Influence of environmental conditions on the interfacial organisation of fengycin, a bioactive lipopeptide produced by Bacillus subtilis.

The effect of the environmental conditions both on the behaviour of fengycin at the air-aqueous interface and on its interaction with DPPC was studied using surface pressure-area isotherms and AFM. The ionisation state of fengycin is at the origin of its monolayer interfacial properties. The most organised interfacial arrangement is obtained when fengycin behaves as if having zero net charge (pH 2). In a fully ionised state (pH 7.4), the organisation and the stability of fengycin monolayers depend on the ionic strength in the subphase. This can modulate the surface potential of fengycin and consequently the electrostatic repulsions inside the interfacial monolayer, as well as the lipopeptide interaction with the layer of water molecules forming the air-water interface. Intermolecular interactions of fengycin with DPPC are also strongly affected by the ionisation state of lipopeptide and the surface pressure (Pi) of the monolayer. A better miscibility between both interfacial components is observed at pH 2, while negatively charged lipopeptide molecules are segregated from the DPPC phase. A progressive desorption of fengycin from the interface is observed at pH 7.4 when Pi increases while at pH 2, fengycin desorption brutally occurs when Pi rises above Pi value of the intermediate plateau.