Membrane orientation of the N-terminal segment of alamethicin determined by solid-state 15N NMR

Alamethicin was synthesized with 15N incorporated into alanine at position 6 in the peptide sequence. In dispersions of hydrated dimyristoylphosphatidylcholine, solid-state 15N NMR yields an axially symmetric powder pattern indicating that the peptide is reorienting with a single axis of symmetry when associated with lamellar lipids. When incorporated into bilayers that are uniformly oriented with the bilayer normal parallel to the B(o) field, the position of the observed 15N chemical shift is 171 ppm. This is coincident with the sigma parallel to edge of the axially symmetric powder pattern for non-oriented hydrated samples. Thus the axis of motional averaging lies along the bilayer normal. Two-dimensional separated local field spectra were obtained that provide a measure of the N-H dipolar coupling in one dimension and the 15N chemical shift in the other. These data yield a dipolar coupling of 17 kHz corresponding to an average angle of 24 degrees for the N-H bond with respect to the B(o) field axis. An analysis of the possible structures and orientations that could produce the observed spectral parameters show that these values are consistent with an alpha-helical conformation inserted along the bilayer normal.