1,720,998 research outputs found
Symmetry in the design of NMR multiple-pulse sequences
No full textThe symmetry principles of NMR pulse-sequence design are summarized. The discussion is guided by an analogy with tiling schemes in the decorative arts. The symmetry operations for NMR pulse sequences are discussed in terms of excitation field modifiers and temporal modifiers. The quantum operators which describe the effect of these modifiers on the excitation field spin Hamiltonian are provided. The symmetry transformations of spin propagators, and the different types of pulse-sequence elements are discussed. The common types of symmetry expansion are treated using the propagator transformations and the Euler angles for the excitation field propagators. The selection rules associated with symmetrical pulse sequences are discussed using average Hamiltonian theory
J-Stabilization of singlet states in the solution NMR of multiple-spin systems
No full textLong-lived singlet states have been observed in the solution NMR of spin systems containing more than two coupled spins, despite the fact that the singlet state is expected to be quenched by small long-range J-couplings. We show that the stability of localized singlet states may be explained by taking into account the intra-pair J-coupling between the two spins which participate in the singlet state. The relatively strong intra-pair J-coupling protects the singlet state against quenching by weaker out-of-pair J-couplings.<br/
Signal enhancement in the triple-quantum magic-angle spinning NMR of spins-3/2 in solids: the FAM-RIACT-FAM sequence
No full textWe achieve a significant signal enhancement for the triple-quantum magic-angle spinning NMR of a spin-3/2 system, by using an amplitude-modulated radiofrequency field, followed by a selective 90° pulse and a phase-shifted strong rf field, for the triple-quantum excitation, and an amplitude-modulated radiofrequency field for the conversion of triple-quantum coherence to observable single-quantum coherence. The experiment is demonstrated on the Rb-87 NMR of polycrystalline rubidium nitrate
Beyond the T-1 limit: Singlet nuclear spin states in low magnetic fields
No full textLow-field nuclear spin singlet states may be used to store nuclear spin order in a room temperature liquid for a time much longer than the spin-lattice relaxation time constant T-1. The low-field nuclear spin singlets are unaffected by intramolecular dipole-dipole relaxation, which is generally the predominant relaxation mechanism. We demonstrate storage of nuclear spin order for more than 10 times longer than the measured value of T-1. This phenomenon may facilitate the development of nuclear spin hyperpolarization methods and may allow the study of motional processes which occur too slowly for existing NMR techniques. This is the first time that the memory of nuclear spins has been extended well beyond the T-1 limit in a system lacking intrinsic magnetic equivalence
High-resolution 1H NMR in the solid state using symmetry-based pulse sequences
No full textWe demonstrate new rotor synchronised pulse sequences for obtaining high-resolution H-1 NMR spectra in the presence of fast magic angle spinning (MAS). The new sequences exploit selection rules generated by appropriate synchronisation of the radio-frequency (rf) field modulations and the sample rotation. We show preliminary results demonstrating the feasibility of high-resolution proton NMR spectroscopy in the presence of fast MAS, and also demonstrate the resolution of H-1-C-13 J-couplings in the solid state
Double-quantum solid-state NMR of 13C spin pairs coupled to 14N
No full textWe examine the double-quantum magic angle spinning NMR spectra of pairs of 13C nuclei coupled to one or more 14N nuclei. The experimental spectra of 13C(2)-glycine and glycyl-[13C(2)]-glycyl-glycine are used to demonstrate the sensitivity of the spectra to the orientation of 14N quadrupole interaction tensors and to the molecular torsional angles
Modulation-aided signal enhancement in the magic angle spinning NMR of spin-5/2 nuclei
No full textWe report signal enhancement schemes using fast amplitude modulated pulses for the one-dimensional (I D) nuclear magnetic resonance (NMR) of spin-5/2 nuclei under magic-angle spinning. Signal enhancement by a factor of around 2.5 is observed when amplitude modulated pulses precede selective excitation of the central transition. This enhancement is a result of the redistribution of energy level populations through partial saturation of the satellite transitions. Results are shown for Al-27 and O-17. The gain in signal intensity is very useful for the observation of weak signals from low abundance quadrupolar nuclei. The scheme works for wide ranges of quadrupole interactions and rf powers
Bidirectional band-selective magnetization transfer along the protein backbone doubles the information content of solid-state NMR correlation experiments
No full textResonance assignment is the first stage towards solving the structure of a protein. This is normally achieved by the employment of separate inter and intra residue experiments. By utilising the mixed rotation and rotary recoupling (MIRROR) condition it is possible to double the information content through the efficient bidirectional transfer of magnetization from the CO to its adjacent Ca and the Ca of the subsequent amino acid. We have incorporated this into a 3Dexperiment, a 3D-MIRROR NCOCA, where correlations present in the 3D spectrum permit the sequential assignment of the protein backbone from a single experiment as we have demonstrated on a microcrystalline preparation of GB3. Furthermore, the low power requirements of the MIRROR recoupling sequence facilitate the development of a low-power 3D-NCOCA experiment. This has enabled us to realise significant reductions in acquisition times, allowing the acquisition of a single 3D-NCOCA spectrum suitable for a full backbone resonance assignment of GB3 in less than 24 hours
Conformation of the glycosidic linkage in a disaccharide investigated by double-quantum solid-state NMR
No full textDouble-quantum heteronuclear local field NMR is performed on a sample of a 13C2-labeled disaccharide, in which the two 13C spins are located on opposite sides of the glycosidic linkage. The evolution of the double-quantum coherences is found to be consistent with the solid-state conformation of the molecule, as previously determined by X-ray diffraction. The dependence of the double-quantum evolution on the glycosidic torsional angles is examined by using a graphical molecular manipulation program interfaced to a numerical spin simulation module
1H homonuclear dipolar decoupling using rotor-synchronised pulse sequences: Towards pure absorption phase spectra
No full textWe demonstrate a pulse sequence using symmetry-based rotor-synchronised sequences for homonuclear dipolar decoupling that achieves pure absorption phase high-resolution 1H spectra in solid-state NMR. This sequence is compared with the phase-modulated Lee-Goldburg scheme. Experimental results are shown for samples of glycine and l-histidine·HCl·H2O for magic-angle-spinning frequencies in the range of 14–30 kHz and at two different magnetic fields.<br/
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