Interfacial properties of surfactin

Abstract

Surfactin is a lipopeptide produced by various strains of Bacillus subtilis and is a very powerful surfactant. Here we present the first report on the interfacial behavior of surfactin. The adsorption, Γ max, of surfactin at the interface of diluted solutions (5 × 10 −8 to 5 × 10 −7 M) is around 3 × 10 18 molecule m −2, a value indicating that surfactin molecules are in a very packed situation. Surfactin spreads readily at the air water interface: the equilibrium spreading pressure π e ≈ 30 mN m −1 reaches 45 mN m −1 when electrolytes (KCl or CaCl 2) are dissolved in the alkaline subphase. We have plotted the compression isotherm curves and determined surface parameters. These parameters vary only a little with temperature but are very affected by the pH of the subphase. Plotting the transition pressure value π t as a function of pH results in a titration curve from which one can deduce the p K value of surfactin at the interface. This value, p K s ≈ 6 is around 2 pH units higher than the p K of surfactin in solution. The addition of electrolytes ( I = 0.15 M) in the alkaline subphase leads to the neutralization of the surfactin monolayer (protonation of the acidic residues LGlu 1 and LAsp 5 of the peptide cycle). This neutralization is complete in the case of Ca ++ ions but not in the case of monovalent cations (Na + or K +). When surfactin monolayers are subjected to successive compression—expansion cycles we observe a reproducible hysteresis loop. The surface parameters of surfactin are compared to those of iturins, lipopeptides also extracted from B. subtilis, the structure and properties of which are very similar.