High-resolution solid-state NMR of anisotropically mobile molecules under very low-power 1H decoupling and moderate magic-angle spinning

We show that for observing high-resolution heteronuclear NMR spectra of anisotropically mobile systems with order parameters less than 0.25, moderate magic-angle spinning (MAS) rates of similar to 11 kHz combined with H-1 decoupling at 1-2 kHz are sufficient. Broadband decoupling at this low H-1 nutation frequency is achieved by composite pulse sequences such as WALTZ-16. We demonstrate this moderate MAS low-power decoupling technique on hydrated POPC lipid membranes, and show that 1 kHz H-1 decoupling yields spectra with the same resolution and sensitivity as spectra measured under 50 kHz H-1 decoupling when the same acquisition times (similar to 50 ms) are used, but the low-power decoupled spectra give higher resolution and sensitivity when longer acquisition times (>150 ms) are used, which are not possible with high-power decoupling. The limits of validity of this approach are explored for a range of spinning rates and molecular mobilities using more rigid membrane systems Such as POPC/cholesterol mixed bilayers. Finally, we show N-15 and C-13 spectra of a uniaxially diffusing membrane peptide assembly, the influenza A M2 transmembrane domain, under 11 kHz MAS and 2 kHz H-1 decoupling. The peptide N-15 and C-13 intensities at low-power decoupling are 70-80% of the high-power decoupled intensities. Therefore, it is possible to study anisotropically mobile lipids and membrane peptides using liquid-state NMR equipment, relatively large rotors, and moderate MAS frequencies. (C) 2009 Elsevier Inc. All rights reserved.