MEMBRANE ORIENTATION OF THE N-TERMINAL SEGMENT OF ALAMETHICIN DETERMINED BY SOLID-STATE N-15 NMR

Alamethicin was synthesized with N-15 incorporated into alanine at pos ition 6 in the peptide sequence. In dispersions of hydrated dimyristoy lphosphatidylcholine, solid-state N-15 NMR yields an axially symmetric powder pattern indicating that the peptide is reorienting with a sing le axis of symmetry when associated with lamellar lipids. When incorpo rated into bilayers that are uniformly oriented with the bilayer norma l parallel to the B-0 field, the position of the observed N-15 chemica l shift is 171 ppm. This is coincident with the sigma(parallel to) edg e of the axially symmetric powder pattern for non-oriented hydrated sa mples. Thus the axis of motional averaging lies along the bilayer norm al. Two-dimensional separated local field spectra were obtained that p rovide a measure of the N-H dipolar coupling in one dimension and the N-15 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-0 field axis. An analysis of the possible s tructures and orientations that could produce the observed spectral pa rameters show that these values are consistent with an alpha-helical c onformation inserted along the bilayer normal.