Interfacial assembly of cationic peptidesurfactants

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 Å inner layer containing about 50% peptide close to the oxide surface and a loose 40 Å 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.