No abstract

Application of rapid sample rotation and radiofrequency irradiation in magic angle spinning (MAS) NMR of lipid bilayers can significantly increase the sample temperature. In this work, we studied the extent of heating during the acquisition of 1H-decoupled 13C MAS spectra of hydrated dimyristoylphosphatidylcholine (DMPC) in the L(alpha) phase. First, we describe a simple procedure for determining the increase in temperature by observing the shift of the 1H water signal. The method is then used to identify and assess the various factors that contribute to the sample heating. The important factors discussed in this paper include: (i) the spinning speed, (ii) the variable-temperature gas pressure, (iii) the rotor geometry, (iv) the power, duration and frequency of the radiofrequency irradiation and (v) the hydration level. A comparison of different heteronuclear decoupling schemes in terms of their ability to produce highly resolved 13C spectra of DMPC is also reported.

We report translational diffusion coefficients in a columnar phase of a discotic liquid crystal formed by a triphenylene-based compound. The experiments were performed using 2H stimulated-echo-type pulsed-field-gradient spin-echo NMR applied to a chain-deuterated sample. The diffusion coefficients were found in the range of 1x10(-14)-4x10(-14) m2/s, three orders of magnitude lower than in the isotopic phase of the same compound. This, together with the high activation energy obtained in columnar phase, indicates that the diffusion is dominated by solidlike jump processes.

Recently (Dvinskikh et al., J. Magn. Reson., 2003, 164, 165 and Dvinskikh et al., J. Magn. Reson., 2004, 168, 194), some of us introduced two efficient solid-state NMR techniques for the determination of heteronuclear dipolar couplings under magic-angle spinning (MAS). These two-dimensional (2D) recoupling methods have been applied previously to simple amino acids, and to columnar systems with high positional and orientational order. In this work, we show that the 2D MAS sequences produce unparalleled 1H-13C dipolar resolution in unoriented lipid membranes. The recoupling experiments were applied to hydrated dimyristoylphosphatidylcholine (DMPC) in the liquid-crystalline Lalpha phase, and the results agreed well with previous NMR investigations using specifically deuterated phospholipids.

Bicelles are increasingly being used as membrane mimicking systems in NMR experiments to investigate the structure of membrane proteins. In this study, we demonstrate the effectiveness of a 2D solid-state NMR approach that can be used to measure the structural constraints, such as heteronuclear dipolar couplings between 1H, 13C, and 31P nuclei, in bicelles without the need for isotopic enrichment. This method does not require a high radio frequency power unlike the presently used rotating-frame separated-local-field (SLF) techniques, such as PISEMA. In addition, multiple dipolar couplings can be measured accurately, and the presence of a strong dipolar coupling does not suppress the weak couplings. High-resolution spectra obtained from magnetically aligned DMPC:DHPC bicelles even in the presence of peptides suggest that this approach will be useful in understanding lipid-protein interactions that play a vital role in shaping up the function of membrane proteins.

No abstract

Molecular reorientation in the two amorphous phases of triphenyl phosphite, namely the supercooled liquid (phase aI) and the newly discovered second amorphous phase (phase aII), was investigated by dielectric relaxation and by two-dimensional (2D) nuclear magnetic resonance spectroscopy (NMR) in the time and frequency domain. Whereas phase aI exhibits the relaxational features typical of supercooled liquids, the molecular motion in phase aII is characterized by an extremely broad dielectric loss and by a pronounced nonexponential reorientational correlation function. Using a Gaussian distribution of correlation times, both dielectric and NMR data reveal consistently correlation times on the order of seconds. The quantitative analysis of the 2D spectra favors the interpretation that molecular motion in phase aII leads to an isotropic distribution of molecular orientation on the surface of a sphere. In addition, we find a secondary relaxation process that shows basically the same features in both phases. We conclude that the newly discovered phase is a second liquid phase with a very unusual reorientational correlation function. However, a nematic liquid crystal cannot completely be ruled out.

The diffusion of ions in an ionic liquid crystal exhibiting a bicontinuous cubic liquid-crystalline phase has been investigated by NMR spectroscopy in order to examine the behaviour of ions in an ordered nanostructure.