The location of the biosurfactant surfactin in phospholipid bilayers supported on silica using neutron reflectometry

We have investigated the formation of supported surfactin-phospholipid mixed bilayers using neutron reflectometry. Micellar mixtures of phospholipid (diphosphatidyl choline, DPPC), surfactin, and beta-d-dodecyl maltoside were used to make the deposition. When the surfactin concentration is at its critical micelle concentration (CMC = 6 x 10(-6) M) in the bulk solution, there is no adsorption at all on the silica. When the surfactin concentration is lowered below the CMC, a mixed bilayer of surfactin and DPPC is formed. Since surfactin does not adsorb on silica from solutions of surfactin alone, this shows that there is a strong attraction between surfactin and DPPC. The variation of adsorbed amount, composition, and structure of the adsorbed layer are consistent with the attractive interaction between surfactin and DPPC and with their respective negative and positive affinities for the silica surface. Three phospholipid isotopic contrasts were measured and used to define the composition and structure of the surfactin-phospholipid bilayer. The maximum amount of surfactin in the bilayer reaches a mole fraction of about 0.2 and this is located in the outer leaflet of the bilayer within the headgroup and part of the adjacent chain region.