Multifrequency Electron Paramagnetic Resonance Spectroscopy Research Articles

Light-Induced Charge Separation in Photosystem I from Different Biological Species Characterized by Multifrequency Electron Paramagnetic Resonance Spectroscopy

Light-Induced Charge Separation in Photosystem I from Different Biological Species Characterized by Multifrequency Electron Paramagnetic Resonance Spectroscopy

Spin-Lattice Relaxation Decoherence Suppression in Vanishing Orbital Angular Momentum Qubits.

Multifrequency electron paramagnetic resonance spectroscopy on oriented single crystals of magnetically dilute Gd(III) ions in Gd0.004Y0.996(trensal) is used to determine the Hamiltonian parameters of the ground 8S7/2 term and its phase memory time, Tm, characterizing its coherent spin dynamics. The vanishing orbital angular momentum of the 8S7/2 term makes it relatively insensitive to spin-lattice relaxation mediated by magnetoelastic coupling and leads to a Tm of 12 μs at 3 K, which is not limited by spin-lattice relaxation.

Pulsed Multifrequency Electron Paramagnetic Resonance Spectroscopy Reveals Key Branch Points for One- vs Two-Electron Reactivity in Mn/Fe Proteins.

Traditionally, the ferritin-like superfamily of proteins was thought to exclusively use a diiron active site in catalyzing a diverse array of oxygen-dependent reactions. In recent years, novel redox-active cofactors featuring heterobimetallic Mn/Fe active sites have been discovered in both the radical-generating R2 subunit of class Ic (R2c) ribonucleotide reductases (RNRs) and the related R2-like ligand-binding oxidases (R2lox). However, the protein-specific factors that differentiate the radical reactivity of R2c from the C-H activation reactions of R2lox remain unknown. In this work, multifrequency pulsed electron paramagnetic resonance (EPR) spectroscopy and ligand hyperfine techniques in conjunction with broken-symmetry density functional theory calculations are used to characterize the molecular and electronic structures of two EPR-active intermediates trapped during aerobic assembly of the R2lox Mn/Fe cofactor. A MnIII(μ-O)(μ-OH)FeIII species is identified as the first EPR-active species and represents a common state between the two classes of redox-active Mn/Fe proteins. The species downstream from the MnIII(μ-O)(μ-OH)FeIII state exhibits unique EPR properties, including unprecedented spectral breadth and isotope-dependent g-tensors, which are attributed to a weakly coupled, hydrogen-bonded MnIII(μ-OH)FeIII species. This final intermediate precedes formation of the MnIII/FeIII resting state and is suggested to be relevant to understanding the endogenous reactivity of R2lox.

The Transient Complex of Cytochrome c and Cytochrome c Peroxidase: Insights into the Encounter Complex from Multifrequency EPR and NMR Spectroscopy.

We present a novel approach to study transient protein‐protein complexes with standard, 9 GHz, and high‐field, 95 GHz, electron paramagnetic resonance (EPR) and paramagnetic NMR at ambient temperatures and in solution. We apply it to the complex of yeast mitochondrial iso‐1‐cytochrome c (Cc) with cytochrome c peroxidase (CcP) with the spin label [1‐oxyl‐2,2,5,5‐tetramethyl‐Δ3‐pyrroline‐3‐methyl)‐methanethiosulfonate] attached at position 81 of Cc (SL−Cc). A dissociation constant KD of 20±4×10−6 M (EPR and NMR) and an equal amount of stereo‐specific and encounter complex (NMR) are found. The EPR spectrum of the fully bound complex reveals that the encounter complex has a significant population (60 %) that shares important features, such as the Cc‐interaction surface, with the stereo‐specific complex.

EPR Spectroscopy of Iron- and Nickel-Doped [ZnAl]-Layered Double Hydroxides: Modeling Active Sites in Heterogeneous Water Oxidation Catalysts.

Iron-doped nickel layered double hydroxides (LDHs) are among the most active heterogeneous water oxidation catalysts. Due to interspin interactions, however, the high density of magnetic centers results in line-broadening in magnetic resonance spectra. As a result, gaining atomic-level insight into the catalytic mechanism via electron paramagnetic resonance (EPR) is not generally possible. To circumvent spin-spin broadening, iron and nickel atoms were doped into nonmagnetic [ZnAl]-LDH materials and the coordination environments of the isolated Fe(III) and Ni(II) sites were characterized. Multifrequency EPR spectroscopy identified two distinct Fe(III) sites (S = 5/2) in [Fe:ZnAl]-LDH. Changes in zero field splitting (ZFS) were induced by dehydration of the material, revealing that one of the Fe(III) sites was solvent-exposed (i.e., at an edge, corner, or defect site). These solvent-exposed sites featured an axial ZFS of 0.21 cm-1 when hydrated. The ZFS increased dramatically upon dehydration (to -1.5 cm-1), owing to lower symmetry and a decrease in the coordination number of iron. The ZFS of the other ("inert") Fe(III) site maintained an axial ZFS of 0.19-0.20 cm-1 under both hydrated and dehydrated conditions. We observed a similar effect in [Ni:ZnAl]-LDH materials; notably, Ni(II) (S = 1) atoms displayed a single, small ZFS (±0.30 cm-1) in hydrated material, whereas two distinct Ni(II) ZFS values (±0.30 and ±1.1 cm-1) were observed in the dehydrated samples. Although the magnetically dilute materials were not active catalysts, the identification of model sites in which the coordination environments of iron and nickel were particularly labile (e.g., by simple vacuum drying) is an important step toward identifying sites in which the coordination number may drop spontaneously in water, a probable mechanism of water oxidation in functional materials.

Electron Paramagnetic Resonance Spectroscopic Identification of the Fe-S Clusters in the SPASM Domain-Containing Radical SAM Enzyme PqqE.

Pyrroloquinoline quinone (PQQ) is an important redox active quinocofactor produced by a wide variety of bacteria. A key step in PQQ biosynthesis is a carbon-carbon cross-link reaction between glutamate and tyrosine side chains within the ribosomally synthesized peptide substrate PqqA. This reaction is catalyzed by the radical SAM enzyme PqqE. Previous X-ray crystallographic and spectroscopic studies suggested that PqqE, like the other members of the SPASM domain family, contains two auxiliary Fe-S clusters (AuxI and AuxII) in addition to the radical SAM [4Fe-4S] cluster. However, a clear assignment of the electron paramagnetic resonance (EPR) signal of each Fe-S cluster was hindered by the isolation of a His6-tagged PqqE variant with an altered AuxI cluster. In this work, we are able to isolate soluble PqqE variants by using a less disruptive strep-tactin chromatographic approach. We have unambiguously identified the EPR signatures for four forms of Fe-S clusters present in PqqE through the use of multifrequency EPR spectroscopy: the RS [4Fe-4S] cluster, the AuxII [4Fe-4S] cluster, and two different clusters ([4Fe-4S] and [2Fe-2S]) bound in the AuxI site. The RS [4Fe-4S] cluster, the AuxII [4Fe-4S] cluster, and the [2Fe-2S] cluster form in the AuxI site can all be reduced by sodium dithionite, with g tensors of their reduced form determined as [2.040, 1.927, 1.897], [2.059, 1.940, 1.903], and [2.004, 1.958, 1.904], respectively. The AuxI [4Fe-4S] cluster that is determined on the basis of its relaxation profile can be reduced only by using low-potential reductants such as Ti(III) citrate or Eu(II)-DTPA to give rise to a g1 = 2.104 signal. Identification of the EPR signature for each cluster paves the way for further investigations of SPASM domain radical SAM enzymes.

Cyan titania nanowires: Spectroscopic study of the origin of the self-doping enhanced photocatalytic activity

We report a detailed spectroscopic study of Ti3+ self-doped TiO2 nanowires prepared by high temperature annealing of the protonated titanate nanowires (TiONW) in hydrogen atmosphere. Hydrogenation causes a color change from white to cyan and a major increase of photocatalytic activity. Combination of synchrotron X-ray diffraction and Rutherford backscattering spectrometry revealed that the bulk of the protonated titanate and Ti3+ self-doped titania is identical. Raman spectroscopy combined with local laser heating indicated the presence of optically active states in the hydrogenated TiO2 nanowires. Using multi-frequency electron paramagnetic resonance spectroscopy (EPR), the presence of Ti3+ surface states on the cyan titania nanowires is confirmed. We argue that these surface point defects are responsible for enhanced visible light absorption and improved their photocatalytic performance.

Identification of Free Radicals Generated by Different Curing Modes in a Dental Resin Cement

The polymerization process of the dual cure resin cement AllCem (FGM, Joinvile, Brazil) was investigated theoretically and experimentally. The free radicals responsible for polymerization continuity were characterized by multi-frequency EPR spectroscopy (X-, Q- and W-band) and spectra simulation. It was observed by multifrequency electron paramagnetic resonance spectroscopy and spectra simulation that the paramagnetic species occur simultaneously and independently for both self-cure and photo-cure modes. The paramagnetic species generating the EPR signal are the same for both photo-cure and self-cure polymerization mechanisms.

Investigation of cutaneous penetration properties of stearic acid loaded to dendritic core-multi-shell (CMS) nanocarriers

Dendritic core-multi shell (CMS) particles are polymer based systems consisting of a dendritic polar polyglycerol polymer core surrounded by a two-layer shell of nonpolar C18 alkyl chains and hydrophilic polyethylene glycol. Belonging to nanotransport systems (NTS) they allow the transport and storage of molecules with different chemical characters. Their amphipihilic character CMS-NTS permits good solubility in aqueous and organic solutions. We showed by multifrequency electron paramagnetic resonance (EPR) spectroscopy that spin-labeled 5-doxyl stearic acid (5DSA) can be loaded into the CMS-NTS. Furthermore, the release of 5DSA from the carrier into the stratum corneum of porcine skin was monitored ex vivo by EPR spectroscopy. Additionally, the penetration of the CMS-NTS into the skin was analyzed by fluorescence microscopy using indocarbocyanine (ICC) covalently bound to the nanocarrier. Thereby, no transport into the viable skin was observed, whereas the CMS-NTS had penetrated into the hair follicles down to a depth of 340μm±82μm. Thus, it could be shown that the combined application of fluorescence microscopy and multi-frequency EPR spectroscopy can be an efficient tool for investigating the loading of spin labeled drugs to nanocarrier systems, drug release and penetration into the skin as well as the localization of the NTS in the skin.

Direct measurement of dysprosium(III)···dysprosium(III) interactions in a single-molecule magnet.

Lanthanide compounds show much higher energy barriers to magnetic relaxation than 3d-block compounds, and this has led to speculation that they could be used in molecular spintronic devices. Prototype molecular spin valves and molecular transistors have been reported, with remarkable experiments showing the influence of nuclear hyperfine coupling on transport properties. Modelling magnetic data measured on lanthanides is always complicated due to the strong spin-orbit coupling and subtle crystal field effects observed for the 4f-ions; this problem becomes still more challenging when interactions between lanthanide ions are also important. Such interactions have been shown to hinder and enhance magnetic relaxation in different examples, hence understanding their nature is vital. Here we are able to measure directly the interaction between two dysprosium(III) ions through multi-frequency electron paramagnetic resonance spectroscopy and other techniques, and explain how this influences the dynamic magnetic behaviour of the system.

A high-frequency electron paramagnetic resonance spectrometer for multi-dimensional, multi-frequency, and multi-phase pulsed measurements.

We describe instrumentation for a high-frequency electron paramagnetic resonance (EPR) and pulsed electron-electron double resonance (PELDOR) spectroscopy. The instrumentation is operated in the frequency range of 107-120 GHz and 215-240 GHz and in the magnetic field range of 0-12.1 T. The spectrometer consisting of a high-frequency high-power solid-state source, a quasioptical system, a phase-sensitive detection system, a cryogenic-free superconducting magnet, and a (4)He cryostat enables multi-frequency continuous-wave EPR spectroscopy as well as pulsed EPR measurements with a few hundred nanosecond pulses. Here we discuss the details of the design and the pulsed EPR sensitivity of the instrumentation. We also present performance of the instrumentation in unique experiments including PELDOR spectroscopy to probe correlations in an insulating electronic spin system and application of dynamical decoupling techniques to extend spin coherence of electron spins in an insulating solid-state system.

A study of hydroxyapatite nanocrystals by the multifrequency EPR and ENDOR spectroscopy methods

Specimens of powders of hydroxyapatite (Ca10(PO4)6(OH)2) with average crystallite sizes in the range of 20–50 nm synthesized by the wet precipitation method have been investigated by the multifrequency (9 and 94 GHz) electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) methods. In specimens subjected to X-ray irradiation at room temperature, EPR signals that are caused by nitrogen compounds have been observed. Numerical calculations performed in terms of the density functional theory show that the observed EPR signal is caused by the occurrence of paramagnetic centers, the structure of which is NO32− and which replace the positions of PO43− in the hydroxyapatite structure.

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.

Photo-Induced Electron Spin Polarization in Chemical and Biological Reactions: Probing Structure and Dynamics of Transient Intermediates by Multifrequency EPR Spectroscopy

In this minireview, modern multifrequency electron paramagnetic resonance (EPR) spectroscopy, in particular, at high magnetic fields, is shown to provide detailed information about structure, motional dynamics and spin chemistry of transient radicals and radical pairs occurring in photochemical reactions. Examples discussed comprise spin-polarized radicals and radical pairs in disordered systems, such as ultraviolet-irradiated quinone and ketone compounds in fluid alcohol solutions, green-light initiated electron transfer in biomimetic porphyrin–quinone donor–acceptor model systems in frozen solution, aiming at artificial photosynthesis, and red-light initiated electron transfer in natural photosynthetic reaction-center protein complexes. The transient paramagnetic states exhibit characteristic electron polarization (CIDEP) effects originating from a triplet mechanism, a radical-pair mechanism or a correlated coupled radical-pair mechanism. They contain valuable information about structure and dynamics of the short-lived reaction intermediates. Moreover, the CIDEP effects can be exploited for signal enhancement. Continuous-wave and pulsed versions of time-resolved high-field EPR spectroscopy, such as transient EPR and electron spin-echo experiments, are compared with respect to their advantages and limitations for the specific photoreaction under study. Furthermore, orientation resolving W-band pulsed electron-electron double resonance (PELDOR) experiments on the spin-correlated coupled radical pair $$ {\text{P}}_{865}^{ \cdot + } $$ $$ {\text{Q}}_{\text{A}}^{ \cdot - } $$ in frozen solution reaction centers from the purple photosynthetic bacterium Rb. sphaeroides reveal details of distance and orientation of the pair partners in their charge-separated transient state. The results are compared with those of the ground-state P865QA. In conjunction with Q-band proton electron-nuclear double resonance (ENDOR) experiments the W-band PELDOR results provide decisive evidence that the local structure of the QA binding site does not change under photoreduction of the quinone—in agreement with earlier FTIR studies. The examples given demonstrate that multifrequency EPR experiments on disordered systems add heavily to the capabilities of “classical” spectroscopic and diffraction techniques for determining structure–dynamics–function relations of biochemical processes, since short-lived intermediates can be observed in real time while staying in their working states at biologically relevant time scales.

A multi-frequency EPR spectroscopy approach in the detection of boson peak excitations

The influence of boson peak (BP) excitations on low-temperature spin–lattice relaxation rate of a paramagnetic center embedded in a glassy matrix is investigated in the context of multi-frequency electron paramagnetic resonance (EPR) detection. In the theoretical analysis, the transition rate of spin one-half in the presence of a phonon field is calculated within the approximation of Fermi’s golden rule. Several phonon densities of states are compared, among which one originating from a model of quasi-localized vibrations has been introduced into electron spin relaxation formalism for the first time. The respective frequency dependencies of spin–lattice relaxation rates are predicted which should lead to observable effects of BP modes if a multi-frequency study at very low temperatures is performed.

Manganese-doped(1−x)BiScO3–xPbTiO3high-temperature ferroelectrics: Defect structure and mechanism of enhanced electric resistivity

The effect of multivalency manganese doping on the defect structure and enhanced electrical resistivity is studied for the high-temperature piezoelectric ($1\ensuremath{-}x$)BiScO${}_{3}$--$x$PbTiO${}_{3}$ (BSPT) solid-solution system by means of multifrequency electron paramagnetic resonance spectroscopy combined with conductivity measurements. The results show that manganese is rather incorporated on a scandium than a titanium site as an isovalent substitute (${\mathrm{Mn}}_{\mathrm{Sc}}^{\ifmmode\times\else\texttimes\fi{}}$) instead of acceptor-type centers, such as ${\mathrm{Mn}}_{\mathrm{Ti}}^{\ensuremath{'}}$, ${\mathrm{Mn}}_{\mathrm{Ti}}^{\ensuremath{'}\ensuremath{'}}$, or ${\mathrm{Mn}}_{\mathrm{Sc}}^{\ensuremath{'}}$. The enhanced electric resistivity is found being on the one hand due to the trapping of conduction electrons at the manganese functional center sites (${\mathrm{Mn}}_{\mathrm{Sc}}^{\ifmmode\times\else\texttimes\fi{}}+{e}^{\ensuremath{'}}{\mathrm{Mn}}_{\mathrm{Sc}}^{\ensuremath{'}}$). On the other hand, through the formation of ${({\mathrm{Mn}}_{\mathrm{Sc}}^{\ensuremath{'}}\ensuremath{-}{V}_{\mathrm{O}}^{\ifmmode\bullet\else\textbullet\fi{}\ifmmode\bullet\else\textbullet\fi{}})}^{\ifmmode\bullet\else\textbullet\fi{}}$ defect complexes the ionic conductivity is reduced. Concerning the overall mechanism of charge compensation in that material, both kinds of defects mutually compensate.

Characterizing Induced-Conformational Changes in Intrinscially Disordered Proteins via Multi-Frequency EPR Spectroscopy

Intrinsically disordered proteins (IDPs) contain little to no secondary or tertiary structure and are often essential in biological systems. Many IDPs undergo a conformational change, where structure is induced upon binding to its target protein. Due to their very nature, structural studies of IDPs are often challenging. Here, we show how a multi-frequency approach to site-directed spin-labeling (SDSL) electron paramagnetic resonance (EPR) spectroscopy can be utilized to characterize the mobility and conformational changes of IDPs. We have applied this method to IA3, which is a 68 residue IDP whose unstructured-to-α-helical conformational transition has been extensively characterized by various biophysical techniques. We monitored the induced conformational change in the presence of the secondary structural stabilizer 2,2,2-trifluoroethanol (TFE), at both X-, and W-band frequencies. Analyses of the X-band EPR spectral line shapes reveal that the data report on global correlation time changes consistent with a two-state model of an unstructured system and the tumbling of a rigid helix; more detailed analyzes of the X-band spectral line shapes can provide site-specific information on the residue level. Analysis of the W-band EPR spectral line shapes, however, more directly reveal site-specific structural changes. Line shape simulations of the data at both frequencies should provide further information on the site-specific conformational changes occurring in the presence of TFE and are currently underway. Using IA3 as a model system, we show multi-frequency EPR can provide insight into structural changes occurring in IDP systems that are otherwise difficult to characterize.

Study of the Kramers rare earth ions ground multiplet with a large orbital contribution by multifrequency EPR spectroscopy: in scintillator

Abstract A multifrequency Electron Paramagnetic Resonance (EPR) investigation of Ce 3 + impurities in PbWO 4 single-crystals at the conventional microwave frequency (CMF) (X-band: 9.43 GHz) and at the high frequencies/fields (HF) 95, 190 and 285 GHz was carried out. The resulting spectra are well described at all frequencies by an axial spin-Hamiltonian corresponding to an effective spin one-half system in a tetragonal site symmetry. The diagonal values of the effective g matrix of the lowest doublet of the ground multiplet, g ‖ and g ⊥ , are frequency dependent at high fields. For the magnetic field perpendicular to the tetragonal axis, the g ⊥ -parameter exhibits also a small azimuthal angular dependence, which is frequency dependent, corresponding to the tetragonal S 4 symmetry. These HF effects are associated with the mixing by the large Zeeman interaction of some of the upper-lying doublets of the ground multiplet into the lowest-lying doublet states. The CMF and multifrequency HF-EPR analysis gives a good description of the magnetic properties and allows an estimation of the crystal field splitting of the ground multiplet of Ce 3 + ions with tetragonal symmetry S 4 in the PbWO 4 scintillator.

Erratum: “Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd3+ in PbWO4 single-crystals” [J. Chem. Phys. 131, 034505 (2009)

Erratum: “Study of the ground multiplet of Kramers rare earth ions in solid matrices by multifrequency electron paramagnetic resonance spectroscopy: Nd3+ in PbWO4 single-crystals” [J. Chem. Phys. 131, 034505 (2009)

The Reaction of Synechocystis Catalase–Peroxidase (KatG) with Isoniazid Investigated by Multifrequency (9–285 GHz) EPR Spectroscopy

Three distinct electron paramagnetic resonance (EPR) spectra of radical intermediates formed as reactive intermediates in the catalytic cycle of Synechocystis catalase–peroxidase were identified. Multifrequency EPR spectroscopy, combined with site-directed mutagenesis and selective deuterium labeling of Trp and Tyr residues, allowed us to unequivocally assign such intermediates to an [Fe(IV) = O Por·+] species, the first committed intermediate in monofunctional peroxidases and two protein-based radicals, identified as Trp106· and a Tyr·, formed subsequently to the [Fe(IV) = O Por·+] species by intramolecular electron transfer. Our recent characterization of the Mycobacterium tuberculosis catalase–peroxidase showed that the Trp· sites differ among these enzymes, and that the [Fe(IV) = O Trp·] species was the reactive intermediate with the prodrug isoniazid. Accordingly, the question to address was whether the dissimilarity in the sites for the formation of the Trp· intermediates and in the geometry of the distal side was reflected by differences in the peroxidase-like reaction of Synechocystis and Mycobacterium tuberculosis catalase–peroxidases with the prodrug isoniazid. Our findings show that in the Synechocystis enzyme, the isoniazid substrate can get closer to the heme distal side and can react readily with the [Fe(IV) = O Por·+] species, at variance to the situation in the M. tuberculosis catalase–peroxidase. These results indicate that, as in the case of monofunctional peroxidases, the difference in the sites for the formation of the Trp· as alternative reactive intermediates to the [Fe(IV) = O Por·+] species is correlated to differences in substrate binding sites.