Hyperfine Shifts Research Articles

Effects of optical absorption onGa71optically polarized NMR in semi-insulating GaAs: Measurements and simulations

The intensity and the hyperfine shift of optically polarized NMR (OPNMR) signals of $^{71}\mathrm{Ga}$ in semi-insulating GaAs have been found to depend on the photon energy and the helicity of light used for optical pumping. Single-crystal GaAs wafers of two different thicknesses, 400 and $175\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$, were examined. The maximum intensity of the OPNMR signals was observed well below the band gap energy, and this maximum OPNMR signal shifted to higher photon energies for the thinner sample. In the range of photon energies for which the maximum OPNMR signals are obtained, there is little or no hyperfine shift of the $^{71}\mathrm{Ga}$ OPNMR resonance. Hyperfine shifts with the largest magnitude are recorded for photon energies at or above the band gap. At a given photon energy, asymmetric OPNMR signals were observed, with ${\ensuremath{\sigma}}^{+}$ light producing a more intense emissive signal than the corresponding absorptive signal coming from ${\ensuremath{\sigma}}^{\ensuremath{-}}$ light. We developed a model that accounts for optical absorptivity of GaAs in order to simulate the observed OPNMR intensity, the change in the OPNMR maximum for the thinner sample, and the hyperfine shift dependence on photon energy. This model also accounts for the asymmetric OPNMR intensity for the two ${\ensuremath{\sigma}}^{+}$ and ${\ensuremath{\sigma}}^{\ensuremath{-}}$ helicities of light. The intensity dependence of the OPNMR signals arises as a consequence of two competing factors: the optical absorptivity of GaAs which directly impacts nuclear polarization $⟨{I}_{Z}⟩$, and the number of accessible nuclear spins. The magnitude of the hyperfine shift of the OPNMR signals reflects the probability of occupation of optically relevant defects in the semiconductor, also related to optical absorptivity. Finally, the asymmetry in the OPNMR signals arises from the sign of electron spin polarization produced in the optical pumping process.

Nonlinear effects in resonance conversion as a test of QED

Various phenomena such as hyperfine shift of 209Bi82+, nuclear decay of 229Th, and 169Yb69+ are considered as cross-invariant channels of the internal conversion process from the viewpoint of testing QED. The way of accelerating nuclear decay in a laser field through merging nuclear and laser photons is discussed. Discovering this fundamental process would give a convincing test of both QED along with our experimental capabilities.

Effects of ground-state hyperfine shifts in quantum computing with rare-earth-metal ions in solids

We present an investigation of the effects of constant but random shifts of the ground hyperfine qubit states in the setting of quantum computing with ion doped crystals. Complex hyperbolic secant pulses can be used to transfer ions reliably to electronically excited states, and a perturbative approach is used to analyse the effect of ground state hyperfine shifts. This analysis shows that the errors due to the hyperfine shift are dynamically supressed during gate operation, a fact we attribute to the AC Stark shift. Furthermore we present an implementation of a controlled phase gate which is resilient to the effects of the hyperfine shift. Decoherence and decay effects are included in simulations in order to show that a demonstration of quantum gates is feasible over the relevant range of system parameters.

Two-fermion relativistic bound states: hyperfine shifts

We discuss the hyperfine shifts of the Positronium levels in a relativistic framework, starting from a two fermion wave equation where, in addition to the Coulomb potential, the magnetic interaction between spins is described by a Breit term. We write the system of four first order differential equations describing this model. We discuss its mathematical features, mainly in relation to possible singularities that may appear at finite values of the radial coordinate. We solve the boundary value problems both in the singular and non singular cases and we develop a perturbation scheme, well suited for numerical computations, that allows to calculate the hyperfine shifts for any level, according to well established physical arguments that the Breit term must be treated at the first perturbative order. We discuss our results, comparing them with the corresponding values obtained from semi-classical expansions.

Light-induced hyperfineGa69shifts in semi-insulatingGaAsobserved by optically polarized NMR

We report the observation of $^{69}\mathrm{Ga}$ NMR light induced hyperfine shifts at $6\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in semi-insulating $\mathrm{GaAs}$ detected by optically polarized nuclear magnetic resonance in a magnetic field of $4.7\phantom{\rule{0.3em}{0ex}}\mathrm{T}$. The main features of the observed shift are a systematic change in the absolute shift value as the irradiation time is increased, and the sign of the hyperfine shift changes with polarization. The shift dependence on irradiation time can be understood as a combination of the hyperfine interaction of localized electrons with the surrounding nuclear spins and nuclear spin diffusion.

Preparation, crystal structures and NMR characterization of substituted-benzoate complexes Nickel(II)-N 3-macrocycles

Substituted-benzoate complexes of nickel(II) of the types bidentate [Ni(mcN 3)(Bz)](PF 6) and monodentate [Ni(mcN 3)(Bz)(H 2O)](PF 6) have been prepared by acid-base reaction between the hydroxo complexes [Ni(mcN 3)(μ-OH)] 2(PF 6) 2 (mcN 3 = 2,4,4-trimethyl-1,5,9-triazacyclododec-1-ene (Me 3-mcN 3) or its 9-methyl derivative (Me 4-mcN 3)) and the corresponding benzoic acid. The paramagnetic nickel(II) complexes have been characterized in solution by NMR spectroscopy. The influence of the substituents on the hyperfine shift patterns for substituted-benzoate complexes of nickel(II) has been studied. The substituent effects upon the coordination mode of substituted benzoates have been established by X-ray diffraction.

Solution 1H NMR characterization of the axial bonding of the two His in oxidized human cytoglobin.

Solution 1H NMR spectroscopy has been used to determine the relative strengths (covalency) of the two axial His-Fe bonds in paramagnetic, S = 1/2, human met-cytoglobin. The sequence specific assignments of crucial portions of the proximal and distal helices, together with the magnitude of hyperfine shifts and paramagnetic relaxation, establish that His81 and His113, at the canonical positions E7 and F8 in the myoglobin fold, respectively, are ligated to the iron. The characterized complex (approximately 90%) in solution has protohemin oriented as in crystals, with the remaining approximately 10% exhibiting the hemin orientation rotated 180 degrees about the alpha-, gamma-meso axis. No evidence could be obtained for any five-coordinate complex (<1%) in equilibrium with the six-coordinate complexes. Extensive sequence-specific assignments on other dipolar shifted helical fragments and loops, together with available alternate crystal coordinates for the complex, allowed the robust determination of the orientation and anisotropies of the paramagnetic susceptibility tensor. The tilt of the major axis is controlled by the His-Fe-His vector, and the rhombic axes are controlled by the mean of the imidazole orientations for the two His. The anisotropy of the paramagnetic susceptibility tensor allowed the quantitative factoring of the hyperfine shifts for the two axial His to reveal an indistinguishable pattern and magnitudes of the contact shifts or pi spin densities, and hence, indistinguishable Fe-imidazole covalency for both Fe-His bonds.

Testing QED with resonance conversion

Various phenomena: hyperfine shift of 209Bi82+, nuclear decay of 229Th and 169Yb69+ are considered as cross-invariant channels of the internal conversion process from the viewpoint of testing QED. The way of accelerating nuclear decay in laser field via merging the nuclear and laser photons is discussed. Discovering this fundamental process would give a convincing test of both QED along with our experimental abilities.

A justification for using NMR model‐free methods when investigating the solution structures of rhombic paramagnetic lanthanide complexes

The detailed analysis of the 1H NMR hyperfine shifts according to the model-free methods shows that the semi-rigid monometallic complexes [Ln(L)(NO3)3] (Ln = Eu-Yb) are isostructural in solution. The associated separation of contact and pseudo-contact contributions to the hyperfine NMR shifts in each rhombic lanthanide complex at room temperature provides paramagnetic susceptibility tensors whose principal magnetic axes match the expected symmetry requirements. Moreover, both axial (Delta chi(ax)) and rhombic (Delta chi(rh)) paramagnetic anisotropies display satisfactory linear dependence on Bleaney's factors, a correlation predicted by the approximate high-temperature expansion of the magnetic susceptibility limited to T(-2). Consequently, the simple, and chemically attracting NMR model-free methods are not limited to axial systems, and can be safely used for the investigation of the solution structures of any lanthanide complexes. Molecular-based structural criteria for the reliable estimation of paramagnetic susceptibility tensors by NMR are discussed, together with the assignment of the labels of the crystal-field and magnetic axes within Bleaney's approach.

Solution NMR study of environmental effects on substrate seating in human heme oxygenase: Influence of polypeptide truncation, substrate modification and axial ligand

Solution proton NMR has been used here to show that, as either the high-spin ferric, protohemin (PH) substrate complex at neutral pH, or the low-spin ferric, cyanide-inhibited PH substrate complex, the active site electronic and molecular structure of the 233- and 265-residue recombinant constructs of human heme oxygenase-1, hHO, are essentially indistinguishable. It is shown, moreover, that the equilibrium PH orientational isomerism about the α,γ- meso axis is 1:1 in the water-ligated, resting-state complex, but changes to a 4:1 equilibrium ratio as the cyanide-inhibited complex, with the minor species in solution corresponding to the only one found in crystals. The introduction of significant PH orientational preference in the cyanide over the aquo complex is rationalized by the crystallographic observation for the same H 2O and CN ligated complexes of rat heme oxygenase (rHO), where the steric tilt of the Fe–CN unit resulted in a ∼1 Å transition of PH into the hydrophobic interior, and stronger interaction of the vinyls with the HO matrix [M. Sugishima, H. Sakamoto, M. Noguchi, K. Fukugama, Biochemistry 42 (2003) 9898–9905]. 1H NMR spectra of the cyanide-inhibited PH complex are the most used, and most useful, for determining the distribution of orientational isomerism for PH in complexes of HO. Hence, it is imperative that the time-course of the spectra after sample preparation be considered in order to reach conclusions that relate isomeric seating of the heme with variable isomeric biliverdin products. The natural orientational isomerism of PH leads to spectral congestion that has prompted the use of a synthetic, twofold symmetric substrate, 2,4-dimethyldeuterohemin, DMDH. While the hyperfine shift pattern for non-ligated residues are very similar and are consistent with largely conserved molecular structure with the alternate substrates, the steric tilt of the Fe–CN vector towards the protein interior, as determined by the orientation of the major magnetic axes, is 2° smaller for DMDH than PH, and is rationalized by the substrate translating even further into the hydrophobic interior in the cyanide complex when the bulky vinyl groups are replaced by methyl groups.

Dielectronic Resonance Method for Measuring Isotope Shifts

Long standing problems in the comparison of very accurate hyperfine-shift measurements to theory were partly overcome by precise measurements on few-electron highly charged ions. Still the agreement between theory and experiment is unsatisfactory. In this Letter, we present a radically new way of precisely measuring hyperfine shifts, and demonstrate its effectiveness in the case of the hyperfine shift of 4s1/2 and 4p1/2 in 207Pb53+. It is based on the precise detection of dielectronic resonances that occur in electron-ion recombination at very low energy. This allows us to determine the hyperfine constant to around 0.6 meV accuracy which is on the order of 10%.

Heme Axial Methionine Fluxion in Pseudomonas aeruginosa Asn64Gln Cytochrome c551

Heme axial methionine ligands in ferricytochromes c552 from Hydrogenobacter thermophilus (HT) and Nitrosomonas europaea, both members of the cyt c8 family, display fluxional behavior. The ligand motion, proposed to be inversion at sulfur, results in an unusually small range of hyperfine shifts for heme substituents in these proteins. Herein, heme axial Met fluxion is induced in a structurally homologous cytochrome c551 from Pseudomonas aeruginosa (PA) by substituting heme pocket residue Asn64 with Gln. The mutant, PA-N64Q, displays a highly compressed range of heme substituent hyperfine shifts, temperature-dependent heme methyl resonance line broadening, low rhombic magnetic anisotropy, and a magnetic axes orientation consistent with Met orientational averaging. Analysis of NMR properties of PA-N64Q demonstrates that the heme pocket of the mutant resembles that of HT. This result confirms the importance of peripheral interactions and, in particular, residue 64 in determining axial Met orientation and heme electronic structure in proteins in the cyt c8 family.

Investigating Sorption on Iron−Oxyhydroxide Soil Minerals by Solid-State NMR Spectroscopy: A 6Li MAS NMR Study of Adsorption and Absorption on Goethite

High-resolution 2H MAS NMR spectra can be obtained for nanocrystalline particles of goethite (alpha-FeOOH, particle size approximately 4-10 nm) at room temperature, facilitating NMR studies of sorption under environmentally relevant conditions. Li sorption was investigated as a function of pH, the system representing an ideal model system for NMR studies. 6Li resonances with large hyperfine shifts (approximately 145 ppm) were observed above the goethite point of zero charge, providing clear evidence for the presence of Li-O-Fe connectivities, and thus the formation of an inner sphere Li+ complex on the goethite surface. Even larger Li hyperfine shifts (289 ppm) were observed for Li+-exchanged goethite, which contains lithium ions in the tunnels of the goethite structure, confirming the Li assignment of the 145 ppm Li resonance to the surface sites.

NMR structures of paramagnetic metalloproteins

Metalloproteins represent a large share of the proteome and many of them contain paramagnetic metal ions. The knowledge, at atomic resolution, of their structure in solution is important to understand processes in which they are involved, such as electron transfer mechanisms, enzymatic reactions, metal homeostasis and metal trafficking, as well as interactions with their partners. Formerly considered as unfeasible, the first structure in solution by nuclear magnetic resonance (NMR) of a paramagnetic protein was obtained in 1994. Methodological and instrumental advancements pursued over the last decade are such that NMR structure of paramagnetic proteins may be now routinely obtained. We focus here on approaches and problems related to the structure determination of paramagnetic proteins in solution through NMR spectroscopy. After a survey of the background theory, we show how the effects produced by the presence of a paramagnetic metal ion on the NMR parameters, which are in many cases deleterious for the detection of NMR spectra, can be overcome and turned into an additional source of structural restraints. We also briefly address features and perspectives given by the use of 13C-detected protonless NMR spectroscopy for proteins in solution. The structural information obtained through the exploitation of a paramagnetic center are discussed for some Cu2+ -binding proteins and for Ca2+ -binding proteins, where the replacement of a diamagnetic metal ion with suitable paramagnetic metal ions suggests novel approaches to the structural characterization of proteins containing diamagnetic and NMR-silent metal ions.

Suppression of Axial Methionine Fluxion in Hydrogenobacter thermophilus Gln64Asn Cytochrome c552

Proteins in the cytochrome c (cyt c) family with His-Met heme axial ligation display diverse heme electronic structures as revealed by the NMR spectra of their oxidized (paramagnetic) forms. These variations in electronic structure are thought to result primarily from differences in heme axial Met orientation among cyt c species. The factors determining Met orientation in cyts c, however, remain poorly understood. An additional layer of complexity was revealed with the recent finding that the axial Met in Hydrogenobacter thermophilus cytochrome c(552) (Ht cyt c(552)) is fluxional, sampling two conformations rapidly on the NMR time scale, resulting in an unusual compressed range of heme substituent hyperfine shifts [Zhong, L., Wen, X., Rabinowitz, T. M., Russell, B. S., Karan, E. F., and Bren, K. L. (2004) Proc.Natl. Acad. Sci. U.S.A. 101, 8637-8642]. In this work, the (1)H NMR hyperfine shift pattern of Ht cyt c(552) is drastically altered by making the conservative heme pocket mutation Gln64Asn. The mutant (Ht Q64N) displays a pattern of heme hyperfine shifts with a remarkable resemblance to that of structurally homologous Pseudomonas aeruginosa cyt c(551), which has Asn at position 64 and a single heme axial Met conformation. NMR analysis reveals that Asn64 in Ht Q64N is positioned to interact with the axial Met61, whereas the Gln64 in wild-type Ht cyt c(552) is not. It also is found that the heme axial Met is not fluxional in Ht Q64N and has an orientation similar to that in P. aeruginosa cyt c(551). These results indicate that peripheral interactions with the axial Met play an important role in determining axial Met orientation and heme electronic structure in cyts c.

Paramagnetic NMR Spectroscopy and Density Functional Calculations in the Analysis of the Geometric and Electronic Structures of Iron−Sulfur Proteins

Paramagnetic NMR spectroscopy has been underutilized in the study of metalloproteins. One difficulty of the technique is that paramagnetic relaxation broadens signals from nuclei near paramagnetic centers. In systems with low electronic relaxation rates, this makes such signals difficult to observe or impossible to assign by traditional methods. We show how the challenges of detecting and assigning signals from nuclei near the metal center can be overcome through the combination of uniform and selective 2H, 13C, and 15N isotopic labeling with NMR experiments that utilize direct one-dimensional (2H, 13C, and 15N) and two-dimensional (13C-X) detection. We have developed methods for calculating NMR chemical shifts and relaxation rates by density functional theory (DFT) approaches. We use the correspondence between experimental NMR parameters and those calculated from structural models of iron-sulfur clusters derived from X-ray crystallography to validate the computational approach and to investigate how structural differences are manifested in these values. We have applied this strategy to three iron-sulfur proteins: Clostridium pasteurianum rubredoxin, Anabaena [2Fe-2S] ferredoxin, and human [2Fe-2S] ferredoxin. Provided that an accurate structural model of the iron-sulfur cluster and surrounding residues is available from diffraction data, our results show that DFT calculations can return NMR observables with excellent accuracy. This suggests that it might be possible to use calculations to refine structures or to generate structural models of active sites when crystal structures are unavailable. The approach has yielded insights into the electronic structures of these iron-sulfur proteins. In rubredoxin, the results show that substantial unpaired electron spin is delocalized across NH...S hydrogen bonds and that the reduction potential can be changed by 77 mV simply by altering the strength of one of these hydrogen bonds. In reduced [2Fe-2S] ferredoxins, hyperfine shift data have provided quantitative information on the degree of valence trapping. The approach described here for iron-sulfur proteins offers new avenues for detailed studies of these and other metalloprotein systems.

Magnetic Properties of the Diamond Chain Compound Cu3(CO3)2(OH)2

Azurite (Cu 3 (CO 3 ) 2 (OH) 2 ) is a model substance of a diamond spin chain, one of frustrated antiferromagnetic quantum spin chains. The high field magnetization up to 60 T, magnetic susceptibility, magnetic entropy, muon spin relaxation and 1 H-NMR of azurite have been measured using single crystals. A distinct 1/3 magnetization plateau is confirmed to be present in the magnetization curve. Two resonance peaks of 1 H-NMR. were observed. By analyzing temperature dependencies of their hyperfine shift. we found two different local susceptibilities which correspond to susceptibility of dimer and monomer spins, respectively. New field induced phase transition was found to occur at a critical field where the magnetic plateau onsets. A short range magnetic ordering developed in a peculiar two-stage process, which was revealed by temperature dependence of the magnetic specific heat and susceptibility.

Structural Transitions and Magnetic Structure in NH4CuCl3via14N-NMR

We report results of 14N-NMR experiments on NH4CuCl3 at the magnetic field of 7 T, where the 1/4-magnetization plateau is observed at low temperatures. The quadrupole coupling parameter νz splits below 70 K, indicating a structural phase transition. At 4.2 K, eight N sites with distinct values of both νz and the magnetic hyperfine shift Kz are resolved in the NMR spectrum for general field directions. We then conclude that the magnetic structure in the 1/4-plateau does not break the symmetry of the crystal. Based on the NMR and the recent neutron scattering results by Rüegg et al. [Phys. Rev. Lett. 93 (2004), 037207], we propose that triplet dimers in the 1/4-plateau are formed not between the nearest neighbor pairs but over different chains.

Folding, Conformational Changes, and Dynamics of CytochromescProbed by NMR Spectroscopy

NMR spectroscopy has become a vital tool for studies of protein conformational changes and dynamics. Oxidized Fe(III)cytochromes c are a particularly attractive target for NMR analysis because their paramagnetism (S = (1)/(2)) leads to high (1)H chemical shift dispersion, even for unfolded or otherwise disordered states. In addition, analysis of shifts induced by the hyperfine interaction reveals details of the structure of the heme and its ligands for native and nonnative protein conformational states. The use of NMR spectroscopy to investigate the folding and dynamics of paramagnetic cytochromes c is reviewed here. Studies of nonnative conformations formed by denaturation and by anomalous in vivo maturation (heme attachment) are facilitated by the paramagnetic, low-spin nature of native and nonnative forms of cytochromes c. Investigation of the dynamics of folded cytochromes c also are aided by their paramagnetism. As an example of this analysis, the expression in Escherichia coli of cytochrome c(552) from Nitrosomonas europaea is reported here, along with analysis of its unusual heme hyperfine shifts. The results are suggestive of heme axial methionine fluxion in N. europaea ferricytochrome c(552). The application of NMR spectroscopy to investigate paramagnetic cytochrome c folding and dynamics has advanced our understanding of the structure and dynamics of both native and nonnative states of heme proteins.

Quantum chemical studies of protein structure.

Quantum chemical methods now permit the prediction of many spectroscopic observables in proteins and related model systems, in addition to electrostatic properties, which are found to be in excellent accord with those determined from experiment. I discuss the developments over the past decade in these areas, including predictions of nuclear magnetic resonance chemical shifts, chemical shielding tensors, scalar couplings and hyperfine (contact) shifts, the isomer shifts and quadrupole splittings in Mössbauer spectroscopy, molecular energies and conformations, as well as a range of electrostatic properties, such as charge densities, the curvatures, Laplacians and Hessians of the charge density, electrostatic potentials, electric field gradients and electrostatic field effects. The availability of structure/spectroscopic correlations from quantum chemistry provides a basis for using numerous spectroscopic observables in determining aspects of protein structure, in determining electrostatic properties which are not readily accessible from experiment, as well as giving additional confidence in the use of these techniques to investigate questions about chemical bonding and chemical reactions.