Friday, March 21, 2008

JMR - March 2008

Journal of Magnetic Resonance - up to March 2008

Includes two articles on 19F MAS NMR of amino acids by Anne Ulrich, effectiveness of 1H decoupling in 13C MAS NMR spectra of paramagnetic solids by Willans and Wasylishen, NMR expeirments using hybrid magnetic fields by Gan and co-workers, and Malcolm Levitt's ideas about singlet state NMR.

Journal of Magnetic Resonance
Vol: 191, Issue: 1, March, 2008 pp. 7-15

Title: Solid state 19F NMR parameters of fluorine-labeled amino acids. Part I: Aromatic substituents
Authors: Durr, U.H.N.; Grage, S.L.; Witter, R.; Ulrich, A.S.
Keywords: Fluorine; Solid state NMR; Amino acid; 19F chemical shift anisotropy
Abstract (English): Structural parameters of peptides and proteins in biomembranes can be directly measured by solid state NMR of selectively labeled amino acids. The ^1^9F nucleus is a promising label to overcome the low sensitivity of 2H, 13C or 15N, and to serve as a background-free reporter group in biological compounds. To make the advantages of solid state 19F NMR fully available for structural studies of polypeptides, we have systematically measured the chemical shift anisotropies and relaxation properties of the most relevant aromatic and aliphatic 19F-labeled amino acids. In this first part of two consecutive contributions, six different 19F-substituents on representative aromatic side chains were characterized as polycrystalline powders by static and MAS experiments. The data are also compared with results on the same amino acids incorporated in synthetic peptides. The spectra show a wide variety of lineshapes, from which the principal values of the CSA tensors were extracted. In addition, temperature-dependent T1 and T2 relaxation times were determined by 19F NMR in the solid state, and isotropic chemical shifts and scalar couplings were obtained in solution.
# Item Identifier: S1090-7807(07)00357-6
ISSN: 1090-7807

Journal of Magnetic Resonance
Vol: 191, Issue: 1, March, 2008 pp. 16-23

Title: Solid state 19F NMR parameters of fluorine-labeled amino acids. Part II: Aliphatic substituents
Authors: Grage, S.L.; Durr, U.H.N.; Afonin, S.; Mikhailiuk, P.K.; Komarov, I.V.; Ulrich, A.S.
Keywords: Flourine; Solid state NMR; Amino acid; 19F chemical shift anisotropy
Abstract (English): A representative set of amino acids with aliphatic 19F-labels has been characterized here, following up our previous compilation of NMR parameters for single 19F-substituents on aromatic side chains. Their isotropic chemical shifts, chemical shift tensor parameters, intra-molecular 19F dipole-dipole couplings and temperature-dependent T1 and T2 relaxation times were determined by solid state NMR on twelve polycrystalline amino acid samples, and the corresponding isotropic 19F chemical shifts and scalar couplings were obtained in solution. Of particular interest are amino acids carrying a trifluoromethyl-group, because not only the 19F chemical shift but also the intra-CF3 homonuclear dipolar coupling can be used for structural studies of 19F-labeled peptides and proteins. The CF3-groups are further compared with CH2F-, CD2F-, and CD3-groups, using both 19F and 2H NMR to describe their motional behavior and to examine the respective linebroadening effects of the protonated and deuterated neighbors. We have also characterized two unnatural amino acids in which a CF3-label is rigidly connected to the backbone by a phenyl or bicyclopentyl moiety, and which are particularly well suited for structure analysis of membrane-bound polypeptides. The 19F NMR parameters of the polycrystalline amino acids are compared with data from the correspondingly labeled side chains in synthetic peptides.
# Item Identifier: S1090-7807(07)00356-4
ISSN: 1090-7807

Journal of Magnetic Resonance
Vol: 191, Issue: 1, March, 2008 pp. 31-46

Title: The effectiveness of 1H decoupling in the 13C MAS NMR of paramagnetic solids: An experimental case study incorporating copper(II) amino acid complexes
Authors: Willans, M.J.; Sears, D.N.; Wasylishen, R.E.
Keywords: Heteronuclear decoupling; 13C solid-state NMR spectroscopy; Paramagnetic metal-organic complexes; Copper; Amino acids
Abstract (English): The use of continuous-wave (CW) 1H decoupling has generally provided little improvement in the 13C MAS NMR spectroscopy of paramagnetic organic solids. Recent solid-state 13C NMR studies have demonstrated that at rapid magic-angle spinning rates CW decoupling can result in reductions in signal-to-noise and that 1H decoupling should be omitted when acquiring 13C MAS NMR spectra of paramagnetic solids. However, studies of the effectiveness of modern 1H decoupling sequences are lacking, and the performance of such sequences over a variety of experimental conditions must be investigated before 1H decoupling is discounted altogether. We have studied the performance of several commonly used advanced decoupling pulse sequences, namely the TPPM, SPINAL-64, XiX, and eDROOPY sequences, in 13C MAS NMR experiments performed under four combinations of the magnetic field strength (7.05 or 11.75T), rotor frequency (15 or 30kHz), and 1H rf-field strength (71, 100, or 140kHz). The effectiveness of these sequences has been evaluated by comparing the 13C signal intensity, linewidth at half-height, LWHH, and coherence lifetimes, T2', of the methine carbon of copper(II) bis(dl-alanine) monohydrate, Cu(ala)2.H2O, and methylene carbon of copper(II) bis(dl-2-aminobutyrate), Cu(ambut)2, obtained with the advanced sequences to those obtained without 1H decoupling, with CW decoupling, and for fully deuterium labelled samples. The latter have been used as model compounds with perfect 1H decoupling and provide a measure of the efficiency of the 1H decoupling sequence. Overall, the effectiveness of 1H decoupling depends strongly on the decoupling sequence utilized, the experimental conditions and the sample studied. Of the decoupling sequences studied, the XiX sequence consistently yielded the best results, although any of the advanced decoupling sequences strongly outperformed the CW sequence and provided improvements over no 1H decoupling. Experiments performed at 7.05T demonstrate that the XiX decoupling sequence is the least sensitive to changes in the 1H transmitter frequency and may explain the superior performance of this decoupling sequence. Overall, the most important factor in the effectiveness of 1H decoupling was the carbon type studied, with the methylene carbon of Cu(ambut)2 being substantially more sensitive to 1H decoupling than the methine carbon of Cu(ala)2.H2O. An analysis of the various broadening mechanisms contributing to 13C linewidths has been performed in order to rationalize the different sensitivities of the two carbon sites under the four experimental conditions.
# Item Identifier: S1090-7807(07)00350-3
ISSN: 1090-7807

Journal of Magnetic Resonance
Vol: 191, Issue: 1, March, 2008 pp. 135-140

Title: High-field NMR using resistive and hybrid magnets
Authors: Gan, Z.; Kwak, H.T.; Bird, M.; Cross, T.; Gor'kov, P.; Brey, W.; Shetty, K.
Keywords: High field; High resolution; MAS; Ferromagnetic shim; Field homogeneity; Stability; HENPEC; NHMFL; Resistive; Hybrid magnet
Abstract (English): Resistive and resistive-superconducting hybrid magnets can generate dc magnetic fields much higher than conventional superconducting NMR magnets but the field spatial homogeneity and temporal stability are usually not sufficient for high-resolution NMR experiments. Hardware and technique development addressing these issues are presented for high-resolution NMR at magnetic fields up to 40T. Passive ferromagnetic shimming and magic-angle spinning are used effectively to reduce the broadening from inhomogeneous magnetic field. A phase correction technique based on simultaneous heteronuclear detection is developed to compensate magnetic field fluctuations to achieve high spectral resolution.
# Item Identifier: S1090-7807(07)00373-4
ISSN: 1090-7807

Journal of Magnetic Resonance
Vol: 191, Issue: 1, March, 2008 pp. 148-155

Title: Isotropic filtering using polyhedral phase cycles: Application to singlet state NMR
Authors: Pileio, G.; Levitt, M.H.
Keywords: Isotropic filtering; Singlet states; Long-lived spin states; Spherical tensor operators; Phase cycling
Abstract (English): A technique is described for filtering out the components of an NMR signal that have passed through an isotropic spin order term. The method involves a coordinated cycle of three radiofrequency phase angles, where two of the phases correspond to the polar angles describing the vertices of a regular polyhedron, and the third angle is stepped around a circle. The most economical isotropic filtering scheme involves a 12-step phase cycle based on tetrahedral symmetry. The method is used to filter out NMR signals that have passed through singlet populations in a solution NMR experiment.
# Item Identifier: S1090-7807(07)00361-8
ISSN: 1090-7807

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