Fast acquisition of multi-dimensional spectra in solid-state NMR enabled by ultra-fast MAS
The advantages offered by ultra-fast (>60 kHz) magic angle spinning (MAS) rotation for the Study of biological samples, notably containing paramagnetic centers are explored. It is shown that optimal conditions for performing solid-state C-13 NMR under 60 kHz MAS are obtained with low-power CW H-1 decoupling, as well as after a low-power H-1,C-13 cross-polarization step at a double-quantum matching condition. Acquisition with low-power decoupling highlights the existence of rotational decoupling sidebands. The sideband intensities and the existence of first and second rotary conditions are explained in the framework of the Floquet-van Vleck theory. As a result, optimal C-13 spectra of the oxidized, paramagnetic form of human copper zinc superoxide dismutase (SOD) can be obtained employing rf-fields which do not exceed 40 kHz during the whole experiment. This enables the removal of unwanted heating which can lead to deterioration of the sample. Furthermore, combined with the short H-1 T(1)s, this allows the repetition rate of the experiments to be shortened from 3 s to 500 ms, thus compensating for the sensitivity loss due to the smaller sample volume in a 1.3 mm rotor. The result is that 2D C-13-C-13 correlation could be acquired in about 24 h on less than 1 mg of SOD sample. (C) 2008 Elsevier Inc. All rights reserved.