Residual Quadrupolar Splitting Research Articles

23Na MRI of human skeletal muscle using long inversion recovery pulses

23Na inversion recovery (IR) imaging allows for a weighting toward intracellular sodium in the human calf muscle and thus enables an improved analysis of pathophysiological changes of the muscular ion homeostasis. However, sodium signal-to-noise ratio (SNR) is low, especially when using IR sequences. 23Na has a nuclear spin of 3/2 and therefore experiences a strong electrical quadrupolar interaction. This results in very short relaxation times as well as in possible residual quadrupolar splitting. Consequently, relaxation effects during a radiofrequency pulse can no longer be neglected and even allow for increasing SNR as has previously been shown for human brain and knee. The aim of this work was to increase the SNR in 23Na IR imaging of the human calf muscle by using long inversion pulses instead of the usually applied short pulses. First, the influence of the inversion pulse length (1 to 20 ms) on the SNR as well as on image contrast was simulated for different model environments and verified by phantom measurements. Depending on the model environment (agarose 4% and 8%, xanthan 2% and 3%), SNR values increased by a factor of 1.15 up to 1.35, while NaCl solution was successfully suppressed. Thus, image contrast between the non-suppressed model compartments changes with IR pulse length. Finally, in vivo measurements of the human calf muscle of ten healthy volunteers were conducted at 3 Tesla. On average, a 1.4-fold increase in SNR could be achieved by increasing the inversion pulse length from 1 ms to 20 ms, leaving all other parameters – including the scan time – constant. This enables 23Na IR MRI with improved spatial resolution or reduced acquisition time.

Long-lived spin state of a tripeptide in stretched hydrogel

The longitudinal (T 1), transverse (T 2), and singlet state (T s) relaxation times of the geminal backbone protons (CH2) of L-Leu-Gly-Gly were studied by NMR spectroscopy at 9.4 T in a bovine hide gelatin gel composed in D2O at 25 °C. Gelatin granules were dissolved in a hot solution of the tripeptide and then the solution was allowed to gel inside a flexible silicone tubing. With increases in gelatin content, the T 2 and T s of the CH2 protons correspondingly decreased (T s/T 2 ~ constant), while the change in T 1 was relatively small. The largest observed T s/T 1 value was 3.3 at 46% w/v gelatin that was the lowest gelatin content examined. Stretching the tubing, and hence the gel, brought about anisotropic alignment of the constituents resulting in residual quadrupolar splitting of the resonance from D2O in (2)H NMR spectra, and residual dipolar splitting of the CH2 resonance in (1)H NMR spectra. WALTZ-16 decoupling during the relaxation intervals extended the singlet state relaxation time, but the efficacy diminished as the gels were stretched. Theoretically predicted T 1, T 2, and T s values, assuming intramolecular dipolar coupling as the only source of relaxation, were within the same order of magnitude as the experimentally observed values. Overall we showed that it is possible to observe a long-lived spin state in an anisotropic medium when T 2 is shorter than T 1 in the presence of non-zero residual dipolar couplings.

Multiscale Water Dynamics within Dense Clay Sediments Probed by 2H Multiquantum NMR Relaxometry and Two-Time Stimulated Echo NMR Spectroscopy

2H NMR spectroscopy, relaxometry, and two-time correlation measurements are used to investigate the structural and dynamical properties of water molecules confined within the multiscale porous network of dense clay sediment. The residual quadrupolar splitting detected by 2H NMR spectroscopy is the fingerprint of the specific orientation of the water molecules pertaining to the first hydration shell of clay lamellae. Multiquantum 2H NMR relaxation measurements are used to quantify the distribution of the clay platelet orientations within the dense sediment. The average residence time of the water molecules confined within the clay interlamellar space is determined by exploiting 2H multiquantum NMR relaxation measurements under spin-locking conditions. Finally, long-time scale diffusion of the water molecules within the multiscale porous network of the clay sediment is quantified by measuring the attenuation of the 2H NMR two-time stimulated echo.

Optic nerve: Separating compartments based on 23Na TQF spectra and TQF-diffusion anisotropy

We present a triple quantum filtered (TQF) sodium spectroscopy study of an excised bovine optic nerve. By choosing proper experimental parameters, this technique allowed us to independently observe the satellite transitions originating from the various compartments in the tissue. TQF-based diffusion experiments provided further characterization of the compartments in terms of their geometry. As a result, the peak that exhibited the smallest residual quadrupolar splitting, and the largest diffusion anisotropy was assigned to axons. Two other pairs of satellite peaks were assigned to extra-cellular compartments on the basis of either the size of their quadrupolar splitting or the diffusion properties.

2H NMR Spectroscopy and Multiquantum Relaxometry as a Probe of the Magnetic-Field-Induced Ordering of Clay Nanoplatelets within Aqueous Dispersions

2H NMR spectroscopy and relaxometry were used to investigate the orientation of nontronite clay nanoplatelets induced by the static magnetic field within dilute aqueous dispersions by exploiting the residual quadrupolar splitting resulting from the specific orientation of heavy water molecules physisorbed at the clay surface. A careful analysis of the variation of the residual 2H splitting as a function of clay concentration and magnetic field strength was required to extract the intrinsic clay ordering induced by the magnetic field. The variation of clay ordering as a function of clay concentration clearly indicated two concentration regimes, corresponding to free and collective orientations of the clay platelets, respectively. Multiquantum NMR relaxation measurements were further used to identify the main NMR relaxation mechanism whose temperature variation is compatible with a fast exchange, at the NMR time scale, between free and physisorbed water molecules.

Orientational Microdynamics and Magnetic-Field-Induced Ordering of Clay Platelets Detected by2H NMR Spectroscopy

The orientation of montmorillonite clays induced by a static magnetic field is quantified by using (2)H NMR spectroscopy. Indeed, the residual quadrupolar splitting of the (2)H resonance line measured for heavy water is a direct consequence of the specific orientation of the clay platelets in the static magnetic field. In the dilute regime, this residual splitting increases linearly with clay concentration, which confirms that the clay/clay electrostatic repulsions remain negligible by comparison with the diamagnetic coupling of these anisotropic platelets. At higher concentration, the electrostatic repulsion between clay particles markedly enhances the detected splitting. Such enhancement is well predicted by numerical simulations. By varying the size of the clay platelets and the strength of the static magnetic field, it is possible to evaluate the order of magnitude of the diamagnetic susceptibility of these anisotropic colloids.

A comparison of proton‐detected 13C local field experiments with deuterium NMR at natural abundance for studying liquid crystals

A comparison is made between the information which can be derived on structure, conformation and orientational order of the molecules in a nematic liquid crystal 4‐pentyl‐4′‐cyanobiphenyl (5CB) from the NMR spectra of the deuterium atoms at natural abundance (NAD NMR), and the two‐dimensional proton‐detected 13C local field experiment (PDLF). The nine residual quadrupolar splittings, Δν k , obtained experimentally have been compared with quadrupolar tensors and a geometry and conformational potentials calculated by the DFT method B3LYP/6‐311G**. The PDLF experiment yielded 42 scaled 13C–1H residual dipolar couplings, kD CiHj . The scaling factor, k, is determined experimentally by comparing unscaled and scaled residual dipolar couplings in a sample of fluorobenzene dissolved in a nematic liquid crystalline solvent. The corrected residual dipolar couplings, D CiHj , are used to investigate the structure and rotational potentials about each bond in the molecule.

Apparatus for rapid adjustment of the degree of alignment of NMR samples in aqueous media: Verification with residual quadrupolar splittings in 23Na and 133Cs spectra

NMR spectra of 23Na + and 133Cs + in gelatine in a silicone rubber tube that was stretched to various extents showed remarkably reproducible resonance multiplicity. The relative intensities of the components of the split peaks had ratios, 3:4:3, and 7:12:15:16:15:12:7, respectively, that conformed with those predicted using a Mathematica program. The silicone-rubber tube was sealed at its lower end by a small rubber stopper and placed inside a thick-walled glass tube. Gelatine was injected in solution into the silicone tube and ‘set’ by cooling below 30 °C. A plastic thumb-screw held the silicone tube at various degrees of extension, up to ∼2-fold. After constituting the gel in buffers containing NaCl and CsCl, both 23Na and 133Cs NMR spectroscopy revealed that after stretching the initial single Lorentzian line was split into a well-resolved triplet and a heptet, respectively. This was interpreted as being due to coupling between the electric quadrupoles of the nuclei and the average electric field gradient tensor of the collagen molecules of gelatine; these molecules became progressively more aligned in the direction of the main magnetic field, B 0, of the vertical bore magnet, as the gel was stretched. This apparatus provides a simple way of demonstrating fundamental physical characteristics of quadrupolar cations, some characteristics of gelatine under stretching, and a way to invoke static distortion of red blood cells. It should be useful with these and other cell types, for studies of metabolic and membrane transport characteristics that may change when the cells are distorted, and possibly for structural studies of macromolecules.

Quadrupolar interaction study of various cations confined in porous charged polymer film of sPI ionomers

The structure of a sulfonated polyimide (sPI) ionomer membranes was investigated via the transport properties of various confined cations (7Li+, 23Na+, 87Rb+, 133Cs+). Their NMR spectra show large residual quadrupolar splitting depending on the orientation of the film in the static magnetic field B0. This behavior is the fingerprint of a macroscopic nematic ordering of charged interfaces. This is also confirmed by the anisotropy of the self-diffusion tensor measured by 1H and 7Li PGSE experiments on N(CH3)4+ and Li+ cations, respectively.

Magnetic field dependence of residual dipolar couplings measured in dilute liquid crystalline media

The effect of the magnetic field strength on the magnitude of residual dipolar couplings introduced by dilute liquid crystal media was investigated. One-bond heteronuclear residual dipolar couplings and residual deuterium quadrupolar splitting of the water were monitored at various static magnetic fields. It is suggested that the value of residual anisotropic NMR parameters resulting from exchange between ordered and non-ordered molecules decreases with increasing magnetic fields.

The interaction of water molecules with purple membrane suspension using 2H double-quantum filter, 1H and 2H diffusion nuclear magnetic resonance.

Bacteriorhodopsin is a membrane protein of the purple membrane (PM) of Halobacterium salinarum, which is isolated as sheets of highly organized two-dimensional hexagonal microcrystals and for which water molecules play a crucial role that affects its function as a proton pump. In this paper we used single- and double-quantum (2)H NMR as well as (1)H and (2)H diffusion NMR to characterize the interaction of water molecules with the PM in D(2)O suspensions. We found that, under the influence of a strong magnetic field on a concentrated PM sample (0.61 mM), the PM sheets affect the entire water population and a residual quadrupolar splitting (upsilon(q) approximately 5.5 Hz, 298 K, at 11.7 T) is observed for the D(2)O molecules. We found that the residual quadrupolar coupling, the creation time in which a maximal DQF signal was obtained (tau(max)), and the relative intensity of the (2)H DQF spectrum of the water molecules in the PM samples (referred to herein as NMR order parameters) are very sensitive to temperature, dilution, and chemical modifications of the PM. In concentrated PM samples in D(2)O, these NMR parameters seem to reflect the relative organization of the PM. Interestingly, we have observed that some of these parameters are sensitive to the efficiency of the trimer packing, as concluded from the apo-membrane behavior. The data for dionized blue membrane, partially delipidated sample, and detergent-treated PM show that these D(2)O NMR order parameters, which are magnetic field dependent, are sensitive to the structural integrity of the PM. In addition, we revealed that heating the PM sample inside or outside the NMR magnet has, after cooling, a different effect on the NMR characteristics of the water molecules in the concentrated PM suspensions. The difference in the D(2)O NMR order parameters for the PM samples, which were heated and cooled in the presence and in the absence of a strong magnetic field, corroborates the conclusions that the above D(2)O order parameters are indirect reflections of both microscopic and macroscopic order of the PM samples. In addition, (1)H NMR diffusion measurements showed that at least three distinct water populations could be identified, based on their diffusion coefficients. These water populations seem to correlate with different water populations previously reported for the PM system.

Structural Ambiguities Revisited in Two Bridged Ring Systems Exhibiting Enantiotopic Elements, Using Natural Abundance Deuterium NMR in Chiral Liquid Crystals

The natural abundance deuterium 2D Q-COSY NMR spectra of two apolar bridged ring systems, norbornene (Cs symmetry) and quadricyclane (C2v symmetry), oriented in a chiral liquid crystal made of an organic solution of poly-γ-benzyl-l-glutamate (PBLG), are analyzed. In such a chiral oriented solvent, enantiotopic nuclei or directions are nonequivalent. Consequently, it is possible to measure many more anisotropic interactions compared to those obtained from NMR spectra in nonchiral nematic solvents. From the measurement of all residual quadrupolar splittings, ΔνQ, and one-bond carbon−proton residual dipolar couplings, 1DC-H, all the elements of the second rank order tensor, Sαβ, were calculated. Knowledge of the Sαβ values allows all deuterons and subsequently proton NMR resonances to be assigned unambiguously. The reason is that there exists a one-to-one mathematical relationship linking the orientational order parameters of a solute molecule, the molecular geometry, and the anisotropic interactions measured on oriented spectra. In the case of norbornene, it was possible to assign nuclei to each enantiotopic face in this prochiral molecule. Such an analytical approach yields original stereochemical information probing the diastereotopicity and/or enantiotopicity of molecules, and is revealed to be a very useful alternative to conventional 2D-NMR experiments in isotropic solvents.

Solution-state cross-polarization for selective excitation of scalar-coupled quadrupolar nuclei S>1/2 in the presence of a residual quadrupolar splitting

We show that solution-state cross-polarization in nuclear magnetic resonance is a valuable technique for the selective observation of scalar-coupled quadrupolar nuclei in anisotropic phases. Numerical simulations show that coherence transfer remains efficient provided the quadrupolar splitting is smaller than the scalar-coupling constant. The method is applied to a mixture of beryllium fluoride complexes. Despite severe overlap in the 9Be spectrum these results clearly demonstrate a quadrupolar splitting for BeF 3 − whereas no such splitting is resolved for BeF 4 2−.

NMR relaxation of spin‐3/2 nuclei: Effects of structure, order, and dynamics in aqueous heterogeneous systems

AbstractThe mathematical expressions for interpreting the results of spin‐lattice relaxation, transverse relaxation, spin‐lattice relaxation in the rotating frame, and Ostroff–Waugh multiple‐pulse sequence relaxation experiments on spin‐3/2 nuclei with electric quadrupole interaction in an isotropic environment are biexponential. A protocol for applying these expressions to investigate quadrupole interactions and ionic motions in complex aqueous heterogeneous systems such as biopolymer gels and biological tissue is described. Experimental evidence for at least two different environments for hydrated sodium ions, including an isotropic liquid aqueous phase and an anisotropic interfacial phase at the macromolecules, is reviewed. The use of the basic equations and experimental protocol are illustrated for systems of increasing complexity, ranging from simple NaCl aqueous solutions, highly concentrated agarose gels, gelatin gels, and gellan gels to xanthan gels that contain ordered macromolecules. Many different experiments on various biopolymer samples indicate a correlation time of approximately 1 ns for ions at the interface that dominates in determining spin‐lattice relaxation and also the long component of the transverse relaxation that arises from the central transitions. The transverse relaxation of the satellite transitions is dominated by much longer correlation times that may be related to the geometry of the macromolecular matrix in which the hydrated aqueous sodium ions diffuse. In the case of xanthan, the macromolecular ordering causes residual quadrupolar splitting and enables the measurement of several very long correlation times. One of these appears to agree with the Kuhn length of xanthan that was determined by light scattering experiments on dilute solutions. © 2001 John Wiley & Sons, Inc. Concepts Magn Reson 13: 294–325, 2001

A model for the dynamics of spins 3/2 in biological media: signal loss during radiofrequency excitation in triple-quantum-filtered sodium MRI.

We have derived the differential equations that describe the dynamics of spin-3/2 nuclei in the presence of radiofrequency (RF) fields and both static and fluctuating quadrupolar interactions. The formalism presented was used to predict the sodium triple-quantum-filtered (TQ-filtered) signal loss in a whole-body scanner, where the widths of the hard 90 degrees RF pulses are on the same order of magnitude as the transverse relaxation times. A small piece of bovine nasal cartilage, known for exhibiting residual quadrupolar splittings, was used to test the theory. The sample was modeled as consisting of small domains, each characterized by a static quadrupolar interaction constant, with an overall Gaussian distribution across the sample. An increase of about 15% in the TQ-filtered signal strength, as the 90 degrees RF pulse width was decreased from 500 to 100 micros, was predicted and demonstrated experimentally for this particular sample.

Sodium Ions in Ordered Environments in Biological Systems: Analysis of 23Na NMR Spectra

Experimentsthat selectively excite I = 32 nuclei exhibiting residual quadrupolar splittings are used to acquire 23Na NMR spectra from a range of biologically relevant samples containing sodium in ordered environments. Three complementary approaches to the analysis of such spectra are described: (i) measurement of relaxation rates, (ii) extraction of homogeneous linewidths from two-dimensional Jeener–Broekaert spectra, and (iii) simultaneous fitting of detailed theoretical functions to a series of one-dimensional Jeener–Broekaert spectra. Analysis of relaxation rates provides evidence for compartmentation in bovine nasal cartilage. Each approach is used to demonstrate the presence of anisotropy in transverse relaxation in porcine tendon. For certain samples containing collagen, a good theoretical fit to the spectra was obtained using a model that allows for anisotropic relaxation by including the effects of slow lateral and radial diffusion.

17O NMR of water in ordered environments

Two NMR experiments are designed for selective excitation of spin I=5/2 nuclei that exhibit residual quadrupolar splittings. The I=5/2 Jeener–Broekaert experiment is preferred to the four-quantum filtration experiment as it is shown to be a more sensitive technique in experimental practice. Both techniques are applied to 17O-enriched water in biological systems. The occurrence of water which displays a residual 17O quadrupolar splitting is demonstrated for the first time in a model biological system and an excised tissue sample. The resulting 17O NMR spectra are shown to have the characteristics predicted in computer-simulated I=5/2 NMR spectra.

Interaction of water with clay minerals as studied by 2H nuclear magnetic resonance spectroscopy

The interaction of water with a series of Ca 2+/Na + , Ca 2+/NH 4 +, and NH 4 +/Na +-exchanged hectorite and saponite clays in water- and clay-dominated systems was studied using 2H NMR. Variabletemperature Nuclear Magnetic Resonance (NMR) (in the range of 189–299 K) was also used to determine the frequency of exchange and the rotation activation energy between water molecules hydrating the cations and the free water of the tactoid. The 2H quadrupolar splitting and spin-lattice relaxation time ( T 1) are related to the ratio of clay to adsorbed water in these systems. In water-dominated systems (where clay/water ratio ≪ 1), motional averaging occurs between water hydrating the cations and the “free” or bulk water of the tactoid. In these systems, the spin-lattice relaxation time ( T 1) is dominated by the relaxation of the “free” water on the external surface of the tactoids and has a value close to that for D 2O (0.436 s), with the rotational correlation time ( τ c) approaching 2.93 × 10 −12 s. A more recent article places τ c at 1.95 × 10 −12 s. The clay systems in this study have clay/ water ratios (=0.0238 g-clay/mL-D 2O), and the quadrupolar splittings are of the order of 100 Hz or less. Splittings of <100 Hz indicate that approximately 1 water molecule is bound to the clay for every 1.7 × 10 3 “free” water molecules. In clay-dominated systems with extremely low water contents (clay/water ratio > 1 g-clay/mL-D 2O), less motional averaging occurs. Rotational correlation times are slower and greater residual quadrupolar splittings are also observed. Increased residual quadrupolar splittings are observed with increasing clay/ water ratios. For a Na-exchanged hectorite sample (5.31 g-clay/mL-D 2O), the quadrupolar splittings are of the order of 12 kHz, and for a Na-exchanged saponite (4.05 g-clay/mL-D 2O), the quadrupolar splittings are of the order of 11 kHz. Rotational activation energies (E a) of 37.8 kJ-mole −1 (9.0 kcal · mole −1) for adsorbed water molecules on Na-exchanged hectorite and 27.2 kJ · mole −1 (6.5 kcal · mole” −1) for Na-exchanged saponite were calculated from spin-lattice relaxation ( T 1) measurements. These values compare with calculated activation energies of rotation (8.5 kcal · mole −1) for water of hydration adsorbed to a Na-exchanged vermiculite.

Interaction of nonionic polymers at a clay interface

The structure and dynamics at the interface of swelling clays (smectites) are crucial to such diverse applications as drilling for oil, cosmetics, paints, water treatment, or chemical reactions. From our previous NMR studies on clay suspensions, we have shown how the orientation of interfacial molecules is affected by the clay structure, 1 by the nature of counterions, 2 or by the composition of the liquid phase. 3 These results have been mainly obtained from the analysis of residual quadrupolar splittings of nuclei from interfacial species. In this work, we report our studies on a clay montmorillonite suspended in aqueous solutions of non ionic polymers. The polymer perturbs the structure within the interfacial region as deduced from the variations of the splittings of water quadrupolar nuclei and of sodium-23 relaxation parameters in terms of the polymer concentration. Proton self-diffusion coefficients and carbon-13 relaxation times are measured to describe the influence of the clay on the polymer mobility. The dependence of the polymer nature on these parameters is also investigated.

Extraction of Homogeneous23Na NMR Linewidths from Two-Dimensional Jeener–Broekaert Spectra*

The Jeener–Broekaert experiment has recently been applied to heterogeneous biological systems for selective observation of spinI= 3/2 nuclei which exhibit residual quadrupolar splittings. This paper describes the use of a two-dimensional version of this experiment to extract the homogeneous linewidths of such nuclei. The powderlike spectra produced by the one-dimensional Jeener–Broekaert experiment have lineshapes which depend on both homogeneous and inhomogeneous quadrupolar broadening. In two-dimensional Jeener–Broekaert spectra, the inhomogeneous broadening appears along the ω1= ω2and ω1= −ω2diagonal axes, with the result that the two broadenings are separated. The homogeneous linewidth can then be extracted by fitting cross sections through analytically generated lineshapes to the experimental spectra. The reliability of this technique is evaluated by computer simulation, and23Na NMR linewidths are measured experimentally in disordered liquid crystalline and heterogeneous biological samples. The possibility of measuring frequency-dependent variations in the homogeneous linewidth is also discussed.