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Biophys J, October 1999, p. 2304-2310, Vol. 77, No. 4

Nanometer Scale Organization of Mixed Surfactin/Phosphatidylcholine Monolayers

Magali Deleu,* Michel Paquot,* Philippe Jacques,# Philippe Thonart,§ Yasmine Adriaensen, and Yves F. Dufrêne

 *Unité de Chimie Biologique Industrielle and  #Unité de Bioindustrie, Faculté Universitaire des Sciences Agronomiques de Gembloux, Passage des Déportés, 2, B-5030 Gembloux;  §Centre Wallon de Biologie Industrielle, Université de Liège, 4000 Liège; and  Unité de Chimie des Interfaces, Université catholique de Louvain, Place Croix du Sud 2/18, 1348 Louvain-la-Neuve, Belgium

Mixed monolayers of the surface-active lipopeptide surfactin-C15 and of dipalmitoyl phosphatidylcholine (DPPC) were deposited on mica and their nanometer scale organization was investigated using atomic force microscopy (AFM) and x-ray photoelectron spectroscopy (XPS). AFM topographic images revealed phase separation for mixed monolayers prepared at 0.1, 0.25, and 0.5 surfactin molar ratios. This was in agreement with the monolayer properties at the air-water interface indicating a tendency of the two compounds to form bidimensional domains in the mixed systems. The step height measured between the surfactin and the DPPC domains was 1.2 ± 0.1 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. The N/C atom concentration ratios obtained by XPS for pure monolayers were compatible 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. These results illustrate the complementarity of AFM and XPS to directly probe the molecular organization of multicomponent monolayers.

Biophys J, October 1999, p. 2304-2310, Vol. 77, No. 4
© 1999 by the Biophysical Society   0006-3495/99/10/2304/07  $2.00


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