Channel-forming activity of lactophoricins I and II in mercury-supported tethered bilayer lipid membranes

Lactophoricin-I (LPcin-I), an antimicrobial cationic peptide with 23 amino acid residues isolated from bovine milk, and LPcin-II, which lacks six N-terminal amino acids with respect to LPcin-I, were investigated at a mercury-supported tethered bilayer lipid membrane (tBLM) consisting of a 2,3,di-O-phytanyl-sn-glycerol-1-tetraethylene-glycol-d,l-? lipoic acid ester thiolipid (DPTL) with a dioleoylphosphatidylcholine (DOPC) or dioleoylphosphatidylserine (DOPS) monolayer on top. The two LPcins were incorporated via scans of electrochemical impedance spectra (EISs) regularly distributed from - 0.30 to - 1.00 V and vice versa. During the pristine negative-going EIS scan the distal lipid monolayer is disrupted via a carpet mechanism, as revealed by an EIS exhibiting inductive behavior, whereas it heals during the positive-going scan. LPcin-I forms ion channels yielding voltage-gated cyclic voltammograms (CVs) at pH 3 and 5.4 in DPTL/DOPC tBLMs, but only at pH 3 in DPTL/DOPS tBLMs; no ion channels are formed at pH 6.8. LPcin-II does not form ion channels in DPTL/DOPC tBLMs at the three pH values investigated; however, in DPTL/DOPS tBLMs it forms ion channels at pH 5.4 and 6.8. This overall behavior is explained by dual electrostatic interactions between the zwitterionic polar heads of DOPC or DOPS and neighboring pairs of oppositely charged residues in the peptide chain.