Amphiphilic properties of bacterial lipopeptides: self-association and monolayer studies of iturins

Abstract Iturins are a special class of bacterial lipopeptides with antifungal activity. They are amphiphilic compounds with a heptapeptidic cycle linked to a C 10 , C 12 alkyl chain. We have studied the self-association in aqueous solution of iturin A and its O -methyltyrosine derivative that is biologically inactive. We have determined the concentration at which aggregates appear by using diphenylhexatriene as a fluorescent probe or by following the intrinsic fluorescence of the tyrosyl residue. The methylated derivative aggregated at a concentration 10 times lower than iturin A. Iturins formed monolayers at the air—water interface, isotherms of which have been studied. The monolayers of iturin A were not very stable. The isotherm curve presented a transition region that was interpreted as the passage to a tridimensional structure. The monolayers of O -methyltyrosine iturin A were more stable: the equilibrium spreading pressure π c was 27 m N m −1 as opposed to 9 m N m −1 for iturin A. For both compounds, compression—expansion cycles resulted in nonreproducible hysteresis.