Biomedical subjects

Malcolm H Levitt

Publications and source records attributed to Malcolm H Levitt.

7 recordsLinked to original sources

PubMed2003

A test for the number of coupled spins I = 1/2 in magic-angle-spinning solids: zero-quantum recoupling of multiple-quantum coherences.

Current methodologies for estimating the number of coupled spins I = 1/2 in solids are based upon the maximum multiple-quantum order that can be observed. This strategy establishes a clear lower bound on the number of coupled spins I = 1/2. However, it is difficult to ascertain the exact number of coupled spins, since the absence of a peak could be due either to the limited size of the spin system or to the experimental difficulty of exciting high-quantum orders and recovering those coherences into detectable signals. Herein, a supplementary test is proposed that allows one to determine whether a given coherence has the highest possible order in the spin system. The sample is subjected to magic-angle spinning and the behaviour of the coherence under a rotor-synchronised spin-echo sequence is compared to its behaviour under a zero-quantum recoupling sequence. A similar decay of the coherence in these two experiments is strong evidence for the coherence order being the maximum possible. We propose applications to biomolecular solid-state NMR spectroscopy.

Journal Article↗

PubMed2003

Multiplex phase cycling.

We discuss a new class of phase cycling procedures, in which a set of individual phase-shifted transients are stored separately in the computer and processed afterwards to yield the separated NMR signals from two or more coherence transfer pathways. In the case of two-dimensional double-quantum spectroscopy, this multiplex acquisition procedure allows the acquisition of pure-absorption spectra in only 62.5% of the time needed by previous methods.

Journal Article↗

PubMed2003

Enhanced triple-quantum excitation in 13C magic-angle spinning NMR.

We describe a new method for exciting triple-quantum coherences in 13C-labelled powder samples under MAS. The proposed method combines selective double-quantum excitation with rotational resonance and frequency-selective composite pulses. The spin dynamics of this new method are described theoretically. Numerical calculations of the spin dynamics are compared to experimental results on fully 13C-labelled L-alanine. The observed triple-quantum filtering efficiency is around 10% for the most intense spectral peak. The method is also demonstrated on other fully 13C-labelled compounds, including a uniformly 13C-labelled amino acid.

Alanine↗

PubMed2003

Application of amplitude-modulated radiofrequency fields to the magic-angle spinning NMR of spin-7/2 nuclei.

We report pulse sequences for the sensitivity enhancement of magic-angle spinning and multiple-quantum magic-angle spinning spectra of spin-72 systems. Sensitivity enhancement is obtained with the use of fast amplitude-modulated (FAM) radiofrequency pulses. In one-dimensional magic-angle spinning experiments, signal enhancement of 3 is obtained by a FAM pulse followed by a soft 90 degrees pulse. In two-dimensional multiple-quantum magic-angle spinning experiments, FAM pulses are used for both the excitation of multiple-quantum coherences and for their conversion into observable single-quantum coherences. The observed signal enhancements are 2.2 in 3Q experiments, 3.1 in 5Q experiments, and 4.1 in 7Q experiments, compared to the conventional two-pulse scheme. The pulse schemes are demonstrated on the 45Sc NMR of Sc2(SO4)3 x 5H2O and the 139La NMR of LaAlO3. We also demonstrate the generation of FAM pulses by double-frequency irradiation.

Journal Article↗

PubMed2002

Cogwheel phase cycling.

A new method for constructing phase cycles is described. The new schemes apply to experiments involving several consecutive coherence transfer steps. The radiofrequency phases of two or more irradiation blocks are incremented simultaneously, as opposed to the traditional "nested" scheme, in which the block phases are incremented independently. In many cases, the "cogwheel" phase cycles achieve the same selectivity as traditional phase cycles, using fewer steps. Significant time savings are achievable in a wide range of NMR experiments.

Magnetic Resonance Spectroscopy↗

PubMed2002

Homonuclear zero-quantum recoupling in fast magic-angle spinning nuclear magnetic resonance.

Solid-state magic-angle-spinning NMR pulse sequences which implement zero-quantum homonuclear dipolar recoupling are designed with the assistance of symmetry theory. The pulse sequences are compensated on a short time scale by the use of composite pulses and on a longer time scale by the use of supercycles. (13)C dipolar recoupling is demonstrated in powdered organic solids at high spinning frequencies. The new sequences are compared to existing pulse sequences by means of numerical simulations. Experimental two-dimensional magnetization exchange spectra are shown for [U-(13)C]-L-tyrosine.

Alanine↗

PubMed2002

Solid-state NMR determination of sugar ring pucker in (13)C-labeled 2'-deoxynucleosides.

The H3'-C3'-C4'-H4' torsional angles of two microcrystalline 2'-deoxynucleosides, thymidine and 2'-deoxycytidine.HCl, doubly (13)C-labeled at the C3' and C4' positions of the sugar ring, have been measured by solid-state magic-angle-spinning nuclear magnetic resonance (NMR). A double-quantum heteronuclear local field experiment with frequency-switched Lee-Goldberg homonuclear decoupling was used. The H3'-C3'-C4'-H4' torsional angles were obtained by comparing the experimental curves with numerical simulations, including the two (13)C nuclei, the directly bonded (1)H nuclei, and five remote protons. The H3'-C3'-C4'-H4' angles were converted into sugar pucker angles and compared with crystallographic data. The delta torsional angles determined by solid-state NMR and x-ray crystallography agree within experimental error. Evidence is also obtained that the proton positions may be unreliable in the x-ray structures. This work confirms that double-quantum solid-state NMR is a feasible tool for studying sugar pucker conformations in macromolecular complexes that are unsuitable for solution NMR or crystallography.

Carbohydrates↗