Lipid shape is a key factor for membrane interactions of amphipathic helical peptides

The membrane alignment of the amphiphilic alpha-helical model peptide MSI-103 (sequence [KIAGKIA]3-NH2) was examined by solid state 2H-NMR in different lipid systems by systematically varying the acyl chain length and degree of saturation, the lipid head group type, and the peptide-to-lipid molar ratio. In liquid crystalline phosphatidylcholine (PC) lipids with saturated chains, the amphiphilic helix changes its orientation from a surface-bound "S-state" to a tilted "T-state" with increasing peptide concentration. In PC lipids with unsaturated chains, on the other hand, the S-state is found throughout all concentrations. Using phosphatidylethanolamine lipids with a small head group or by addition of lyso-lipids with only one acyl chain, the spontaneous curvature of the bilayer was purposefully changed. In the first case with a negative curvature only the S-state was found, whereas in systems with a positive curvature the peptide preferred the obliquely immersed T-state at high concentration. The orientation of MSI-103 thus correlates very well with the shape of the lipid molecules constituting the membrane. Lipid charge, on the other hand, was found to affect only the initial electrostatic attraction to the membrane surface but not the alignment preferences. In bilayers that are "sealed" with 20% cholesterol, MSI-103 cannot bind in a well-oriented manner and forms immobilized aggregates instead. We conclude that the curvature properties of a membrane are a key factor in the interactions of amphiphilic helical peptides in general, whose re-alignment and immersion preferences may thus be inferred in a straightforward manner from the lipid-shape concept.