Thursday, July 03, 2008

Solid-State Nuclear Magnetic Resonance - Vol 33 Issue 4

1H NMR signal broadening in spectra of alkane molecules adsorbed on MFI-type zeolites
Ekaterina E. Romanova, Cordula B. Krause, Alexander G. Stepanov, Ursula Wilczok, Wolfgang Schmidt, Jasper M. van Baten, Rajamani Krishna, André Pampel, Jörg Kärger and Dieter Freud
The anisotropic behavior of C1–C6 alkane molecules adsorbed in MFI zeolite was studied by 1H nuclear magnetic resonance (NMR) using single-pulse excitation, Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence, Hahn echo (HE) pulse sequence, and magic-angle spinning. The molecular order parameter was obtained by both static 2H NMR spectroscopy and molecular simulations. This yields an order parameter in the range of 0.28–0.42 for linear alkanes in MFI zeolite, whereas the parameter equals zero for FAU zeolite with a cubic symmetry. Thus, in the case of a zeolite with a non-cubic symmetry like MFI, the mobility of the molecules in one crystallite cannot fully average the dipolar interaction. As a consequence, transverse nuclear magnetization as revealed in the echo attenuation notably deviates from a mono-exponential decay. This information is of particular relevance for the performance of pulsed field gradient (PFG) NMR diffusion experiments, since the occurrence of non-exponential magnetization attenuation could be taken as an indication of the existence of different molecules or of molecules in different states of mobility.

Solid-state 17O NMR spectroscopy of a phospholemman transmembrane domain protein: Implications for the limits of detecting dilute 17O sites in biomaterials
Alan Wong, Andrew J. Beevers, Andreas Kukol, Ray Dupree and Mark E. Smith
The 17O-‘diluted’ glycine-14 sites in a phospholemman (PLM) transmembrane domain protein are characterized by solid-state 17O NMR spectroscopy. The PLM transmembrane domain is an -helical tetramer unit of four 28-residue peptides and is rigidly embedded in a bilayer where each -helix has an average tilt of 7.3° against the membrane normal. The PLM sample investigated here consists of a high lipid/peptide molar ratio (25:1) with one glycine residue in each helix enriched to <40% 17O; thus, this is a very dilute 17O-sample and is the most dilute 17O-membrane protein to date to be characterized by solid-state 17O NMR spectroscopy. Based on the spectral analysis of 17O magic angle spinning (MAS) at 14.1 and 18.8 T, the PLM transmembrane domain protein consists of multiple crystallographic gly14 sites, suggesting that the tetramer protein is an asymmetric unit with either C2- or C1-rotational symmetry along the bilayer normal.

Determination of 27Al–29Si connectivities in zeolites with 2D 27Al→29Si RAPT–CPMG–HETCOR NMR
Gordon J. Kennedy, Jerzy W. Wiench and Marek Pruski
The recently introduced concept of the combined use of rotor assisted population transfer (RAPT) and Carr–Purcell–Meiboom–Gill (CPMG) techniques to boost the sensitivity of cross polarization (CP) based NMR experiments is applied to a synthetic zeolite (ZSM-4). The sensitivity was increased by a factor of 4, which enabled acquisition of a high quality two-dimensional 27Al–29Si HETCOR (heteronuclear correlation) spectrum. By separating the resonances in two dimensions, through-space connectivities between spins were revealed and the effective resolution was improved in both dimensions, which allowed determination of the existing ambiguities in spectral assignments in this material. The spectra provided clear indication of random distribution of aluminum and silicon within the ZSM-4 network. Additionally, unexpected correlations were observed between different components of inhomogeneously broadened 29Si and 27Al lines, which are most likely due to differences in the second coordination sphere environments.

Probing amide bond nitrogens in solids using 14N NMR spectroscopy
Sasa Antonijevic and Nicholas Halpern-Manners
A novel two-dimensional nuclear magnetic resonance (NMR) experiment is proposed for indirect observation of 14N nuclei in various types of nitrogen-containing solids. In a method somewhat similar to the heteronuclear single-quantum correlation (HSQC) experiment widely used for protein structure determination in solutions, this technique correlates spin S=1/2 nuclei, e.g., 1H, 13C, with the 14N spin I=1 nucleus in solids. The present experiment, however, transfers coherence from neighboring 1H or 13C nuclei to 14N via a combination of J-couplings and residual dipolar splittings (RDS). Projections of the two-dimensional NMR spectra onto the 14N dimension yield powder patterns that reflect the 14N quadrupolar interaction, which can be used to study molecular structure and dynamics. Indirect detection of amide nitrogen-14 via 1H and 13C is shown experimentally on a model compound of N-acetyl-glycine.

Determination of the orientations for the 17O NMR tensors in a polycrystalline l-alanine hydrochloride
Kazuhiko Yamada, Tadashi Shimizu, Toshio Yamazaki and Shinobu Ohki
We report a solid-state 17O NMR study of the 17O electric-field-gradient (EFG) and chemical shielding (CS) tensors for the carboxyl oxygen in an l-alanine hydrochloride. Using [17O]– and [13C,17O]–l-alanine hydrochlorides, both the magnitudes and the orientations in the molecular frame of the 17O EFG and CS tensors could be determined by the analysis of the 17O magic-angle spinning (MAS) and stationary NMR spectra. For the carbonyl oxygen, the smallest EFG tensor component, VXX, and the largest EFG component, VZZ, roughly lies in the carboxyl molecular plane and the direction of VXX is parallel to the dipolar vector between 13C and 17O, that is, the direction of CO bond. The angles between the intermediate EFG component, VYY, and δ33 component, and between δ22 component and VZZ are found to be approximately 10° and 35°, respectively. We also present the results of the quantum chemical calculations for a theoretical hydrogen-bonding model, indicating that hydrogen-bonding strengths make it possible to vary both magnitudes and orientations of the carbonyl 17O EFG tensors in amino acid hydrochlorides.

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