Alexander G. Maryasov

Pulsed electron double resonance (PELDOR) and its applications in free-radicals research

The papers related to the theoretical background and experimental investigations by pulsed electron double resonance (PELDOR) are reviewed. The main aim of this pulsed ESR application is to study the dipole-dipole spin interaction. In PELDOR the ESR spectrum is excited by two ESE pulses at frequencyωa and additional pumping pulse atωb. Decay functionV(T) of the ESE signal, when the time intervalT between the first ESE pulse and pumping pulse is varied, contains the information on dipole-dipole couplings in the spin system. The kinetics ofV(T) decay strongly depends upon distance, mutual orientation inside interacting spin pairs and space distribution of radicals throughout the sample. The distances between spins which were measured or estimated using PELDOR in the papers reviewed are in the range of 15 ÷ 130 Å. This pulsed ESR technique turns now to be a powerful supplement to conventional ESE in studying the free radicals space distribution..

Superslow rotations of nitroxide radicals studied by pulse EPR spectroscopy

Abstract The rotation-induced saturation transfer in EPR spectra has been studied in pulse experiments using an electron spin-echo spectrometer supplied with a magnetic field modulation coil. After the action of a saturating pulse (40 nsec width, 5 G amplitude) a rectangular pulse of current is applied to the modulation coil which shifts the resonant magnetic field by 0 to 65 G. This method makes it possible to study the time evolution of the longitudinal magnetization at various parts of an EPR spectrum, the magnetization being detected utilizing a 90–180° pulse sequence. The range of rotation times that can be studied has a lower limit of several microseconds and an upper limit of the order of magnitude of the radical spin-lattice relaxation time T 1 . The saturation transfer in an EPR spectrum for nitroxide radicals in dibutyl phthalate has been investigated at temperatures below 210 K. The effects observed have been analyzed theoretically in detail. Theory coincides with experiment only for the model of rotation by large-angle jumps. The model of continuous diffusion contradicts experimental results. Rotation correlation times can be easily determined from experimental data. The temperature dependence of the rotation time slows down sharply in the vicinity of the glass transition point.

Echo-induced EPR spectra of nitroxides in organic glasses : model of orientational molecular motions near equilibrium position

Abstract The shapes of echo-induced EPR spectra (i.e of the field dependence of ESE signal intensity) for 15 N nitroxides in organic glasses are shown to be in good agreement with those computed for the model where magnetic relaxation is caused by stochastic orientational motion of a molecule near its equilibrium position.

Visualization of distance distribution from pulsed double electron-electron resonance data

Double electron-electron resonance (DEER), also known as pulsed electron-electron double resonance (PELDOR), is a time-domain electron paramagnetic resonance method that can measure the weak dipole-dipole interactions between unpaired electrons. DEER has been applied to discrete pairs of free radicals in biological macromolecules and to clusters containing small numbers of free radicals in polymers and irradiated materials. The goal of such work is to determine the distance or distribution of distances between radicals, which is an underdetermined problem. That is, the spectrum of dipolar interactions can be readily calculated for any distribution of free radicals, but there are many, quite different distributions of radicals that could produce the same experimental dipolar spectrum. This paper describes two methods that are useful for approximating the distance distributions for the large subset of cases in which the mutual orientations of the free radicals are uncorrelated and the width of the distribution is more than a few percent of its mean. The first method relies on a coordinate transformation and is parameter-free, while the second is based on iterative least-squares with Tikhonov regularization. Both methods are useful in DEER studies of spin-labeled biomolecules containing more than two labels.

PULSED ELDOR IN SPIN-LABELED POLYPEPTIDES

Abstract The pulsed electron–electron double-resonance (PELDOR) technique was applied to obtain information about the structure of the synthetic polypeptide–biradical in a frozen glassy solution. From the concentration dependence of the PELDOR signal, the effects of intermolecular and intramolecular interactions were separated. It was found that the intramolecular dipole–dipole interactions in the biradical peptide led to the modulation effects in the PELDOR signal decay. This may be attributed to the existence of a conformational population having a distance between the two unpaired electrons of ∼20 A with a distribution of (∼2 A). Its fraction is estimated as about 25%.

Weakly coupled radical pairs in solids: ELDOR in ESE structure studies

Possibilities of the structure determination of radical pairs having fixed geometry with the help of ELDOR in ESE technique are considered. It is demonstrated that one can obtain information on relative orientation of paramagnetic centers in weakly coupled pairs in addition to the energy parameters of the spin Hamiltonian. Appropriate requirements for such experiments are formulated.

Formation of the pulsed electron-electron double resonance signal in the case of a finite amplitude of microwave fields

A theoretical study of the effect of microwave (MW) fields of finite amplitude on the process of pulsed electron-electron double resonance (PELDOR) signal formation is carried out. It is shown that the behavior of the experimentally observed values can be described by four vectors of partial magnetizations whose motion is reduced to precession in effective magnetic fields. In the case of strong spin-spin interaction, the PELDOR effect can be observed when a sufficiently powerful MW field is applied at pumping frequency to affect both components of the Pake doublet. The possibility of a “two-frequency” spin echo to appear under the action of two pulses with different carrier frequencies in the system where the inhomogeneous broadening of the electron spin resonance line contour is mainly determined by the dipole-dipole interaction is demonstrated.

Librational dynamics of nitroxide molecules in a molecular glass studied by echo-detected EPR

Nitroxides 2,2,6,6-tetramethyl-4-piperidone N-oxide (tempone), 3-carboxy-proxyl and potassium peroxylamine disulfonate (Fremy salt) in glycerol solution were studied in a wide temperature range near the glass transition temperatureTg. The echo-detected (ED) electron paramagnetic resonance (EPR) lineshape reveals strong dependence on the time interval τ between the echo-forming microwave pulses which is readily explained by anisotropic phase relaxation. Employing a librational model of molecular motion and the Redfield relaxation theory, spectra were simulated for the τ’s varying in a large interval. The anisotropic relaxation rate increases with temperature increase and it is larger for nitroxide with a larger molecular size. The mean-squared amplitude of motion, obtained from reduced hyperfine splitting in continuous-wave EPR, near Tg linearly depends on temperature which is characteristic of harmonic solids. For tempone in a host crystal 2,2,4,4-tetramethyl-cyclobutan-1, 3-dione the anisotropic spin relaxation rate decreases with temperature increase so the found feature solely belongs to a glassy state. A new approach is proposed for modeling slow wobbling motion in a restricted angular space.

Spin-polarization effects on the phase relaxation induced by dipole-dipole interactions

Abstract Spin-polarization effects on the phase relaxation due to dipole-dipole interactions of particles with 1 2 spin are studied theoretically for magnetically diluted solids. Paramagnetic centers with anisotropic g tensor are considered, random modulation of the dipole-dipole interaction by spin flip-flops in the process of spin-lattice relaxation ( T 1 process) is taken into account. The T 1 process is shown to narrow the dipolar contour at the center of the line, the limiting dipolar width being proportional to the spin polarization p at T 1 → 0. The angular dependence of the dipolar broadening is investigated. The signal decay kinetics are calculated for free induction and electron spin echo at arbitrary T 1 and p . Spin polarization is demonstrated to reduce the contribution from the dipole-dipole interaction to the irreversible spin dephasing by the spectral diffusion mechanism and to increase the contribution from the instantaneous diffusion mechanism to the echo signal decay.

Dipole-dipole interactions of high-spin paramagnetic centers in disordered systems

Dipole-dipole interactions between distant paramagnetic centers (PCs) where at least one PC has spinS>1/2 are examined. The results provide a basis for the application of pulsed electron-electron double resonance method to the measurement of distances between PCs involving high-spin species. A projection operator technique based on spectral decomposition of the secular Hamiltonian is used to calculate electron paramagnetic resonance (EPR) line splitting caused by the dipole coupling. This allows calculation of operators projecting an arbitrary wave function onto high-spin PC eigenstates when the eigenvectors of the Hamiltonian are not known. The effective spin vectors — that is, the expectation values for vector spin operators in the PC eigenstates — are calculated. The dependence of these effective spin vectors on the external magnetic field is calculated. There is a qualitative difference between pairs having at least one integer spin (non-Kramers PC) and pairs of two half-integer (Kramers PC) spins. With the help of these effective spin vectors, the dipolar line shape of EPR lines is calculated. Analytical relations are obtained for PCs with spinS=1/2 and 1. The dependence of Pake patterns on variations of zero-field splitting, Zeeman energy, temperature and dipolar coupling are illustrated.