Attenuated total reflectance Fourier transform infrared spectroscopy (ATR FT-IR) has been used to monitor alterations in phospholipid organization in thin layers of 1,2-dipalmitoylphosphatidylcholine (DPPC) and l-palmitoyl-2-oleoylphosphatidylcholine (POPC), induced by the membrane lytic peptide melittin, its fragments 1–15 (hydrophobic fragment) and 16—26 (hydrophilic fragment), and δ-hemolysin. In addition, the secondary structures of the peptides and the orientation of helical fragments were determined with respect to the bilayer. The insertion of melittin into POPC caused large perturbations in the order and increased rates of motion of the acyl chains, as monitored by the frequency and half-width of the symmetric CH2 stretching vibration near 2850 cm−1, as well as by the ATR dichroic ratio for this mode. Changes in DPPC organization were less and were consistent with peptide-induced static disordering (gauche rotamer formation) in the acyl chains. Melittin adopted primarily an α-helical secondary structure, although varying small proportions of ß and/or aggregated forms were noted. The helical segments were preferentially oriented perpendicular to the bilayer plane. Several modes of melittin/lipid interaction were considered in an attempt to semiquantitatively understand the observed dichroic ratios. By considering the peptide as a bent rigid rod, a plausible model for its lytic properties has been developed. The hydrophilic fragment in DPPC showed a secondary structure with little α-helix present. As judged by its effect on phospholipid acyl chain organizational parameters, the fragment did not penetrate the bilayer substantially. The hydrophobic fragment in DPPC gave amide I spectral patterns consistent with a mixture of predominantly ß-antiparallel pleated sheet with a smaller fraction of α-helix. Disordering of the lipid acyl chains was induced by this fragment. δ-Hemolysin exhibited predominantly α-helical secondary structure, with only moderate orientational preference. It is suggested to lie randomly within the bilayer.
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