Interfacial Films Formed by a Biosurfactant Modularized with a Silken Tail

Abstract

This paper reports the dynamic interfacial behavior of a new interfacially active peptide AM-S, which was designed based on a peptide surfactant AM1 modularized with an additional silk-derived hydrophobic tail to enhance anchoring to air–water interfaces. AM-S peptide shows a random coil conformation in bulk solution similar to AM1 as determined by circular dichroism spectroscopy, which facilitates rapid adsorption at the air–water interface, reducing interfacial tension from 72 to 52 mN/m within 300 s at a low concentration of 10 μM. Although the interfacial films formed by AM-S demonstrated low tensile stress as compared to AM1, the AM-S films in the presence of Zn(II), but not in its absence, show significant resistance against compression, as peptides were unable to desorb quickly under the compression conditions imposed by the Cambridge interfacial tensiometer (CIT). These results indicate that AM-S peptides tend to undergo a multilayer adsorption at the interfaces, in contrast to AM1 peptide that only forms an interfacial monolayer, demonstrating a distinct physical effect of the silk tail. The multilayer structure of AM-S in the presence of Zn(II) was also apparent on a thin-film pressure balance experiment. The thin films formed by AM-S peptide were thicker than the films formed by AM1 peptide, thus enabling stabilization of the films against increased critical air pressure as well as the self-assembly of the AM-S peptides. Also, AM-S peptide was shown to be capable of forming dense foams with small bubble size and maintained foam stability comparable to AM1 peptide. This study demonstrated that addition of a silk tail peptide to a biosurfactant can significantly modify interfacial adsorption behavior at a fluid–fluid interface, which may guide further molecular design strategies.