Publications by authors named "Leonid V Kulik"

22 Publications

  • Page 1 of 1

Enhancement of Volumetric Capacitance of Binder-Free Single-Walled Carbon Nanotube Film via Fluorination.

Nanomaterials (Basel) 2021 Apr 27;11(5). Epub 2021 Apr 27.

Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, 630090 Novosibirsk, Russia.

Robust electrode materials without the addition of binders allow increasing efficiency of electrical storage devices. We demonstrate the fabrication of binder-free electrodes from modified single-walled carbon nanotubes (SWCNTs) for electrochemical double-layer capacitors (EDLCs). Modification of SWCNTs included a sonication in 1,2-dichlorobenzene and/or fluorination with gaseous BrF at room temperature. The sonication caused the shortening of SWCNTs and the splitting of their bundles. As a result, the film prepared from such SWCNTs had a higher density and attached a larger amount of fluorine as compared to the film from non-sonicated SWCNTs. In EDLCs with 1M HSO electrolyte, the fluorinated films were gradually defluorinated, which lead to an increase of the specific capacitance by 2.5-4 times in comparison with the initial values. Although the highest gravimetric capacitance (29 F g at 100 mV s) was observed for the binder-free film from non-modified SWCNT, the fluorinated film from the sonicated SWCNTs had an enhanced volumetric capacitance (44 F cm at 100 mV s). Initial SWCNT films and defluorinated films showed stable work in EDLCs during several thousand cycles.
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http://dx.doi.org/10.3390/nano11051135DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8146156PMC
April 2021

Analysis of Natural Gas Using a Portable Hollow-Core Photonic Crystal Coupled Raman Spectrometer.

Appl Spectrosc 2020 Dec 9;74(12):1496-1504. Epub 2020 Oct 9.

Institute of Solid State Physics Russian Academy of Sciences, Chernogolovka, Russian Federation.

The low accessibility of natural gas fields and transporting pipelines requires portable online analyzers of the composition of natural gas, ensuring nearly chromatographic precision and capable of in situ analysis of a wide range of gases, including infrared-inactive ones (hydrogen, oxygen, nitrogen, chlorine). We have developed an express method of gas analysis meeting all the requirements for analysis of natural gas and its derivative mixtures using a portable 532 nm Raman spectrometer rigidly connected to a hollow-core crystal photonic fiber.
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http://dx.doi.org/10.1177/0003702820915535DOI Listing
December 2020

Flavodoxin with an air-stable flavin semiquinone in a green sulfur bacterium.

Photosynth Res 2019 Nov 13;142(2):127-136. Epub 2019 Jul 13.

Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow, Russia, 119234.

Flavodoxins are small proteins with a non-covalently bound FMN that can accept two electrons and accordingly adopt three redox states: oxidized (quinone), one-electron reduced (semiquinone), and two-electron reduced (quinol). In iron-deficient cyanobacteria and algae, flavodoxin can substitute for ferredoxin as the electron carrier in the photosynthetic electron transport chain. Here, we demonstrate a similar function for flavodoxin from the green sulfur bacterium Chlorobium phaeovibrioides (cp-Fld). The expression of the cp-Fld gene, found in a close proximity with the genes for other proteins associated with iron transport and storage, increased in a low-iron medium. cp-Fld produced in Escherichia coli exhibited the optical, ERP, and electron-nuclear double resonance spectra that were similar to those of known flavodoxins. However, unlike all other flavodoxins, cp-Fld exhibited unprecedented stability of FMN semiquinone to oxidation by air and difference in midpoint redox potentials for the quinone-semiquinone and semiquinone-quinol couples (- 110 and - 530 mV, respectively). cp-Fld could be reduced by pyruvate:ferredoxin oxidoreductase found in the membrane-free extract of Chl. phaeovibrioides cells and photo-reduced by the photosynthetic reaction center found in membrane vesicles from these cells. The green sulfur bacterium Chl. phaeovibrioides appears thus to be a new type of the photosynthetic organisms that can use flavodoxin as an alternative electron carrier to cope with iron deficiency.
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http://dx.doi.org/10.1007/s11120-019-00658-1DOI Listing
November 2019

Cationic penetrating antioxidants switch off Mn cluster of photosystem II in situ.

Photosynth Res 2019 Nov 13;142(2):229-240. Epub 2019 Jul 13.

Faculty of Biology, M.V. Lomonosov Moscow State University, Moscow, Russia, 119234.

Mitochondria-targeted antioxidants (also known as 'Skulachev Ions' electrophoretically accumulated by mitochondria) exert anti-ageing and ROS-protecting effects well documented in animal and human cells. However, their effects on chloroplast in photosynthetic cells and corresponding mechanisms are scarcely known. For the first time, we describe a dramatic quenching effect of (10-(6-plastoquinonyl)decyl triphenylphosphonium (SkQ1) on chlorophyll fluorescence, apparently mediated by redox interaction of SkQ1 with Mn cluster in Photosystem II (PSII) of chlorophyte microalga Chlorella vulgaris and disabling the oxygen-evolving complex (OEC). Microalgal cells displayed a vigorous uptake of SkQ1 which internal concentration built up to a very high level. Using optical and EPR spectroscopy, as well as electron donors and in silico molecular simulation techniques, we found that SkQ1 molecule can interact with Mn atoms of the OEC in PSII. This stops water splitting giving rise to potent quencher(s), e.g. oxidized reaction centre of PSII. Other components of the photosynthetic apparatus proved to be mostly intact. This effect of the Skulachev ions might help to develop in vivo models of photosynthetic cells with impaired OEC function but essentially intact otherwise. The observed phenomenon suggests that SkQ1 can be applied to study stress-induced damages to OEC in photosynthetic organisms.
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http://dx.doi.org/10.1007/s11120-019-00657-2DOI Listing
November 2019

EPR evidence for a fast-relaxing iron center in Na-translocating NADH:quinone-oxidoreductase.

J Inorg Biochem 2018 07 6;184:15-18. Epub 2018 Apr 6.

Belozersky Institute of Physico-Chemical Biology, Lomonosov Moscow State University, Moscow 119234, Russia. Electronic address:

A paramagnetic Cys[Fe] center was detected by pulse EPR in Na-translocating NADH:quinone-oxidoreductase (Na-NQR) by influence of this center on transverse and longitudinal spin relaxation of Na-NQR flavin radicals. The oxidation state of the Cys[Fe] center was Fe in the oxidized and Fe in the reduced Na-NQR, as deduced from the temperature dependence of spin relaxation rates of different flavin radicals. A high-spin state of iron in the Cys[Fe] center was assigned to both forms of Na-NQR.
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http://dx.doi.org/10.1016/j.jinorgbio.2018.04.004DOI Listing
July 2018

Time-domain shape of electron spin echo signal of spin-correlated radical pairs in polymer/fullerene blends.

J Magn Reson 2017 03 23;276:86-94. Epub 2017 Jan 23.

Voevodsky Institute of Chemical Kinetics and Combustion of the Siberian Branch of the Russian Academy of Sciences, Institutskaya 3, Novosibirsk 630090, Russia; Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia. Electronic address:

Temporal shape of electron spin echo (ESE) signal of photoinduced spin-correlated radical pairs (SCRP) in composite of conductive polymer P3HT and substituted fullerene PCBM is studied in details. ESE signals of radical pairs (RP) P3HT/PCBM are calculated in realistic model, taking into account finite microwave pulse length. Inhomogeneous broadening of resonant lines and interradical distance distribution are included. Experimentally observed ESE time-domain shape was found to contradict predictions of conventional SCRP theory, which would be valid in the case of very fast electron transfer. Thus, instantaneous formation of singlet SCRP is not the case for P3HT/PCBM pair, and spin system has enough time to evolve coherently during sequential electron transfer. While it is impossible to reproduce experimental data within simple singlet SCRP model, assumption of presence of additional - with respect to what is predicted by singlet SCRP theory - AE (absorption/emission) spin polarization gives convincing accordance with the experiment. Density matrix of RP P3HT/PCBM is a superposition of two contributions, namely the parts reflecting (i) antiphase polarization of original singlet-born SCRP and (ii) additional AE-polarization which is generated during initial stage of charge separation. AE-polarization affects experimental ESEEM (electron spin echo envelope modulation) traces, as well as ESE shape, making impossible their interpretation via simple singlet SCRP model. However, this effect can be eliminated by averaging of ESEEM traces over EPR spectral positions. Finally, choosing the optimal gate for ESE time-domain integration and proper microwave detection phase tuning are considered.
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http://dx.doi.org/10.1016/j.jmr.2017.01.016DOI Listing
March 2017

Light-induced charge separation in a P3HT/PCBM composite as studied by out-of-phase electron spin echo spectroscopy.

Phys Chem Chem Phys 2016 Oct;18(41):28585-28593

Voevodsky Institute of Chemical Kinetics and Combustion of Siberian Branch of Russian Academy of Sciences, Institutskaya 3, 630090 Novosibirsk, Russia. and Novosibirsk State University, Pirogova 2, 630090 Novosibirsk, Russia.

A composite material of semiconducting polymer P3HT and fullerene derivative PCBM was studied by means of electron spin echo (ESE) spectroscopy. The out-of-phase ESE signal was observed under laser irradiation of the composite at low temperature. We assume that during the charge separation process firstly the spin-correlated radical pairs in the singlet-polarized spin state are formed, and then the net polarization of radical pairs arises due to spin evolution. Both types of polarizations contribute to the out-of-phase ESE signal in the case of non-ideal microwave pulses. Analytical calculation of the echo shape for both types of initial polarization revealed that the contribution of the net polarization becomes zero after averaging over the whole EPR spectrum of the radical pair. This behavior was experimentally confirmed; thus the analysis of the out-of-phase ESE signal was simplified. Interspin distance distributions in the charge transfer state were obtained by modeling the out-of-phase ESE envelope modulation measured at different delays after laser flash T from 300 ns to 3.3 μs at a temperature of 65 K. Due to geminate recombination and diffusion of the radicals from the interface the distribution becomes significantly broader with larger distances prevailing at longer T values. The average distance between charges increases from 3.5 nm to 5.6 nm with an increase in T.
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http://dx.doi.org/10.1039/c6cp05389kDOI Listing
October 2016

Higher triplet state of fullerene C70 revealed by electron spin relaxation.

J Chem Phys 2015 Dec;143(24):244314

Voevodsky Institute of Chemical Kinetics and Combustion, Siberian Branch of Russian Academy of Sciences, Institutskaya St. 3, 630090 Novosibirsk, Russia.

Spin-lattice relaxation times T1 of photoexcited triplets (3)C70 in glassy decalin were obtained from electron spin echo inversion recovery dependences. In the range 30-100 K, the temperature dependence of T1 was fitted by the Arrhenius law with an activation energy of 172 cm(-1). This indicates that the dominant relaxation process of (3)C70 is described by an Orbach-Aminov mechanism involving the higher triplet state t2 which lies 172 cm(-1) above the lowest triplet state t1. Chemical modification of C70 fullerene not only decreases the intrinsic triplet lifetime by about ten times but also increases T1 by several orders of magnitude. The reason for this is the presence of a low-lying excited triplet state in (3)C70 and its absence in triplet C70 derivatives. The presence of the higher triplet state in C70 is in good agreement with the previous results from phosphorescence spectroscopy.
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http://dx.doi.org/10.1063/1.4938417DOI Listing
December 2015

Out-of-Phase Electron Spin Echo Studies of Light-Induced Charge-Transfer States in P3HT/PCBM Composite.

J Phys Chem B 2015 Oct 22;119(43):13543-8. Epub 2015 Apr 22.

Voevodsky Institute of Chemical Kinetics and Combustion of the Siberian Branch of the Russian Academy of Sciences , Institutskaya 3, Novosibirsk 630090, Russia.

The light-induced charge-transfer (CT) state in the composite of the conductive polymer poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) has been studied by electron spin echo (ESE) spectroscopy. The out-of-phase ESE signal corresponding to the spin-correlated radical pair P3HT(+)/PCBM(-) has been observed in this composite material. The time-domain ESE shape for different delays between the laser flash and the microwave pulse sequence has been analyzed. In order to explain the evolution of the out-of-phase ESE signal as a function of the delay between the microwave pulses, a model of the CT state is proposed. The hole is assumed to be delocalized on the P3HT chain over several thiophene subunits, while the point-dipole approximation is used to describe the interaction with the electron on PCBM. The distribution of distances between the positive and negative charges in the CT state has been evaluated.
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http://dx.doi.org/10.1021/acs.jpcb.5b02142DOI Listing
October 2015

Theory of pulsed Reaction Yield Detected Magnetic Resonance.

Phys Chem Chem Phys 2012 Oct;14(38):13325-31

International Tomography Center SB RAS, Institutskaya 3a, Novosibirsk 630090, Russia.

We propose pulse sequences for Reaction Yield Detected Magnetic Resonance (RYDMR), which are based on refocusing the zero-quantum coherences in radical pairs by non-selective microwave pulses and using the population of a radical pair singlet spin state as an observable. The new experiments are analogues of existing EPR experiments such as the primary echo, Carr-Purcell, ESEEM, stimulated echo and Mims ENDOR. All pulse sequences are supported by analytical results and numerical calculations. The pulse sequences can be used for more efficient and highly detailed characterization of intermediates of chemical reactions and charge carriers in organic semiconductors.
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http://dx.doi.org/10.1039/c2cp42117hDOI Listing
October 2012

Sodium-dependent movement of covalently bound FMN residue(s) in Na(+)-translocating NADH:quinone oxidoreductase.

Biochemistry 2012 Jul 25;51(27):5414-21. Epub 2012 Jun 25.

Department of Molecular Energetics of Microorganisms, A.N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, Russia.

Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) is a component of respiratory electron-transport chain of various bacteria generating redox-driven transmembrane electrochemical Na(+) potential. We found that the change in Na(+) concentration in the reaction medium has no effect on the thermodynamic properties of prosthetic groups of Na(+)-NQR from Vibrio harveyi, as was revealed by the anaerobic equilibrium redox titration of the enzyme's EPR spectra. On the other hand, the change in Na(+) concentration strongly alters the EPR spectral properties of the radical pair formed by the two anionic semiquinones of FMN residues bound to the NqrB and NqrC subunits (FMN(NqrB) and FMN(NqrC)). Using data obtained by pulse X- and Q-band EPR as well as by pulse ENDOR and ELDOR spectroscopy, the interspin distance between FMN(NqrB) and FMN(NqrC) was found to be 15.3 Å in the absence and 20.4 Å in the presence of Na(+), respectively. Thus, the distance between the covalently bound FMN residues can vary by about 5 Å upon changes in Na(+) concentration. Using these results, we propose a scheme of the sodium potential generation by Na(+)-NQR based on the redox- and sodium-dependent conformational changes in the enzyme.
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http://dx.doi.org/10.1021/bi300322nDOI Listing
July 2012

Electronic structure of a weakly antiferromagnetically coupled Mn(II)Mn(III) model relevant to manganese proteins: a combined EPR, 55Mn-ENDOR, and DFT study.

Inorg Chem 2011 Sep 11;50(17):8238-51. Epub 2011 Aug 11.

Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany.

An analysis of the electronic structure of the [Mn(II)Mn(III)(μ-OH)-(μ-piv)(2)(Me(3)tacn)(2)](ClO(4))(2) (PivOH) complex is reported. It displays features that include: (i) a ground 1/2 spin state; (ii) a small exchange (J) coupling between the two Mn ions; (iii) a mono-μ-hydroxo bridge, bis-μ-carboxylato motif; and (iv) a strongly coupled, terminally bound N ligand to the Mn(III). All of these features are observed in structural models of the oxygen evolving complex (OEC). Multifrequency electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) measurements were performed on this complex, and the resultant spectra simulated using the Spin Hamiltonian formalism. The strong field dependence of the (55)Mn-ENDOR constrains the (55)Mn hyperfine tensors such that a unique solution for the electronic structure can be deduced. Large hyperfine anisotropy is required to reproduce the EPR/ENDOR spectra for both the Mn(II) and Mn(III) ions. The large effective hyperfine tensor anisotropy of the Mn(II), a d(5) ion which usually exhibits small anisotropy, is interpreted within a formalism in which the fine structure tensor of the Mn(III) ion strongly perturbs the zero-field energy levels of the Mn(II)Mn(III) complex. An estimate of the fine structure parameter (d) for the Mn(III) of -4 cm(-1) was made, by assuming the intrinsic anisotropy of the Mn(II) ion is small. The magnitude of the fine structure and intrinsic (onsite) hyperfine tensor of the Mn(III) is consistent with the known coordination environment of the Mn(III) ion as seen from its crystal structure. Broken symmetry density functional theory (DFT) calculations were performed on the crystal structure geometry. DFT values for both the isotropic and the anisotropic components of the onsite (intrinsic) hyperfine tensors match those inferred from the EPR/ENDOR simulations described above, to within 5%. This study demonstrates that DFT calculations provide reliable estimates for spectroscopic observables of mixed valence Mn complexes, even in the limit where the description of a well isolated S = 1/2 ground state begins to break down.
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http://dx.doi.org/10.1021/ic200767eDOI Listing
September 2011

Isotropic reorientations of fullerene C70 triplet molecules in solid glassy matrices revealed by light-induced electron paramagnetic resonance.

J Chem Phys 2011 Aug;135(5):054507

Institute of Chemical Kinetics and Combustion, Russian Academy of Sciences, Novosibirsk, Russia.

Continuous-wave X-band electron paramagnetic resonance (EPR) of fullerene C(70) molecules excited to a triplet state by continuous light illumination was studied in molecular glasses of o-terphenyl and cis/trans-decaline and in the glassy polymers polymethylmethacrylate (PMMA) and polystyrene (PS). Above ∼100 K, a distinct narrowing of EPR lineshape of the triplet was observed, which was very similar for all systems studied. EPR lineshape was simulated reasonably well within a framework of a simple model of random jumps, which implies that the C(70) molecule performs isotropic orientational motion by sudden jumps of arbitrary angles. In simulations, a single correlation time τ(c) was used, varying in the range of 10(-7)-10(-8) s. Near and below 100 K electron spin echo (ESE) signals were also obtained which were found to decay exponentially. Correlation times τ(c) obtained from simulation of the EPR spectra in the slow-motion limit (τ(c) close to 10(-7) s) turned out to be in good agreement with the phase memory times T(M) of the ESE decay, which additionally supports the employed simple model. The observed motional effects provide evidence that the nanostructure of the solid glassy media of different origins is soft enough to allow a large asymmetric C(70) molecule to reorient rapidly. Except for the EPR spectra of the triplet, in the center of the spectra, a small admixture of a narrow line was also observed; its possible nature is briefly discussed.
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http://dx.doi.org/10.1063/1.3618738DOI Listing
August 2011

The electronic structures of the S(2) states of the oxygen-evolving complexes of photosystem II in plants and cyanobacteria in the presence and absence of methanol.

Biochim Biophys Acta 2011 Jul 22;1807(7):829-40. Epub 2011 Mar 22.

Max-Planck-Institut für Bioanorganische Chemie, D-45470 Mülheim an der Ruhr, Germany.

The electronic properties of the Mn(4)O(x)Ca cluster in the S(2) state of the oxygen-evolving complex (OEC) were studied using X- and Q-band EPR and Q-band (55)Mn-ENDOR using photosystem II preparations isolated from the thermophilic cyanobacterium T. elongatus and higher plants (spinach). The data presented here show that there is very little difference between the two species. Specifically it is shown that: (i) only small changes are seen in the fitted isotropic hyperfine values, suggesting that there is no significant difference in the overall spin distribution (electronic coupling scheme) between the two species; (ii) the inferred fine-structure tensor of the only Mn(III) ion in the cluster is of the same magnitude and geometry for both species types, suggesting that the Mn(III) ion has the same coordination sphere in both sample preparations; and (iii) the data from both species are consistent with only one structural model available in the literature, namely the Siegbahn structure [Siegbahn, P. E. M. Accounts Chem. Res.2009, 42, 1871-1880, Pantazis, D. A. et al., Phys. Chem. Chem. Phys.2009, 11, 6788-6798]. These measurements were made in the presence of methanol because it confers favorable magnetic relaxation properties to the cluster that facilitate pulse-EPR techniques. In the absence of methanol the separation of the ground state and the first excited state of the spin system is smaller. For cyanobacteria this effect is minor but in plant PS II it leads to a break-down of the S(T)=½ spin model of the S(2) state. This suggests that the methanol-OEC interaction is species dependent. It is proposed that the effect of small organic solvents on the electronic structure of the cluster is to change the coupling between the outer Mn (Mn(A)) and the other three Mn ions that form the trimeric part of the cluster (Mn(B), Mn(C), Mn(D)), by perturbing the linking bis-μ-oxo bridge. The flexibility of this bridging unit is discussed with regard to the mechanism of O-O bond formation.
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http://dx.doi.org/10.1016/j.bbabio.2011.03.002DOI Listing
July 2011

Effects of spin transitions degeneracy in pulsed EPR of the fullerene C70 triplet state produced by continuous light illumination.

Spectrochim Acta A Mol Biomol Spectrosc 2011 May 2;78(5):1548-52. Epub 2011 Feb 2.

Institute of Chemical Kinetics and Combustion, Novosibirsk, Russia.

X-band echo-detected electron paramagnetic resonance (ED EPR) spectra of triplet state of fullerene C(70) generated by continuous light illumination were found to correspond below 30K to a non-equilibrium electron spin polarization. Above 30K spectra are characteristic of Boltzmann equilibrium. Spectra were simulated fairly well with zero-field splitting parameters D=153 MHz and E and distributed within the range of 6-42 MHz. The origin of E distribution is attributed to the Jahn-Teller effect, which in glassy matrix is expected to depend on the local surrounding of a fullerene molecule (a so-called E-strain). In the center of ED EPR spectra a narrow hole was observed. With increase of the microwave pulse turning angle this hole transforms into a single narrow absorptive line. Numerical simulations by density matrix formalism confirm that central hole originates from a simultaneous excitation of both allowed electron spin transitions of the triplet (T(0)↔T(+) and T(0)↔T(-)), because of their degeneracy at this spectral position. Also explanations are given why this hole has not been observed in the previously reported experiments on continuous wave EPR and on ED EPR under laser pulse excitation.
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http://dx.doi.org/10.1016/j.saa.2011.01.047DOI Listing
May 2011

Pulse EPR and ENDOR study of 1,2,3-benzodithiazolyl, 2,1,3-benzothiaselenazolyl and 1,2,3-benzodiselenazolyl radicals.

Phys Chem Chem Phys 2011 Mar 4;13(9):3873-80. Epub 2011 Jan 4.

Institute of Chemical Kinetics and Combustion, Siberian Branch of Russian Academy of Sciences, 630090 Novosibirsk, Russia.

1,2,3-Benzodithiazolyl, 2,1,3-benzothiaselenazolyl and 1,2,3-benzodiselenazolyl radicals were generated by the reduction of the corresponding cations and investigated by pulse EPR and ENDOR in frozen CHCl(3) solutions at 30 and 80 K. These methods, in combination with density functional theory calculations, were used to study the magnetic parameters of the radicals, namely the principal values of the nitrogen and proton hyperfine interactions and g-tensors. The spin density distribution was shown to be nearly the same for all investigated radicals and, therefore, replacement of sulfur by selenium leads to a limited perturbation of the radicals' electronic structure. A high anisotropy of the g-tensors was found for the selenium-containing radicals.
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http://dx.doi.org/10.1039/c0cp02051fDOI Listing
March 2011

Spin relaxation of fullerene C(70) photoexcited triplet in molecular glasses: Evidence for onset of fast orientational motions of molecules in the matrix near 100 K.

J Chem Phys 2009 Oct;131(14):144501

Institute of Chemical Kinetics and Combustion, Institutskaya 3, Novosibirsk 630090, Russia.

Electron spin echo (ESE) was applied to study transversal spin relaxation of photoexcited triplet state of fullerene C(70) molecules in glassy o-terphenyl and cis-/trans-decalin matrices (glass transition temperatures of 243 and 137 K, respectively). The relaxation rate T(2) (-1) was found to increase sharply above 110 K in o-terphenyl and above 100 K in decalin. It is suggested that this increase arises from interaction of (3)C(70) pseudorotation with fast molecular librations in the matrix. Both these types of motion involve atomic vibrations and are uniaxial in their nature, the known literature data on Raman light scattering and others indicate that molecular librations may be thermally activated in glasses just near 100 K. The increase in T(2) (-1) near 100 K is not observed for photoexcited triplet state of fullerene C(60), for which pseudorotation is not uniaxial. As the fullerene molecule has a size much larger than that for glass solvent molecules, it is likely that molecular librations in the matrix are of collective nature.
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http://dx.doi.org/10.1063/1.3244983DOI Listing
October 2009

Redox properties of the prosthetic groups of Na(+)-translocating nadh:quinone oxidoreductase. 1. Electron paramagnetic resonance study of the enzyme.

Biochemistry 2009 Jul;48(27):6291-8

Department of Molecular Energetics of Microorganisms, A. N. Belozersky Institute of Physico-Chemical Biology, Moscow State University, Moscow 119992, Russia.

Redox properties of all EPR-detectable prosthetic groups of Na(+)-translocating NADH:quinone oxidoreductase (Na(+)-NQR) from Vibrio harveyi were studied at pH 7.5 using cryo-EPR spectroelectrochemistry. Titration shows five redox transitions. One with E(m) = -275 mV belongs to the reduction of the [2Fe-2S] cluster, and the four others reflect redox transitions of flavin cofactors. Two transitions (E(m)(1) = -190 mV and E(m)(2) = -275 mV) originate from the formation of FMN anion radical, covalently bound to the NqrC subunit, and its subsequent reduction. The remaining two transitions arise from the two other flavin cofactors. A high potential (E(m) = -10 mV) transition corresponds to the reduction of riboflavin neutral radical, which is stable at rather high redox potentials. An E(m) = -130 mV transition reflects the formation of FMN anion radical from a flavin covalently bound to the NqrB subunit, which stays as a radical down to very low potentials. Taking into account the EPR-silent, two-electron transition of noncovalently bound FAD located in the NqrF subunit, there are four flavins in Na(+)-NQR all together. Defined by dipole-dipole magnetic interaction measurements, the interspin distance between the [2Fe-2S](+) cluster and the NqrB subunit-bound FMN anion radical is found to be 22.5 +/- 1.5 A, which means that for the functional electron transfer between these two centers another cofactor, most likely FMN bound to the NqrC subunit, should be located.
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http://dx.doi.org/10.1021/bi900524mDOI Listing
July 2009

Anisotropic pseudorotation of the photoexcited triplet state of fullerene C60 in molecular glasses studied by pulse EPR.

J Phys Chem A 2008 Mar 26;112(12):2519-25. Epub 2008 Feb 26.

Novosibirsk State University, 630090 Pirogova 2, Novosibirsk, Russia.

Spin-polarized echo-detected electron paramagnetic resonance (EPR) spectra and the transversal relaxation rate T2(-1) of the photoexcited triplet state of fullerene C60 molecules were studied in o-terphenyl, 1-methylnaphthalene, and decalin glassy matrices. The model is composed of a fast (correlation time approximately 10(-12) s) pseudorotation of (3)C60 in a local anisotropic potential created by interaction of the fullerene molecule with the surrounding matrix molecules. In simulations, this potential is assumed to be axially symmetric around some axis of a preferable orientation in a matrix cage. The fitted value of the potential was found to depend on the type of glass and to decrease monotonically with a temperature increase. A sharp increase of the T2(-1) temperature dependence was found near 240 K in glassy o-terphenyl and near 100 K in glassy 1-methylnaphthalene and decalin. This increase probably is related to the influence on the pseudorotation of the onset of large-amplitude vibrational molecular motions (dynamical transition in glass) that are known for glasses from neutron scattering and molecular dynamics studies. The obtained results suggest that molecular and spin dynamics of the triplet fullerene are extremely sensitive to molecular motions in glassy materials.
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http://dx.doi.org/10.1021/jp0765291DOI Listing
March 2008

Focusing the view on nature's water-splitting catalyst.

Philos Trans R Soc Lond B Biol Sci 2008 Mar;363(1494):1167-77; discussion 1177

Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany.

Nature invented a catalyst about 3Gyr ago, which splits water with high efficiency into molecular oxygen and hydrogen equivalents (protons and electrons). This reaction is energetically driven by sunlight and the active centre contains relatively cheap and abundant metals: manganese and calcium. This biological system therefore forms the paradigm for all man-made attempts for direct solar fuel production, and several studies are underway to determine the electronic and geometric structures of this catalyst. In this report we briefly summarize the problems and the current status of these efforts and propose a density functional theory-based strategy for obtaining a reliable high-resolution structure of this unique catalyst that includes both the inorganic core and the first ligand sphere.
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http://dx.doi.org/10.1098/rstb.2007.2212DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2614100PMC
March 2008

Electronic structure of the Mn4OxCa cluster in the S0 and S2 states of the oxygen-evolving complex of photosystem II based on pulse 55Mn-ENDOR and EPR spectroscopy.

J Am Chem Soc 2007 Nov 10;129(44):13421-35. Epub 2007 Oct 10.

Max Planck Institute for Bioinorganic Chemistry, Stiftstrasse 34-36, Mülheim an der Ruhr, Germany.

The heart of the oxygen-evolving complex (OEC) of photosystem II is a Mn4OxCa cluster that cycles through five different oxidation states (S0 to S4) during the light-driven water-splitting reaction cycle. In this study we interpret the recently obtained 55Mn hyperfine coupling constants of the S0 and S2 states of the OEC [Kulik et al. J. Am. Chem. Soc. 2005, 127, 2392-2393] on the basis of Y-shaped spin-coupling schemes with up to four nonzero exchange coupling constants, J. This analysis rules out the presence of one or more Mn(II) ions in S0 in methanol (3%) containing samples and thereby establishes that the oxidation states of the manganese ions in S0 and S2 are, at 4 K, Mn4(III, III, III, IV) and Mn4(III, IV, IV, IV), respectively. By applying a "structure filter" that is based on the recently reported single-crystal EXAFS data on the Mn4OxCa cluster [Yano et al. Science 2006, 314, 821-825] we (i) show that this new structural model is fully consistent with EPR and 55Mn-ENDOR data, (ii) assign the Mn oxidation states to the individual Mn ions, and (iii) propose that the known shortening of one 2.85 A Mn-Mn distance in S0 to 2.75 A in S1 [Robblee et al. J. Am. Chem. Soc. 2002, 124, 7459-7471] corresponds to a deprotonation of a mu-hydroxo bridge between MnA and MnB, i.e., between the outer Mn and its neighboring Mn of the mu3-oxo bridged moiety of the cluster. We summarize our results in a molecular model for the S0 --> S1 and S1 --> S2 transitions.
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November 2007

55Mn pulse ENDOR at 34 GHz of the S0 and S2 states of the oxygen-evolving complex in photosystem II.

J Am Chem Soc 2005 Mar;127(8):2392-3

Max-Planck-Institut für Bioanorganische Chemie, Stiftstrasse 34-36, D-45470 Mülheim/Ruhr, Germany.

55Mn pulse ENDOR experiments at 34 GHz (Q-band) are reported for the S0 and S2 states of the oxygen-evolving complex of photosystem II. Their numerical analysis (i) shows that in both states all four Mn ions are magnetically coupled, (ii) allows a refinement of the hyperfine interaction (HFI) parameters obtained earlier for the S2 state at X-band (Peloquin, J. M.; Campbell, K. A.; Randall, D. W.; Evanchik, M. A.; Pecoraro, V. L.; Armstrong, W. H.; Britt, R. D. J. Am. Chem. Soc. 2000, 122, 10926-10942), (iii) provides the first reliable 55Mn HFI tensors for the S0 state, and (iv) leads to the suggestion that the Mn oxidation states in S0 and S2 are Mn4(III, III, III, IV) and Mn4(III, IV, IV, IV), respectively. In addition, a Q-band EPR spectrum is reported for the S0 state, and inversion-recovery experiments at 4.5 K directly show that the electron spin-lattice relaxation for the S0 state is about 2 orders of magnitude faster than that for the S2 state.
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March 2005
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