Structure and Orientation of the Pore-forming Peptide Melittin, in Lipid Bilayers

Ten analogues of the 26-residue, bee venom peptide, melittin (H 3N +-GIGAVLTTGL-PALISWIKRKRQQ-CONH 2), were synthesized, each with 13C enrichment of a single peptide carbonyl carbon. These peptides were incorporated into bilayers of the diether lipid, ditetradecylphosphatidylcholine, aligned between stacked glass plates. Solid-state 13C nuclear magnetic resonance spectra were obtained as a function of the angle between the bilayer planes and the magnetic field of the spectrometers, and at temperatures above and below the lipid gel-to-liquid crystalline transition temperature, T c. For bilayers aligned with the normal along the applied magnetic field there was no shift in the carbonyl resonances of residues Ile2, Ala4, Leu9, Leu13, or Ala15, with minor changes for residues Va18 and Ile20, and small changes at Va15, Leu6 and Ile17 on immobilization of the peptide below T c. In contrast, the spectra for bilayers aligned at right angles to the field showed greatly increased anisotropy below T c for all analogues. From these experiments it was evident that the peptide was well-aligned in the bilayers and reoriented about the bilayer normal. The observed reduced chemical shift anisotropies and the chemical shifts were consistent with melittin adopting a helical conformation with a transbilayer orientation in the lipid membranes. With the exception of Ile17, there was no apparent difference between the behavior of residues in the two segments that form separate helices in the water-soluble form of the peptide of the peptide, suggesting that in membranes the angle between the helices is greater than the 120° observed in the crystal form.