Micellar Structure And Aggregation Behavior Of Surfactin In Aqueous Solution

Surfactin is an efficient biosurfactant excreted by different strains of Bacillus subtilis and it has a broad application in oil exploitation, environment recovery, pharmaceutics and food as well as cosmetics industry. It has limited research in the aspects of the structure of aggregates and aggregation behavior due to the special structure of the surfactin. Our study provides a molecular view on the micellar structure and the aggregation behavior of surfactin in aqueous solution by molecular dynamic simulation, the mainly parts in this dissertation are:The difference of the inner structures of the micelles constructed of unionized iso-C16and n-C15surfactins was focused. They differ in distribution of the hydrocarbon tails in the micelles and the hydrocarbon tails of iso-C16surfactin have higher rigidity. The branched carbon group partly limited the movement of the tail.The temperature dependence of the structure and the interfacial properties of unionized surfactin micelles have been studied. The overall size and shape, the surface area, the radial density distribution of the micelles, the conformation of the hydrocarbon chain, and the intramolecular/intermolecular hydrogen bonds formed in surfactin molecules were investigated. The micelles were mostly in sphere shapes and the radius of surfactin micelle was found to be around2.20±0.01nm; the peptide rings occupied most of the surface of the micelles, and they seldom adopted horse-saddle conformation; when temperature increased, the solvent accessible surface area decreased, the shape of the peptide rings became more plane-flatted and circular and some surfactin molecules in the micelle changed their molecular orientation; in addition, the stability of the hydrogen bond interactions in the micelles decreased with the increasing temperature.The behaviors of the interactions between surfactin molecules and hydration water at four temperatures were also examined. The hydration numbers and the hydrogen bonding properties of the surfactin micelles were measured and the detailed analyses of the lifetime dynamics of surfactin-water (SW) hydrogen bond have been carried out. It shows that temperature dehydration occurred at the interfaces of the micelles with the temperature increasing and the number of the hydration water decreased from340.5±13.0at277K to245.0=12.0at343K; the average hydrogen bond life decreased with the temperature increasing; the average activation energy (EA) of the SW hydrogen bonds ranges from5.35±0.11kJ mol-1in a temperature range from277to343K.The effect of the counterion (Na+, Ca+) on the preassembled sphere micelle has been investigated as well. It shows that the shape of the aggregates became irregular. The ion environment had great influence on the structure of the peptide ring, the peptide ring adopted planes of cycle rings instead of the horse-saddle conformations. The peptide ring had greater deformation in Na+aqeous solution. The ionized surfactin had more hydration water, and the hydration water more strongly interacted with the ionized surfactin than that with the unionized surfactin.Spontaneous aggregation of iso-C16surfactins of1.50×10’4M in homogenous aqueous solution above critical micellar concentration has been conducted by a300ns molecular dynamic simulation. The iso-C16surfactin molecule was transformed and modeled by coarse-grained model. Then, the kinetics of the aggregation process is analyzed. The process of aggregation of surfactin molecules proceeded and the disassociation of the surfactin aggregate simultaneously happened at the same time and there exists single surfactin molecule in the aqueous solution all the time. The sizes of the aggregates are small and the biggest aggregation number is7.In order to obtain deuterated amino acids which could replace the amino acid residues in the surfactin molecule, glutamic acid as the target amino acid was deuterated in acidic conditions. The experiment of kinetics of deuteration in acidic condition was conducted and the kinetic equations of deuteration of y-H were established and the value of Ea of both a and y were acquired.