Plasmon-Waveguide Resonance and Impedance Spectroscopy Studies of the Interaction between Penetratin and Supported Lipid Bilayer Membranes

The interaction between the cell-penetrating peptide, penetratin, and solid-supported lipid bilayer membranes consisting of either egg phosphatidylcholine (PC) or a 75/25 mol% mixture of egg PC and palmitoyloleylphosphatidylglycerol has been studied by simultaneously measuring plasmon-waveguide resonance (PWR) spectra and impedance spectra of lipid-peptide mixtures. When penetratin was incorporated into an egg PC + palmitoyloleylphosphatidylglycerol bilayer, PWR measurements showed a hyperbolic increase in the average refractive index and the refractive index anisotropy, with no change in membrane thickness, over a concentration range between 0 and 2 μM peptide. In the case of an egg PC bilayer, a biphasic dependence was observed, with a decrease in average refractive index and anisotropy and no thickness change occurring between 0 and 5 μM peptide, and an increase in membrane thickness occurring between 5 and 15 μM peptide with no further change in the refractive index parameters. For both membranes, the impedance spectroscopy measurements demonstrated that the electrical resistance was not altered by peptide incorporation, whereas a decrease in membrane capacitance occurred with the same concentration dependence as observed in the PWR experiments, although for the PC membrane no further changes in electrical properties were observed in the higher concentration range. A structural interpretation of these results is described, in which the peptide binds electrostatically within the headgroup region of the bilayer and influences the headgroup conformation, amount of bound water, and the lipid-packing density, without perturbing the hydrocarbon core of the bilayer.