show that multi‐frequency continuous‐wave (CW) and pulse EPR provides detailed information about disordered systems.     

全硫取代三苯甲基自由基酯基衍生物与牛血清白蛋白的相互作用

采用荧光光谱、电子顺磁共振（EPR）波谱、紫外-可见吸收光谱和分子对接等技术研究了全硫取代三苯甲基（TAM）自由基酯基衍生物ET-03与牛血清白蛋白（BSA）的相互作用,发现ET--03与BSA能自发发生结合作用;主要以疏水作用力结合在BSA亚结构域ⅡA（位点Ⅰ）和亚结构域ⅢA（位点Ⅱ）上;ET-03对BSA的荧光猝灭效应为动态、静态混合猝灭机制,且可能存在非辐射能量转移.研究结果表明,酯基衍生化TAM自由基与白蛋白能自发结合,有望用于蛋白构效关系研究;同时也提示将TAM自由基酯基衍生物用于活体成像或自旋标记物时应考虑其与蛋白相互作用的影响.     

Accurate Extraction of Nanometer Distances in Multimers by Pulse EPR

Pulse electron paramagnetic resonance (EPR) is gaining increasing importance in structural biology. The PELDOR (pulsed electron–electron double resonance) method allows extracting distance information on the nanometer scale. Here, we demonstrate the efficient extraction of distances from multimeric systems such as membrane‐embedded ion channels where data analysis is commonly hindered by multi‐spin effects.     

Approximate methods for the fast computation of EPR and ST-EPR spectra. I. A perturbation approach

The electron paramagnetic resonance (EPR) and saturation transfer electron paramagnetic resonance (ST-EPR) spectra of nitroxide spin labels are theoret     

Distance measurements on spin-labelled biomacromolecules by pulsed electron paramagnetic resonance

The biological function of protein, DNA, and RNA molecules often depends on relative movements of domains with dimensions of a few nanometers. This length scale can be accessed by distance measurements between spin labels if pulsed electron paramagnetic resonance (EPR) techniques such as electron-electron double resonance (ELDOR) and double-quantum EPR are used. The approach does not require crystalline samples and is well suited to biomacromolecules with an intrinsic flexibility as distributions of distances can be measured. Furthermore, oligomerization or complexation of biomacromolecules can also be studied, even if it is incomplete. The sensitivity of the technique and the reliability of the measured distance distribution depend on careful optimization of the experimental conditions and procedures for data analysis. Interpretation of spin-to-spin distance distributions in terms of the structure of the biomacromolecules furthermore requires a model for the conformational distribution of the spin labels.     

Dual-mode EPR study of Mn(III) salen and the Mn(III) salen-catalyzed epoxidation of cis-beta-methylstyrene

Dual-mode electron paramagnetic resonance (EPR), in which an oscillating magnetic field is alternately applied parallel or perpendicular to the static magnetic field, is a valuable technique for studying both half-integer and integer electron spin systems and is particularly useful for studying transition metals involved in redox chemistry. We have applied this technique to the characterization of the Mn(III) salen (salen = N,N'-ethylene bis(salicylideneaminato)) complex [(R,R)-(-)-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediaminomanganese(III)], with an S = 2 integer electron spin system. Furthermore, we have used dual-mode EPR to study the Mn salen complex during the Mn(III) salen-catalyzed epoxidation of cis-beta-methylstyrene. Our study shows that the additives N-methylmorpholine N-oxide (NMO) and 4-phenylpyridine-N-oxide (4-PPNO), which are used to improve epoxidation yields and enantioselection, bind to the Mn(III) center prior to the epoxidation reaction, as evidenced by the alteration of the Mn(III) parallel mode EPR signal. With these additives as ligands, the axial zero-field splitting values and (55)Mn hyperfine splitting of the parallel mode signal are indicative of an axially elongated octahedral geometry about the Mn(III) center. Since the dual-mode EPR technique allows the observation of both integer and half-integer electron spin systems, Mn oxidation states of II, III, IV, and potentially V can be observed in the same sample as well as any radical intermediates or Mn(III,IV) dinuclear clusters formed during the Mn(III) salen-catalyzed epoxidation reaction. Indeed, our study revealed the formation of a Mn(III,IV) dinuclear cluster in direct correlation with expoxide formation. In addition to showing the possible reaction intermediates, dual-mode EPR offers insight into the mechanism of catalyst degradation and formation of unwanted byproducts. The dual-mode technique may therefore prove valuable for elucidating the mechanism of Mn(III) salen catalyzed reactions and ultimately for designing optimum catalytic conditions (solvents, oxidants, and additives such as NMO or 4-PPNO).     

Principles and applications of ENDOR spectroscopy for structure determination in solution and disordered matrices

Both electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) spectroscopies are extremely powerful and versatile methods for the characterisation of paramagnetic systems in biology, chemistry, and physics. However, by comparison to EPR, ENDOR remains a less widely used technique. In this tutorial review the basic principles of continuous wave ENDOR are described. The theory of orientation selective ENDOR, for structure determination in frozen solutions and powders, is then described. A range of examples, illustrating the type of information obtained from the ENDOR spectrum, is finally presented.     

Theoretical investigations of the EPR g factors and the local structure for Er3+ in BaWO4

The electron paramagnetic resonance (EPR) parameters g factors g(parallel) and g(perpendicular) as well as the local structure for Er3+ in scheelite-type BaWO4 are theoretically investigated by using the perturbation formulas of the EPR parameters for a 4f11 ion under tetragonal symmetry. In these formulas, the contributions to the EPR parameters arising from the second-order perturbation terms and the admixture of different states are included. It is found that the impurity-ligand bonding angles (or the polar angles) related to the fourfold axis in the tetragonal Er3+ center are about 1.5 degrees smaller than those in pure crystal. The calculated EPR parameters are in agreement with the observed values. The validity of the results is discussed.     

Multi‐frequency (S,X, Q and W‐band) EPR and ENDOR Study of Vanadium(IV) Incorporation in the Aluminium Metal–Organic Framework MIL‐53

Doping the well‐known metal–organic framework MIL‐53(Al) with vanadium(IV) ions leads to significant changes in the breathing behaviour and might have repercussions on the catalytic behaviour as well. To understand the properties of such a doped framework, it is necessary to determine where dopant ions are actually incorporated. Electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) are applied to reveal the nearest environment of the paramagnetic vanadium(IV) dopant ions. EPR spectra of as‐synthesised vanadium‐doped MIL‐53 are recorded at S‐, X‐, Q‐ and W‐band microwave frequencies. The EPR spectra suggest that at low dopant concentrations (1.0–2.6 mol %) the vanadium(IV) ions are well dispersed in the matrix. Varying the vanadium dopant concentration within this range or the dopant salt leads to the same dominant EPR component. In the ENDOR spectra, hyperfine (HF) interactions with ¹H, ²⁷Al and ⁵¹V nuclei are observed. The HF parameters extracted from simulations strongly suggest that the vanadium(IV) ions substitute Al in the framework.     

Discriminative Detection of Biothiols by Electron Paramagnetic Resonance Spectroscopy using a Methanethiosulfonate Trityl Probe

Biothiols, such as glutathione (GSH), homocysteine (Hcy), and cysteine (Cys), coexist in biological systems with diverse biological roles. Thus, analytical techniques that can detect, quantify, and distinguish between multiple biothiols are desirable but challenging. Herein, we demonstrate the simultaneous detection and quantitation of multiple biothiols, including up to three different biothiols in a single sample, using electron paramagnetic resonance (EPR) spectroscopy and a trityl‐radical‐based probe (MTST). We term this technique EPR thiol‐trapping. MTST could trap thiols through its methanethiosulfonate group to form the corresponding disulfide conjugate with an EPR spectrum characteristic of the trapped thiol. MTST was used to investigate effects of l ‐buthionine sulfoximine (BSO) and pyrrolidine dithiocarbamate (PDTC) on the efflux of GSH and Cys from HepG2 cells.     

面向金属离子电池研究的固体核磁共振和电子顺磁共振方法

电池,尤其是锂离子电池的快速发展极大改变了我们的生活。从移动电子设备到新能源汽车再到电网储能,电池应用于多个领域且目前在能量密度和功率密度方面难以被取代。电池技术的向前发展要求我们对其电化学反应机理有完整的认识,这需要来自不同领域研究人员的交叉碰撞。磁共振波谱技术包括核磁共振波谱(NMR)和电子顺磁共振波谱(EPR),前者适合于研究Li、Na、P、O等电池材料中常见的轻元素,后者适合于研究Co、Mn、Fe、V等电池材料中常见的过渡金属。加上它们具有对样品无损、对结晶度无要求、能够定量分析等优点,NMR和EPR在过去三十年的电池研究中不断进步,日益成为电池表征的重要角色。本文从磁共振方法的角度出发,首先概述了固体NMR和EPR中的主要相互作用及其哈密顿表达形式,接着概述了固体NMR和EPR常用的重要方法及其在金属离子电池研究领域的代表性应用。本文有助于让我们直观地了解磁共振技术本身在金属离子电池研究领域的重要价值,并有望为解决利用固体NMR和EPR进行电池研究的过程遇到的困难提供指导。     

Elektronenspinresonanz‐Spektroskopie

During the last two decades electron paramagnetic resonance (EPR) witnessed a tremendous methodological and instrumental development, in particular in the areas of pulsed EPR and double resonance techniques. Often in combination with site‐directed spin‐labeling these techniques are applied within different subdisciplines in chemistry and in particular in biostructure research. Applications on the intrinsically disordered protein Alpha‐Synuclein associated with Parkinson's disease are used to illustrate the potential of modern EPR spectroscopy.