Lipid-peptide interface: valine conformation and dynamics in the gramicidin channel

High-resolution dynamic and structural characterizations have been achieved for each of the valine side chains of the gramicidin channel while solubilized in hydrated lipid bilayers. The characterizations have been achieved by 2H NMR spectra of both oriented and unoriented samples obtained at 36 and 5 degrees C, respectively. Powder patterns displaying intermediate time frame averaging provide dynamic information, and quadrupole splittings from aligned samples provide orientational constraints for the side chain structure. Librational amplitudes for each site throughout the side chain have also been characterized. Val6 and Val8 are shown to be fixed in rotameric states, potentially constraining two of the indole rings and the functionally important indole dipole moment orientations. Val1 and Val7 undergo three-state jump motions. The jump frequencies increase from the microsecond to nanosecond time frame upon increasing the temperature through the lipid phase transition. For the same temperature range, there is no evidence for changes in conformational state populations. Despite small differences in the substate populations for the two residues, the motions may be loosely coupled as indicated by the high-resolution structure.