Physical properties and surface activity of surfactant-like membranes containing the cationic and hydrophobic peptide KL4

ABSTRACT

Surfactant-like membranes containing the 21-residue peptide KLLLLKLLLLKLLLLKLLLLK (KL₄), have been clinically tested as a therapeutic agent for respiratory distress syndrome in premature infants. The aims of this study were to investigate the interactions between the KL₄ peptide and lipid bilayers, and the role of both the lipid composition and KL₄ structure on the surface adsorption activity of KL₄-containing membranes. We used bilayers of three-component systems [1,2-dipalmitoyl-phosphatidylcholine/1-palmitoyl-2-oleoyl-phosphatidylglycerol/palmitic acid (DPPC/POPG/PA) and DPPC/1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC)/PA] and binary lipid mixtures of DPPC/POPG and DPPC/PA to examine the specific interaction of KL₄ with POPG and PA. We found that, at low peptide concentrations, KL₄ adopted a predominantly α-helical secondary structure in POPG- or POPC-containing membranes, and a β-sheet structure in DPPC/PA vesicles. As the concentration of the peptide increased, KL₄ interconverted to a β-sheet structure in DPPC/POPG/PA or DPPC/POPC/PA vesicles. Ca²⁺ favored α⇆β interconversion. This conformational flexibility of KL₄ did not influence the surface adsorption activity of KL₄-containing vesicles. KL₄ showed a concentration-dependent ordering effect on POPG- and POPC-containing membranes, which could be linked to its surface activity. In addition, we found that the physical state of the membrane had a critical role in the surface adsorption process. Our results indicate that the most rapid surface adsorption takes place with vesicles showing well-defined solid/fluid phase co-existence at temperatures below their gel to fluid phase transition temperature, such as those of DPPC/POPG/PA and DPPC/POPC/PA. In contrast, more fluid (DPPC/POPG) or excessively rigid (DPPC/PA) KL₄-containing membranes fail in their ability to adsorb rapidly onto and spread at the air–water interface.