Interaction with phospholipid bilayers, ion channel formation, and antimicrobial activity of basic amphipathic...

Basic amphipathic alpha-helical peptides Ac-(Leu-Ala-Arg-Leu)3 or 4-NHCH3 (4(3) or 4(4)) and H-(Leu-Ala-Arg-Leu)3-(Leu-Arg-Ala-Leu)2 or 3-OH (4(5) or 4(6)) were synthesized and studied in terms of their interactions with phospholipid membranes, biological activity, and ion channel-forming ability. CD study of the peptides showed that they form alpha-helical structures in the presence of phospholipid liposomes and thus they have amphipathic distribution of the side chains along the axis of the helix. A leakage study of carboxyfluorescein encapsulated in phospholipid vesicles indicated that the peptides possess a highly potent ability to perturb the membrane structure. Membrane current measurements using the planar lipid bilayer technique revealed that the peptide 4(6), which was long enough to span the lipid bilayer in the alpha-helical structure, formed cation-selective ion channels at a concentration of 0.5 microM in a planar diphytanoylphosphatidylcholine bilayer. In contrast, other shorter peptides failed to form discrete and stable channels though they occasionally induced an increase in the membrane current with erratic conductance levels. The probability of detecting a conductance increase was in the order of 4(6) greater than 4(5) greater than 4(4) greater than 4(3), which corresponds to the order of the peptide chain lengths. Furthermore, 4(6) but not 4(5) showed an antimicrobial activity against both Gram-positive and -negative bacteria. The structure of ion channels formed by 4(6) and the relationship between the peptide chain length and biological activity of the synthetic peptides are discussed.