The kinetics of the primary electron donor P700+ and the quinone acceptor A1– redox transitions were simultaneously studied for the first time in the time range of 200 μs-10 ms using high-frequency pulse Q-band EPR spectroscopy at cryogenic temperatures in various complexes of photosystem I (PSI) from the cyanobacterium Synechocystis sp. PCC 6803. In the A1-core PSI complexes that lack 4Fe4S clusters, the kinetics of the A1– and P700+ signals disappearance at 100 K were similar and had a characteristic time of τ ≈ 500 μs, caused by charge recombination in the P700+A1A– ion-radical pair in the A branch of redox cofactors. The kinetics of the backward electron transfer from A1B– to P700+ in the B branch of redox cofactors with τ < 100 μs could not be resolved due to time limitations of the method. In the native PSI complexes with a full set of redox cofactors and in the FX-core complexes, containing the 4Fe4S cluster FX, the kinetics of the A1– signal was significantly faster than that of the P700+ signal. The disappearance of the A1– signal had a characteristic time of 280-350 μs; it was suggested that, in addition to the backward electron transfer from A1A– to P700+ with τ ≈ 500 μs, its kinetics also includes the forward electron transfer from A1A– to the 4Fe4S cluster FX, which had slowed down to 150-200 μs. In the kinetics of P700+ reduction, it was possible to distinguish components caused by the backward electron transfer from A1– (τ ≈ 500 μs) and from 4Fe4S clusters (τ = 1 ms for the FX-core and τ > 5 ms for native complexes). These results are in qualitative agreement with the data on the kinetics of P700+ reduction obtained previously using pulse absorption spectrometry at cryogenic temperatures.

In this work, the nutation of the spins of unpaired electrons in the nitroxide biradical of bis-methano[60] fullerene was experimentally studied. Nutation frequencies were found in a wide range of microwave field power. To interpret the obtained results, numerical calculations of the nutation of biradicals were carried out for a set of parameters of the spin-spin interaction of a pair of unpaired electrons and for different values of the Rabi frequency of the microwave field. At comparing numerical results with experimental data, we also used the results of analytical calculations of nutation for some model situations. As a result of the analysis of experimental data on nutation, an estimate of the exchange and dipole-dipole interactions for the studied biradical was obtained. They are consistent with the results obtained from analysis of the shape of the EPR spectrum for a given biradical.

The observation of the exchange narrowing effect is a routine event in EPR spectroscopy. In this paper, I want to draw attention to the fact that every time we observe an exchange narrowing of the EPR spectrum, we have a gas of identical bosons, where the prerequisites of BEC formation are satisfied at room temperature. Imagine the possibility to create BEC without the need to cooling down to nano Kelvin temperatures to get a gas of identical bosons because the phenomenon of exchange narrowing of the EPR spectrum can be easily observed at room temperatures. This article provides a detailed explanation of how a gas of identical bosons can be formed at room temperature in dilute solutions of paramagnetic particles with a discrete EPR spectrum of individual particles. There are still many questions about the Bose–Einstein condensation of a spin polariton gas, which is created under conditions of exchange narrowing of EPR spectra. With the expectation of obtaining additional information about the condensation of bosons in the situation under consideration, this article proposes the protocol of one EPR experiment to prove that the effect of exchange narrowing of EPR spectra can be instrumental in creating BEC in dilute solutions at room temperature.

Experimental confirmation of the manifestations of new spin exchange paradigm in EPR spectra of 14N nitroxide radical solutions is presented. It was shown that in the region of relatively low concentrations of radicals, the two side components of the spectrum have a mixed shape (the sum of the absorptive line and dispersive line). The dispersion contributions in these two lines have opposite signs. As the concentration of radicals increases, the contribution of dispersion passes through an extremum and in the region of maximum contribution of dispersion, the contribution of absorption to these two lines changes sign. In the region of high concentrations of radicals, when one homogeneously broadened line is practically observed, it turns out that these side components have resonant frequencies that do not coincide with the frequency of the center of gravity of the spectrum.

We celebrate 80 years of EPR with a special issue of Applied Magnetic Resonance featuring both reviews and regular research articles. The focus is new opportunities for application of EPR and new directions for development of EPR. This introduction concisely surveys the scope of EPR and hints at future developments.

The phase of the primary electron spin echo signal was analyzed on model radical pairs (RPs) in a singlet spin state. An interpretation is given of the fact known in the literature that the echo signal of such pairs may not have a component with a “normal” phase, which is expected for the echo signal of non-interacting unpaired electrons in the RP. For the first time, it has been theoretically shown that forbidden EPR transitions can make a significant contribution to the phase shift of the magnetization of RP spins, caused by the dipole–dipole hyperfine interaction of unpaired RP electrons with magnetic nuclei. The “anomalous” phase of the spin echo signal of interacting spins in radical pairs is a demonstration that spin dynamics is accompanied by reversible transitions of dipole polarization (spin magnetization) and quadrupole spin polarization.

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Spin exchange caused by the exchange interaction during bimolecular collisions of paramagnetic particles in dilute solutions causes several effects: broadens the resonance lines of the EPR spectrum, changes the resonance frequencies, changes the shape of the resonance lines of the spectrum, and causes the effect of the exchange narrowing of the spectrum. The well-established belief is that the dipole–dipole interaction between paramagnetic particles only broadens the resonance lines. According to the new paradigm of spin exchange, the dipole–dipole interaction causes effects similar to the effects of spin exchange. In this article, a detailed quantitative analysis of the effect of the dipole–dipole interaction on the shape of the EPR spectra of dilute solutions of paramagnetic particles is carried out for the model system. It is shown that the contribution of the dipole–dipole interaction to the spin coherence transfer between particles makes it possible to more accurately determine the rate of spin exchange and, as a result, the rate of bimolecular collisions of molecules from the analysis of the shape of the EPR spectra. An experimental protocol is proposed that definitely highlights the contribution of the dipole–dipole interaction to the transfer of spin coherence.

Spin exchange during random bimolecular collisions of paramagnetic particles in dilute solutions leads to a surprising effect. Collective modes of motion of the average values of the transverse magnetization components (spin coherences) of subensembles of radicals with different resonant frequencies are formed. The elementary excitations of these modes can be considered as quasiparticles. As a result of interactions with the microwave field, these quasiparticles form spin polaritons. The theoretical prediction for the formation of spin polaritons was made on the basis that the resonance frequencies observed in the EPR experiment depend on the power of the microwave field. In this work, we present an experimental confirmation of the resonant frequency dependence of the spin ensemble on the microwave power for nitroxide radical [15N]-4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl in toluene.

The continuous-wave saturation (CWS) behavior of several measureable parameters in an EPR spectrum of a nitroxide free radical is carried out theoretically and tested experimentally with 15N-d17-4-Hydroxy-2,2,6,6-tetramethylpiperidine 1-oxyl in 60% aqueous-glycerol solutions from 273 to 340 K. The theory explicitly includes spin relaxation due to electron spin exchange and dipole–dipole interactions, but no other spectral diffusion pathways of paramagnetic relaxation. A primary objective of this work was to study the CWS of the dispersion signal induced by spin exchange and dipole–dipole interactions, a signal that is superimposed upon the absorption. This was rendered possible because, using modern least-squares fitting methods, the two signals may also be separated to high precision. Using a new rigorous theory, theoretical spectra were simulated and were also separated into absorption and dispersion components. The absorption components, both experimental and theoretical, may be analyzed using the traditional Bloch equations and compared with literature results. The dispersion components may only be analyzed by direct comparison of parameters derived from the theoretical and experimental spectra. A comparison of the amplitudes of the absorption and the dispersion components provides a severe test of the theory and the experimental results are good agreement with the theory. The experimental values of $${T}_{1}$$ both from The Bloch equations and the direct comparison of the absorption components were comparable to the limited and scattered literature results where both pulsed- and CWS methods were employed. From the theory, the same value of $${T}_{1}$$ was found from the CWS of the Lorentzian line width and the doubly-integrated intensity; however, there were discrepancies in the values of $${T}_{1}$$ for these two parameters experimentally.

It is shown that Torry’s theory of nutation based on the Bloch equations for the magnetization vector cannot be used for describing the “nutation” of interacting spins (including the splitting of spin energy levels in zero magnetic field). The Bloch equations presume that the magnetic moment vector of spins fully determines the spin state. However, this is true of only noninteracting particles with spin S = 1/2. Systematic analysis is performed for the response (“nutation”) of spins to the instantaneous application of an alternating magnetic field for the simplest system with spin S = 1 as an example. The dependence of spin nutation on the spin–spin interaction and the pattern of excitation of spins by an alternating field is analyzed in detail. In the conditions when the spin–spin interactions are comparable with of the spin interaction with the alternating field, the motion of spin magnetization is described as the sum of contributions oscillating with different frequencies, which are equal to the frequencies of transitions between the eigenstates of the spin Hamiltonian in a rotating coordinate system. For the first time, the spin “nutation” is described using the Heisenberg mathematical apparatus. In this approach, the equations of motion are written directly for quantities measured in experiment. The complete orthogonal set of quantities for spins consists of the dipole moment and multipole polarizations. For demonstrating the potential of this description of “nutation,” the specific case of paramagnetic particles with spin S = 1 is considered. Coupled equations of motion for the dipole and quadrupole moments are obtained with account for the energy of splitting in zero magnetic field. These equations can be referred to as generalized equations for the magnetic polarization of spins. The equations show that in the presence of spin–spin interactions, the reversible mutual conversion of the dipole and quadrupole moments occurs. This leads to oscillations of the length of the spin magnetization vector, the projection of which is usually observed in experiment. Therefore, the experimentally observed oscillations of the magnetization projections reflect the nutation of the magnetization vector as well as the modulation of the length of this vector due to mutual conversion of the dipole and quadrupole polarizations.

In photosynthetic reaction centers of intact photosystem I (PSI) complexes from cyanobacteria, electron transfer at room temperature occurs along two symmetrical branches of redox cofactors A and B at a ratio of 3 : 1 in favor of branch A. Previously, this has been indirectly demonstrated using pulsed absorption spectroscopy and more directly by measuring the decay modulation frequencies of electron spin echo signals (electron spin echo envelope modulation, ESEEM), which allows to determine the distance between the separated charges of the primary electron donor P700+ and phylloquinone acceptors A1A– and A1B– in the symmetric redox cofactors branches A and B. In the present work, these distances were determined using ESEEM in PSI complexes lacking three 4Fe–4S clusters, FX, FA, and FB, and the PsaC protein subunit (the so-called P700–A1 core), in which phylloquinone molecules A1A and A1B serve as the terminal electron acceptors. It was shown that in the P700–A1 core preparations, the average distance between the centers of the P700+A1– ion-radical pair at a temperature of 150 K in an aqueous glycerol solution and in a dried trehalose matrix, as well as in a trehalose matrix at 280 K, is 25.5 Å, which corresponds to the symmetrical electron transfer along the A and B branches of redox cofactors at a ratio of 1 : 1. Possible reasons for the change in the electron transfer symmetry in PSI upon removal of the PsaC subunit and 4Fe–4S clusters FX, FA, and FB are discussed.

Quantum processing units (QPU) in theory propose a computational supremacy in a significant number of tasks. Quantum programs are well suited for vector and matrix data processing. The greatest concern is whether physical implementations will step over the noise and decoherence limitations: today in the noisy intermediate-scale quantum (NISQ) era the bigger the problem, the less reliable the results are. In this work, we show that even NISQ computers can be used to obtain reliable results in processing experimental data. We perform quantum Fourier transform (QFT) of PELDOR oscillation at IonQ trapped ion QPU followed by spin labels' distance measurement by analysis of oscillation data. For 4 and 5-qubit programs, we show the results comparable to Fourier analysis on a classical computer.

On the occasion of my 85th birthday, I share insights about my life and science. This autobiography has been written in an interesting period of my life. Two years ago I formulated a new paradigm in a particular scientific discipline: spin exchange. This work has helped me to see that the paradigm formulation is an effective practical tool for increasing the effectiveness of scientific research. Today I am full of plans to use this tool to continue scientific research.

The Carr–Purcell–Meiboom–Gill (CPMG) multipulse protocol is actively used in magnetic resonance to study decoherence processes. However, its use in electron paramagnetic resonance spectroscopy is often related to manifestations of unwanted contributions from other signals. In this study, expressions for the observed echo signals in the CPMG protocol are obtained in an explicit form, taking into account the superposition of other signals at the time of observation. Separating the contributions of different signals by modifying the CPMG protocol is experimentally demonstrated.

For a model system of spins with two frequencies, a detailed analysis of the spin exchange in the EPR spectrum under saturation conditions in dilute solutions of paramagnetic particles is performed. For an arbitrary power of the microwave field, explicit analytical formulas are obtained for the frequency and width of the spectral lines, and for the contribution of the dispersion to the observed lines in the region of relatively slow spin exchange. A formula is obtained for the integral intensity of the spectrum for an arbitrary microwave field strength and spin exchange rate. The analysis of the dependence of the width of the spectrum lines and the contribution of the dispersion to the spectrum on the spin coherence transfer rate and on the microwave field power revealed the "destructive interference” of these factors in terms of their manifestation in the EPR spectra. It is shown that in a wide range of spin exchange rates up to the collapse of the spectrum, the dependence of the line broadening on the spin exchange rate is given by a straight line, the slope of which does not depend on the power of the microwave field. Extrapolating this straight line to the zero spin exchange rate gives a value that depends on the microwave field strength and the relaxation times of the longitudinal and transverse magnetization. The results obtained can be used to find the magnetic resonance parameters of the spins, including the spin–lattice relaxation time, from the continuous wave EPR spectroscopy data with high accuracy.