Cooperative tuneable interactions between a designed peptide biosurfactant and positional isomers of SDOBS at the air - Water interface

Rationally designed peptide biosurfactant AMI was mi×ed with sodium dodecyl benzene sulfonate (SDOBS) to self-assemble a mi×ed surfactant-biosurfactant layer at the air-water interface. Under optimal conditions in the presence of Zn²⁺, the interfacial elasticity of the mi×ed layer was appro×imately 5-fold higher than for biosurfactant alone. Two head positional isomers, SDOBS-2 and SDOBS-6, were compared for their ability to enhance interfacial film strength. SDOBS-6 forms a stronger layer with AMI than does SDOBS-2. The highest interfacial elasticity of the AM1/SDOBS-6 layer was 640 mN m^-1 whereas the ma×imum value for the AM1/SDOBS-2 layer was 440 mN m^-1. Neutron reflection was used to investigate the structure of AM1/SDOBS films at varied bulk SDOBS concentrations. Both deuterated and nondeuterated SDOBS-2 and SDOBS-6 were used to provide contrast variation. It was shown that there is cooperative interaction between AM 1 and SDOBS at low SDOBS concentration in the presence of 100 μM Zn²⁺, promoting AM 1 adsorption at the interface to form a two-layered structure of AM 1 resulting in a mechanically strong interfacial film. In the presence of EDTA, only a single AM 1 layer was formed at the same SDOBS concentration, and the film did not show lateral force transmission capability. Further increasing the SDOBS concentration to a molar e×cess of > 10 × decreased the peptide population at the interface and resulted in a mechanically weak layer. Compared to SDOBS-6, SDOBS-2 depletes AM 1 at a lower bulk concentration. These results demonstrate that the film strength of a self-assembled surfactant - biosurfactant mi×ed layer can be fine tuned by changing the isomer type and concentration of surfactant and by adding or removing metal ions.