High-resolution solid-state NMR of anisotropically mobile molecules under very low-power (1)H 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 approximately 11kHz combined with (1)H decoupling at 1-2kHz are sufficient. Broadband decoupling at this low (1)H 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 1kHz (1)H decoupling yields spectra with the same resolution and sensitivity as spectra measured under 50kHz (1)H decoupling when the same acquisition times (approximately 50ms) are used, but the low-power decoupled spectra give higher resolution and sensitivity when longer acquisition times (>150ms) 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 (15)N and (13)C spectra of a uniaxially diffusing membrane peptide assembly, the influenza A M2 transmembrane domain, under 11kHz MAS and 2kHz (1)H decoupling. The peptide (15)N and (13)C 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.