Investigation of Molecular Motions by Lee-Goldburg Cross-Polarization NMR Spectroscopy

Abstract

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 2H NMR, which, however, has limited site resolution and requires site-specific isotopic labeling. Using a 2D LG-CP technique, we resolve the 13C−1H or 15N−1H dipolar couplings according to the 13C or 15N isotropic chemical shift. Applications to systems undergoing 180° phenylene ring flips show spectral line shapes reflecting the geometry of the motion. Using this LG-CP technique, we measured the 13C−1H and 15N−1H dipolar couplings in the water-soluble and membrane-bound states of the colicin Ia channel domain. The backbone motions of the membrane-bound colicin scale both the Cα−Hα and N−H couplings similarly, thus ruling out rotation of the α-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 1H−13C cross-polarization step, and the site resolution of the 15N−1H LG-CP spectra can be enhanced by 13C indirect detection.