Tuning micelles of a bioactive heptapeptide biosurfactant via extrinsically induced conformational transition of surfactin assembly

We have studied the effects of extrinsic environmental conditions on the conformation of surfactin, a heptapeptide biosurfactant from Bacillus subtilis , in aqueous solutions. It has been made clear that temperature, pH, Ca 2+ ions and the synthetic nonionic surfactant hepta‐ethylene glycol (C 12 E 7 ) affect the conformation of surfactin in aqueous solutions. The β ‐sheet formation reached a maximum at 40°C both in presence and absence of (C 12 E 7 ) and the nonionic surfactant enhances the β ‐sheet formation even at 25°C. Ca 2+ induced the formation of a‐helices and caused this transition at 0.3 m m with surfactin monomers or at 0.5 m m with surfactin micelles, but above these transition concentrations of Ca 2+ β ‐sheets were observed. In micellar solution the β ‐sheet structure was stabilized at pH values below 7 or upon addition of Ca 2+ in concentrations above 0.5 m m . Our results indicated that the bioactive conformation of surfactin is most likely the β ‐sheets when the molecules are assembled in micelles. The β ‐sheet structure in micelles could be retained by tuning the micelles. Surfactin micelles could be tuned in the bioactive conformation by manipulating pH, temperature, Ca 2+ or (C 12 E 7 ) concentrations in surfactin solutions. Our results strongly indicated that Ca 2+ and other molecules (such as C 12 E 7 ) may function as directing templates in the assembly and conformation of surfactin in micelles. Thus, we suggest environmental manipulation and template‐aided micellation (TAM) as a new approach for preparing predesigned micelles, microemulsions or micro‐spheres for specific application purposes. © 1998 European Peptide Society and John Wiley & Sons, Ltd.