Tryptophan dynamics and structural refinement in a lipid bilayer environment: solid state NMR of the gramicidin channel.

Tryptophans in the gramicidin channel are important for defining the conformation and the orientation with respect to the bilayer normal and for facilitating cation conductance. Here, high-resolution structure and dynamics of these rings are characterized by solid state NMR. Both oriented and unoriented lipid bilayer preparations are used. Fast frozen lipid bilayer preparations of unoriented samples have been used to obtain static characterizations of nuclear spin interaction tensors. The temperature dependence of these unoriented samples and the spectral features of fast frozen oriented samples were used to experimentally define the local motions of the individual indole rings. Local motions were shown to have amplitudes as high as +/- 29 degrees, and the motions were dominated by libration about the chi 2 axis. The high-resolution structure has been achieved by interpreting seven precise (+/- 0.3 degree) orientational constraints from 2H and 15N NMR for each indole ring in light of the motionally averaged interaction tensors. Each of the four indoles is restricted to a unique orientation of the ring with respect to the bilayer normal and one of two possible rotameric states. The side chain torsion angles for each residue are very similar, generating similar electric dipole moment orientations with respect to the channel.