AVS 47th International Symposium
    Biomaterial Interfaces Monday Sessions
       Session BI+MC-MoA

Paper BI+MC-MoA7
Probing the Spatial Organization of Mixed Lipopeptide/Phospholipid Monolayers : Complementarity of AFM and XPS

Monday, October 2, 2000, 4:00 pm, Room 202

Session: Characterization of Biomaterial Interfaces
Presenter: Y.F. Dufrene, Universite Catholique de Louvain, Belgium
Authors: Y.F. Dufrene, Universite Catholique de Louvain, Belgium
M. Deleu, Faculte Universitaire des Sciences Agronomiques de Gembloux, Belgium
P. Jacques, Faculte Universitaire des Sciences Agronomiques de Gembloux, Belgium
P. Thonart, Centre Wallon de Biologie Industrielle, Belgium
M. Paquot, Faculte Universitaire des Sciences Agronomiques de Gembloux, Belgium
Correspondent: Click to Email
Surfactin is a surface-active bacterial lipopeptide, with important biological properties, which is known to interact with lipid membranes. To gain insight into the spatial organization (miscibility, molecular orientation) of mixed surfactin/dipalmitoyl phosphatidylcholine (DPPC) monolayers, the morphology and chemical composition of mixed monolayers transferred on mica were determined by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS), respectively. AFM topographic and friction images revealed phase-separation for mixed monolayers prepared at 0.1, 0.25 and 0.5 surfactin molar ratios. The step height measured between the surfactin and the DPPC domains was about 1.2 nm, pointing to a difference in molecular orientation: while DPPC had a vertical orientation, the large peptide ring of surfactin was lying on the mica surface. These data were in excellent agreement with the monolayer properties at the air-water interface and with computer simulation data. The N/C atom concentration ratios obtained by XPS for pure monolayers were consistent with two distinct geometric models: a random layer for surfactin and for DPPC, a layer of vertically-oriented molecules in which the polar headgroups are in contact with mica. XPS data for mixed systems were accounted for by a combination of the two pure monolayers, considering respective surface coverages that were in excellent agreement with those measured by AFM. Finally, exciting new possibilities offered by dynamic AFM imaging modes (force modulation, phase imaging) to investigate the film nanomechanical properties will be presented. This work demonstrates the complementarity of AFM imaging and XPS analysis to directly probe the molecular organization of multicomponent monolayers.