When and how the melittin ion channel exhibits ohmic behavior

Melittin exhibits an ohmic behavior in a lipid bilayer having a DOPC distal leaflet and interposed between a 2.25 nm tetraethyleneoxy chain tethered to a mercury drop and an aqueous solution. This behavior is induced in a pH 6.8 buffer solution of 0.8 μg/mL melittin by a pretreatment consisting of series of electrochemical impedance spectroscopy measurements at bias potentials varied by 50 mV steps over a transmembrane potential range from about + 250 to − 250 mV. This metastable ohmic behavior remains unchanged for hours, even after acidifying the solution to pH 3. The midpoint potential E1/2 between the positive and negative peaks of the resulting cyclic voltammogram is almost coincident with that of the ohmic channels gramicidin and syringopeptin 25A and shifts by the same amount toward more positive potentials with a pH decrease from 6.8 to 3. This common cyclic voltammetry behavior is explained by a tilt of the DOPC polar heads around the channel mouth of these three peptides and is simulated by a modelistic approach. The ohmic behavior of melittin is explained by the persistence of the peptide orientation initially assumed at trans-negative potentials even after application of trans-positive ones, at sufficiently high peptide-to-lipid molar ratios.