Interaction of Indolicidin with Model Lipid Bilayer: Quartz Crystal Microbalance and Atomic Force Microscopy Study

Ever since a tridecapeptide amide, indolicidin, was isolated by Selsted et al.¹ from the cytoplasmic granules of bovine neutrophils, its biological activity has been studied extensively.^2-4 The primary structure of indolicidin is H-Ile-Leu-Pro-Trp-Lys-Trp-Pro-Trp-Trp-Pro-Trp-Arg-Arg-NH². Indolicidin is known to possess antimicrobial activity,5,6 but its detailedmechanismis not yet understood clearly. The mode of action may be similar to that of other small peptides such as magainins, cecropins, and melittin. For the latter peptides, the antibiotic activity has been claimed to be due to the formation of ion channels or pores in the lipid membranes.^7-9 On this ground, we have recently examined the interaction of indolicidin with model lipid bilayers using Fourier transform infrared attenuated total reflection (FTIR-ATR) spectroscopy.¹⁰ On the basis of the results of cyclic voltammetry (CV) measurements, indolicidin molecules were found to be incorporated inside the acyl chains of the lipid bilayers upon coming into contact with Langmuir-Blodgett (LB) lipid bilayers. In accordance with the circular dichroism (CD) measurements, it was also concluded that indolicidin, consisting of an ill-defined random coil structure in an aqueous medium, adopts a 3₁₀-helical conformation in LB lipid bilayers. It was very intriguing, however, that the perturbation of C-H stretching bands of model lipid molecules was not substantial even though indolicidin was incorporated clearly inside the lipid bilayers.We have performed in this work quartz crystal microbalance(QCM) and atomic force microscopy (AFM) studies, hoping to reveal how many indolicidin molecules are incorporated inside the lipid bilayer and to know the size of the aggregate that indolicidin molecules form upon incorporation into the lipid bilayer.