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At least 937 records · Page 52Linked to original sources
PubMed2004

Precision 1H-1H distance measurement via 13C NMR signals: utilization of 1H-1H double-quantum dipolar interactions recoupled under magic angle spinning conditions.

We applied the POST-C7 DQ-dipolar recoupling pulse sequence to the measurement of (1)H-(1)H distances with high precision. The spectral resolution is enhanced by detecting the (1)H magnetization via (13)C signals. A least-squares fitting of the build-up curve of the transferred magnetization to the exact numerical simulations yielded a (1)H(alpha)-(1)H(beta) distance of 248 +/- 4 pm for fully (13)C-labeled L-valine. This distance agrees with the neutron diffraction study. The negative transferred magnetization clearly indicates that the direct DQ (1)H-(1)H dipolar couplings have the largest effect. The signal for the magnetization transfer builds up rapidly by the direct (1)H-(1)H dipolar coupling, and decreases to zero at longer mixing time when the relayed magnetization transfer becomes significant. This large intensity change of the signal leads to the high precision in the distance measurement. We inspected factors that limit the effective bandwidth of the POST-C7 recoupling for the (1)H and (13)C homonuclear spin systems. The spin interactions at times shorter than the cycle time of the C7 sequence were also evaluated to measure the distances. The carbon-detected 2D (1)H DQ mixing experiment was demonstrated for the measurement of multiple (1)H-(1)H distances.

Carbon Isotopes↗
PubMed2004

Multiple acquisitions with global inversion cycling (MAGIC): a multislice technique for vascular-space-occupancy dependent fMRI.

Recently, a new fMRI technique, termed vascular-space-occupancy (VASO), was introduced that uses T1-based blood nulling to detect cerebral blood volume (CBV) changes during brain activity. However, similar to other T1-preparation methods, this technique is hampered by the fact that there is only one zero-crossing on the relaxation curve, presently limiting its application to single-slice studies. A multislice VASO-fMRI method is presented that employs a series of nonselective 180 degrees pulses to periodically invert the magnetization and maintain it around zero, while acquiring slices in between. The effects of magnetization transfer and signal contamination by stimulated echoes are discussed. Solutions to reduce the effect of T1-signal decay as a function of slice number are provided. Phantom data show excellent agreement between experiments and numerical simulations. Multislice VASO-fMRI images of visual stimulation show effective blood nulling in all slices and appropriate functional activations in all volunteers (n=4).

Blood Volume↗
PubMed1998

MR-microscopic visualization of anisotropic internal cartilage structures using the magic angle technique.

NMR microscopic studies of articular cartilage at 7.1 T are presented. Using a special experimental design, T2-weighted spin-echo images of cartilage-bone plugs were taken under variable angles with respect to the static magnetic field B0 to visualize the angular-dependent representation of internal matrix structures mediated by the collagen network arrangement. To quantify the observed orientational effect in the MR images, exact measurements of the transverse relaxation time T2 were taken using the CPMG sequence. The NMR experiments show the strong influence of the cartilage orientation with respect to the static magnetic field on the inhomogeneous appearance of the articular cartilage in the MR image. Additionally performed polarization light microscopic investigations demonstrate the direct relation between the oriented collagenous structures and the anisotropic regions observed in the MR images. A simple cartilage matrix model derived from the experimental findings is proposed, and consequences for the clinical assessment of the articular joint are discussed.

Aged↗
PubMed1997

A High-Resolution 3D Separated-Local-Field Experiment by Means of Magic-Angle Turning

A 3D separated-local-field (SLF) experiment based on the 2D PHORMAT technique is described. In the 3D experiment, the conventional 2D SLF powder pattern for each chemically inequivalent carbon is separated according to their different isotropic chemical shifts. The dipolar coupling constant of a C-H pair, hence the bond distance, and the relative orientation of the chemical-shift tensor to the C-H vector can all be determined for the protonated carbons with a single measurement. As the sample turns at only about 30 Hz in a MAT experiment, the SLF patterns obtained approach those of a stationary sample, and an accuracy in the measurement similar to that obtained on a stationary sample is expected. The technique is demonstrated on 2,6-dimethoxynaphthalene, where the 13 C-1 H separated-local-field powder patterns for the six chemically inequivalent carbons are clearly identified and measured. The observed dipolar coupling for the methoxy carbon is effectively reduced by the fast rotation of the group about its C3 symmetry axis. The average angle between the C-H bond direction and the C3 rotation axis in the OCH3 group is found to be about 66°.

Journal Article↗
PubMed1998

Pulsed field gradient selection in two-dimensional magic angle spinning NMR spectroscopy of dipolar solids

The utility of gradient selection in MAS spectroscopy of dipolar solids is explored in two examples. In the first, rotor-synchronized gradients of appropriate strength and duration are applied to select 1H double-quantum coherences. The resulting DQ MAS spectrum of adamantane is compared with that acquired by the corresponding phase-cycling technique. As a second example, a 1H 2D exchange MAS experiment is performed on an elastomer sample. In this experiment, a gradient is applied to remove undesired coherences that would otherwise distort the spectrum for short mixing times. The diagonal-peak intensities in the resulting spectrum show a linear decrease with increasing mixing time indicating cross-relaxation by slow chain motions as the relevant process. Both types of experiments demonstrate the potential of gradient-selection techniques for MAS spectroscopy of dipolar solids. Copyright 1998 Academic Press.

Journal Article↗
PubMed1999

Origin of the residual NMR linewidth of a peptide bound to a resin under magic angle spinning.

The tetrapeptide Ala-lle-Gly-Met bound to a Wang resin via the methionine residue was studied by NMR under MAS conditions and compared to the same peptide in solution. The bound peptide exhibits average linewidths superior to those observed for the peptide in solution. The origin of the residual NMR linewidth observed for the bound form was investigated. The dynamics of the peptide is shown to be only marginally responsible for the increased linewidth; the major cause of the line broadening appears to be nonaveraged magnetic susceptibility differences.

Anisotropy↗
PubMed1999

Magic angle spinning NMR spectroscopy with composite RF pulses.

Using numerical optimization procedures it is shown that it is possible to design composite 180 degrees RF pulses for MAS NMR spectroscopy by explicitly taking into account the variation of the resonance offset of each crystallite during the application of the RF pulses. When using composite RF pulses in experiments such as TOSS, where the delays between the RF pulses have to be critically adjusted, an optimization of these delays can lead to the desired performance characteristics.

Magnetic Resonance Spectroscopy↗
PubMed2000

Compound radiofrequency-driven recoupling pulse sequences for efficient magnetization transfer by homonuclear dipolar interaction under magic-angle spinning conditions

The maximum of the transferred magnetization in rotating powdered solids under the radiofrequency-driven recoupling (RFDR) pulse sequence is enhanced by reducing the orientation dependence of the effective recoupled homonuclear dipolar interaction. The compound RFDR (CRFDR) pulse sequence for this enhancement consists of RFDR pulse units (tau(i)-pi-tau(R)-pi-1171;tau(i)) with different tau(i), where tau(R) is the sample rotation period, tau(i) and 1171;tau(i) (=tau(R) - tau(i)) are delays, and pi is a 180 degrees pulse. The delay tau(i) modifies the zero-quantum spin operators and the sample rotation-angle dependence of the recoupled dipolar Hamiltonian. The CRFDR pulse sequences were optimized for mixing by varying tau(i). Numerical simulation for the two-spin system only with a dipolar interaction and isotropic chemical shifts indicates that the transfer efficiency of CRFDR averaged over the powder is about 70%, which is 30% higher than the efficiency of the RFDR pulse over a broad range of about 1/tau(R) in resonance frequency difference. The CRFDR sequences need about 60% longer mixing times to maximize the transferred magnetizaion in comparison with the original RFDR sequence. Chemical shift anisotropy, the other dipolar interactions, and relaxation generally reduce the enhancement by CRFDR. Experiments for fully (13)C-labeled alanine, however, show that the maximum of the magnetization transferred with CRFDR from the carboxyl to alpha carbon is about 15% greater than that with RFDR. Copyright 2000 Academic Press.

Journal Article↗
PubMedDate not supplied

An optimal strategy for recovering the deuterium (2H) quadrupolar interaction under magic-angle spinning NMR.

By exploiting the homology in the form of the truncated high-field homonuclear dipole-dipole and quadrupole coupling Hamiltonians, we have previously demonstrated that a simple adaptation of a rotor-synchronized pulse sequence (DRAMA) used for the recovery of dipole-dipole couplings can also be used to resurrect quadrupole couplings (QUADRAMA). In the canonical implementation of these recovery pulse sequences, the couplings are not significantly scaled down from their static sample values. While such minimal scaling is of course desirable in the recovery of typical homonuclear dipolar couplings (< or =2 kHz) and small quadrupole couplings, it is clearly not ideal for the recovery of the much larger quadrupole couplings (20-200 kHz) often encountered in solid-state 2H NMR. In such a case, some prior knowledge of the order of magnitude of the coupling is required to optimize the experimental conditions for QUADRAMA. In order to overcome this drawback, in this study, we have developed a general and optimized strategy for implementing the QUADRAMA technique which does not require any knowledge of the size of the coupling vQ. Experimental tests of the optimized protocol demonstrate that by judicious choices of a combination of scaling factors and recoupling times, 2H quadrupole couplings ranging over an order of magnitude from 3 to 42 kHz can be measured. Since this optimized protocol can reliably be used to recover couplings over a broad range, it expands the range of systems accessible to study by 2H NMR into a realm where static sample NMR and simple MAS NMR may fail.

Journal Article↗
PubMed2005

Black magic and EPR oximetry: from lab to initial clinical trials.

EPR oximetry is a technique that can make repeated non-invasive measurements of the PO2 in tissues. To extend the application of EPR oximetry to humans, India ink is the probe of choice because appropriate India inks have EPR signals whose line widths are sensitive to changes in oxygen concentrations, and, most importantly, India ink already has been used extensively in humans as a marker in the skin, lymphatics, various organs during surgery, tumors, and for decoration as tattoos. We have developed an India ink that has good sensitivity to oxygen, high stability in tissues, good signal intensity, and minimal toxicity. In this article we describe the various properties of this India ink, results obtained from our animal experiments, and our first preliminary clinical results, which are part of the first systematic clinical use of EPR oximetry. The clinical results indicate that it is possible to do repeated measurements over several months and probably years after the injection of the ink, indicating that long-term follow-up studies are feasible. We are very encouraged with these results and are confident that EPR oximetry using India ink will be a non-invasive, fast, and reliable technique for pO2 measurements in clinical studies.

Animals↗