Paramagnetic Interactions Research Articles

Electron-nuclear interactions as probes of domain motion in proteins

Long range interactions between nuclear spins and paramagnetic ions can serve as a sensitive monitor of internal motion of various parts of proteins, including functional loops and separate domains. In the case of interdomain motion, the interactions between the ion and NMR-observable nuclei are modulated in direction and magnitude mainly by a combination of overall and interdomain motions. The effects on observable parameters such as paramagnetic relaxation enhancement (PRE) and pseudocontact shift (PCS) can, in principle, be used to characterize motion. These parameters are frequently used for the purpose of structural refinements. However, their use to probe actual domain motions is less common and is lacking a proper theoretical treatment from a motional perspective. In this work, a suitable spin Hamiltonian is incorporated in a two body diffusion model to produce the time correlation function for the nuclear spin-paramagnetic ion interactions. Simulated observables for nuclei in different positions with respect to the paramagnetic ion are produced. Based on these simulations, it demonstrated that both the PRE and the PCS can be very sensitive probes of domain motion. Results for different nuclei within the protein sense different aspects of the motions. Some are more sensitive to the amplitude of the internal motion, others are more sensitive to overall diffusion rates, allowing separation of these contributions. Experimentally, the interaction strength can also be tuned by substitution of different paramagnetic ions or by varying magnetic field strength (in the case of lanthanides) to allow the use of more detailed diffusion models without reducing the reliability of data fitting.

Study of Zn 1− xCo xO (0.02≤ x≤0.08) dilute magnetic semiconductor prepared by mechanosynthesis route

Single-phase Zn 1− x Co x O (0.02≤ x≤0.08) dilute magnetic semiconductor is prepared by mechanical milling process. The shift of XRD peaks towards the higher angle and a redshift in the band gap compared to the undoped ZnO ensure the incorporation of Co 2+ ions in the semiconductor host lattice. Pure Zn x Co 1− x O phases show the paramagnetic behavior in the temperature range 80 K≤ T≤300 K. The room temperature volume magnetic susceptibility ( χ v) estimated in case of Zn 0.96Co 0.04O is ∼10 −5 emu/Oe cm 3. The temperature dependence of susceptibility χ v can be fitted well with Curie law confirming the paramagnetic interaction. The observed crystal-field splitting of 3d levels of Co 2+ ions inside Zn 1− x Co x O has been successfully interpreted using Curie law.

Iron-Sulfur Cluster N5 Is Coordinated by an HXXXCXXCXXXXXC Motif in the NuoG Subunit of Escherichia coli NADH:Quinone Oxidoreductase (Complex I)

NADH:quinone oxidoreductase (complex I) plays a central role in cellular energy metabolism, and its dysfunction is found in numerous human mitochondrial diseases. Although the understanding of its structure and function has been limited, the x-ray crystal structure of the hydrophilic part of Thermus thermophilus complex I recently became available. It revealed the localization of all redox centers, including 9 iron-sulfur clusters and their coordinating ligands, and confirmed the predictions mostly made by Ohnishi et al. (Ohnishi, T., and Nakamaru-Ogiso, E. (2008) Biochim. Biophys. Acta 1777, 703-710) based on various EPR studies. Recently, Yakovlev et al. (Yakovlev, G., Reda, T., and Hirst, J. (2007) Proc. Natl. Acad. Sci. U. S. A. 104, 12720-12725) claimed that the EPR signals from clusters N4, N5, and N6b were misassigned. Here we identified and characterized cluster N5 in the Escherichia coli complex I whose EPR signals had never been detected by any group. Using homologous recombination, we constructed mutant strains of H101A, H101C, H101A/C114A, and cluster N5 knock-out. Although mutant NuoEFG subcomplexes were dissociated from complex I, we successfully recovered these mutant NuoCDEFG subcomplexes by expressing the His-tagged NuoCD subunit, which had a high affinity to NuoG. The W221A mutant was used as a control subcomplex carrying wild-type clusters. By lowering temperatures to around 3 K, we finally succeeded in detecting cluster N5 signals in the control for the first time. However, no cluster N5 signals were found in any of the N5 mutants, whereas EPR signals from all other clusters were detected. These data confirmed that, contrary to the misassignment claim, cluster N5 has a unique coordination with His(Cys)(3) ligands in NuoG.

Paramagnetic hyperfine splitting in theE151uMössbauer spectra ofCaF2:Eu2+

(151)Eu Mössbauer spectra in zero magnetic field of highly dilute (0.1 mol%) Eu(2+) ions in CaF(2) showed an almost temperature-independent asymmetrically split pattern, arising from the paramagnetic hyperfine interaction AS. I in a cubic crystal field with slow electron spin relaxation; in a small external magnetic field B of 0.2 T such that gμ(B)B>A an almost symmetrical pattern was observed. Both the spectra with and without external field are well described using the spin Hamiltonian and previous electron paramagnetic resonance data. A more concentrated (2 mol% Eu(2+)) sample exhibited a strongly broadened symmetrical resonance line due to an increased Eu-Eu spin relaxation rate; in an external magnetic field of 0.2 T the Mössbauer spectra exhibited further broadening and additional magnetic structures due to the reduced relaxation rate. When a large field of 6 T was applied such that gμ(B)B is much larger than the crystal field splitting, a fully resolved hyperfine pattern was observed at 2.5 K, with an effective field at the Eu nuclei of -33.7 T; at higher temperatures superimposed patterns originating from excited electronic states were observed in the spectra. The present results on the highly dilute CaF(2) : 0.1%Eu(2+) sample deliver a straightforward explanation for previous observations of a seemingly large dependence of the Eu(2+) isomer shift on europium concentration.

An EPR and NMR Study of Supramolecular Effects on Paramagnetic Interaction between a Nitroxide Incarcerated within a Nanocapsule with a Nitroxide in Bulk Aqueous Media

A 15N-labeled nitroxide was incarcerated into an octa acid nanocapsule, which was confirmed by 1H NMR and EPR spectroscopy. Electron paramagnetic interaction between the 15N-labeled incarcerated nitroxide and a 14N-labeled free nitroxide in the external aqueous solution was observed by EPR spectroscopy. The observation of spin-spin interaction, through the walls of the cancer and is reflected in the simultaneous line-broadening of both the 15N-labeled and 14N-labeled nitroxides. The computer-assisted analysis of the EPR data further provides direct information on the motion and the polarity of both the incarcerated paramagnetic nitroxide and the nitroxides in the external bulk aqueous phase. We also show how communication between an incarcerated guest and molecules in the bulk solvent can be enhanced or inhibited by supramolecular factors such as Coulombic attraction or repulsion between a charged guest@host complex (incarcerated 15N nitroxide) and charged molecules in the aqueous phase.

Progress in C13 and H1 solid-state nuclear magnetic resonance for paramagnetic systems under very fast magic angle spinning

High-resolution solid-state NMR (SSNMR) of paramagnetic systems has been largely unexplored because of various technical difficulties due to large hyperfine shifts, which have limited the success of previous studies through depressed sensitivity/resolution and lack of suitable assignment methods. Our group recently introduced an approach using "very fast" magic angle spinning (VFMAS) for SSNMR of paramagnetic systems, which opened an avenue toward routine analyses of small paramagnetic systems by (13)C and (1)H SSNMR [Y. Ishii et al., J. Am. Chem. Soc. 125, 3438 (2003); N. P. Wickramasinghe et al., ibid. 127, 5796 (2005)]. In this review, we discuss our recent progress in establishing this approach, which offers solutions to a series of problems associated with large hyperfine shifts. First, we demonstrate that MAS at a spinning speed of 20 kHz or higher greatly improves sensitivity and resolution in both (1)H and (13)C SSNMR for paramagnetic systems such as Cu(II)(DL-alanine)(2)H(2)O (Cu(DL-Ala)(2)) and Mn(acac)(3), for which the spectral dispersions due to (1)H hyperfine shifts reach 200 and 700 ppm, respectively. Then, we introduce polarization transfer methods from (1)H spins to (13)C spins with high-power cross polarization and dipolar insensitive nuclei enhanced by polarization transfer (INEPT) in order to attain further sensitivity enhancement and to correlate (1)H and (13)C spins in two-dimensional (2D) SSNMR for the paramagnetic systems. Comparison of (13)C VFMAS SSNMR spectra with (13)C solution NMR spectra revealed superior sensitivity in SSNMR for Cu(DL-Ala)(2), Cu(Gly)(2), and V(acac)(3). We discuss signal assignment methods using one-dimensional (1D) (13)C SSNMR (13)C-(1)H rotational echo double resonance (REDOR) and dipolar INEPT methods and 2D (13)C(1)H correlation SSNMR under VFMAS, which yield reliable assignments of (1)H and (13)C resonances for Cu(Ala-Thr). Based on the excellent sensitivity/resolution and signal assignments attained in the VFMAS approach, we discuss methods of elucidating multiple distance constraints in unlabeled paramagnetic systems by combing simple measurements of (13)C T(1) values and anisotropic hyperfine shifts. Comparison of experimental (13)C hyperfine shifts and ab initio calculated shifts for alpha- and beta-forms of Cu(8-quinolinol)(2) demonstrates that (13)C hyperfine shifts are parameters exceptionally sensitive to small structural difference between the two polymorphs. Finally, we discuss sensitivity enhancement with paramagnetic ion doping in (13)C SSNMR of nonparamagnetic proteins in microcrystals. Fast recycling with exceptionally short recycle delays matched to short (1)H T(1) of approximately 60 ms in the presence of Cu(II) doping accelerated 1D (13)C SSNMR for ubiquitin and lysozyme by a factor of 7.3-8.4 under fast MAS at a spinning speed of 40 kHz. It is likely that the VFMAS approach and use of paramagnetic interactions are applicable to a variety of paramagnetic systems and nonparamagnetic biomolecules.

Solid state NMR as a new approach for the structural characterization of rare-earth doped lead lanthanum zirconate titanate laser ceramics

To facilitate the design of laser host materials with optimized emission properties, detailed structural information at the atomic level is essential, regarding the local bonding environment of the active ions (distribution over distinct lattice sites) and their extent of local clustering as well as their population distribution over separate micro- or nanophases. The present study explores the potential of solid state NMR spectroscopy to provide such understanding for rare-earth doped lead lanthanum zirconate titanate (PLZT) ceramics. As the NMR signals of the paramagnetic dopant species cannot be observed directly, two complementary approaches are utilized: (1) direct observation of diamagnetic mimics using 45Sc NMR and (2) study of the paramagnetic interaction of the constituent host lattice nuclei with the rare-earth dopant, using 207Pb NMR lineshape analysis. 45Sc MAS NMR spectra of scandium-doped PLZT samples unambiguously reveal scandium to be six-coordinated, suggesting that this rare-earth ion substitutes in the B site. Static 207Pb spin echo NMR spectra of a series of Tm-doped PLZT samples reveal a clear influence of paramagnetic rare-earth dopant concentration on the NMR lineshape. In the latter case high-fidelity spectra can be obtained by spin echo mapping under systematic incrementation of the excitation frequency, benefiting from the signal-to-noise enhancement afforded by spin echo train Fourier transforms. Consistent with XRD data, the 207Pb NMR lineshape analysis suggests that statistical incorporation into the PLZT lattice occurs at dopant levels of up to 1 wt.% Tm 3+, while at higher levels the solubility limit is reached.

Electrodynamics of Fulde-Ferrell-Larkin-Ovchinnikov superconducting state

The Ginzburg-Landau theory of the vortex lattice for clean isotropic three-dimensional superconductors is developed, including the space variation of currents and fields, in the limit when the critical field is mainly determined by the paramagnetic depairing effect and the orbital effect is of minor importance. Then, in addition to the Abrikosov vortex lattice, the formation of a Fulde-Ferrell-Larkin-Ovchinnikov state is favored. In this case, the diamagnetic superfluid currents mainly come from paramagnetic interaction of electron spins with the local magnetic field, and not from the kinetic energy response to the external field as usual. We find that the stable vortex lattice keeps its triangular structure as in the habitual Abrikosov mixed state, while the internal magnetic field acquires components perpendicular to the applied magnetic field. Experimental possibilities related to this prediction are discussed.

A semiquinone intermediate generated at the Q o site of the cytochrome bc 1 complex: Importance for the Q-cycle and superoxide production

The cytochrome bc1 and related complexes are essential energy-conserving components of mitochondrial and bacterial electron transport chains. They orchestrate a complex sequence of electron and proton transfer reactions resulting in the oxidation of quinol, the reduction of a mobile electron carrier, and the translocation of protons across the membrane to store energy in an electrochemical proton gradient. The enzyme can also catalyze substantial rates of superoxide production, with deleterious physiological consequences. Progress on understanding these processes has been hindered by the lack of observable enzymatic intermediates. We report the first direct detection of a semiquinone radical generated by the Q(o) site using continuous wave and pulsed EPR spectroscopy. The radical is a ubisemiquinone anion and is sensitive to both specific inhibitors and mutations within the Q(o) site as well as O2, suggesting that it is the elusive intermediate responsible for superoxide production. Paramagnetic interactions show that the new semiquinone species is buried in the protein, probably in or near the Q(o) site but not strongly interacting with the 2Fe2S cluster. The semiquinone is substoichiometric, even with conditions optimized for its accumulation, consistent with recently proposed models where the semiquinone is destabilized to limit superoxide production. The discovery of this intermediate provides a critical tool to directly probe the elusive chemistry that takes place within the Q(o) site.

Relaxation-allowed nuclear magnetic resonance transitions by interference between the quadrupolar coupling and the paramagnetic interaction

Of the various ways in which nuclear spin systems can relax to their ground states, the processes involving an interference between different relaxation mechanisms, such as dipole-dipole coupling and chemical shift anisotropy, have become of great interest lately. The authors show here that the interference between the quadrupolar coupling and the paramagnetic interaction (cross-correlated relaxation) gives rise to nuclear spin transitions that would remain forbidden otherwise. In addition, frequency shifts arise. These would be reminiscent of residual anisotropic interactions when there are none. While interesting from a fundamental point of view, these processes may become relevant in magnetic resonance imaging experiments which involve quadrupolar spins, such as (23)Na, in the presence of contrast agents. Geometrical constraints in paramagnetic molecule structures may likewise be derived from these interference effects.

Influence of pitch composition and surface properties of petroleum coke on their interaction during the preparation of carbon/carbon composites

The influence of pitch composition and surface properties of petroleum coke on the interaction of pitch and coke in the composite is studied. The adsorption results and EPR data indicate that recombination of paramagnetic centers and interaction of oxygen functional groups occur during the preparation of the composite. As a result of interaction between petroleum coke and pitch, polar compounds are concentrated in the adsorption layer. The baking criterion is used for the characterization of the baking ability of the pitches obtained as a result of different oxidation treatments of commercial coal tar pitch. It is determined that physico-chemical properties of the obtained pitches influence the value of the “baking criterion”. The kind of thermo-oxidation treatment of the parent pitch should be chosen hence achieving the maximum value of the baking criterion. For this purpose, the maximal part of the pitch has to take part in the adsorption layer.

Study of water dynamics and distances in paramagnetic solids by variable-temperature two-dimensional H2 NMR spectroscopy

A recently proposed two-dimensional (2)H NMR experiment is used to measure the (2)H (spin I=1) quadrupolar and paramagnetic shift anisotropy interactions in powdered CuCl(2).2D(2)O as a function of temperature. The principal components of the quadrupolar and paramagnetic shift anisotropy tensors and the Euler angles describing the orientations of the tensors in the molecular frame are determined at each temperature. For this purpose an analytical approach is introduced to extract desired parameters from motionally averaged two-dimensional line shapes where the averaging is introduced by rapid 180 degrees flips around C(2) axes of D(2)O molecules. This approach can be readily applied to study various materials containing water of crystallization. It is also clearly shown that the rapid continuous rotation of D(2)O molecules around their C(2) axes is not taking place in the studied solid in the range of temperatures between 209 and 344 K. Once the paramagnetic shift anisotropy of a deuterium atom is measured accurately it is used to estimate the distance between deuterium and the nearest copper atom bearing an unpaired electron. Excellent agreement is found between structural parameters obtained in this study and those provided by neutron and x-ray diffraction, showing that the paramagnetic shift anisotropy is a sensitive probe of distances in paramagnetic solids.

Effects of strong paramagnetic interactions on solid-state deuterium NMR spectra

The method for analyzing the 2H NMR spectrum affected by strong paramagnetic interaction is discussed. The 2H NMR spectral simulation including the effects of paramagnetic shift and paramagnetic spin–spin relaxation was performed for several molecular motions. The contribution of distant paramagnetic ions to the paramagnetic shift and the effect of anisotropic spin–spin relaxation on the lineshape of the 2H NMR spectrum were investigated by the spectral simulation. The temperature variation of 2H NMR spectrum of [Mn(H 2O) 6][SiF 6] observed by the Exorcycled quadrupole-echo sequence was well reproduced using the proposed method.

Inverse melting and inverse freezing: A spin model

Systems of highly degenerate ordered or frozen state may exhibit inverse melting (reversible crystallization upon heating) or inverse freezing (reversible glass transition upon heating). This phenomenon is reviewed, and a list of experimental demonstrations and theoretical models is presented. A simple spin model for inverse melting is introduced and solved analytically for infinite range, constant paramagnetic exchange interaction. The random exchange analogue of this model yields inverse freezing, as implied by the analytic solution based on the replica trick. The qualitative features of this system (generalized Blume-Capel spin model) are shown to resemble a large class of inverse melting phenomena. The appearance of inverse melting is related to an exact rescaling of one of the interaction parameters that measures the entropy of the system. For the case of almost degenerate spin states, perturbative expansion is presented, and the first three terms correspond to the empiric formula for the Flory-Huggins chi parameter in the theory of polymer melts. The possible microscopic origin of this chi parameter and the limitations of the Flory-Huggins theory where the state degeneracy is associated with the different conformations of a single polymer or with the spatial structures of two interacting molecules are discussed.

6/7Li NMR study of the Li1-zNi1+zO2 phases

A series of Li1-zNi1+zO2 materials have been synthesised by the coprecipitation route. An X-ray diffraction study was carried out on these materials using the Rietveld method to determine the departure from the ideal stoichiometry z, which ranges from 0 to 0.138. The actual Li/Ni ratio was also checked by chemical analyses using inductively coupled plasma (ICP) for each sample. The stoichiometric sample (z approximately 0) was obtained using a 15% Li excess. (6/7)Li NMR results from LiNiO2 (z approximately 0) show that the asymmetric shape of the NMR signal is due to anisotropy. Calculations give evidence that the paramagnetic dipolar interaction from the electron spins carried by Ni is anisotropic but does not completely explain the experimental anisotropy. (6)Li MAS NMR (magic angle spinning NMR) experiments and temperature standardisation NMR measurements unambiguously assign the isotropic position at +726 ppm. The static-echo NMR spectra of the non-stoichiometric Li1-zNi1+zO2 phases also exhibit an asymmetric shape whose width increases with the departure from the ideal stoichiometry z. (6/7)Li static and MAS NMR show that the 2zNi(2+) ions thus formed modify the dipolar interaction within the materials and also affect the Fermi contact interaction, since a distribution of Li environments is observed using (6)Li NMR for non-stoichiometric samples.

Spin model for inverse melting and inverse glass transition.

A spin model that displays inverse melting and inverse glass transition is presented and analyzed. Strong degeneracy of the interacting states of an individual spin leads to entropic preference of the "ferromagnetic" phase, while lower energy associated with the noninteracting states yields a "paramagnetic" phase as temperature decreases. An infinite range model is solved analytically for constant paramagnetic exchange interaction, while for its random exchange analogous results based on the replica symmetric solution are presented. The qualitative features of this model are shown to resemble a large class of inverse melting phenomena. First and second order transition regimes are identified.

Energetics and mechanism of Ca2+ displacement by lanthanides in a calcium binding protein.

The displacement of Ca(2+) by trivalent lanthanide ions (Tm(3+)) in a protozoan (Entamoeba histolytica) Ca(2+) binding protein has been studied by NMR and isothermal calorimetry (ITC). The study provides a basis for understanding the behavior of lanthanides when used as a substitute for Ca(2+), the pattern of sequential binding, the structural changes involved, the range and magnitude of paramagnetic interaction, and the associated energetics and mechanism. The progressive Ca(2+) displacement from site III first, followed by displacement from site II, I, and IV, as observed during the NMR titration experiments, is interpreted in the light of ITC data to provide a deeper insight into the intradomain and, for the first time, interdomain cooperativity and information about the statistical phenomenon involved in it. A theoretical model governing Ca(2+) displacement is provided. The small structural changes involved in Ca(2+) displacement by a diamagnetic lanthanide (La(3+)) has also been monitored.

Dynamics and disposition of benzene guest molecules in the micropore channels of a flexible metal-organic framework studied by 2H NMR and X-ray crystallography

The dynamics and disposition of benzene guest molecules included in the microporous channel structure of β-[CuL 2] (L is 1,1,1-trifluoro-5,5-dimethyl-5-methoxyacetylacetonate, {CF 3COCHCOC(CH 3) 2OCH 3} −) were studied as a function of temperature and total guest content. Two samples, with ∼100% and ∼35% of the sorption sites in the channels occupied by benzene, were studied with single-crystal XRD at 173 K. In the fully occupied sample, the benzene molecules are located in the alternating broader and narrower wide portions of the channels, and these sites are populated nearly equally, producing dense 1D packing of the guest molecules along the channel. The molecules in the broader channel site are almost perpendicular to the channel axis (⊥), while the molecules in the narrower channel site are almost aligned along the channel (∥) with their thermal parameters about twice as large. In the sample with 35% occupancy, the benzenes have thermal parameters that are twice those in the fully occupied sample and are distributed over ⊥ and ∥ sites in about a 1:2 ratio, suggesting an energetic preference for the smaller site. 2H NMR spectra of C 6D 6 were obtained for the 100% occupancy sample as a function of temperature. The lines are skewed slightly by paramagnetic interactions with the Cu II centers. Line shape analysis indicated a sequence of three types of benzene motion with increasing temperature: there is already rapid in-plane motion at 77 K, followed at slightly higher temperatures by the onset of 2-site jumps between the ⊥ and ∥ orientations. Before this reaches the fast motion limit, the onset of reorientation about the 3-fold axis of the channel commences in the 175 K region. At room temperature all of these dynamic processes are rapid, as they are for samples with lower benzene loadings, but in these cases the reduced ⊥:∥ ratio results in progressive line narrowing.

A solid-state 51V NMR characterization of vanadium sites in LiCo xNi 1− xVO 4

LiCo x Ni 1− x VO 4 compounds ( x=0, 0.2, 0.5, 0.8, 1.0) have been prepared for lithium ion batteries by different preparation methods, namely, wet-chemistry (WC) and solid-state (SS) routes. 51V nuclear magnetic resonance (NMR) has been employed to accurately assess the vanadium environments in these materials and results show that vanadium atoms are distributed in octahedral and tetrahedral sites. The fractions of vanadium atoms in tetrahedral/octahedral sites depends on composition and sample preparation conditions. The tetrahedral site, which is particularly dominant in samples produced via the WC route, is more affected by paramagnetic interactions and its 51V NMR spectrum is essentially Gaussian in shape. The octahedral site, on the other hand, exhibits first-order quadrupolar broadened satellites. Even though tetrahedral sites for a given x are comparable for WC and SS materials, there are some differences amongst octahedral environments, namely in the distributions of quadrupolar broadened satellites. The larger satellite distribution observed for WC materials is indicative of a larger distribution of structural defects for these as compared to SS materials.

A solid-state 51V NMR characterization of vanadium sites in LiCoxNi1$minus;xVO4

LiCoxNi1−xVO4 compounds (x=0, 0.2, 0.5, 0.8, 1.0) have been prepared for lithium ion batteries by different preparation methods, namely, wet-chemistry (WC) and solid-state (SS) routes. 51V nuclear magnetic resonance (NMR) has been employed to accurately assess the vanadium environments in these materials and results show that vanadium atoms are distributed in octahedral and tetrahedral sites. The fractions of vanadium atoms in tetrahedral/octahedral sites depends on composition and sample preparation conditions. The tetrahedral site, which is particularly dominant in samples produced via the WC route, is more affected by paramagnetic interactions and its 51V NMR spectrum is essentially Gaussian in shape. The octahedral site, on the other hand, exhibits first-order quadrupolar broadened satellites. Even though tetrahedral sites for a given x are comparable for WC and SS materials, there are some differences amongst octahedral environments, namely in the distributions of quadrupolar broadened satellites. The larger satellite distribution observed for WC materials is indicative of a larger distribution of structural defects for these as compared to SS materials.