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Volume 231 Issue 3 - Special Issue dedicated to Kev Salikhov on the occasion of his 80th birthday / Guest Editor: Gerd Buntkowsky, Konstantin L. Ivanov and Hans-Martin Vieth / Part II: Spin dynamics and spin hyperpolarization

Issue of Zeitschrift für Physikalische Chemie

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Journal Overview

Publicly Available February 8, 2017

Frontmatter

Page range: i-iv

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February 8, 2017

Intramolecular Electron Transfer from Tryptophan to Guanosyl Radicals in a Linked System as a Model of DNA Repair

Olga B. Morozova, Natalya N. Fishman, Alexandra V. Yurkovskaya

Page range: 479-495

Abstract

As a model of chemical DNA repair, intramolecular electron transfer from tryptophan to the radical of the purine base guanosine combined into a conjugate by a flexible linker was studied by time-resolved chemically induced dynamic nuclear polarization (CIDNP). The guanosyl radicals were photochemically generated in the quenching reaction of the triplet excited dye 2,2′-dipyridyl. The CIDNP kinetics was obtained by detection of NMR spectra containing anomalously enhanced signals of diamagnetic products that are formed during a variable period after excitation by a laser pulse. The kinetic data obtained for the protons located on the guanosyl and tryptophanyl moieties of the conjugate were compared to those obtained in photoreactions of the molecules containing the same linker, but with only one of the two reactive moieties of the conjugate – tryptophanyl or guanosyl. Strong differences between the CIDNP kinetics of different conjugates were revealed and explained by a rapid intramolecular electron transfer from tryptophan to the guanosyl radical in the conjugate. Model simulations of the CIDNP kinetics allowed for determination of the rate constant of intramolecular electron transfer at (1.0±0.5)×10 6 s –1 indicating a high potential of chemical repair of the guanosyl radical by means of electron transfer from the tryptophanyl moiety in a surrounding protein pool that can provide rather efficient protection of oxidized DNA bases from pathological damage on a submicrosecond time scale.

August 26, 2016

SABRE Hyperpolarization of Bipyridine Stabilized Ir-Complex at High, Low and Ultralow Magnetic Fields

Andrey N. Pravdivtsev

Page range: 497-511

Abstract

A strong limitation of nuclear magnetic resonance is its low inherent sensitivity that can be overcome by using an appropriate hyperpolarization technique. Presently, dynamic nuclear polarization and spin-exchange optical pumping are the only hyperpolarization techniques that are used in applied medicine. However, both are relatively complex in use and expensive. Here we present a modification of the signal amplification by reversible exchange (SABRE) hyperpolarization method – SABRE on stabilized Ir-complexes. A stabilized Ir-complex (here we used bipyridine for stabilization) can be hyperpolarized in a wide range of magnetic fields from a few μT upto 10 T with 15 N polarization of about 1–3%. Moreover, the investigated complex can be incorporated into biomolecules or other bulky molecules; in this situation exchange with para -hydrogen will allow one to continuously generate hyperpolarization.

August 26, 2016

Relaxation of Multiple Quantum NMR Coherences in Quasi-One-Dimensional Spin Systems

Georgy A. Bochkin, Edward B. Fel’dman, Sergey G. Vasil’ev

Page range: 513-525

Abstract

Dynamics and relaxation of the multiple quantum (MQ) NMR coherences of the zeroth and second orders are studied experimentally and theoretically in a quasi-one-dimensional chain of nuclear spins 19 F in calcium fluorapatite. The dependencies of the intensities of those coherences on the time of the preparation period of the MQ NMR experiment is obtained. A good agreement of the experiment with theoretical predictions is demonstrated. Dipolar relaxation of the MQ NMR coherences is investigated on the evolution period of the MQ NMR experiment. A theory of dipolar relaxation of the MQ NMR coherences is developed for the model in which only the ZZ part of the secular dipole–dipole interactions is taken into account (ZZ model). It is shown that the MQ NMR coherence of the zeroth order is not subject to dipolar relaxation in the ZZ model. The experimental data qualitatively agree with the results of the developed theory for the MQ NMR coherence of the second order.

September 1, 2016

Exciting Wide NMR Spectra of Static Solid Samples with Weak Radiofrequency Fields

Diego Carnevale, Srinivas Chinthalapalli, Geoffrey Bodenhausen

Page range: 527-543

Abstract

Trains of short pulses in the manner of ‘delays alternating with nutations for tailored excitation’ (DANTE) have been applied to the Pake patterns of protons of water molecules trapped in a static powdered sample of barium chlorate monohydrate. The spin dynamics in the course of such experiments have been investigated by means of numerical simulations and compared with the ideal refocusing that can be achieved under magic-angle spinning (MAS). Solid echoes yield essentially undistorted lineshapes, in contrast to direct excitation without refocusing that leads to severe dispersions of the phases because of inhomogeneous interactions such as homonuclear dipolar couplings and anisotropic chemical shifts. The selectivity of DANTE sequences allows one to access ‘slices’ of the Pake pattern that can be related to particular crystallite orientations. Single-crystal spectra can therefore be extracted from powder spectra. A similar behavior is expected for both dipolar and quadrupolar echoes.

August 29, 2016

Spin Dynamics and Establishing of Internal Quasi-Equilibrium in Dilute Paramagnetic Solids

Vadim A. Atsarkin, Fridrikh S. Dzheparov

Page range: 545-560

Abstract

A brief review is given of the development and experimental verification of the two-temperature quasi-equilibrium concept as applied to EPR in dilute paramagnetic solids. Existing experimental data on spectral diffusion in a dipolar broadened EPR line and excitation transfer inside the electron dipolar reservoir are newly considered. The discussion is supplemented with theoretical arguments based on the concentration and cluster expansion techniques. Conditions and limitations of applicability of the two-temperature quasi-equilibrium model to electron spin systems with random distribution of paramagnetic centers are cleared up.

August 26, 2016

Multi-Frequency Pulsed Overhauser DNP at 1.2 Tesla

Philipp Schöps, Philipp E. Spindler, Thomas F. Prisner

Page range: 561-573

Abstract

Dynamic nuclear polarization (DNP) is a methodology to increase the sensitivity of nuclear magnetic resonance (NMR) spectroscopy. It relies on the transfer of the electron spin polarization from a radical to coupled nuclear spins, driven by microwave excitation resonant with the electron spin transitions. In this work we explore the potential of pulsed multi-frequency microwave excitation in liquids. Here, the relevant DNP mechanism is the Overhauser effect. The experiments were performed with TEMPOL radicals in aqueous solution at room temperature using a Q-band frequency (1.2 T) electron paramagnetic resonance (EPR) spectrometer combined with a Minispec NMR spectrometer. A fast arbitrary waveform generator (AWG) enabled the generation of multi-frequency pulses used to either sequentially or simultaneously excite all three 14 N-hyperfine lines of the nitroxide radical. The multi-frequency excitation resulted in a doubling of the observed DNP enhancements compared to single-frequency microwave excitation. Q-band free induction decay (FID) signals of TEMPOL were measured as a function of the excitation pulse length allowing the efficiency of the electron spin manipulation by the microwave pulses to be extracted. Based on this knowledge we could quantitatively model our pulsed DNP enhancements at 1.2 T by numerical solution of the Bloch equations, including electron spin relaxation and experimental parameters. Our results are in good agreement with theoretical predictions. Whereas for a narrow and homogeneous single EPR line continuous wave excitation leads to more efficient DNP enhancements compared to pulsed excitation for the same amount of averaged microwave power. The situation is different for radicals with several hyperfine lines or in the presence of inhomogeneous line broadening. In such cases pulsed single/multi-frequency excitation can lead to larger DNP enhancements.

September 24, 2016

Gas-Phase Hydrogenation with Parahydrogen Over Immobilized Vaska’s Complex

Ivan V. Skovpin, Vladimir V. Zhivonitko, Igor P. Prosvirin, Dzhalil F. Khabibulin, Igor V. Koptyug

Page range: 575-592

Abstract

Generating parahydrogen-induced polarization (PHIP) of nuclear spins with immobilized transition metal complexes as hydrogenation catalysts allows one to produce pure hyperpolarized substances, which can open new revolutionary perspectives for PHIP applications. A major drawback of immobilized complexes is their low stability under reaction conditions. In the present work we studied an immobilized iridium complex, Ir/SiO 2 P , synthesized by a covalent anchoring of Vaska’s complex on phospine-modified silica gel. This complex was used to obtain hyperpolarized gasses in the gas phase hydrogenation of propene, propyne and 1-butyne with parahydrogen in PASADENA and ALTADENA experiments. It was found that, in contrast to other immobilized complexes, Ir/SiO 2 P is stable under reaction conditions at up to 140°C, and the reduction of iridium does not occur according to XPS analysis. Moreover, the application of Ir/SiO 2 P catalyst allowed us to generate continuous flow of hyperpolarized propene and 1-butene with (300–500)-fold NMR signal enhancement which is significantly higher than commonly observed for most supported metal catalysts. The shape of polarized propene signals in PASADENA experiment has indicated that parahydrogen addition to propyne occurs non-stereospecifically, i.e. PHIP was observed for all protons of the vinyl fragment of propene. The analysis of the polarized signals has shown that syn pairwise addition dominates, which was confirmed by spectra simulations. It was found that storage of Ir/SiO 2 P under Ar atmosphere leads to a decrease in PHIP amplitude and an increase in the activity of the catalyst. This observation is discussed in terms of the interaction of Ir/SiO 2 P with trace amounts of oxygen in Ar, leading to partial oxidation of triphenylphosphine ligand to triphenylphosphine oxide accompanied by the activation of the immobilized complex. It was also found that the interaction of Ir/SiO 2 P with alkenes likely leads to formation of stable monohydride complexes, decreasing the production of PHIP in hydrogenations. At the same time, stable substrate complexes are likely formed in alkyne hydrogenations, leading to a significant decrease in the monohydride complex formation and to an increased production of PHIP.

October 8, 2016

Triplet Charge Recombination in Heliobacterial Reaction Centers Does Not Produce a Spin-Polarized EPR Spectrum

Bryan Ferlez, Alessandro Agostini, Donatella Carbonera, John H. Golbeck, Art van der Est

Page range: 593-607

Abstract

In photosynthetic reaction centers, reduction of the secondary acceptors leads to triplet charge recombination of the primary radical pair (RP). This process is spin selective and in a magnetic field it populates only the T 0 state of the donor triplet state. As a result, the triplet state of the donor has a distinctive spin polarization pattern that can be measured by transient electron paramagnetic resonance (TREPR) spectroscopy. In heliobacterial reaction centers (HbRCs), the primary donor, P 800 , is composed of two bacteriochlorophyll g′ molecules and its triplet state has not been studied as extensively as those of other reaction centers. Here, we present TREPR and optically detected magnetic resonance (ODMR) data of 3 P 800 and show that although it can be detected by ODMR it is not observed in the TREPR data. We demonstrate that the absence of the TREPR spectrum is a result of the fact that the zero-field splitting (ZFS) tensor of 3 P 800 is maximally rhombic, which results in complete cancelation of the absorptive and emissive polarization in randomly oriented samples.

October 14, 2016

Photoinduced Electron Transfer in Dyads with (R)-/(S)-Naproxen and (S)-Tryptophan

Ekaterina A. Khramtsova, Alexandra A. Ageeva, Alexander A. Stepanov, Viktor F. Plyusnin, Tatyana V. Leshina

Page range: 609-623

Abstract

Short-lived intermediates arising from the donor-acceptor interaction of non-steroidal anti-inflammatory drug (NSAID) – (S)-naproxen (NPX) and its (R)-enantiomer with the tryptophan amino acid residue (Trp) have been studied by spin chemistry and photochemistry methods. The donor-acceptor interaction has caried out in a model linked system – dyad under the UV-irradiation. Interest in the NPX-Trp dyad diastereomers is connected with the possibility of using them as models of ligand-enzyme binding as long as amino acid residues are located at the enzyme’s active centers. It is these residues that interact with NSAID during the binding. It is widely thought that charge transfer processes are involved in the process of drug-enzyme binding. Withing this framework the role of charge transfer in NPX-Trp excited state quenching have been investigated. The analysis of the chemically induced dynamic nuclear polarization (CIDNP), as well as fluorescence kinetics and quantum yield in different polarity media has shown that the main channel of NPX fluorescence quenching is the intramolecular electron transfer between NPX and Trp fragments. Electron transfer rate constants and fluorescence quantum yields of diastereomers have demonstrated stereodifferentiation.

October 15, 2016

Toward Matching Optically and NMR Active Volumes for Optimizing the Observation of Photo-Induced Reactions by NMR

Eduard Stadler, Anna Eibel, Dmytro Neshchadin, Georg Gescheidt

Page range: 625-636

Abstract

In the recent years photo-induced reactions are becoming increasingly popular in many fields of chemistry comprising biological conversions, material/environmental science and synthesis. NMR monitoring of such reactions has been shown being advantageous and several strategies of providing an efficient irradiation of the NMR sample have been developed and reported. Here we show that adjusting the optical properties of the investigated solution to the active volume detected by the NMR experiment is valuable. This is shown with the help of three examples comprising photo-isomerization, photo-induced polymerization and CIDNP-detected bond cleavage. Adjusting the photo-active volume to the NMR-detectable portion of the sample provides a substantially more realistic kinetic information, background suppression and reduction of thermal and diffusional effects.

October 21, 2016

Pulsed EPR Signals from Triplets

Michael K. Bowman, Hanjiao Chen, Alexander G. Maryasov

Page range: 637-652

Abstract

Pulsed EPR experiments have proven to be an important tool for measuring EPR spectra, kinetics and relaxation rates of free radicals and triplet molecules. The EPR frequencies and selection rules from CW-EPR spectra also govern pulsed EPR-experiments, but pulsed excitation provides much greater control over spin dynamics and allows clean separation and measurement of many properties of the spin system. Most pulsed EPR measurements of triplet molecules have been made in the selective pulse limit where only one EPR transition of a molecule is excited by microwave pulses and its EPR spectroscopy behaves like that of a radical with spin of S =1/2. However, some important classes of systems with S =1, such as molecular photoconversion devices, donor-acceptor complexes, biradicaloids, biradicals and triplet states with high symmetry, can have all their EPR transitions excited simultaneously, the so-called hard-pulse limit. The pulsed EPR signals in this limit have many similarities, but also some important differences from those in the selective pulse limit or from radicals with S =1/2. The dependence of pulsed EPR signals on microwave pulse phase and turning angle is examined in both limits for S =1 systems in thermal equilibrium or with electron spin polarization.

October 27, 2016

³¹P-Solid-State NMR Characterization and Catalytic Hydrogenation Tests of Novel heterogenized Iridium-Catalysts

Torsten Gutmann, Safaa Alkhagani, Niels Rothermel, Hans-Heinrich Limbach, Hergen Breitzke, Gerd Buntkowsky

Page range: 653-669

Abstract

The synthesis of novel robust and stable iridium-based immobilized catalysts on silica-polymer hybrid materials (Si-PB-Ir) is described. These catalysts are characterized by a combination of 1D 31 P CP-MAS and 2D 31 P- 1 H HETCOR and J-resolved multinuclear solid state NMR experiments. Different binding situations such as singly and multiply coordinated phosphines are identified. Density functional theory (DFT) calculations are performed to corroborate the interpretation of the experimental NMR data, in order to propose a structural model of the heterogenized catalysts. Finally, the catalytic activity of the Si-PB-Ir catalysts is investigated for the hydrogenation of styrene employing para-enriched hydrogen gas.

October 26, 2016

Multi-Gaussian Monte Carlo Analysis of PELDOR Data in the Frequency Domain

Anna G. Matveeva, Yulia V. Yushkova, Sergei V. Morozov, Igor A. Grygor’ev, Sergei A. Dzuba

Page range: 671-688

Abstract

Pulsed double electron–electron resonance technique (PELDOR or DEER) is often applied to study conformations and aggregation of spin-labelled macromolecules. Because of the ill-posed nature of the integral equation determining the distance distribution function, a regularization procedure is required to restrict the smoothness of the solution. In this work, we performed PELDOR measurements for new flexible nitroxide biradicals based on trolox, which is the synthetic analogue of α -tocopherol; spin-labelled trolox derivatives are investigated as potential anti-cancer drugs. We use regularization by an approximation of the solution with a sum of limited number of Gaussians, by varying their positions, widths and amplitudes. Their best-fitted values were found by a completely random Monte Carlo process. The use of the frequency-domain PELDOR data allowed diminution of the artifacts induced by spin–spin electron–nuclear and intermolecular electron–electron interactions. It was found that for the all biradicals studied, the use of three Gaussians was enough for good agreement with the experiments. The number of trials for obtaining satisfactory result was found to be quite reasonable, which is explained by presence of the singularity in the core of integral equation. The maxima of inter-spin distance distribution for different biradicals were found to vary between 1.5 and 2.3 nm, depending on the linkers between the Trolox core and nitroxides. The distance distributions around these positions reflect flexibility of the biradicals.

November 9, 2016

Concept of Phase Cycling in Pulsed Magnetic Resonance Using Sinusoidal Magnetic Field Modulation

Mark Tseytlin

Page range: 689-703

Abstract

In conventional pulsed magnetic resonance suppression of unwanted signals is achieved by changing pulse phases with respect to the reference signal and spin magnetization phase. This method is called phase cycling. An alternative approach is suggested to separate the unwanted signals from the spin echo by using magnetic field modulation. Precession frequency of the spins, and therefore phases of free indication decays and echo signals, can be controlled by the selection of modulation parameters. This enables phase cycling. Since the signal is detected in the presence of the changing magnetic field, which drives spin precession, the echo signal is frequency-modulated. Numerical transformation into an accelerating reference frame associated with the Larmor frequency restores the signal to a form that would have been observed in the absence of modulation. The suggested phase cycling method is analyzed in detail for the two pulse spin echo case.

About this journal

Objective
Zeitschrift für Physikalische Chemie (ZPC), founded in 1887, covers the main developments in physical chemistry with emphasis on experimental and theoretical research. It represents a combination of reaction kinetics and spectroscopy, quantum theory, surface research and electrochemistry, thermodynamics and structure analysis of matter in its various conditions. Short times for peer review and publication can be guaranteed for high quality submissions.

Topics

reaction kinetics
spectroscopy
surface research
electrochemistry
thermodynamics
structure analysis

Article formats
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Volume 231 Issue 3 - Special Issue dedicated to Kev Salikhov on the occasion of his 80th birthday / Guest Editor: Gerd Buntkowsky, Konstantin L. Ivanov and Hans-Martin Vieth / Part II: Spin dynamics and spin hyperpolarization

Issue of Zeitschrift für Physikalische Chemie

Frontmatter

Preface

Intramolecular Electron Transfer from Tryptophan to Guanosyl Radicals in a Linked System as a Model of DNA Repair

Abstract

SABRE Hyperpolarization of Bipyridine Stabilized Ir-Complex at High, Low and Ultralow Magnetic Fields

Abstract

Relaxation of Multiple Quantum NMR Coherences in Quasi-One-Dimensional Spin Systems

Abstract

Exciting Wide NMR Spectra of Static Solid Samples with Weak Radiofrequency Fields

Abstract

Spin Dynamics and Establishing of Internal Quasi-Equilibrium in Dilute Paramagnetic Solids

Abstract

Multi-Frequency Pulsed Overhauser DNP at 1.2 Tesla

Abstract

Gas-Phase Hydrogenation with Parahydrogen Over Immobilized Vaska’s Complex

Abstract

Triplet Charge Recombination in Heliobacterial Reaction Centers Does Not Produce a Spin-Polarized EPR Spectrum

Abstract

Photoinduced Electron Transfer in Dyads with (R)-/(S)-Naproxen and (S)-Tryptophan

Abstract

Toward Matching Optically and NMR Active Volumes for Optimizing the Observation of Photo-Induced Reactions by NMR

Abstract

Pulsed EPR Signals from Triplets

Abstract

31P-Solid-State NMR Characterization and Catalytic Hydrogenation Tests of Novel heterogenized Iridium-Catalysts

Abstract

Multi-Gaussian Monte Carlo Analysis of PELDOR Data in the Frequency Domain

Abstract

Concept of Phase Cycling in Pulsed Magnetic Resonance Using Sinusoidal Magnetic Field Modulation

Abstract

³¹P-Solid-State NMR Characterization and Catalytic Hydrogenation Tests of Novel heterogenized Iridium-Catalysts