The penetratin peptide, a 16 amino acid sequence extracted from Antennapedia homeodomain, is able to translocate across a neural cell membrane through an unknown mechanism, most likely a non-specific interaction with membrane lipids. Beyond its potential application as vector targeting small hydrophilic molecules and enabling them to reach a cell nucleus, this observation raises intriguing questions concerning the physico-chemistry of peptide-lipid interactions. Here we present a study of the role of lipid surface pressure and head charge on the mechanism of interaction. This was performed using optical techniques: surface infrared spectroscopy and ellipsometry, applied to a monolayer of phospholipids deposited at the air-water interface. Determination of the structure and orientation of peptides and lipids (separately or together) evidenced that electrostatic rather than amphiphilic interactions determine the peptide adsorption and its action on lipids.
Polarization-modulated infrared reflection-absorption spectroscopy (PM-IRRAS), ellipsometry, and shear elastic constant measurements were used to study the adsorption and the behavior of ovalbumin and S-ovalbumin at the air-water interface at different values of the subphase pH. Native and S-ovalbumin exhibited similar behaviors, with a maximum plateau value of the shear elastic constant near the isoelectric pH of the protein. However, higher surface concentration values were reached with S-ovalbumin in low net charge conditions, which suggest adsorption of aggregates or multilayer adsorption. For both proteins, the statistical analysis of PM-IRRAS spectra demonstrated that the aging of the interfacial film and the increase of the shear elastic constant were correlated with a significant increase in the relative contribution of intermolecular β-sheets in the amide I band with time. This increase was significantly faster at low pH values. At the same pH value and age of the interface, the relative contribution of intermolecular β-sheets was significantly higher for S-ovalbumin.
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