The regularities of the mechanical activation of α-Bi 2 O 3 , the nature and thermal stability of defects resulting from the activation, and an increase in the reactivity of the oxide have been analyzed with the use of X-ray diffraction, measurement of specific surface area, and synchronous thermal analysis combined with mass spectrometry. The process of Bi 2 O 3 mechanical activation may be divided into two stages. At the stage of the fracture of particles, their specific surface area grows to S = 3.2 m 2 /g, while the particle size and size L of the coherent-scattering region decrease to 100 and 40 nm, respectively. At the stage of friction, S somewhat decreases, while L remains unchanged. After grinding in air, a phase of Bi 2 O 2 CO 3 is observed in addition to the main phase of monoclinic α-Bi 2 O 3 , with the former phase resulting from sorption of CO 2 from air. When an activated sample is heated, bismutite decomposes with CO 2 liberation in a wide temperature range. For an activated sample of nanosized oxide, heat absorption due to the α-Bi 2 O 3 → δ-Bi 2 O 3 phase transition begins at a temperature that is 10°C lower than the usual one. The reactivity of activated Bi 2 O 3 has been determined by the example of its reduction in the atmosphere of CO. The mechanical activation increases Bi 2 O 3 conversion upon reduction at 600°C by 2.5 times and decreases the temperature of the reduction onset by nearly 100°C.

The regularities of the mechanical activation of α-Bi2O3, the nature and thermal stability of defects resulting from the activation, and an increase in the reactivity of the oxide have been analyzed with the use of X-ray diffraction, measurement of specific surface area, and synchronous thermal analysis combined with mass spectrometry. The process of Bi2O3 mechanical activation may be divided into two stages. At the stage of the fracture of particles, their specific surface area grows to S = 3.2 m2/g, while the particle size and size L of the coherent-scattering region decrease to 100 and 40 nm, respectively. At the stage of friction, S somewhat decreases, while L remains unchanged. After grinding in air, a phase of Bi2O2CO3 is observed in addition to the main phase of monoclinic α-Bi2O3, with the former phase resulting from sorption of CO2 from air. When an activated sample is heated, bismutite decomposes with CO2 liberation in a wide temperature range. For an activated sample of nanosized oxide, heat absorption due to the α-Bi2O3 → δ-Bi2O3 phase transition begins at a temperature that is 10°C lower than the usual one. The reactivity of activated Bi2O3 has been determined by the example of its reduction in the atmosphere of CO. The mechanical activation increases Bi2O3 conversion upon reduction at 600°C by 2.5 times and decreases the temperature of the reduction onset by nearly 100°C.

ABSTRACTMultiferroic (MF) composites based on nanoparticles consisting of a silica core and a shell of spin‐variable Fe(III) complexes in a polymer matrix (polystyrene) were synthesized and characterized by different methods. The nanoparticles had the formula 80SiO2·20{Fe[OSi(Me)(OEt)2]3}, and their particle size was on the order of 5–7 nm. Dielectric and electron spin resonance studies showed the presence of two types of Fe ions in the nanocomposite. Iron ions in the low‐spin state [Fe(III)‐LS] and iron ions in the high‐spin state [Fe(III)‐HS], which were bound by indirect exchange interactions through oxygen and silicon atoms {[Fe(III)‐LS]─O─Si─O─[Fe(III)‐HS]} were responsible for the MF properties of the composites with core–shell nanoparticles. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 136, 47681.

The synthesis, structure, and electrophysical properties of a polymer-inorganic biocompatible composite based on unsaturated chitosan ether, namely, allyl chitosan, and vinyltriethoxysilane are studied. During composite synthesis, allyl chitosan forms an individual nanophase with vinyltriethoxysilane and its condensation products in the polymer matrix of allyl chitosan. The size of nanoparticles embedded in a polymer matrix increases from 50 to 1000 nm as the fraction of the added vinyltriethoxysilane grows. Under exposure to UV radiation, both homopolycondensation and heteropolycondensation occur in the composite films via crosslinking according to the radical mechanism and the composite becomes insoluble in water. It has been shown that the resulting composites feature ionic conductivity under application of both direct current and high-frequency electric fields to the sample. Conductivity is provided by a proton–electron ensemble that concentrates at the nanoparticle/polymer matrix interface.

The combination of high-field electron paramagnetic resonance (EPR) with site-directed spin labeling (SDSL) techniques employing nitroxide radicals has turned out to be particularly powerful in revealing subtle changes of the polarity and proticity profiles in proteins enbedded in membranes. This information can be obtained by orientation-selective high-field EPR resolving principal components of the nitroxide Zeeman (g) and hyperfine ( A) tensors of the spin labels attached to specific molecular sites. In contrast to the g- and A-tensors, the (14)N ( I = 1) quadrupole interaction tensor of the nitroxide spin label has not been exploited in EPR for probing effects of the microenvironment of functional protein sites. In this work it is shown that the W-band (95 GHz) high-field electron spin echo envelope modulation (ESEEM) method is well suited for determining with high accuracy the (14)N quadrupole tensor principal components of a nitroxide spin label in disordered frozen solution. By W-band ESEEM the quadrupole components of a five-ring pyrroline-type nitroxide radical in glassy ortho-terphenyl and glycerol solutions have been determined. This radical is the headgroup of the MTS spin label widely used in SDSL protein studies. By DFT calulations and W-band ESEEM experiments it is demonstrated that the Q(yy) value is especially sensitive to the proticity and polarity of the nitroxide environment in H-bonding and nonbonding situations. The quadrupole tensor is shown to be rather insensitive to structural variations of the nitroxide label itself. When using Q(yy) as a testing probe of the environment, its ruggedness toward temperature changes represents an important advantage over the g xx and A(zz) parameters which are usually employed for probing matrix effects on the spin labeled molecular site. Thus, beyond measurenments of g xx and A(zz) of spin labeled protein sites in disordered solids, W-band high-field ESEEM studies of (14)N quadrupole interactions open a new avenue to reliably probe subtle environmental effects on the electronic structure. This is a significant step forward on the way to differentiate between effects from matrix polarity and hydrogen-bond formation.

We report on recent 95 and 360 GHz high-field electron paramagnetic resonance (EPR), electron-nuclear double resonance (ENDOR) and pulsed electron-electron double resonance (PELDOR) studies of wild-type and mutant reaction centers (RCs) from the photosynthetic bacteriumRhodobacter sphaeroides. Taking advantage of the excellent spectral and temporal resolution of EPR at 95 and 360 GHz, the electron-transfer (ET) cofactors radical ions and spin-correlated radical pairs were characterized by theirg- and hyperfine-tensor components, their anisotropicT 2 relaxation as well as by the dipolar interaction between P 865 •+ Q A •− radical pairs. The goal of these studies is to better understand the dominant factors determining the specificity and directionality of transmembrane ET processes in photosynthetic RC proteins. In particular, our multifrequency experiments elucidate the subtle cofactor-protein interactions, which are essential for fine-tuning the ET characteristics, e.g., the unidirectionality of the light-induced ET pathways along the A branch of the RC protein. By our high-field techniques, frozen-solution RCs of novel site-specific single and double mutants ofR. sphaeroides were studied to modulate the ET characteristics, e.g., even to the extent that dominant B branch ET prevails. The presented multifrequency EPR work culminates in first 360 GHz ENDOR results from organic nitroxide radicals as well as in first 95 GHz high-field PELDOR results from orientationally selected spin-polarized radical pairs P 865 •+ Q A •− , which allow to determine the full geometrical structure of the pairs even in frozen-solution RCs.

Distance and relative orientation of functional groups within protein domains and their changes during chemical reactions determine the efficiency of biological processes. In this work on disordered solid-state electron-transfer proteins, it is demonstrated that the combination of pulsed high-field EPR spectroscopy at the W band (95 GHz, 3.4 T) with its extensions to PELDOR ( pulsed electron-electron double resonance) and RIDME (relaxation-induced dipolar modulation enhancement) offers a powerful tool for obtaining not only information on the electronic structure of the redox partners but also on the three-dimensional structure of radical-pair systems with large interspin distances ( up to about 5 nm). Strategies are discussed both in terms of data collection and data analysis to extract unique solutions for the full radical-pair structure with only a minimum of additional independent structural information. By this novel approach, the three- dimensional structure of laser-flash-induced transient radical pairs P(865)(.+)Q(A)(.-) in frozen-solution reaction centers (RCs) from the photosynthetic bacterium Rhodobacter (Rb.) sphaeroides is solved. The measured positions and relative orientations of the weakly coupled ion radicals P-865(.+) and Q(A)(.-) are compared with those of the precursor cofactors P-865 and Q(A) known from X-ray crystallography. A small but significant reorientation of the reduced ubiquinone Q(A) is revealed and interpreted as being due to the photosynthetic electron transfer. In contrast to the large conformational change of Q(B)(.-) upon light illumination of the RCs, the small light-induced reorientation of Q(A)(.-) had escaped previous attempts to detect structural changes of photosynthetic cofactors upon charge separation. Although small, they still may be of functional importance for optimizing the electronic coupling of the redox partners in bacterial photosynthesis both for the charge-separation and charge-recombination processes.

In the last decade, joint efforts of biologists, chemists and physicists have helped in understanding the dominant factors determining specificity and directionality of transmembrane transfer processes in proteins. In this endeavor, electron paramagnetic resonance (EPR) spectroscopy has played an important role. Characteristic examples of such determining factors are hydrogen-bonding patterns and polarity effects of the microenvironment of protein sites involved in the transfer process. These factors may undergo characteristic changes during the reaction and, thereby, control the efficiency of biological processes, e.g. light-induced electron and proton transfer across photosynthetic membranes or ion-channel formation of bacterial toxins. In case the transfer process does not involve stable or transient paramagnetic species or states, site-directed spin labeling with suitable nitroxide radicals still allows EPR techniques to be used for studying structure and conformational dynamics of the proteins in action. By combining site-directed spin labeling with high-field/high-frequency EPR, unique information on the proteins is revealed, which is complementary to that of X-ray crystallography, solid-state NMR, FRET, fast infrared and optical spectroscopic techniques. The main object of this publication is twofold: (i) to review our recent spin-label high-field EPR work on the bacteriorhodopsin light-driven proton pump from Halobacterium salinarium and the Colicin A ion-channel forming bacterial toxin produced in Escherichia coli, (ii) to report on novel high-field EPR experiments for probing site-specific pK(a) values in protein systems by means of pH-sensitive nitroxide spin labels. Taking advantage of the improved spectral and temporal resolution of high-field EPR at 95 GHz/3.4 T and 360 GHz/12.9 T, as compared to conventional X-band EPR (9.5 GHz/0.34 T), detailed information on the transient intermediates of the proteins in biological action is obtained. These intermediates can be observed and characterized while staying in their working states on biologically relevant timescales. The paper concludes with an outlook of ongoing high-field EPR experiments on site-specific protein mutants in our laboratories at FU Berlin and Osnabruck. Copyright (c) 2005 John Wiley & Sons, Ltd.

A vacuum-tube device for the generation of pulsed microwave radiation in the submillimeter range (up to 380 GHz) is presented, designed for use as a source in a 360 GHz high-field/high-frequency electron paramagnetic resonance (EPR) spectrometer—the pulsed Orotron. Analogous to the known continuous wave (cw) version, in the pulsed Orotron microwave radiation is generated by the interaction of a nonrelativistic electron beam with a diffraction grating (stimulated Smith–Purcell radiation) in feedback with an open Fabry–Pérot resonator construction. The presented design extends the cw Orotron by a gate electrode and a high-voltage pulsing unit to control the electron beam current. The generated pulses at 360 GHz have pulse lengths from 100 ns–10 μs and a pulse power of (22±5) mW. The output in a broader frequency band between 320 and 380 GHz ranges from 20 up to 60 mW. Within a 10 μs time slot, incoherent pulse trains of arbitrary duration can be generated. The pulsed Orotron has been incorporated in the quasioptical microwave bridge of a heterodyne induction mode EPR spectrometer. The first free induction decay measurements at a microwave frequency of 360 GHz and a magnetic field of 12.8 T on a polycrystalline perylenyl–ion sample are presented and future applications and extensions of Orotron-EPR spectroscopy are discussed.

Free radicals formed in the radiolysis and photolysis of wood and lignin were studied using X-band and D-band EPR measurements. It was found that singlet spectra at g ≈ 2, which appeared upon the low-temperature (77 K) γ- and UV irradiation of wood and lignin, or singlets detected in a posteffect on heating the irradiated samples belong to radicals having conjugated carbon–carbon bonds. Formyl radicals in γ-irradiated wood and peroxide radicals in γ- and UV-irradiated wood were detected for the first time using EPR spectra. The radiation-chemical and quantum yields of radical formation reactions were determined. In wood at 77 K, G R ≈ 3.2 1/100 eV and ϕ R ≈ 2 × 10 –3 .

The aim of this book is to highlight the state of an exciting field that has been developing rapidly in the last decade: Very High Frequency EPR (VHF EPR) or sometimes called Very High Field EPR (conveniently, also abbreviated as VHF EPR). This introductory chapter covers the early period of the development of High Frequency EPR especially in the group of Yakov Lebedev, from its initiation in 1970 to till 1988. While this introduction provides historical aspects and some insight about the people that have been involved in the development of this field, the rest of the volume is devoted to current developments. The scientific achievements of that period are summarized in several earliest overviews (Grinberg et al., 1981 in Russian) and (Grinberg et al., 1983; Lebedev et al., 1992, available in English). This chapter has been written to highlight the driving forces and restraints, and the steps forward as well as the difficulties that occurred during this exciting period, and solutions that were found to overcome those difficulties. Also it provides additional personal aspects beyond the scientific results.

A field-jump device for fast stepping the electron paramagnetic resonance magnetic field around 3.4 T during pulsed electron-electron double resonance experiments at W-band (95 GHz) is described. Field jumps up to ±160 G and submicrosecond times for the full field-jump cycle allow precession frequency transfer experiments to be made for the determination of the nanometer distance and the orientation of nitroxide spin-label pairs in disordered samples.

The natural paramagnetism of lignocellulose materials and its variation in the treatment of materials with chemicals (NaOH, HCl, Cl-2) used in the technological processes of delignification and bleaching were studied by X- and W-band ESR spectroscopy. It was found that most singlet spectra at g similar to 2 are due to radicals with conjugated carbon-carbon bonds. The action of an alkali on lignin produces o-semiquinone radicals, whose spectrum was first recorded with a W-band ESR spectrometer. When the chlorination temperature of lignin is lowered (170-200 K), chlorine adds primarily to double bonds of macromolecules; as a result, the yield of toxic chlorophenols considerably decreases and the bleaching process becomes more effective because of the degradation of polyconjugated systems.

The possibility of the mechanochemical synthesis of metal-containing mono-, bi-, and triradicals under mechanical pulse action on mixtures of metal oxides (CuO, ZnO, CdO, PbO, Al 2 O 3 , Ga 2 O 3 , Sb 2 O 3 , Bi 2 O 3 , Cr 2 O 3 , TiO 2 , GeO 2 , ZrO 2 , or SnO 2 ) with quinone and pyrocatechol was examined. The reaction products are formed both on the surface of metal oxides and as individual solid phases. The mechanism of chemical processes that occur under the action of a single elastic wave pulse is proposed.

The kinetics of sequential generation of mono-, bi-, and triradical centers in heterogeneous reactions of toluene solutions of orthoquinones, catechols, and their mixtures with γ-Al2O3 was studied. The localization of mono- and biradical centers on the solid oxide surface and the transfer of triradical Al3+ (Q−)3 complexes (Q− is the corresponding 3,6-di-tert-butylsemiquinone radical) to a solution (extraction of Al from the solid phase) were proved. The number of the triradical complexes extracted is considerably higher than that of active surface centers accessible for coordination with radical ligands. The conditions favoring the formation of the radical complexes on the Al ions were established. Possible mechanisms of these processes were proposed.