Structure and orientation of antibiotic peptide alamethicin in phospholipid bilayers as revealed by chemical shift oscillation analysis of solid state nuclear magnetic resonance and molecular dynamics

The structure, topology and orientation of membrane-bound antibiotic alamethicin were studied using solid state nuclear magnetic resonance (NMR) spectroscopy. C-13 chemical shift interaction was observed in [1-C-13]-labeled alamethicin. The isotropic chemical shift values indicated that alamethicin forms a helical structure in the entire region. The chemical shift anisotropy of the carbonyl carbon of isotopically labeled alamethicin was also analyzed with the assumption that alamethicin molecules rotate rapidly about the bilayer normal of the phospholipid bilayers. It is considered that the adjacent peptide planes form an angle of 100 degrees or 120 degrees when it forms alpha-helix or 3(10)-helix, respectively. These properties lead to an oscillation of the chemical shift anisotropy with respect to the phase angle of the peptide plane. Anisotropic data were acquired for the 4 and 7 sites of the N- and C-termini, respectively. The results indicated that the helical axes for the N- and C-termini were tilted 17 degrees and 32 degrees to the bilayer normal, respectively. The chemical shift oscillation curves indicate that the N- and C-termini form the alpha-helix and 3(10)-helix, respectively. The C-terminal 3(10)-helix of alamethicin in the bilayer was experimentally observed and the unique bending structure of alamethicin was further confirmed by measuring the internuclear distances of [1-C-13] and [N-15] doubly-labeled alamethicin. Molecular dynamics simulation of alamethicin embedded into dimyristoyl phophatidylcholine (DMPC) bilayers indicates that the helical axes for alpha-helical N- and 3(10)-helical C-termini are tilted 12 degrees and 32 degrees to the bilayer normal, respectively, which is in good agreement with the solid state NMR results. (C) 2015 Elsevier B.V. All rights reserved.