Micellization Activity Of Surfactin In Aqueous Solution

Surfactin is a kind of lipopeptide produced basically by microorganisms. Surfactin has excellent surface and biological activities, which can be used in scientific and industrial fields such as pharmaceutics, foods, cosmetics and environment remediation.In this paper, the samples were originally obtained from the cell free broth of Bacillus subtilis HSO121 in our laboratory. The surfactins were isolated and purified by reversed-phase high performance liquid chromatography (HPLC). The structural analysis indicated that the isolated surfactins include surfactin-C12, surfactin-C13, surfactin-C14, surfactin-C15 and surfactin-C16. And the micellization activities of these surfactins were studied.The micellization activities of surfactin-C16 isoform were systematic investigated by fluorescence, dynamic light-scattering (DLS) and freeze-fracture transmission electron microscopy (FF-TEM) and surface tension measurements. The critical micelle concentration (CMC) of surfactin-C16 was about 6×10-6 mol/L, which was determined by surface tension and fluorescence measurements. The adsorption parameters of surfactin-C16 at the air-aqueous solution interface were calculated from the surface tension data, and the critical packing parameter (CPPs) indicated that surfactin-C16 tended to form spherical micelles. The micelle size distribution and micelle morphology of surfactin-C16 in aqueous solution were determined by DLS and FF-TEM. In terms of the relative scattered volume, the distribution of micelle size was bimodal, with one peak at around 100 nm and another peak between 100 nm and hundreds nm. FF-TEM photos further confirmed that the surfactin-C16 micelles coexisted with some large aggregations.The effect of counterions (Li+, Na+, K+, Ca2+, Mg2+ and Al3+) on micellization behaviors of surfactin-C16 was further researched. The results indicated that counterions enhanced the surface activity of surfactin-C16 and reduced its CMC because of the electrostatic repulsions screening from counterions. The high valence ions effect the CMC values more than the low valence ones. The addition of univalent concentrations reduced micelle micropolarity, increased the microviscosity, and tended to form small and spherical micelle particles. Divalent counterions had a special effect on surfactin-C16 micellization activity, so with low concentration of divalent cations, they had strong interaction with surfactin-C16 micelles. There are not only small micelles but also large layered aggregates in surfactin-C16 solution.The micellization activities of a group surfactin isforms with different fatty acids chain produced by Bacillus subtilis HSO121 was studies. The results indicated that the surface and interfacial activities of surfactins increased with the increase of chain length. For theβ-sheet structure of surfactin micelle, the short fatty acid chain surfactin tended to form small micelles, and surfactin tended to form larger micelle aggregates with the increase of chain length.