On the interaction of the highly charged peptides casocidins with biomimetic membranes

Casocidin I and II (CI and CII) are structurally related antimicrobial peptides made of 39 and 31 amino acids, respectively, which derive from natural proteolytic processing of αs2-casein and adopt an ordered α-helical structure in biomimetic membranes. Their putative membrane-permeabilizing activity was investigated at Hg-supported self-assembled monolayers (SAMs) and at tethered bilayer lipid membranes (tBLMs); the latter consisted of a monolayer of 2,3,di-O-phytanyl-sn-glycerol-1-tetraethylene-glycol-d,l-α lipoic acid ester thiolipid (DPTL), with a dioleoylphosphatidylcholine (DOPC) or dioleoylphosphatidylserine (DOPS) monolayer on top of it. Interaction of CI and CII with these biomimetic membranes was studied by four electrochemical techniques at pH 3, 5.4 and 6.8. Peptide incorporation in tBLMs was attempted via scans of electrochemical impedance spectra. Experiments demonstrated that CI and CII penetrate SAMs as well as the distal DOPC monolayer of DPTL/DOPC tBLMs, but not the proximal phytanyl monolayer, with the only exception of CII at pH 5.4. Conversely, CII permeabilized DPTL/DOPS tBLMs to a moderate extent at all investigated pH values by forming holes across the membrane, but not ion channels. Structural distribution of charged residues seemed to prevent CII from having a hydrophobic side of the α-helix capable of stabilizing a regular ion channel in the lipid matrix.