Matvey V. Fedin

Ultrafast Photoswitching in a Copper‐Nitroxide‐Based Molecular Magnet

Molecular compounds with photoswitchable magnetic properties have been intensively investigated over the last decades due to their prospective applications in nanoelectronics, sensing and magnetic data storage. The family of copper-nitroxide-based molecular magnets represents a new promising type of photoswitchable compounds. We report the first study of these appealing systems using femtosecond optical spectroscopy. We unveil the mechanism of ultrafast (<50 fs) spin state photoswitching and establish its principal differences compared to other photoswitchable magnets. On this basis, we propose potential advantages of copper-nitroxide-based molecular magnets for the future design of ultrafast magnetic materials.

Development and Application of Spin Traps, Spin Probes, and Spin Labels.

This chapter focuses on major achievements of the last decade in the synthesis and applications of spin traps, spin probes, and spin labels. Our discussion on spin trapping is mainly concerned with novel aspects of nitrones used as spin traps and with the kinetics caused by bioreductants. The second part of the chapter deals with recent developments in site-directed spin labeling (SDSL) for studying structure and functions of proteins and nucleic acids. We focus on SDSL EPR distance measurements using advanced trityl and nitroxide labels, on new approaches for incorporation of spin labels in biomolecules, and finally, on recent room/physiological temperature measurements made feasible by these novel spin labels.

Room-temperature electron spin relaxation of nitroxides immobilized in trehalose: Effect of substituents adjacent to NO-group

Trehalose has been recently promoted as efficient immobilizer of biomolecules for room-temperature EPR studies, including distance measurements between attached nitroxide spin labels. Generally, the structure of nitroxide influences the electron spin relaxation times, being crucial parameters for room-temperature pulse EPR measurements. Therefore, in this work we investigated a series of nitroxides with different substituents adjacent to NO-moiety including spirocyclohexane, spirocyclopentane, tetraethyl and tetramethyl groups. Electron spin relaxation times (T1, Tm) of these radicals immobilized in trehalose were measured at room temperature at X- and Q-bands (9/34GHz). In addition, a comparison was made with the corresponding relaxation times in nitroxide-labeled DNA immobilized in trehalose. In all cases phase memory times Tm were close to 700ns and did not essentially depend on structure of substituents. Comparison of temperature dependences of Tm at T=80-300K shows that the benefit of spirocyclohexane substituents well-known at medium temperatures (∼100-180K) becomes negligible at 300K. Therefore, unless there are specific interactions between spin labels and biomolecules, the room-temperature value of Tm in trehalose is weakly dependent on the structure of substituents adjacent to NO-moiety of nitroxide. The issues of specific interactions and stability of nitroxide labels in biological media might be more important for room temperature pulsed dipolar EPR than differences in intrinsic spin relaxation of radicals.

Complementary-addressed site-directed spin labeling of long natural RNAs

Nanoscale distance measurements by pulse dipolar Electron paramagnetic resonance (EPR) spectroscopy allow new insights into the structure and dynamics of complex biopolymers. EPR detection requires site directed spin labeling (SDSL) of biomolecule(s), which remained challenging for long RNAs up-to-date. Here, we demonstrate that novel complementary-addressed SDSL approach allows efficient spin labeling and following structural EPR studies of long RNAs. We succeeded to spin-label Hepatitis C Virus RNA internal ribosome entry site consisting of ≈330 nucleotides and having a complicated spatial structure. Application of pulsed double electron–electron resonance provided spin–spin distance distribution, which agrees well with the results of molecular dynamics (MD) calculations. Thus, novel SDSL approach in conjunction with EPR and MD allows structural studies of long natural RNAs with nanometer resolution and can be applied to systems of biological and biomedical significance.

Theoretical and experimental studies of chemically induced electron-nuclear polarization in low magnetic fields

Chemically induced electron-nuclear polarization at low magnetic fields is considered theoretically for a radical with one magnetic nucleus. It is shown that large non-equilibrium populations of the radical spin levels are expected to exist under low magnetic fields. This large electron-nuclear polarization has been observed experimentally during the photolysis of two phosphine oxides using a modified L-band TREPR setup. The TREPR spectra and kinetics of dimethoxyphosphonyl and diphenylphosphonyl radicals have been measured and analysed in low magnetic fields, and excellent agreement between experiment and theory has been achieved.

Interaction of triarylmethyl radicals with DNA termini revealed by orientation-selective W-band double electron–electron resonance spectroscopy

Spin labels selectively attached to biomolecules allow high-accuracy nanoscale distance measurements using pulsed electron paramagnetic resonance (EPR), in many cases providing the only access to the structure of complex biosystems. Triarylmethyl (TAM) radicals have recently emerged as a new class of spin labels expanding the applicability of the method to physiological temperatures. Along with other factors, the accuracy of the obtained distances crucially relies on the understanding of interactions between biomolecules and spin labels. In this work, we consider such crucial interactions and their impact on pulsed EPR distance measurements in TAM-labeled DNAs. Using orientation-selective high-frequency (94 GHz) double electron-electron resonance (DEER) we demonstrate strong specific interactions between DNA termini and TAM labels, leading to a significant restriction of their conformational mobility. An understanding of such interactions guides the way to select optimum TAM-labeling strategies, thus refining nanoscale EPR distance measurements in nucleic acids and their complexes under physiological conditions.

Reaction of Paramagnetic Synthon, Lithiated 4,4,5,5-Tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide, with Cyclic Aldonitrones of the Imidazole Series

It was shown that dipole-stabilized paramagnetic carbanion lithiated 4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazol-1-oxyl 3-oxide can be attached in a nucleophilic manner to either isolated or conjugated aldonitrones of the 2,5-dihydroimidazole 3-oxide and 2H-imidazole 1-oxide series to afford adducts the subsequent oxidation of which leads to polyfunctional mono- and diradicals. According to XRD, at least two polymorphic modifications can be formed during crystallization of the resulting paramagnetic compounds, and for each of them, geometric parameters of the molecules are similar. An EPR spectrum of the diradical in frozen toluene has a complicated lineshape, which can be fairly well reproduced by using X-ray diffraction structural analysis and the following set of parameters: D=14.9 mT, E=1.7 mT; tensor a((14) N)=[0.260 0.260 1.625] mT, two equivalent tensors for the nitronyl nitroxide moiety a((14) N)=[0.198 0.198 0.700] mT, and g≈2.007. According to our DFT and ab initio calculations, the intramolecular exchange in the diradical is very weak and most likely ferromagnetic.

Cu(II) complex with nitronyl nitroxide whose paramagnetism is suppressed by temperature decrease and/or pressure increase

The reaction of Cu(hfac)2 with a spirocyclic nitronyl nitroxide 2-(1-ethylpyrazol-4-yl)-4,5-bis(spirocyclopentane)-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (LEt) gave a heterospin complex [Cu(hfac)2LEt]-I, whose solid state is formed by polymer chains with a head-to-tail motif (the bridging function inside the chains is performed by the paramagnetic organic ligand). At 295 K the distance from the Cu atom to the N atom of the pyrazole ring and to the O atom of the NO group is 2.333(2) and 2.451(2) A, respectively. The cooling of the compound to 110 K leads to a structural transition, as a result of which the unit cell volume decreases by more than 7%. The elongated axis of the octahedron changes from N–Cu–ONO to Ohfac–Cu–Ohfac in all coordination units as a result of a significant decrease in the Cu–ONO and Cu–N bond lengths to 2.002(4) and 2.034(5) A, respectively, and an increase in the two Cu–Ohfac distances from 1.950(2) and 1.955(2) A to 2.263(4) and 2.213(4) A, respectively. The [Cu(hfac)2LEt]-I crystals are elastic and stable during the thermally induced spin crossover. The transition from the high-temperature to low-temperature phase is accompanied by a spin transition at 125 K at 1.0 × 10−4 GPa, as a result of which [Cu(hfac)2LEt]-I transforms from a paramagnetic compound to a diamagnetic one due to the complete spin coupling in the {Cu(II)–O˙–N<} exchange clusters inside the polymer chains. The reversible phase transformation is accompanied by a reversible contrastive change of color from blue to dark brown upon cooling and vice versa from dark brown to blue upon heating. The spin crossover temperature is very sensitive to the change of external pressure: when pressure increases from 1.0 × 10−4 GPa to 0.48 GPa, it increases to 325 K. The increase of pressure provokes a transition from a high-spin to low-spin state, which again transforms into the high-spin state when the external load is removed. The [Cu(hfac)2LEt]-I crystals are the first examples of polymer-chain breathing crystals whose paramagnetic properties are suppressed by the lowering of temperature and/or increase of pressure.

Bismuth germanate as a perspective material for dielectric resonators in EPR spectroscopy

High purity bismuth germanate (Bi4(GeO4)3, BGO) is proposed and implemented as an alternative material for dielectric EPR resonators. A significant improvement of the absolute sensitivity can be readily achieved by substituting the alumina insert (ring) by BGO-made one in commercially available X-band EPR probeheads. Four BGO dielectric inserts of 2, 3, 4 and 5mm inner diameter (ID) were made for comparison with standard 5mm inner diameter alumina insert. All inserts were introduced into commercial Bruker EPR resonator ER 4118X-MD-5W1, and their performance was investigated. The Q-values of empty resonators, B1 saturation curves and continuous wave EPR spectra of DPPH (2,2-diphenyl-1-picrylhydrazyl) were measured and analyzed in a temperature range 6-300K. BGO-made resonators were found superior in several important aspects. The background signals arising from BGO are much weaker compared to those of alumina at B=0-0.6T and T=6-300K; this is especially useful for measuring weak signals in the half-field region, as well as those near the central field. Moreover, mechanical properties of BGO allow easy fabrication of dielectric bodies having various shapes and sizes; in particular, small BGO resonators (e.g. ID=2 or 3mm) strongly enhance sensitivity for small samples due to increase of the filling factor. All these advantages have been also inspected in the pulse mode, proving that higher B1 fields and better filling factors can be achieved, contributing to the overall enhancement of the performance.

Probing Microenvironment in Ionic Liquids by Time-Resolved EPR of Photoexcited Triplets

Unusual physicochemical properties of ionic liquids (ILs) open vistas for a variety of new applications. Herewith, we investigate the influence of microviscosity and nanostructuring of ILs on spin dynamics of the dissolved photoexcited molecules. We use two most common ILs [Bmim]PF6 and [Bmim]BF4 (with its close analogue [C10mim]BF4) as solvents and photoexcited Zn tetraphenylporphyrin (ZnTPP) as a probe. Time-resolved electron paramagnetic resonance (TR EPR) is employed to investigate spectra and kinetics of spin-polarized triplet ZnTPP in the temperature range 100-270 K. TR EPR data clearly indicate the presence of two microenvironments of ZnTPP in frozen ILs at 100-200 K, being manifested in different spectral shapes and different spin relaxation rates. For one of these microenvironments TR EPR data is quite similar to those obtained in common frozen organic solvents (toluene, glycerol, N-methyl-2-pyrrolidone). However, the second one favors the remarkably slow relaxation of spin polarization, being much longer than in the case of common solvents. Additional experiments using continuous wave EPR and stable nitroxide as a probe confirmed the formation of heterogeneities upon freezing of ILs and complemented TR EPR results. Thus, TR EPR of photoexcited triplets can be effectively used for probing heterogeneities and nanostructuring in frozen ILs. In addition, the increase of polarization lifetime in frozen ILs is an interesting finding that might allow investigation of short-lived intermediates inaccessible otherwise.