Design of biosurfactant proteins with pH-sensitive reversible surface-active property and their excellent foam and emulsion stability.

Biosurfactant proteins obtained through microbial fermentation have garnered increasing attention as their strong sequence designability and cost-effectiveness. A series of novel biosurfactant proteins (DL21P4, DAFP4 and DAFSP4) as pH-responsive surfactants was designed by a seven-residue repeating motif peptide Lac21 with α-helical structure. These proteins exhibit a conformation composed of α-helices and random coils, which facilitates their stable adsorption at the interface. Compared with DL21P4 and DAFP4, DAFSP4 with special silk-derived hydrophobic tail peptide tends to undergo multilayer adsorption at the interface, and had excellent foam and emulsion stability. The stability of DAFP4 and DAFSP4 on foams and emulsions was significantly enhanced by the addition of Zn2+, the foam half-lives of DAFP4 and DAFSP4 increased by 3.55 times and 3.71 times, while DAFP4 and DAFSP4 showed an increase in ESI of 2.02-fold and 1.68-fold, respectively. These results indicated that laterally intramolecular and intermolecular cross-linking between histidine residues through Zn2+ in DAFP4 and DAFSP4 enhanced the compactness and mechanical strength of the interfacial film, thereby effectively suppressing foam rupture and coalescence of emulsion droplets. In addition, differences in electrostatic repulsion under varying pH conditions imparted the three biosurfactant proteins with pH-sensitive reversible property. The reversible transformation of foams and emulsions can be achieved by adjusting the pH. This research was expected to provide theoretical references and application basis for the development of biosurfactant proteins with high surface performance and intelligent response.