Chemical Shift Anisotropy Mechanism Research Articles

The 103Rh NMR spectroscopy and relaxometry of the rhodium formate paddlewheel complex.

The nuclear magnetic resonance (NMR) spectroscopy of spin-1/2 nuclei with low gyromagnetic ratio is challenging due to the low NMR signal strength. Methodology for the rapid acquisition of 103Rh NMR parameters is demonstrated for the case of the rhodium formate "paddlewheel" complex Rh2(HCO2)4. A scheme is described for enhancing the 103Rh signal strength by polarization transfer from 1H nuclei, which also greatly reduces the interference from ringing artifacts, a common hurdle for the direct observation of low-γ nuclei. The 103Rh relaxation time constants T1 and T2 are measured within 20minby using 1H-detected experiments. The field dependence of the 103Rh T1 is measured. The high-field relaxation is dominated by the chemical shift anisotropy mechanism. The 103Rh shielding anisotropy is found to be very large: |Δσ| = 9900 ± 540ppm. This estimate is compared with density functional theory calculations.

Limits in Proton Nuclear Singlet-State Lifetimes Measured with para-Hydrogen-Induced Polarization.

The synthesis of a hyperpolarized molecule was developed, where the polarization and the singlet state were preserved over two controlled chemical steps. Nuclear singlet-state lifetimes close to 6 min for protons are reported in dimethyl fumarate. Owing to the high symmetry (AA'X3 X3 ' and A2 systems), the singlet-state readout requires either a chemical desymmetrization or a long and repeated spin lock. Using DFT calculations and relaxation models, we further determine nuclear spin singlet lifetime limiting factors, which include the intramolecular dipolar coupling mechanism (proton-proton and proton-deuterium), the chemical shift anisotropy mechanism (symmetric and antisymmetric), and the intermolecular dipolar coupling mechanism (to oxygen and deuterium). If the limit of paramagnetic relaxation caused by residual oxygen could be lifted, the intramolecular dipolar coupling to deuterium would become the limiting relaxation mechanism and proton lifetimes upwards of 26 min could become available in the molecules considered here (dimethyl maleate and dimethyl fumarate).

Band inversion amplifies 31 P-31 P nuclear overhauser effects: Relaxation mechanism and dynamic behavior of ATP in the human brain by 31 P MRS at 7 T.

To develop an improved method to measure the 31 P nuclear Overhauser effect (NOE) for evaluation of adenosine triphosphate (ATP) dynamics in terms of correlation time (τc ), and contribution of dipole-dipole (DD) and chemical shift anisotropy (CSA) mechanisms to T1 relaxation of ATP in human brain. The NOE of ATP in human brain was evaluated by monitoring changes in magnetization in the β-ATP signal following a band inversion of all downfield 31 P resonances. The magnetization changes observed were analyzed using the Bloch-McConnell-Solomon formulation to evaluate the relaxation and motion dynamic parameters that describe interactions of ATP with cellular solids in human brain tissue. The maximal transient NOE, observed as a reduction in the β-ATP signal, was 24 ± 2% upon band inversion of γ- and α-ATP, which is 2-3-fold higher than achievable by frequency-selective inversion of either γ- or α-ATP. The rate of 31 P-31 P cross relaxation (0.21 ± 0.02 s-1 ) led to a τc value of (9.1 ± 0.8) × 10-8 s for ATP in human brain. The T1 relaxation of β-ATP is dominated by CSA over the DD mechanism (60%: 40%). The band inversion method proved effective in amplifying 31 P NOE, and thus facilitating ATP τc and relaxation measurements. This technique renders ATP a potentially useful reporter molecule for cellular environments. Magn Reson Med 77:1409-1418, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Kinetics of Coupled Double Proton and Deuteron Transfer in Hydrogen-Bonded Ribbons of Crystalline Pyrazole-4-carboxylic Acid

Abstract The proton tautomerism of pyrazole-4-carboxylic acid (PCA) has been studied by a combination of 15N CPMAS and 2H NMR spectroscopy and relaxometry. Down to 250 K, PCA forms a hydrogen bonded ribbon where adjacent carboxylic and pyrazole groups are linked by an OH···N and an O···HN hydrogen bond, forming either the tautomeric state A or B. Down to about 250 K, the tautomerism is fast on the NMR timescale and degenerate, corresponding to a phase exhibiting dynamic proton disorder. At lower temperatures, a transition to an ordered phase is observed with localized protons, assigned to an all-syn conformation adopting the sequence of tautomeric states ..ABABA.. The longitudinal 15N relaxation times T 1 of PCA-15N2 have been measured at 9.12 MHz (2.1 T). Because of the low field, a chemical shift anisotropy mechanism could be neglected, and the data were analyzed in terms of a dipolar 1H-15N relaxation mechanism, yielding the rate constants k HH. The rate constants k HD and k DD were obtained from the measurement and analysis of the 2H T 1 values of PCA-15N1-d0.9 and PCA-15N1-d0.1 measured at 46.03 MHz. Within the margin of error, no kinetic isotope effects could be detected, in contrast to previous results reported for the very fast tautomerism of solid benzoic acid dimers and the much slower tautomerism of solid 3,5-diphenyl-4-brompyrazole (DPBrP) dimers. The Arrhenius curves of all three systems were simulated using the Bell–Limbach tunneling model. Evidence for a major heavy atom motion for the tautomerism of PCA is obtained, associated with small angle reorientation of PCA molecules around the molecular axis. The observed proton order-disorder transition and the mechanism of the observed rate process are discussed in terms of a coupling of adjacent tautomeric states.

Direct simulation of magnetic resonance relaxation rates and line shapes from molecular trajectories.

We simulate spin relaxation processes, which may be measured by either continuous wave or pulsed magnetic resonance techniques, using trajectory-based simulation methodologies. The spin-lattice relaxation rates are extracted numerically from the relaxation simulations. The rates obtained from the numerical fitting of the relaxation curves are compared to those obtained by direct simulation from the relaxation Bloch-Wangsness-Abragam-Redfield theory (BWART). We have restricted our study to anisotropic rigid-body rotational processes, and to the chemical shift anisotropy (CSA) and a single spin-spin dipolar (END) coupling mechanisms. Examples using electron paramagnetic resonance (EPR) nitroxide and nuclear magnetic resonance (NMR) deuterium quadrupolar systems are provided. The objective is to compare those rates obtained by numerical simulations with the rates obtained by BWART. There is excellent agreement between the simulated and BWART rates for a Hamiltonian describing a single spin (an electron) interacting with the bath through the chemical shift anisotropy (CSA) mechanism undergoing anisotropic rotational diffusion. In contrast, when the Hamiltonian contains both the chemical shift anisotropy (CSA) and the spin-spin dipolar (END) mechanisms, the decay rate of a single exponential fit of the simulated spin-lattice relaxation rate is up to a factor of 0.2 smaller than that predicted by BWART. When the relaxation curves are fit to a double exponential, the slow and fast rates extracted from the decay curves bound the BWART prediction. An extended BWART theory, in the literature, includes the need for multiple relaxation rates and indicates that the multiexponential decay is due to the combined effects of direct and cross-relaxation mechanisms.

Solvent Dynamical Behavior in an Organogel Phase As Studied by NMR Relaxation and Diffusion Experiments

An organogelation process depends on the gelator-solvent pair. This study deals with the solvent dynamics once the gelation process is completed. The first approach used is relaxometry, i.e., the measurement of toluene proton longitudinal relaxation time T(1) as a function of the proton NMR resonance frequency (here in the 5 kHz to 400 MHz range). Pure toluene exhibits an unexpected T(1) variation, which has been identified as paramagnetic relaxation resulting from an interaction of toluene with dissolved oxygen. In the gel phase, this contribution is retrieved with, in addition, a strong decay at low frequencies assigned to toluene molecules within the gel fibers. Comparison of dispersion curves of pure toluene and toluene in the gel phase leads to an estimate of the proportion of toluene embedded within the organogel (found around 40%). The second approach is based on carbon-13 T(1) and nuclear Overhauser effect measurements, the combination of these two parameters providing direct information about the reorientation of C-H bonds. It appears clearly that reorientation of toluene is the same in pure liquid and in the gel phase. The only noticeable changes in carbon-13 longitudinal relaxation times are due to the so-called chemical shift anisotropy (csa) mechanism and reflect slight modifications of the toluene electronic distribution in the gel phase. NMR diffusion measurements by the pulse gradient spin-echo (PGSE) method allow us to determine the diffusion coefficient of toluene inside the organogel. It is roughly two-thirds of the one in pure toluene, thus indicating that self-diffusion is the only dynamical parameter to be slightly affected when the solvent is inside the gel structure. The whole set of experimental observations leads to the conclusion that, once the gel is formed, the solvent becomes essentially passive, although an important fraction is located within the gel structure.

Behavior of Cesium and Thallium Cations inside a Calixarene Cavity As Probed by Nuclear Spin Relaxation. Evidence of Cation−π Interactions in Water

We have studied the complexes formed between the p-sulfonatocalix[4]arene and cesium or thallium metal cation, first by carbon-13 longitudinal relaxation of the calixarene molecule at two values of the magnetic field B(0). From the longitudinal relaxation times of an aromatic carbon directly bonded to a proton, thus subjected essentially to the dipolar interaction with that proton, we could obtain the correlation time describing the reorientation of the CH bond. The rest of this study has demonstrated that it is also the correlation time describing the tumbling of the whole calixarene assembly. From three non-proton-bearing carbons of the aromatic cycles (thus subjected to the chemical shift anisotropy and dipolar mechanisms), we have been able to determine the variation of the chemical shift anisotropy when going from the free to the complex form of the calixarene. These variations not only provide the location of the cation inside the calixarene cavity but also constitute a direct experimental proof of the cation-pi interactions. These results are complemented by cesium and thallium relaxation measurements performed again at two values of the magnetic field B(0). An estimation of the mean distance between the cation and the calixarene protons could be obtained. These measurements have also revealed an important chemical shift anisotropy of thallium upon complexation.

Analysis of the spin-lattice relaxation rate and reorientational dynamics of fullerene C 70 in chlorobenzene- d5

13C spin-lattice relaxation measurements were performed on C 70 in deuterated chlorobenzene as a function of temperature and field strength to investigate the mechanistic nature of the relaxation process as well as to probe the rotational dynamics of this fullerene. Our measurements indicate that the chemical shift anisotropy (CSA) mechanism dominates the relaxation process up to a temperature of 323 K. At this temperature, the relaxation rates for all carbons are seen to switch directions indicating that the SR mechanism begins to dominate at higher temperatures. Experimental reorientational correlation times, τ C eff, were used to obtain information on the diffusion coefficients, D Z and D X. Diffusion was observed to be anisotropic at the lower temperatures but progressed toward isotropic behavior with increasing temperature. Theoretical predictions generated by the Perrin and Hu–Zwanzig models were found to be significantly different than the experimental observations. The Gillen and Griffiths approach provided mixed success with D X predictions being close to experiment but D Z values being much higher than observed. An intuitive modification of the Giere-Wirtz model generated acceptable predictions to both diffusion constants suggesting that the reorientation of C 70 can be correlated to the diffusion of spheroids whose effective rotational axes is represented by their molecular semi-axes.

Complexes of EDTA in aqueous solutions. Structural aspects from a 13C NMR relaxation study

13 C NMR relaxation times T1 of the carboxylate groups in EDTA and its complexes with Mg(II), Ca(II), Zn(II) and Al(III) ions in aqueous solutions were measured at different magnetic field strengths, allowing the determination of the contribution of the chemical shift anisotropy mechanism to the total relaxation rate and, consequently, the shielding tensor (chemical shift) anisotropy of these nuclei. A discussion of these values in relation to the possible structure of these species in solution and a comparison with the values obtained in the solid state were performed.

Field-Dependent Relaxation and Molecular Reorientation of C60 in Chlorobenzene

To advance our understanding of the 13C spin−lattice relaxation process and rotational dynamics of C60 in various environments, we have acquired relaxation data on this molecule in chlorobenzene-d5 as a function of field strength and at various temperatures. Field-dependent measurements allowed us to separate the contributions arising from the two possible modes for relaxation in this molecule: chemical shift anisotropy (CSA) and spin−rotation (SR). The CSA and SR values were used to extract the reorientational time, τc, and the angular momentum time, τJ, which allowed us to investigate the rotational dynamics of C60 in this solvent. We found that the longitudinal relaxation rate is dominated by the chemical shift anisotropy mechanism and that spin−rotation becomes rapidly more important with rising temperature. This is seen especially at 4.7 T, where SR dominates at the moderate temperature of 333 K. We also compared our field-dependent findings with values derived via the Hubbard relation, and we have ...

Measurement of the protein backbone dihedral angle phi based on quantification of remote CSA/DD interference in inter-residue 13C'(i - 1)-13Calpha(i) multiple-quantum coherences.

A novel triple-resonance NMR method is presented for the measurement of the protein backbone dihedral angle phi based on differential multiple-quantum relaxation induced by relaxation interference between 1Halpha(i)-13Calpha(i) dipolar and 13C'(i - 1) (carbonyl) chemical shift anisotropy mechanisms. The method employs a simultaneous transfer of 15N magnetization to the inter- and intra-residue 13Calpha carbons as well as the directly attached carbonyl carbon 13C'. Results obtained on 13C,15N-labeled ubiquitin demonstrate the potential of the method.

Improved Estimation of CSA–Dipolar Coupling Cross-Correlation Rates from Laboratory-Frame Relaxation Experiments

We have investigated the underlying assumptions in estimating cross-correlation rates between chemical shift anisotropy (CSA) and dipolar coupling mechanisms in a scalar-coupled two-spin IS system, from laboratory frame relaxation experiments. It has been shown that for an arbitrary relaxation delay, the difference in relaxation rates of the individual components of an in-phase (or antiphase) doublet is not related to the CSA–dipolar coupling cross-correlation rate in a simple way. This is especially true in the case where the difference in the decay rates of the in-phase and antiphase terms of the density matrix becomes comparable to the magnitude of the scalar coupling between the two spins. Improved means of extracting cross-correlation rates in these cases are presented.

A Multidimensional NMR Experiment for Measurement of the Protein Dihedral Angle ψ Based on Cross-Correlated Relaxation between 1Hα−13Cα Dipolar and 13C‘ (Carbonyl) Chemical Shift Anisotropy Mechanisms

A high-resolution triple-resonance NMR method is presented for the measurement of the protein backbone dihedral angle ψ based on cross-correlated relaxation between 1Hα−13Cα dipolar and 13C‘ (carbo...

Determination of the (17)O Quadrupolar Coupling Constant and of the (13)C Chemical Shielding Tensor Anisotropy of the CO Groups of Pentane-2,4-dione and beta-Diketonate Complexes in Solution. NMR Relaxation Study.

13C NMR relaxation times T(1) of the carbonyl groups of pentane-2,4-dione and beta-diketonate complexes Al(acac)(3) and Zr(acac)(4) (acac: pentanedionate anion) were measured for various magnetic field strengths, allowing a determination of the contribution of the chemical shift anisotropy mechanism to the total relaxation. NOE and T(1) measurements for the (13)C nucleus of the central methine carbon furnished the correlation time tau(c) for the reorientation of theses species. The chemical shift tensor anisotropy Deltasigma could be deduced and compared to the values obtained in the solid state. The quadrupolar coupling constant (QCC) of the (17)O nucleus could also be determined by measuring the line width of the (17)O NMR signal and using the tau(c) value. QCC values for the complexes are in the same range as for the pentane-2,4-dione molecule, indicating similar electronic distribution and symmetry around the oxygen atom of these different species. Deltasigma for the complexes are close together, and the values obtained in solution are approximately those obtained in the solid state. They are close to the value reported in the literature for tetraacetylethane, which can be considered as a dimer of a beta-diketone, but slight differences are observed for the individual components of the chemical shielding tensor.

Cross-correlation effects in the transverse relaxation of multiple-quantum transitions of heteronuclear spin systems

The relaxation of multiple quantum transitions in a heteronuclear three-spin system is described in terms of Redfield theory including intramolecular dipolar and chemical shift anisotropy (CSA) mechanisms. CSA-dipolar cross-correlations, interference terms between CSA terms at different nuclear sites and cross-correlations between different dipolar interactions are illustrated and discussed. It is shown that cross-correlations between various relaxation mechanisms lead to non-exponential relaxation behaviour of multiple-quantum transitions, thus causing differential line broadening.

Selenium-77 and phosphorus-31 nuclear spin relaxation in tri-(tert-butyl) phosphine selenide

Selenium-77 and phosphorus-31 spin-lattice relaxation times are reported for tri-tert-butylphosphine selenide in chloroform-d, at 303 K and at several different magnetic field strengths. At moderate fields the 31P–1H dipole–dipole, spin-rotation, and chemical shift anisotropy mechanisms contribute significantly towards the 31P T1. At high fields chemical shift anisotropy dominates. The selenium-77 nuclear spin relaxes almost exclusively by spin rotation at low to moderate fields and the chemical shift anisotropy contribution only becomes significant at very high fields. This is due to an unusually small 77Se CSA. The contribution due to 31P–77Se dipole–dipole interactions is small but significant. Key words: 77Se NMR, NMR relaxation, phosphine selenide.

NMR relaxation properties of 77Se-labeled proteins

A 77Se-containing moiety has been attached to cysteine residues in bovine hemoglobin, reduced ribonuclease A, and glutathione by reaction with [77Se]6,6'-diselenobis(3-nitrobenzoic acid). The resultant species contain Se-S linkages that have 77Se NMR absorptions in the range range of 568-580 ppm. Spectra have been recorded at 4.7 and 9.7 tesla (T). For labeled hemoglobin a line width of 250 Hz is seen at 4.7 T and 1000 Hz at 9.4 T. This quadrupling of line width with doubling of observational field strength is consistent with exclusive relaxation by the chemical shift anisotropy (CSA) mechanism. These line widths are greater than expected for a molecule the size of hemoglobin and indicate some aggregation at the high concentrations used. Upon dissociation and partial unfolding of the hemoglobin subunits, the line widths of the selenium resonance decrease to 35 and 120 Hz at 4.7 and 9.4 T, respectively. The spin-lattice relaxation time (T1) for the dissociated hemoglobin at 9.4 T was found to be 220 ms. Together with a value of 377 ms for the spin-spin relaxation time (T2), determined from the line width, an estimate of the CSA was made. This gave a value of 890 ppm, which is in accord with other values for Se(II) linked only by single bonds. When this value for the CSA is used, together with the CSA contribution to the line width, in estimating a correlation time for seleno(3-nitrobenzoic acid) (SeNB)-labeled glutathione, a value of 4 x 10(-11) s is obtained. For SeNB-labeled denatured ribonuclease, four distinct resonances are resolvable at 4.7 T and five resonances at 9.4 T. From T1 values for these resonances and the value of 890 ppm for the CSA, an appropriate correlation time of 0.1 ns was determined, which should result in 77Se resonances of 0.2-1.0 Hz at 4.7 and 9.4 T, respectively. Much greater apparent line widths are observed, which are attributed to microheterogeneity resulting from formation of inter- and intramolecular disulfide linkages. It is concluded that when there are no complications from protein aggregation or chemical exchange, the CSA values anticipated to exist in glutathione peroxidase or other selenoproteins should result in resonances with line widths in the range from 27 to 170 Hz, depending on field strength. These resonances should therefore be observable in the intact protein, if 77Se-enriched material is available.

Carbon‐13 spin–lattice relaxation study on molecular dynamics of some penta‐ and tricarbonylchromium complexes

AbstractCarbon‐13 spin–lattice relaxation times were measured for some penta‐ and tricarbonylchromium complexes of organic ligands. The chemical shift anisotropy of the carbonyl carbons was estimated to be ca. 500 ppm. The contributions of dipole–dipole and chemical shift anisotropy mechanisms to the overall relaxation were evaluated and used to determine the reorientational parameters for the ligand and carbonylchromium moieties. It seems that the internal rotation rate about the ligand—chromium bond is governed mainly by intermolecular interactions.

Nuclear spin relaxation in some phenyl phosphorus compounds

13C and 31P spin-lattice relaxation times ( T1) and nuclear Overhauser effects were measured in triphenylphosphine, triphenylphosphine oxide, triphenylphosphine sulphide, and triphenylphosphine selenide, over a temperature, range −40°C to +55°C. Isotropic reorientation correlation times were calculated from the dipolar relaxation times for phosphorus and that for the para carbon. The spin-rotation mechanism is the dominant relaxation mechanism for phosphorus in triphenylphosphine, whereas the chemical shift anisotropy is the dominant mechanism in triphenylphosphine oxide at low temperatures. In triphenylphosphine sulphide and triphenylphosphine selenide, the dipolar and the chemical shift anisotropy mechanisms are important in phosphorus relaxation. The motional anisotropy of internal rotation of phenyl rings and overall tumbling is determined from the ratio of T 1's of ortho or meta carbon to para carbon, and is found to increase with the decrease in temperature and with the increase in mol. wt. The angular momentum correlation times were calculated from phosphorus spin-rotation relaxation rates. From viscosity data, reduced correlation times were calculated and compared with that calculated using Stokes-Einstein-Debye, Gierer-Wirtz, and Hu-Zwanzig models.

31P NMR relaxation time studies of 2,3-diphosphoglycerate in solution and intact erythrocytes.

The 31P T1 spin-lattice relaxation times and nuclear Overhauser effects of 3-P and 2-P in 2,3-diphosphoglycerate (2,3-DPG) have been measured in pure water solutions and in suspensions of intact erythrocytes. It was found that extremely careful purification of the water solutions from paramagnetic impurities was necessary in order to obtain reproducible results. The dominant relaxation mechanism in purified samples was shown to be the dipole-dipole interaction. Contributions from the chemical-shift anisotropy mechanism were demonstrated to be important at higher magnetic field strengths. Based upon the measurements in water solutions and intact erythrocyte suspensions it was concluded that there could be observed no significant influence of the oxygenation state of hemoglobin on the 31P T1 values of 2,3-DPG.