Rapidly-frozen polypeptide samples for characterization of high definition dynamics by solid-state NMR spectroscopy

A method is described for defining anisotropic local dynamics in polypeptides by solid-state NMR. To avoid conformational heterogeneity introduced by large hexagonal ice crystals in low temperature hydrated samples, a fast-freezing technique is used for sample preparation. For a demonstration of this approach, the backbone librational motions of the gramicidin A channel conformation are studied in hydrated DMPC bilayers. The static 15N chemical shift tensor is characterized at 123 K for the Ala3 site. The temperature dependence of this tensor yields a determination of the librational amplitude and anisotropy of the motionally sampled space. This amplitude represents the sum of nanosecond and picosecond time-frame motions, both of which have a significant amplitude.