Study On The Interaction Between Surfactin And Biomacromolecules

Surfactin is a kind of lipopeptide containing seven amino acid residues and aβ-hydroxy fatty acid residues, which has excellent surface properties and biological activities. Compared with the chemical surfactants, surfactin has some unique advantages such as environment friendly, higher biodegradability. Surfactin isoform (surfactin-C15) with aβ-hydroxy fatty acid chain of 15 carbon atoms is the primary ingredient in surfactin variants. In this paper the aggregation activity of surfactin-C15 and the interaction between surfactin-C15 and biomacromolecule were investigated. The results include as follows.In the first section, surface tension, small-angle neutron scattering (SANS), freeze-fracture transmission electron microscopy (FF-TEM) and circular dichroism(CD) measurements have been used to study the self-aggregation properties of surfactin-C 15 in 0.01 M (pH 7.4) phosphorate buffer solution (PBS). It has been found that critical micelle concentration (CMC) is 1.5×10-5 M, the surface tension is 27.7 mN/m and the area per molecule at air-water interface is 107.8 A2 for surfactin-C 15. surfactin-C15 molecules adopt aβ-sheet conformation making it Surface-active at such low concentrations. From SANS and FF-TEM results it is shown that surfactin-C 15 exhibits strong self-assembly ability to form spherical micelles and some larger aggregates even at the rare low concentration. The aggregation number of spherical micelles (<20) is much smaller than that of conventional surfactants with similar alkyl chain length.In the second section, we studied the interaction of surfactin-C 15 with several representative proteins in PBS solutions by UV-vis spectra, fluorescence spectra, SANS, AFM, FF-TEM and DLS measurements. It is found that the interactions between surfactin-C 15 and different protein species are mainly due to the electrostatic attraction, hydrophobic interaction and hydrogen bonds, which effected by the different structure and charge of protein and the solution environment (pH). With the increase of surfactin-C 15 concentration, the interactions between surfactin-C 15 and different protein lead to the unfolding of protein and take on a'necklace model'microstructure. Additionally, surfactin-C 15 can change the secondary combined with the decrease in a-helix content. Compared to the traditional surfactant, surfactin-C 15 has the gentle disruption effect on the structure of protein and presents different physicochemical behaviors.In the third section:surfactin-C 15 effect on Phosphatidylcholine (PC) liposome (model membrane) was studied by fluorescence, atomic force microscopy (AFM), FF-TEM and dynamic light scattering measurement (DLS). DLS results showed that the hydrodynamic diameter of PC liposome decreased with the surfactin-C 15 addition, which was verified by the decrease of transmittance and micropolarity of this system. These results were because of the microstructure change of PC liposome to PC/surfactin-C 15 mixed micelles. And it is interesting that the dynamic change of PC liposome to PC/surfactin-C15 mixed micelle was recorded by DLS and was further conformed by FF-TEM images. In sum, the process of PC liposome to PC/surfactin-C 15 micelle is the solubilization of PC liposome and the reorganization of PC/surfactin-C 15 aggregates.In the forth section:Aggregation properties and distribution behavior of toluidine blue (TB) in surfactin-C 15 solution have been investigated. It was detected by UV-vis spectra and fluorescence probe that TB can be located in the palisade of surfactin-C 15 micelle, which facilitates the formation of TB aggregates. This is because the dimethylamino group in TB molecule is prone to interact with the two negatively charged amino acid of surfactin-C 15 molecule. The binding constants of TB with surfactin-C 15 micelles and the distribution coefficients between the micelles phase and the aqueous phase have been calculated by Benesi-Hildebrand method and phase separation model, respectively. The values of thermodynamics functions show that electrostatic attractive interaction plays a dominating role at the location of TB in surfactin-C 15 micelles.