Investigation of molecular motions by Lee-Goldburg cross-polarization NMR Spectroscopy

We demonstrate the use of Lee-Goldburg cross-polarization (LG-CP) NMR under fast magic-angle spinning (MAS) to investigate the amplitude and geometry of segmental motions in biomolecular and polymeric solids. Motional geometry information was previously available only from 2 H NMR, which, however, has limited site resolution and requires site-specific isotopic labeling. Using a 2D LG-CP technique, we resolve the C-13-H-1 or N-15-H-1 dipolar couplings according to the C-13 or N-15 isotropic chemical shift. Applications to systems undergoing 180degrees phenylene ring flips show spectral line shapes reflecting the geometry of the motion. Using this LG-CP technique, we measured the C-13-H-1 and N-15-H-1 dipolar couplings in the water-soluble and membrane-bound states of the colicin la channel domain. The backbone motions of the membrane-bound colicin scale both the Calpha-Halpha and N-H couplings similarly, thus ruling out rotation of the alpha-helices around their axes as a specific mechanism of motion. We also show that the sensitivity of the LG-CP spectra can be enhanced by the addition of a phase-inverted H-1-C-13 cross-polarization step, and the site resolution of the N-15-H-1 LG-CP spectra can be enhanced by C-13 indirect detection.