Nuclear Magnetic Double Resonance Research Articles

Preparation, characterization, and structural studies of new sodium gallium tungstate phosphate glasses

A new glass forming region in the quaternary system Na2O-P2O5-WO3-Ga2O3 along the composition line (NaPO3)80-x(Ga2O3)20(Na2WO4)x (x = 0, 5, 10, 15, 20 and 25 mol%) has been prepared and characterized by X-ray diffraction, Differential Scanning Calorimetry (DSC), Raman scattering and 31P, 23Na and 71Ga nuclear magnetic single and double resonance NMR techniques. The partial substitution of the NaPO3 component by Na2WO4 produces a significant increase of molar volume (decreased oxygen packing density), whereas the glass transition temperatures Tg pass through a shallow maximum near x = 10 to 15. While in the x = 0 sample the structure is dominated by Q1 units linked to gallium in four-, five- and six-coordination, the gradual substitution of phosphate by tungstate significantly affects network reconstruction, eliminating P-O-P linkages, the concentration of which is found to be greatly diminished above the x = 10 level. While the structure of these glasses is dominated by Q0 units, connected to both gallium and tungstate species, 71Ga{31P} REDOR data suggest significant decreases in the extent of gallium-phosphorus connectivity with increasing Na2WO4 content. Finally, monotonic changes in both 23Na chemical shifts and 23Na-31P second moment values, M2(Na-P), indicate that the anionic surroundings of the sodium ions evolve from a pure phosphate to a mixed phosphate/tungstate environment.

Reconstruction of Coupled Intra- and Interdomain Protein Motion from Nuclear and Electron Magnetic Resonance.

Proteins composed of multiple domains allow for structural heterogeneity and interdomain dynamics that may be vital for function. Intradomain structures and dynamics can influence interdomain conformations and vice versa. However, no established structure determination method is currently available that can probe the coupling of these motions. The protein Pin1 contains separate regulatory and catalytic domains that sample "extended" and "compact" states, and ligand binding changes this equilibrium. Ligand binding and interdomain distance have been shown to impact the activity of Pin1, suggesting interdomain allostery. In order to characterize the conformational equilibrium of Pin1, we describe a novel method to model the coupling between intra- and interdomain dynamics at atomic resolution using multistate ensembles. The method uses time-averaged nuclear magnetic resonance (NMR) restraints and double electron-electron resonance (DEER) data that resolve distance distributions. While the intradomain calculation is primarily driven by exact nuclear Overhauser enhancements (eNOEs), J couplings, and residual dipolar couplings (RDCs), the relative domain distribution is driven by paramagnetic relaxation enhancement (PREs), RDCs, interdomain NOEs, and DEER. Our data support a 70:30 population of the compact and extended states in apo Pin1. A multistate ensemble describes these conformations simultaneously, with distinct conformational differences located in the interdomain interface stabilizing the compact or extended states. We also describe correlated conformations between the catalytic site and interdomain interface that may explain allostery driven by interdomain contact.

19F Paramagnetic Relaxation-Based NMR for Quaternary Structural Restraints of Ion Channels.

Quaternary distance restraints are essential to define the three-dimensional structures of protein assemblies. These distances often fall within a range of 10-18 Å, which challenges the high and low measurement limits of conventional nuclear magnetic resonance (NMR) and double electron-electron resonance electron spin resonance spectroscopies. Here, we report the use of 19F paramagnetic relaxation enhancement (PRE) NMR in combination with 19F/paramagnetic labeling to equivalent sites in different subunits of a protein complex in micelles to determine intersubunit distances. The feasibility of this strategy was evaluated on a pentameric ligand-gated ion channel, for which we found excellent agreement of the 19F PRE NMR results with previous structural information. The study suggests that 19F PRE NMR is a viable tool in extracting distance restraints to define quaternary structures.

A New Algorithm for the Reactivity Umpolung of Aliphatic Nitro Compounds

A new four-stage algorithm for the reactivity umpolung of aliphatic nitro compounds has been proposed and experimentally confirmed. Bis(trialkylsilyloxy)iminium cations, which are key intermediates in the new scheme, have been qualitatively and quantitatively determined by the method of double nuclear magnetic resonance. Their thermodynamic parameters have been determined. The kinetics of coupling of these cations with π-nucleophiles has been measured experimentally, and the stereodynamics of the interaction of these cations has been discussed using a general model involving anomeric assistance. The utility and novelty of the proposed approach for the development of modern methodology of organic synthesis using available aliphatic nitro compounds as initial reagents have been demonstarted.

Electron density distribution in BaPb1 − x Sb x O3 superconducting oxides studied by double nuclear magnetic resonance methods

The effect of charge disorder on the formation of an inhomogeneous state of the electron system in the conduction band in BaPb1 − xSbxO3 superconducting oxides is investigated experimentally by NMR methods. The NMR spectra of 17O are measured systematically, and the contributions from 17O atoms with different cation nearest surroundings are identified. It is found that microscopic regions with an elevated spin density of charge carriers are formed within two coordination spheres near antimony ions. Nuclei of the superconducting phase of the oxide (regions with an elevated antimony concentration) microscopically distributed over the sample are detected in compounds with x = 0.25 and 0.33. Experiments in which a double resonance signal of the spin echo of 17O-207Pb and 17O-121Sb are measured in the metal phase of BaPb1 − xSbxO3 oxides are carried out for the first time. The constants of indirect heteronuclear spin-spin 17O-207Pb interaction are determined as functions of the local Knight shift 207Ks. The estimates of the constants of the indirect interaction between the nuclei of the nearest neighbors (O-Pb and Pb-Pb atoms) and analysis of evolution of the NMR spectra of 17O upon a change in the antimony concentration are convincing evidence in favor of the development of a microscopically inhomogeneous state of the electron system in the metal phase of BaPb1 − xSbxO3 oxides.

Magnetic field compensation for field-cycling NMR Relaxometry in the ULF band

In setting up field-cycling experiments aimed to study physical phenomena in the low-field region, magnetic field contributions from external sources (earth’s field, environment, other magnets, etc.) become important. Indeed, a compensation of these contributions has successfully been used for the application of field-cycling methods to nuclear magnetic relaxation and double resonance experiments. This feature becomes relevant in samples where local fields are stronly averaged due to motional narrowing, on the ground that relaxation experiments can therefore be extended to lower fields. Compensation of external contributions is also crucial for the study of kinky processes related to internal local fields. In this article we outline NMR field-cycling experiments aimed to detect and quantify the external net magnetic field sensed by the spin-system. Both parallel and normal components with respect to the high-field Zeeman axis can be determined separately by using different experimental protocols.

The spin distribution in low-spin iron porphyrins.

In many low-spin (S = 1/2) iron porphyrin derivatives, electron spin resonance (ESR) spectra indicate that one of the d(pi) orbitals of iron, either a d(xz) or d(yz), depending on the axial ligands of the porphyrin complex as well as their orientation, is essentially singly occupied; the unpaired electron is almost completely located at the metal. In contrast, nuclear magnetic resonance (NMR) and electron nuclear double resonance (ENDOR) spectroscopy convincingly show that a significant share of the unpaired electron is delocalized. This apparent contradiction is explained by the present density-functional-theory (DFT) calculations performed on a heme a model as well as on bis-imidazole-ligated iron porphyrin without substituents. The calculations show that the integrated spin density at the iron atom is nearly one, in agreement with the ESR measurements. However, significant areas with opposite (beta) spin are found along the Fe-N bond axes, thus evoking a need for additional alpha-spin density to be present in the porphyrin ring, ring substituents, and the axial ligands to keep the net amount of unpaired spin exactly one. The gross spin density, that is, the sum of unpaired alpha and beta spins, amounts to about 1.3 electrons. It seems that the degree to which alpha and beta spin dominate in different regions of the heme structure, as evidenced in these calculations, has not been previously observed.

Solid-State Double Nuclear Magnetic Resonance Study of the Local Structure of Calcium Phosphate Nanoparticles Synthesized by a Wet-Mechanochemical Reaction

Calcium phosphate synthesized by a wet-mechanochemical reaction between Ca(OH)2 and H3PO4 is identified by X-ray diffractometry as an apparently single phase of hydroxyapatite (HAp). We examined more detail on the local structure of the product by high-resolution 1H and 31P solid-state double nuclear magnetic resonance (NMR) spectroscopy. Adsorbed and structural waters, as well as two kinds of OH groups in HAp, were observed in the inversion recovery 1H magic angle spinning (MAS) spectra. HAp and other calcium phosphate were observed in 1H→31P cross polarization (CP)/MAS spectra. The 31P differential cross polarization (DCP)/MAS and 1H inversion recovery → 31P CP/MAS measurements revealed different chemical interactions between 1H and 31P and, hence, could distinguish between ordered and disordered HAp. This micro-inhomogenity results from interiors and near-surface of nanoparticles as well as Ca2+-deficiency.

Site-selective electron nuclear double resonance in the exchange-narrowed regime of the ESR in conducting solids.

Electron nuclear magnetic double resonance on conduction electrons reveals the hyperfine interaction hidden by the fast electron spin exchange. We used the Overhauser shift technique to investigate the electron spin density of the conduction band of gallium oxide, beta-Ga(2)O(3). Due to the monoclinic structure, the conduction band of beta-Ga(2)O(3) is anisotropic and it is dominated by contributions from the two nonequivalent Ga sites. The large quadrupole couplings of the two gallium isotopes (69)Ga and (71)Ga (both with I = 3/2) are completely resolved in our double-resonance experiments. This resolved quadrupole interaction allows the determination of the electric field gradients at both gallium sites with high precision and high sensitivity. The resolved quadrupole splitting is the key to the site-selected determination of the hyperfine interaction. The concepts behind these double-resonance techniques are rather general and should be applicable in similar semiconductor systems.

Application of Nuclear Magnetic Double Resonance to Kinetic Studies of Photoinduced Chemical Exchange in Solution. Some Theoretical and Methodological Problems

Application of Nuclear Magnetic Double Resonance to Kinetic Studies of Photoinduced Chemical Exchange in Solution. Some Theoretical and Methodological Problems

Determination of off-center displacements in NaBr:F− by nuclear magnetic double resonance

The measurements of the quadrupole splittings of Na+ ion in NaBr:F− crystal are performed by nuclear magnetic double resonance. The electric field gradients produced by lattice distortion at the Na nuclei in the vicinity of the substitutional F− impurity ion is obtained, and the displacement of Na+ ions at (100) sites is decided. By analyzing the source of the Na− displacement, the magnitude of the off-center displacement of F− ion along 〈110〉 direction in NaBr crystal is determined as 0.45 Å. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 75: 961–965, 1999

Determination of off-center displacements in NaBr:F? by nuclear magnetic double resonance

Determination of off-center displacements in NaBr:F? by nuclear magnetic double resonance

NMR studies of exchanging amide protons in 1‐desamino‐8‐D‐argininevasopressin by selective Hartmann‐Hahn transfer

AbstractProton exchange rates between the solvent H2O and the amidic protons of a nonapeptide (1‐desamino‐8‐D)‐argininevasopressin in aqueous solution were determined by using selective nuclear magnetic double‐resonance methods. Transverse magnetization, the magnitude of which depends on the chemical exchange rate and the relaxation time T1, is transferred via doubly selective homonuclear Hartmann‐Hahn mixing to a scalar coupling partner. With respect to well established two‐dimensional experiments, this one‐dimensional alternative has two advantages: it is much less time consuming and, as a true benefit, it is even applicable when two signals which are in exchange with the same compound (solvent) are overlapping. The evaluation of amide proton exchange rates in the investigated peptide led to the assumption of an intramolecular hydrogen bond.

A Low-Temperature High-Sensitivity Field-Cycling Spectrometer for Quadrupole Double Resonance Spectroscopy

Abstract This paper discusses the design of a variable-temperature high-sensitivity field cycling spectrometer for nuclear magnetic resonance and nuclear quadrupole double resonance experiments. The instrument is capable of detecting N-14 quadrupole resonance signals from samples as small as 0.1 g down to temperatures of 150 K. The magnetic field cycling is performed by mechanical transfer of the sample using a pair of switchable electromagnetic solenoids, in times of about 0.2 s.

A 153Eu NMR Study of EuSe at Ultralow Temperatures

153Eu NMR of EuSe is studied at very low temperatures using a 3He-4He dilution refrigerator. Below 1.8K the free induction decay(A-signal) which comes from the 153Eu nucleus in the antiferromagnetic state is observed without external field. In the small external field the signal (B-signal) which comes from the nucleus in the ferrimagnetic state grows up and at 0.05 Tesla the intensity of A-signal is equal to that of B-signal. In the field between 0.15 and 0.4 Tesla only B-signal is observed and by double nuclear magnetic resonance it is found the B-signal is modified by frequency pulling effect.

Double nuclear magnetic resonance in the weak ferromagnet YCrO3

Applications of 2D-NMR spectroscopy to measurements of the quadrupole splitting in magnetic materials are discussed using the weak ferromagnet YCrO3 as a model system. A simple qualitative dissussion and results from computersimulations are given for various experiments. In systems with distributed electric field gradients the method provides a unique way to measure hyperfine field, quadrupole splitting and correlations between these local parameters.

1H and 13c nmr analysis of lycorine and α-dihydrolycorine

-The 1H and 13C NMR spectra of lycorine and its α-dihydro derivative have been studied. The employment of nuclear magnetic double resonance, nuclear Overhauser effect and acetylated derivatives, allows the assignment of all proton resonances. The assignments of the carbon shifts have been obtained by means of proton noise decoupled, single frequency off-resonance decoupled, single frequency selective decoupling, time dependence nuclear Overhauser effect and by comparison with reference compounds.

1H- 31P nuclear magnetic double resonance study of Escherichia coli lipopolysaccharide mobility

Purified lipopolysaccharide preparations from Escherichia coli K12 were studied by 1H- 31P nuclear magnetic double resonance methods (cross-polarization and dipolar decoupling). At each of the temperatures studied (24, 37 and 46°C), the overall rotational mobility of a sizeable fraction of the phosphorus nuclei was restricted on the time scale of the NMR measurements, approx. 10 −4 s. The fraction of motionally restricted material, as well as the extent of restriction, decreased with increasing temperature. These studies provide the first numerical estimate of the percentage of lipopolysaccharide that is motionally restricted at a given temperature.

Nuclear magnetic double resonance with a doubly modulated radiofrequency field

Efficient broadband decoupling of heteronuclear spin systems is accomplished by a modulation method which successively phase-modulates the irradiating radiofrequency field with two square-wave frequencies that have a frequency ratio of four to one. Experimental observations show this efficient utilization of decoupler power can increase the signal heights by reducing residual splittings which broaden spectral lines. Compared to random-noise decoupling, line narrowing and signal-to-noise improvements of a factor of 3 are observed for proton-decoupled carbon-13 and phosphorus-31 NMR spectra. Efficient spin decoupling with only 1.5 W of decoupling power is demonstrated using this new modulation method.

Nuclear magnetic double resonance for a spin system subject to scalar relaxation of the second kind

The equation of motion of a reduced spin density operator characterizing the state of a group of spins scalar coupled to a second group of spins which are either relaxing rapidly and/or irradiated with a strong radiofrequency field is derived within the framework of the non-viscous liquid approximation. An analytic expression for the spectral densities entering this equation is derived within the extreme narrowing approximation for the case in which the relaxation rates of the nuclei in the second group of spins are much greater than any scalar couplings between them. The resultant equation is used to discuss the conditions needed in a high resolution nuclear magnetic double resonance experiment to decouple a spin-½ nucleus (A) from a rapidly relaxing quadrupolar nucleus (X) in the fast relaxation limit, defined by |2πJAX T 1X | ≪ 1, in which the A resonance consists of a single broadened line. It is found that, if (ω A - ω X )2 ≫ T 1X -2 so that the x and y components of the JAX coupling can be neglected...