Multifrequency Electron Paramagnetic Resonance Research Articles

Electronic structure of positive and negative polarons in functionalized dithienylthiazolo[5,4-d]thiazoles: a combined EPR and DFT study

2,5-Dithienylthiazolo[5,4-d]thiazole (DTTzTz) derivatives have high potential for solution-processed organic field-effect transistors and solar cells, both as electron acceptors and donors. Here, the electronic structure of positive and negative radicals (polarons) of two functionalized DTTzTz materials is studied using multi-frequency and multi-resonance electron paramagnetic resonance (EPR) in combination with density functional theory (DFT). It is shown that the negative and positive DTTzTz polarons can be distinguished on the basis of their characteristic EPR parameters. The chemically induced polarons are compared to light-generated states observed in a blend of one of the DTTzTz derivatives with a donor polymer. The study gives in-depth information about the spread of the electron or hole in the DTTzTz molecules.

Multifrequency EPR Detects Orientation of Calcium Transport Proteins in Lipid Bicelles

We have used electron paramagnetic resonance (EPR) at X and Q band to probe the structural dynamics of the integral membrane protein phospholamban (PLB), as a function of phosphorylation and addition of its regulatory target, the sarcoplasmic reticulum calcium ATPase (SERCA). We found previously that PLB remains bound to SERCA after phosphorylation, suggesting that a structural transition within the SERCA-PLB complex is responsible for relief of inhibition. Our current goal is to elucidate this mechanism through orientation and accessibility EPR, supporting rational design of therapies to improve calcium transport in muscle cells. We used the monomeric mutant AFA-PLB with the rigid spin label TOAC incorporated within the transmembrane domain. This protein was reconstituted with purified SERCA into lipid bicelles and magnetically aligned at both X and Q band field strength, allowing for a robust global fitting approach. We measured changes in PLB tilt upon phosphorylation, in the absence and presence of SERCA. Experiments were performed in the Biophysical Spectroscopy Center at the University of Minnesota. This work was funded by grants from NIH (R01 GM27906, P30 AR0507220, and T32 AR007612).

Sequential Thermal Decomposition of the Shell of Cubic ZnS/Zn(OH)2 Core–Shell Quantum Dots Observed With Mn2+ Probing Ions

Thermally induced changes in the structure and composition of the shell of tightly aggregated cubic ZnS/Zn(OH)2 core–shell quantum dots of 1.9 nm average core size were investigated by multifrequency electron paramagnetic resonance of Mn2+ probing ions. The observed three-steps temperature induced transformation of the Mn2+ surface centers in the 80–450 °C temperature range was attributed to the sequential decomposition by dehydration of the disordered ε-Zn(OH)2 shell into ZnO, with the formation of the Zn2O(OH)2 and Zn4O3(OH)2 intermediate nanocompounds. The presence of a 0.3 to 1.9 nm thick surface layer of disordered nanomaterial separating the cubic ZnS cores and its shrinking to a few atomic layers by mass loss after annealing up to 350 °C was observed by high resolution transmission electron microscopy. Unlike the single step dehydration around 120 °C of the bulk ε-Zn(OH)2, the complex decomposition of the ε-Zn(OH)2 shell is attributed to its nanosized, disordered structure.

EPR study of the effect of ionizing radiation on chromium centers in Mg2SiO4: Cr,Li laser crystals

Forsterite single crystals doped with chromium and lithium and exposed to ionizing radiation have been studied using multifrequency electron paramagnetic resonance (EPR) spectroscopy. It has been found that ionizing irradiation up to a dose of 108 rad does not lead to a significant change in the concentration of single chromium impurity centers. At the same time, γ-ray irradiation of the crystal leads to a decrease in the concentration of active laser centers, which form an associate of trivalent chromium and monovalent lithium in the crystallographic positions M2 and M1, respectively, and to the formation of new centers of divalent chromium. The structure and magnetic properties of the new centers have been discussed.

High-Field Magnetism of theS= 5/2 Triangular-Lattice Antiferromagnet CuFe1-xGaxO2(x=0–0.028)

We have performed multifrequency electron spin resonance (ESR), electric polarization, and magnetization measurements on single crystals of the triangular-lattice antiferromagnet CuFe1-xGaxO2 (x = 0.000–0.028) in high magnetic fields. We compared ESR results with the other results (electric polarization and magnetization measurements) to clarify the origin of the expansion of a ferroelectric and incommensurate (FEIC) phase induced by the doping of nonmagnetic Ga3+ ions. We observed a rapid increase in the ESR linewidth and a large shift of the transition field from a four-sublattice collinear phase to the FEIC phase accompanied with a magnetization jump and a spontaneous electric polarization with slight doping of Ga3+ ions. We have found that the ESR linewidth is highly correlated with this shift. As a result, we have succeeded in explaining the origin of the expansion of the FEIC phase as well as the change of the transition field induced by broadening of the lowest excitation branch with an increase in...

Reversible, Long-Range Radical Transfer in E. coli Class Ia Ribonucleotide Reductase

Ribonucleotide reductases (RNRs) catalyze the conversionof nucleotides to 2'-deoxynucleotides and are classified on the basis of the metallo-cofactor used to conduct this chemistry. The class Ia RNRs initiate nucleotide reduction when a stable diferric-tyrosyl radical (Y•, t1/2 of 4 days at 4 °C) cofactor in the β2 subunit transiently oxidizes a cysteine to a thiyl radical (S•) in the active site of the α2 subunit. In the active α2β2 complex of the class Ia RNR from E. coli , researchers have proposed that radical hopping occurs reversibly over 35 Å along a specific pathway comprised of redox-active aromatic amino acids: Y122• ↔ [W48?] ↔ Y356 in β2 to Y731 ↔ Y730 ↔ C439 in α2. Each step necessitates a proton-coupled electron transfer (PCET). Protein conformational changes constitute the rate-limiting step in the overall catalytic scheme and kinetically mask the detailed chemistry of the PCET steps. Technology has evolved to allow the site-selective replacement of the four pathway tyrosines with unnatural tyrosine analogues. Rapid kinetic techniques combined with multifrequency electron paramagnetic resonance, pulsed electron-electron double resonance, and electron nuclear double resonance spectroscopies have facilitated the analysis of stable and transient radical intermediates in these mutants. These studies are beginning to reveal the mechanistic underpinnings of the radical transfer (RT) process. This Account summarizes recent mechanistic studies on mutant E. coli RNRs containing the following tyrosine analogues: 3,4-dihydroxyphenylalanine (DOPA) or 3-aminotyrosine (NH2Y), both thermodynamic radical traps; 3-nitrotyrosine (NO2Y), a thermodynamic barrier and probe of local environmental perturbations to the phenolic pKa; and fluorotyrosines (FnYs, n = 2 or 3), dual reporters on local pKas and reduction potentials. These studies have established the existence of a specific pathway spanning 35 Å within a globular α2β2 complex that involves one stable (position 122) and three transient (positions 356, 730, and 731) Y•s. Our results also support that RT occurs by an orthogonal PCET mechanism within β2, with Y122• reduction accompanied by proton transfer from an Fe1-bound water in the diferric cluster and Y356 oxidation coupled to an off-pathway proton transfer likely involving E350. In α2, RT likely occurs by a co-linear PCET mechanism, based on studies of light-initiated radical propagation from photopeptides that mimic the β2 subunit to the intact α2 subunit and on [(2)H]-ENDOR spectroscopic analysis of the hydrogen-bonding environment surrounding a stabilized NH2Y• formed at position 730. Additionally, studies on the thermodynamics of the RT pathway reveal that the relative reduction potentials decrease according to Y122 < Y356 < Y731 ≈ Y730 ≤ C439, and that the pathway in the forward direction is thermodynamically unfavorable. C439 oxidation is likely driven by rapid, irreversible loss of water during the nucleotide reduction process. Kinetic studies of radical intermediates reveal that RT is gated by conformational changes that occur on the order of >100 s(-1) in addition to the changes that are rate-limiting in the wild-type enzyme (∼10 s(-1)). The rate constant of one of the PCET steps is ∼10(5) s(-1), as measured in photoinitiated experiments.

Multi-frequency ESR in EuFe2As2

We have performed multi-frequency electron spin resonance (ESR) measurements on EuFe2As2, which is one of the parent compounds of Fe-based superconductors. At T SDW=190 K, this compound shows a spin-density-wave (SDW) transition. At temperatures above T SDW, the relaxation of Eu2+ spins takes place via the coupling of conduction electrons, resulting in a Korringa-type increase in the linewidth. At temperatures below T SDW, however, this compound shows an anisotropic temperature-independent linewidth except at temperatures near the long-range ordering temperature (19 K) of Eu2+ spins, giving evidence for a gap opening at T SDW. This change of physical properties indicates a change in the exchange interaction between localized Eu2+ spins and conduction electrons in the FeAs layers. We evaluate the exchange constant, which was not reported in previous X-band ESR studies, and compare it with the value for another Eu metallic compound.

High-field multi-frequency ESR in the S=2 heisenberg antiferromagnetic chain compound MnCl3(bpy)

We report the results of high-field multi-frequency electron spin resonance (ESR) experiments on a powder sample of MnCl3 (bpy) (bpy=2, 2′-bipyridine). This compound is one of the rare examples of the spin (S) 2 quasi one-dimensional (1D) Heisenberg antiferromagnets (HAFs), and magnetic properties of tiny single crystal samples were reported previously (G. E. Granroth et al., Phys. Rev. Lett. 77, 1616 (1996).). In our previous paper on this compound (M. Hagiwara et al., J. Phys.: Conf. Ser. 400, 032014 (2012).), we reported, for the S=2 1D HAF, good agreement between experiment and calculation for the temperature dependence of magnetic susceptibility and the high-field magnetization process at 1.3 K. In this work, we have observed some high-frequency resonance modes with zero-field gaps of about 800 GHz that may correspond to antiferromagnetic resonance modes with orthorhombic anisotropy in a long-range-ordered phase that emerged at low temperatures above the critical field at which the energy gap (Haldane gap) closes. The single-ion anisotropy constant D evaluated from the analysis of these ESR modes is larger than that reported previously.

Antiferromagnetic resonance in the quasi-one-dimensional magnet IPACu(Cl x Br1−x )3 (x=0.83)

We have performed multi-frequency electron spin resonance (ESR) and magnetization measurements on single crystals of the quasi one-dimensional magnet IPACu(Cl x Br1−x )3 (x=0.83) in magnetic fields H along the b-axis at temperatures below an antiferromagnetic long-range ordering temperature (T N ≅ 13 K). We observed some ESR branches for typical antiferromagnetic modes with biaxial anisotropy at 1.5 K (≪ T N). We have analyzed the magnetization curve and the ESR excitation modes for H∥b by using a two sub-lattice model and obtained good agreement between experiment and calculation. In addition, we show that the origin of a field-induced magnetization jump is a spin-flop transition. These results show that the phase at temperatures below T N in this Cl-Br mixed compound can be described as a simple two sub-lattice antiferromagnet.

Low-energy magnetic excitations in the quasi-one-dimensional spin-1 chain compound SrNi2V2O8

Multi-frequency electron spin resonance (ESR) transmission spectra have been measured as function of temperature and magnetic field on single crystals of the quasi-one-dimensional spin-1 chain compound SrNi2V2O8 in the GHz frequency range. Magnetic resonance modes above 50 K have been observed with an effective g-factor of 2.24 at 100 K. Below 30 K, intra-triplet excitations have been observed in the ESR spectra, which reveal the presence of single-ion anisotropy with D = -0.29 meV.

EPR study of the effect of partial pressure of oxygen in the growth atmosphere on concentration of chromium centers in synthetic forsterite

Samples of synthetic forsterite doped with chromium and grown in an argon atmosphere with different oxygen concentrations (at a partial pressure of oxygen in the range from 0.03 to 0.78 kPa) have been studied using multifrequency electron paramagnetic resonance (EPR) spectroscopy. It has been demonstrated how the oxidizing properties of the atmosphere during the growth of crystals affect the relative concentration of di-, tri-, and tetravalent chromium ions in the samples. The structure of trivalent chromium impurity centers has been discussed.

Correlation of Lattice Disorder with Crystallite Size and the Growth Kinetics of Mn2+ Doped ZnO Nanocrystals Probed by Electron Paramagnetic Resonance

The correlation of the lattice disorder with the nanocrystal average size, in ZnO nanocrystals synthesized by several different methods, has been quantitatively monitored by line shape analysis of the multifrequency electron paramagnetic resonance (EPR) spectra of low concentrations of substitutional Mn2+ probing ions. The observed correlation between the line broadening parameter of the spectrum and the average ZnO nanocrystals size, independent of the synthesis procedure of the ZnO nanocrystals, demonstrates the dominance of the size related strain/disorder. On the basis of this result, a new method for determining the average ZnO nanocrystal size from the quantitative analysis of the EPR spectra of the Mn2+ probes was derived. The nanocrystallization of the disordered ZnO formed by the thermal decomposition of hydrozincite was monitored using this procedure. The observed ZnO nanocrystallite growth kinetics at lower temperatures was described by a structural relaxation mechanism consisting of the local ...

Mn-substituted spinel Li4Ti5O12 materials studied by multifrequency EPR spectroscopy

In this contribution we report on the influence of Mn doping on various physical properties of Li4Ti5−xMnxO12 (x = 0, 0.005, 0.025, 0.01, 0.05, 0.075, 0.1 and 0.2). Spinel-type (space group Fd3m, a = 0.8358 nm) Li4Ti5−xMnxO12 was produced as highly uniform and polyhedral shaped powder in a size range between 200 and 400 nm. The crystal structure of Li4Ti5−xMnxO12, the morphology and the Mn-ion distribution have been investigated by X-ray diffraction (XRD) and scanning and transmission electron microscopies (SEM and TEM). The oxidation state of Mn and the conductivity behavior of Mn-substituted Li4Ti5−xMnxO12 samples have been studied using multi-frequency electron paramagnetic resonance (EPR). Mn is incorporated as Mn2+, and these ions increase the conductivity of Li4Ti5O12; a conductivity value of 0.48 × 10−8 S cm−1 was measured for Li4Ti5−xMnxO12 with x = 0.075, which is ten-fold larger than that of undoped Li4Ti5O12.

1P011 多周波電子スピン共鳴によるスピンラベル変性タンパクのダイナミックス(01A. 蛋白質:構造,ポスター,日本生物物理学会年会第51回(2013年度))

1P011 多周波電子スピン共鳴によるスピンラベル変性タンパクのダイナミックス(01A. 蛋白質:構造,ポスター,日本生物物理学会年会第51回(2013年度))

Dzyaloshinsky-Moriya Interaction Estimated by AFMR of Kagome Like Substance Cu2O(SO4) Observed at 1.8K

Dolerophanite Cu2O(SO4) is one of model substances for S = 1/2 kagome like antiferromagnet. Although many X-ray diffraction studies have been performed on natural mineral Cu2O(SO4), little is known about its magnetic property. Therefore, the multi-frequency electron spin resonance (ESR) measurements of dolerophanite powder sample have been performed using pulsed magnetic fields up to 16 T in the frequency range from 50 GHz to 315 GHz in the temperature range from 1.8 K to 86 K Antiferromagnetic resonance (AFMR) has been observed below TN = 20 K, which is consistent with the previous magnetic susceptibility result by Asano et al. Moreover, the frequency-field relation at 1.8 K suggests typical easy-plane type AFMR with Dzyaloshinsky-Moriya (DM) interaction, which is also consistent with the weak ferromagnetism below TN suggested by Asano et al. As the estimation of DM interaction is very important to discuss the quantum phase transition in kagome antiferromagnet as suggested by Cépas et al. [2008, Phys. Rev. B 78 140405(R)], we have estimated the D term of DM interaction as 6.98 ± 0.01 K from our AFMR results.

Multi-frequency electron spin resonance analysis of interfacial Ge dangling bond defects in condensation-grown (1 0 0)Si/SiO2/Si1–xGex/SiO2

The multi-frequency electron spin resonance (ESR) study of the GePb1 Ge dangling bond (DB) interface defect, appearing identically at both SiO2/GexSi1–x interfaces in condensation-grown SiO2/GexSi1–x/SiO2/(1 0 0)Si heterostructures (0.28 ≤ x ≤ 0.93), shows that the ESR signal width is dominated by inhomogeneous broadening due to a strain-induced spread in g. For the x = 0.73 case, this results in a frequency (ν)-dependent peak-to-peak broadening of ΔBppSB/ν = 0.62 G/GHz for the applied magnetic field B along the g3 principal axis Ge DB direction. Compared to the familiar Si Pb-type interface defects in (1 0 0)Si/SiO2, the enhanced ν-dependent broadening scales with the spin–orbit coupling constant ratio λ(Ge)/λ(Si). The natural inhomogeneous broadening due to unresolved 73Ge hyperfine interaction is found to be below ∼1.56 G. While the results adduce quantitative support for the assignment of GePb1 as a Ge DB-type interface center, it is concluded that the GeSi/SiO2 interface uniformity is of good level.

Terahertz spectroscopy in the pseudo-Kagome system Cu3Bi(SeO3)2O2Br

Terahertz (THz) transmission spectra have been measured as a function of temperature and magnetic field on single crystals of Cu${}_{3}$Bi(SeO${}_{3}$)${}_{2}$O${}_{2}$Br. In time-domain THz spectra without magnetic field, two resonance absorptions are observed below the magnetic ordering temperature ${T}_{N}\ensuremath{\sim}27.4$ K. The corresponding resonance frequencies increase with decreasing temperature and reach energies of 1.28 and 1.23 meV at 3.5 K. Multifrequency electron spin resonance transmission spectra as a function of the applied magnetic field show the field dependence of four magnetic resonance modes, which can be modeled as a ferromagnetic resonance including demagnetization and anisotropy effects.

Conformationally Distinct Five-Coordinate Heme–NO Complexes of Soluble Guanylate Cyclase Elucidated by Multifrequency Electron Paramagnetic Resonance (EPR)

Soluble guanylate cyclase (sGC) is a heme-containing enzyme that senses nitric oxide (NO). Formation of a heme Fe-NO complex is essential to sGC activation, and several spectroscopic techniques, including electron paramagnetic resonance (EPR) spectroscopy, have been aimed at elucidating the active enzyme conformation. Of these, only EPR spectra (X-band ~9.6 GHz) have shown differences between low- and high-activity Fe-NO states, and these states are modeled in two different heme domain truncations of sGC, β1(1-194) and β2(1-217), respectively (Derbyshire et al., Biochemistry 2008, 47, 3892-3899). The EPR signal of the low-activity sGC Fe-NO complex exhibits a broad lineshape that has been interpreted as resulting from site-to-site inhomogeneity, and simulated using g strain, a continuous distribution about the principal values of a given g tensor. This approach, however, fails to account for visible features in the X-band EPR spectra as well as the g anisotropy observed at higher microwave frequencies. Herein we analyze X-, Q-, and D-band EPR spectra and show that both the broad lineshape and the spectral structure of the sGC EPR signal at multiple microwave frequencies can be simulated successfully with a superposition of only two distinct g tensors. These tensors represent different populations that likely differ in Fe-NO bond angle, hydrogen bonding, or the geometry of the amino acid residues. One of these conformations can be linked to a form of the enzyme with higher activity.

Electron spin resonance features of the Ge Pb1 dangling bond defect in condensation-grown (100)Si/SiO2/Si1−xGex/SiO2 heterostructures

A multi-frequency electron spin resonance (ESR) study has been carried out the Ge Pb1 dangling bond (DB)-type interface defect in SiO2/GexSi1−x/SiO2/(100)Si heterostructures of different Ge fraction manufactured by the condensation technique. The notable absence of Si Pb-type centers enables unobscured spectral analysis as function of magnetic field angle, reassured by coinciding multi-frequency ESR data. The center features monoclinic-I (C2v) symmetry with principal g values g1 = 2.0338 ± 0.0003, g2 = 2.0386 ± 0.0006, g3 = 2.0054, with the lowest value, g3, axis 24 ± 2° off a 〈111〉 direction towards the [100] interface normal n. The defect appearance shows a systematic variation as function of x, reaching maximum densities of ∼6.8 × 1012 cm−2 per Si/SiO2 interface for x ∼ 0.7, to disappear for x outside the ]0.45–0.87[ range. The ESR signal width is dominated by inhomogeneous broadening arising from strain-induced Gaussian spread in g, resulting in frequency (ν)-dependent peak-to-peak broadening of ΔBppSB/ν = 0.62 G/GHz and 1.13 G/GHz for the applied magnetic field B//g3 principal axis and B//n, respectively. Compared to the familiar Si Pb-type interface defects in (100)Si/SiO2, the enhanced ν-dependent broadening scales with the spin-orbit coupling constant ratio λ(Ge)/λ(Si). The natural inhomogeneous broadening due to unresolved 73Ge hyperfine interaction is found to be below ∼1.56 G, the inferred residual (ν→0) width. The core of the defect, previously depicted as a Ge Pb1-type center, concerns an asymmetrically back bonded trivalent Ge atom where the total of all data would suggest a back bond arrangement involving two Ge and one Si atoms and/or a strained bond. Initial studies show the defect to exhibit reversal hydrogen passivation/depassivation kinetics, where defects can be substantially, yet only partially, ESR-inactivated by heating in H2. The properties and nature of the center are discussed within the context of the their inherent incorporation as interface mismatch centers, where assessing the defect’s nature and modalities of occurrence may provide a link to unraveling the role of point defects in adapting interfacial mismatch.

Effect of quaternary structure change on the low-lying electronic states of the ferrous heme in deoxy-Hb studied by multi-frequency EPR

The effect of the quaternary structure change in deoxy-Hb on the electronic states of the ferrous high-spin heme was studied by multi-frequency electron paramagnetic resonance (MFEPR). We used a single-pass transmission-type EPR at 70–220GHz and at 1.5K. We compared the EPR spectra of deoxy-Hb at high pH with those at low pH and in the presence of the allosteric effectors, such as inositol hexaphosphate (IHP). Our EPR results imply that the low-lying electronic states of the ferrous heme in deoxy-Hb could be altered by the changes in the heme geometry, which might be caused by the quaternary structure change in Hb. We also measured EPR spectra of deoxy FeMg hybrid Hb to clarify the difference between α and β hemes at low pH with IHP. The additional complicated resonance branches appeared in deoxy-Hb as well as in deoxy FeMg hybrid Hbs at low pH with IHP may reflect the energy states of the ferrous high-spin heme correlated with the low oxygen affinity state. The magnetic moment of the lowest excited state of the α heme seems to be different from that of the β heme in the low oxygen affinity state.