Surfactin and fengycin A properties at liquid/liquid interfaces and in model emulsions | Proprietes de la surfactine et de la fengycine A aux interfaces liquide/liquide et dans des emulsions modeles

The surface-active properties of two lipopeptide classes from Bacillus subtilis S499 (surfactins and fengycins A) were examined at liquid/liquid interfaces and in model emulsions The study of structure-function relationships was carried out. For the surfactins, the interfacial organisation and conformation were also investigated. The dynamic adsorption properties of lipopeptides at water/apolar solvent interface are close to those of low molecular weight surfactants. Their monomolecular films formed at the water/n-dodecan interface have a dilatational elasticity higher than the classical surfactants and close to the flexible proteins. However, at the macroscopic level, the emulsifying activity and the short-term stabilizing properties of lipopeptides are not more effective than those of the sodium dodecylsulfate. The behaviour of lipopeptides at liquid/liquid interface is mainly related to the nature of the peptide ring. A more hydrophobic character (like surfactins) is efficient to reduce the meso-equilibrium interfacial tension, to form smaller droplets and to better stabilize the emulsions in the short term. The lipid chain structure influences slightly the adsorption properties. Lipopeptides with a shorter chain adsorb faster at the interface but those with a longer chain reduce the meso-equilibrium interfacial tension to a lower value. At the macroscopic level, the influence of the lipid chain is not pronounced. All of these properties are strongly influenced by the NaCl presence. The AFM, the XPS, the ATR-IR, the film balance and the molecular modelisation techniques were used to examine the conformation of the surfactins at interfaces. Most of them show that for low compression conditions the peptide ring of the surfactin is lying at the interface and that the lipid chain has a disorganized structure. According to the molecular modelisation, the ring forms a bowl structure with the two acidic residues protruded in the hydrophilic medium. In the models, the most energetic stable conformation presents a folding of the lipid chain on the peptide ring. The ATR-IR technique shows the existence of a beta-turn in the peptide ring. In the models, the orientation of the three homologous molecules at the interface is similar. However, according to the ATR-IR spectra, the secondary structure of the peptide ring is different between the C13 and the C14 and C15 homologous. A critical review of the results is presented. The relationships between models and the fundamental interfacial and emulsifying properties are discussed.