Molecular behavior of a microbial lipopeptide monolayer at the air–water interface

The monolayer of a microbial lipopeptide has been studied as functions of subphase temperature and pH with a Langmuir–Blodgett (LB) film balance. The surface pressure–area (π–A) isotherm and the surface compressibility–area (Cs–A) curve indicate that the lipopeptide monolayer exists in four different physical states classified as: liquid-expanded (LE) film, liquid-condensed (LC) film, LE–LC transition and a plateau region. Physical states of the monolayer are greatly influenced by subphase temperature and pH. Significantly, these two factors exert influences on physical states in different ways. The LE film and LE–LC transition are much affected by subphase temperature, while the shapes of the plateau region are mainly influenced by subphase pH. LE film turns more condensed and LE–LC transition becomes clearly evident when temperature increased from 5 to 15 °C. The plateau region becomes less horizontal and begins at lower pressure when subphase pH increased from 2 to 10. The variation of the effects may be related to that subphase temperature alters the mobility of fatty acid chains and subphase pH impacts the ionization of peptide loop. The lipopeptide films transferred from air–water interface onto mica substrate are also investigated with atomic force microscopy (AFM). AFM images provide direct evidence that the lipopeptide molecules take up distinct organizations at different physical states, and the plateau region in π–A isotherm is a phase transition from monolayer to multilayer on acidic subphase.