Effect of chain length on the structure of aqueous surfactin solutions: Molecular dynamics studies

Surfactin, a highly effective biosurfactant synthesized by various strains of Bacillus subtilis, exhibits remarkable biological and physicochemical properties. Molecular dynamics simulations highlight the influence of hydrocarbon chain length on the micellar structure, aggregation behavior, hydrogen bonding patterns, and secondary structure of surfactin molecules. The findings reveal that the aggregates adopt an increasingly spherical shape as the hydrocarbon chain length increases. The aggregation number of surfactin molecules also increases with longer chain lengths, indicating enhanced clustering tendencies. In the peptide ring, the occurrence of γ-turn, identified by the intramolecular hydrogen bond between Leu2 - Val4 residues, decreases with increasing chain length. The β-turn, identified by hydrogen bond between Leu2-Asp5 does not exhibit a clear trend, though the maximum occurrence probability is observed in systems with surfactin having longer alkyl tail. The surface tension of surfactin solutions increases with chain length due to lower coverage of the surface in case of surfactins with longer tails due to the tilted orientation of their hydrophobic tails, which limits efficient packing.