Interfacial assembly of cationic peptide surfactants

Abstract

Short surfactant-like peptides bearing the molecular architecture of hydrophilic and hydrophobic moieties are attractive for a wide range of biological and technological applications. However, to realize the benefits offered by them we need to understand the basic physiochemical properties arising from their interfacial self-assembly and solution aggregation. In this paper, we use a combined approach of spectroscopic ellipsometry (SE), atomic force microscopy (AFM) and neutron reflection (NR) to characterize the interfacial assembly of cationic peptide V6K2 at the hydrophilic silica–water interface. The SE measurement revealed that the peptide dynamic adsorption was characterised by a fast initial process within the first 3–5 min, followed by a slow molecular reorientation within the next 30–40 min. AFM imaging revealed the formation of a dense peptide layer incorporating defects and some large vesicles. This interfacial structural feature from AFM offered a useful starting point for fitting the neutron reflectivity profiles measured. At 0.05 wt% V6K2, the reflectivity profiles were well fitted using a two-layer model with a dense 40 A inner layer containing about 50% peptide close to the oxide surface and a loose 40 A outer layer containing some 8% peptide on the solution side. With the help of partial deuteration to the peptide (hV6hK2 and dV6hK2 and the solvent isotopic contrasts (D2O, H2O), we found that the densely packed peptide region was comprised of a sandwiched peptide bilayer with their hydrophobic tails (V6) attracted to each other and the cationic head groups (K2) projected towards the oxide surface and the bulk water. This peptide bilayer structure is similar to that formed by conventional cationic surfactants when adsorbed at the same anionic SiO2–water interface, indicating the dominant effect of hydrophobic interaction. This study has demonstrated that the combined measurements provide a useful account of structure and dynamics of interfacial peptide self-assembly.