Electron spin relaxation time of Ni(II) ion in hexapyrazole zinc(II) dinitrate at 300 K

Understanding the electron spin relaxation properties of paramagnetic species is a fundamental requirement to use them as a probe to measure distances between sites in biomolecules by electron paramagnetic resonance (EPR) spectroscopy. Even though Ni(II) ion is an essential trace element for many species, relaxation properties are not well understood. Herein, the polycrystalline sample of Ni(II) ion magnetically diluted in Zn(Pyrazole)₆(NO₃)₂ (Ni/ZPN) has been studied in detail by EPR spectroscopy to explore the electron spin relaxation time. Progressive continuous-wave (CW) EPR power saturation study on Ni/ZPN at 300 K yielded 907 mW as the P_1/2 value. The cavity constant (K_Q) has been calculated using tempol in PVA-BA glass matrix and the product of electron spin-lattice relaxation time (T₁) and spin–spin relaxation time (T₂) for Ni/ZPN at 300 K has been reported for the first time.     

In‐Situ Spin Labeling of His‐Tagged Proteins: Distance Measurements under In‐Cell Conditions

New spin labeling strategies have immense potential in studying protein structure and dynamics under physiological conditions with electron paramagnetic resonance (EPR) spectroscopy. Here, a new spin‐labeled chemical recognition unit for switchable and concomitantly high affinity binding to His‐tagged proteins was synthesized. In combination with an orthogonal site‐directed spin label, this novel spin probe, Proxyl‐trisNTA (P‐trisNTA) allows the extraction of structural constraints within proteins and macromolecular complexes by EPR. By using the multisubunit maltose import system of E. coli: 1) the topology of the substrate‐binding protein, 2) its substrate‐dependent conformational change, and 3) the formation of the membrane multiprotein complex can be extracted. Notably, the same distance information was retrieved both in vitro and in situ allowing for site‐specific spin labeling in cell lysates under in‐cell conditions. This approach will open new avenues towards in‐cell EPR.     

gem-Diethyl Pyrroline Nitroxide Spin Labels: Synthesis,EPR Characterization,Rotamer Libraries and Biocompatibility

The availability of bioresistant spin labels is crucial for the optimization of site-directed spin labeling protocols for EPR structural studies of biomolecules in a cellular context. As labeling can affect proteins’ fold and/or function, having the possibility to choose between different spin labels will increase the probability to produce spin-labeled functional proteins. Here, we report the synthesis and characterization of iodoacetamide- and maleimide-functionalized spin labels based on the gem-diethyl pyrroline structure. The two nitroxide labels are compared to conventional gem-dimethyl analogs by site-directed spin labeling (SDSL) electron paramagnetic resonance (EPR) spectroscopy, using two water soluble proteins: T4 lysozyme and Bid. To foster their use for structural studies, we also present rotamer libraries for these labels, compatible with the MMM software. Finally, we investigate the “true” biocompatibility of the gem-diethyl probes comparing the resistance towards chemical reduction of the NO group in ascorbate solutions and E. coli cytosol at different spin concentrations.     

Light‐Driven Coordination‐Induced Spin‐State Switching: Rational Design of Photodissociable Ligands

The bistability of spin states (e.g., spin crossover) in bulk materials is well investigated and understood. We recently extended spin‐state switching to isolated molecules at room temperature (light‐driven coordination‐induced spin‐state switching, or LD‐CISSS). Whereas bistability and hysteresis in conventional spin‐crossover materials are caused by cooperative effects in the crystal lattice, spin switching in LD‐CISSS is achieved by reversibly changing the coordination number of a metal complex by means of a photochromic ligand that binds in one configuration but dissociates in the other form. We present mathematical proof that the maximum efficiency in property switching by such a photodissociable ligand (PDL) is only dependent on the ratio of the association constants of both configurations. Rational design by using DFT calculations was applied to develop a photoswitchable ligand with a high switching efficiency. The starting point was a nickel–porphyrin as the transition‐metal complex and 3‐phenylazopyridine as the photodissociable ligand. Calculations and experiments were performed in two iterative steps to find a substitution pattern at the phenylazopyridine ligand that provided optimum performance. Following this strategy, we synthesized an improved photodissociable ligand that binds to the Ni–porphyrin with an association constant that is 5.36 times higher in its trans form than in the cis form. The switching efficiency between the diamagnetic and paramagnetic state is efficient as well (72 % paramagnetic Ni–porphyrin after irradiation at 365 nm, 32 % paramagnetic species after irradiation at 440 nm). Potential applications arise from the fact that the LD‐CISSS approach for the first time allows reversible switching of the magnetic susceptibility of a homogeneous solution. Photoswitchable contrast agents for magnetic resonance imaging and light‐controlled magnetic levitation are conceivable applications.     

Spin‐Labeled Heparins as Polarizing Agents for Dynamic Nuclear Polarization

A potentially biocompatible class of spin‐labeled macromolecules, spin‐labeled (SL) heparins, and their use as nuclear magnetic resonance (NMR) signal enhancers are introduced. The signal enhancement is achieved through Overhauser‐type dynamic nuclear polarization (DNP). All presented SL‐heparins show high ¹H DNP enhancement factors up to E=?110, which validates that effectively more than one hyperfine line can be saturated even for spin‐labeled polarizing agents. The parameters for the Overhauser‐type DNP are determined and discussed. A striking result is that for spin‐labeled heparins, the off‐resonant electron paramagnetic resonance (EPR) hyperfine lines contribute a non‐negligible part to the total saturation, even in the absence of Heisenberg spin exchange (HSE) and electron spin‐nuclear spin relaxation (T_1ne). As a result, we conclude that one can optimize the use of, for example, biomacromolecules for DNP, for which only small sample amounts are available, by using heterogeneously distributed radicals attached to the molecule.     

EPR/Spin‐trapping study of free radical intermediates in the photolysis of trifluoromethyl ketones with initiators

Photolysis of trifluoromethyl ketones (TFMKs) 1a–1e versus the non‐fluorinated ketones 2a–2b in the presence of radical initiators by electron paramagnetic resonance spectroscopy has been studied for the first time. The transient radicals generated after irradiation of the ketones were identified by trapping with 2‐methyl‐2‐nitrosopropane (MNP) and 2,4,6‐tri‐tert‐butylnitrosobenzene (TTBNB) as spin traps. TTBNB is a powerful, particularly useful spin trap in these kinds of processes producing anilino and nitroxyl spin adducts due to the ambivalent reactivity on the N and O atoms. In the presence of t‐butylperoxide, short‐chain TFMKs, such as 1,1,1‐trifluoroacetone (1d) and hexafluoroacetone (1e), give rise to detection of the elusive trifluoromethyl radical. In contrast, long‐chain TFMKs did not provide clues to prove formation of the trifluoromethyl radical but instead to radicals derived by abstraction of one α‐methylene proton to the carbonyl. Although TFMKs are quite stable to photodegradation in the absence of initiator, methyl ketone 2b and phenyl ketone 3 produce radicals resulting from abstraction of a γ‐hydrogen to the carbonyl group. Copyright © 2010 John Wiley & Sons, Ltd.     

One-bond 1J(15N─19F) spin–spin coupling constants of cationic fluorinating reagents: Insights from DFT calculations

We have investigated, by means of density functional theory protocols, the one-bond ¹J(¹⁵N─¹⁹F) spin–spin coupling constants in a series of fluorinating reagents, containing the N─F bond, recently studied experimentally. The results of the calculations show a very good linear relationship with the experimental values, even though only the M06-2X(PCM)/pcJ-2//B3LYP/6-311G(d,p) level affords a very low mean absolute error. The calculations allow to analyze the various molecular orbitals contributions to the J coupling and to rationalize the observed positive sign, corresponding to a negative sign of the reduced spin–pin coupling constant K(N─F). Moreover, of the four Ramsey contributions, only the diamagnetic spin orbit is negligible, whereas the paramagnetic spin orbit and spin dipole terms decrease the magnitude of the Fermi contact (FC) term by an amount that goes from a minimum of 35% up to more than 60% of the FC term itself. Several effects have been investigated, namely, the contribution of the long-range solvent reaction field, relativistic corrections, and conformational and vibrational effects.     

High spin/low spin phase transitions of a spin-crossover complex in the emulsion polymerization of trifluoroethylmethacrylate (TFEMA) using PVA as a protective colloid

We have studied the magnetic properties of an Fe(II) spin-crossover complex near its high spin/low spin (HS/LS) phase transition in the emulsion polymerization of trifluoroethylmethacrylate (TFEMA) using poly(vinyl alcohol) (PVA) as a protective colloid, in comparison with sodium lauryl sulfate (SLS). Morphological analysis was used to establish that the nanodispersed spin-crossover complex was incorporated into the cores of polymer particles covered with PVA shells. The obvious bi-stability of the HS/LS phase transition was considered by the identification of multiplet states such as the triplet (S = 1) and quintet (S = 2) states, and the paramagnetic state (S = 1/2), by noting a gradual shift of g-value anisotropy in the electron spin resonance (ESR) spectrum at 5 K. This was thought to have arisen from the exchange interaction as a Jahn–Teller effect in the emulsion particles. Chemical modifications such as ligand substitution, and the nature of the central metal atom in the emulsion particle, especially influenced the HS/LS phase transition.     

Dynamic Nuclear Polarization in Suspensions Consisting of Xylene Isomers and Asphaltene Extracted from MC‐800 Liquid Asphalt

Overhauser effect type dynamic nuclear polarization experiments were performed to study suspensions of asphaltene in the xylene isomers (o‐, m‐, p‐) at a low magnetic field of 1.44 mT and three different temperatures (15, 25, and 35°C). The asphaltene was extracted from MC‐800 liquid asphalt. Intermolecular spin‐spin interactions occur between nuclear spins of hydrogen in the solvent medium and the free electron spins in the asphaltene micelles. The electron paramagnetic resonance spectrum of the asphaltene was obtained and the saturation experiments were applied to the samples prepared in vacuum. For all media, the dipole‐dipole interaction is predominant due to the negative signal enhancements. In all temperatures, the ultimate enhancement is the smallest for the p‐xylene solvent medium which has the lowest electrical dipole moment. The normalized low frequency relaxation components were calculated for 25°C, and the behavior of the nuclear‐electron coupling parameter according to this component is in agreement with the other works in the literature.     

Orientational order of guest molecules in aligned liquid crystal as measured by EPR and UV–vis techniques

Orientational order of guest molecules in aligned liquid crystal 4-cyano-4′-pentylbiphenyl (5CB) is studied via optical dichroism and electron paramagnetic resonance (EPR) spectra measurements. The guest molecules used are bifunctional molecules bearing paramagnetic nitroxide group and photochromic azobenzene moiety. The bifunctional probe with rigidly bonded nitroxide and azobenzene moieties was found to align as a whole, while flexible long spacer between the moieties provides independent alignment for the nitroxide and azobenzene parts. Intermolecular interactions responsible for the alignment of azobenzene and nitroxide moieties of the probe molecules are discussed. The molecules with cis-configuration of azobenzene moiety are able to align in the liquid-crystalline medium, but to a lesser extent than the molecules with trans-configuration. Directions of orientational axes and characteristics of rotational mobility of spin probes are determined. Second, fourth and, in some cases, sixth rank order parameter values are found.     

Recent progress in probing atomic and molecular quantum coherence with scanning tunneling microscopy

Quantum coherent physics and chemistry concern the creation and manipulation of an excited-state manifold that contains the superposition and entanglement of multiple quantum levels. Electromagnetic waves such as light and microwave can be used to generate and probe different quantum coherent phenomena. The recent advances in scanning tunneling microscopy (STM) techniques including ultrafast laser coupled STM and electron spin resonance STM combine electromagnetic excitation with tunneling electron detection, bringing the investigation of quantum coherence down to the atomic and molecular level. Here, we survey the latest STM studies of different quantum coherent phenomena covering molecular vibration, electron transfer, surface plasmon resonance, phonon, spin oscillation, and electronic transition, and discuss the state and promise of characterizing and manipulating quantum coherence at the atomic or molecular scale.     

Pulsed EPR Dipolar Spectroscopy on Spin Pairs with one Highly Anisotropic Spin Center: The Low-Spin FeIII Case

Pulsed electron paramagnetic resonance (EPR) dipolar spectroscopy (PDS) offers several methods for measuring dipolar coupling constants and thus the distance between electron spin centers. Up to now, PDS measurements have been mostly applied to spin centers whose g-anisotropies are moderate and therefore have a negligible effect on the dipolar coupling constants. In contrast, spin centers with large g-anisotropy yield dipolar coupling constants that depend on the g-values. In this case, the usual methods of extracting distances from the raw PDS data cannot be applied. Here, the effect of the g-anisotropy on PDS data is studied in detail on the example of the low-spin Fe³⁺ ion. First, this effect is described theoretically, using the work of Bedilo and Maryasov (Appl. Magn. Reson. 2006 , 30, 683–702) as a basis. Then, two known Fe³⁺/nitroxide compounds and one new Fe³⁺/trityl compound were synthesized and PDS measurements were carried out on them using a method called relaxation induced dipolar modulation enhancement (RIDME). Based on the theoretical results, a RIDME data analysis procedure was developed, which facilitated the extraction of the inter-spin distance and the orientation of the inter-spin vector relative to the Fe³⁺ g-tensor frame from the RIDME data. The accuracy of the determined distances and orientations was confirmed by comparison with MD simulations. This method can thus be applied to the highly relevant class of metalloproteins with, for example, low-spin Fe³⁺ ions.     

Interpretation of EPR and optical spectra of Ni(II) ions in crystalline lattices at ambient temperature

Many biologically important paramagnetic metal ions are characterized by electron paramagnetic resonance (EPR) spectroscopy to use as spin probes to investigate the structure and function of biomolecules. Though nickel(II) ions are an essential trace element and part of many biomolecules, the EPR properties are least understood. Herein, the EPR and optical absorption spectra measured at 300 K for Ni(II) ions diluted in two different diamagnetic hosts are investigated and reported. The EPR spectrum of a polycrystalline Ni/Mg(3-methylpyrazole)₆(ClO₄)₂ [Ni/MMPC] shows two transitions at X-band frequency (~9.5 GHz), suggesting the zero-field splitting parameter (D) is larger than the resonance field of the free electron (H_o). This incomplete and complex spectrum is successfully analyzed to obtain EPR parameters. The EPR spectrum of the polycrystalline Ni/Zn(pyrazole)₆(NO₃)₂ [Ni/ZPN] shows a triplet spectrum indicating D < H_o. A detailed analysis of single-crystal EPR data yielded the spin Hamiltonian parameters. The optical absorption spectra are deconvoluted to understand the symmetry of the coordination environment in the complex.     

Solid-phase mechanochemical incorporation of a spin label into cellulose

Samples of cellulose labeled with stable nitroxyl radicals were prepared by mechanochemical synthesis. The samples were studied by IR and EPR spectroscopies, X-ray phase analysis, and electron microscopy. The EPR spectral patterns indicate a uniform distribution of “grafted” paramagnetic centers over the cellulose macromolecular chains. X-Ray diffraction patterns obtained and the results of crystallinity index calculations for the samples showed that strong bonding of spin labels causes changes in the cellulose structure up to nearly complete amorphization of the material. __________ Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1127–1130, May, 2005.     

The nature and dynamics of nonlinear excitations in conducting polymers. Polyaniline

The advantages of using the multifrequency (3-cm, 8-mm, and 2-mm) EPR method in combination with the modified method of spin label and spin probes and the methods of steady-state microwave saturation and microwave saturation transfer of spin packets in studies of the spin and electrodynamic properties of polyaniline are outlined. The methods of determining the composition of localized and delocalized paramagnetic centers and calculating their main parameters from total ESR spectra are described. Original results of investigations of polyaniline modified with sulfuric, hydrochloric, camphorsulfonic, and 2-acrylamido-2-methyl-l-propanesulfonic acids are reported. The dependences of the nature, electronic relaxation, dynamics of paramagnetic centers, and the charge-transfer mechanism on the method of synthesis, the structure of the acid molecule, and the polyaniline oxidation level are shown. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 205–230, February, 2000.     

Intramolecular Spin Alignment within Mono‐Oxidized and Photoexcited Anthracene‐Based π Radicals as Prototypical Photomagnetic Molecular Devices: Relationships Between Electrochemical,Photophysical, and Photochemical Control Pathways

AnOV is a π‐conjugated radical built from an anthracene (An) unit linked by a p‐phenylene to an oxoverdazyl (OV) moiety. The mono‐oxidized (cationic) form of AnOV was generated both electrochemically and photochemically (in the presence of an electron acceptor). The triplet nature (S=1) of the electronic ground state of AnOV ⁺ was demonstrated by combining spectroelectrochemistry, electron‐spin resonance (ESR) experiments, and ab initio molecular orbital (MO) calculations. The intramolecular spin alignment (ISA) within AnOV ⁺ results from the ferromagnetic coupling (J_electrochem>0) of the two unpaired electrons located on the oxidized electron donor (An⁺) and on the pendant OV radical. The spin‐density distribution pattern of AnOV ⁺ is akin to that of AnOV when photopromoted ( AnOV *) to its high‐spin (HS) lowest excited quartet (S=3/2) state. This high‐spin state results from the ferromagnetic coupling (J_photophys>0) of the triplet locally excited state of An (³An*) with the doublet ground state of OV. As a shared salient feature, AnOV ⁺ and AnOV * (HS) show a spin delocalization within the domain of activated An in either An⁺ or ³An* (nexus states) forms. The present study essentially contributes to establish and clarify relationships between electrochemical, photophysical, and photochemical pathways to achieve ISA processes within AnOV . In particular, we discuss the impact of the spin polarization of the unpaired electron of OV on electronic features of the An electron‐donating subunit. Close analysis of this polarizing interplay allows one to derive a novel functional paradigm to manipulate electron spins at the intramolecular level with light and under an external magnetic field. Indeed, two original functional elements are identified: light‐triggered donors of spin‐polarized electrons and spin‐selective electron acceptors, which are of potential interest for molecular spintronics.     

π‐Extended Dihydrophenazines with Three‐State NIR Electrochromism Involving Large Conformational Changes

π‐Extended dihydrophenazines were successfully prepared by oxidation of 2‐(N ‐arylamino)anthracenes. Their roof‐type conformations were revealed by X‐ray diffraction analysis, and the analysis of the optical properties indicated the presence of intramolecular charge‐transfer processes. Upon chemical oxidation, the electronic absorption dramatically changed in a two‐step fashion. The electron spin resonance (ESR) analysis revealed that, depending on the amount of oxidant added, either a paramagnetic radical cation or a diamagnetic dication was generated. The NMR analysis revealed a conformational change upon oxidation, which was supported by theoretical calculations. A three‐state electrochromic behavior was observed during the electrochemical oxidation and reduction cycles, showing sequential switching between visible and near‐infrared (NIR) absorption properties upon application of electrochemical stimuli.     

Electron Spin Echo Study of Molecular Structure and Dynamics: New Approaches Based on Spontaneous Fluctuations of Magnetic Interactions

Modulation phenomena that take place during electron spin echo signal decay have long been used in structural studies of free radicals and their environment. These phenomena are based on coherent dynamic effects, arising from simultaneous excitation (by microwave pulses) of two or more transitions in the EPR spectrum. Recently, a new source of stimulated electron spin echo (ESE) modulation was discovered due to spontaneous changes in the magnetic parameters of radicals during the operation of the pulse sequence. For monoradicals, these changes are caused by intramolecular motions. For radical pairs, additional mechanisms are longitudinal relaxation of spin counterparts and transformations of the paramagnetic partners during chemical reactions. Promising applications of this phenomenon to structural studies of radicals and radical pairs in solids and to investigations of their mobility and chemical transformations are considered.     

Effect of the chemical structure of a tetradentate equatorial ligand on the spin-crossover properties of the Fe (III) complexes chain structures: Electron paramagnetic resonance study

The effect of the chemical structure of the equatorial ligand on the spin state of the Fe (III) ion in a series of 1-D chain complexes of the general formula [Fe(L)(tvp)]BPh₄·nCH₃OH, where L = dianions of Schiff base containing a different number of aromatic groups: N,N′-ethylenebis (salicylaldimine) (salen) 1 , N,N′-ethylenebis (acetylacetone)2,2′-imine (acen) 2 , N,N′-ethylenebis (benzoylacetylacetone)2,2′-imine (bzacen) 3 , and tvp = 1,2-di(4-pyridyl)ethylene, was studied by ultraviolet–visible (UV–vis) and electron paramagnetic resonance (EPR) methods. The values of exchange interactions, thermodynamic parameters of spin-crossover, and electronic structure features of the Fe (III) complexes were estimated from the EPR data. The substitution of a fragment of the equatorial ligand L in the series salen–acen–bzacen changes the local symmetry of the complex in the 1-D chain, thereby affecting the spin variable properties.