EIS and PM-IRRAS studies of alamethicin ion channels in a tethered lipid bilayer

Alamethicin was reconstituted into DPhPC/DPTL tethered bilayer lipid membranes (tBLMs) to gain a better understanding of ion pore formation. The properties of the ion-conductive alamethicin pores were investigated by electrochemical impedance spectroscopy (EIS) and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) as a function of the peptide concentration within the membrane. At low peptide concentrations (<= 5%), the alamethicin-incorporated tBLM displays the highest membrane resistance since the alamethicin molecules adopt a surface state where the tilt angles of the helices are large with respect to the surface normal. At alamethicin concentrations ranging from 10 to 20 mol%, the tilt angles of the helices decrease to lower values signifying that the alamethicin molecules are inserting into the tBLM. Consequently, the membrane resistivity is also lowered indicating that the peptides are forming ion conductive channels. The results show an excellent correlation between molecular information concerning the orientation and conformation of alamethicin molecules obtained from the PM-IRRAS measurements and electrochemical properties of the membranes measured by EIS. These findings reveal that the alamethicin concentration within the film is directly related to the ion conductance across membrane.