Low-temperature solid-state 15N NMR characterization of polypeptide backbone librations.

The local molecular dynamics of gramicidin A in dimyristoylphosphatidylcholine bilayers were probed by a combination of low-temperature solid-state 15N NMR and rapid freezing of samples using liquid propane. It has previously been shown that this approach leads to sharp discontinuities in powder-pattern spectra of the gramicidin channel in lipid bilayers, and hence, an accurate determination of tensor element magnitudes is achieved. The static-tensor-element magnitudes determined here using hydrated bilayer samples at low temperatures are different from the values obtained from dry-powder samples at 308 K. However, for the one site studied, the orientation of the 15N chemical-shift tensor in the molecular frame is the same for both types of samples. Averaging of the chemical-shift tensor between 200 and 263 K is shown to be anisotropic and consistent with a motional axis parallel with the C alpha-C alpha axis for adjacent residues. The librational amplitudes determined here range from +/- 14 degrees to +/- 22 degrees at 263 K and are significantly larger than the nanosecond librations characterized by relaxation studies. The nanosecond motions are thought to represent correlated motions, while the librations described here are the sum of correlated (nanosecond) and uncorrelated (picosecond) motions. The picosecond librational amplitudes become considerably larger and more isotropic above 0 degree for the residues near the bilayer surface.