Electron cryo-microscopic studies on micellar shape and size of surfactin, an anionic lipopeptide

Abstract Micelles of surfactin, a biosurfactant, in water, were visualized by an ice-embedding technique and transmission electron cryo-microscopy, cooling down the specimen to the temperature of liquid helium. Cryo-micrographs revealed spherical, ellipsoidal and/or cylindrical micelles with an inhomogeneous size distribution at pH 7, 9.5, and 12. The appearance at low pH and disappearance at pH 12 of a characteristic FT-IR band of a lactone group in surfactin suggested that the micelles are formed differently in structures with cyclic and linear head groups at these respective pH values. At pH 9.5, the addition of 100 mM NaCl and 20 mM CaCl 2 to a surfactin solution transformed the cylindrical micelles into spherical and/or ellipsoidal micelles of small size. This small size was explained by a common understanding model with a high or low degree of internal order in the structure of alkyl chains. The large size of the spherical, ellipsoidal, and cylindrical micelles was explained in terms of bilayered unilamellar or multilamellar structures such as vesicles.