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 2 H NMR, which, however, has limited site resolution and requires site-specific isotopic labeling. Using a 2D LG-CP technique, we resolve the 13 C− 1 H or 15 N− 1 H dipolar couplings according to the 13 C or 15 N 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 13 C− 1 H and 15 N− 1 H 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 1 H− 13 C cross-polarization step, and the site resolution of the 15 N− 1 H LG-CP spectra can be enhanced by 13 C indirect detection.