Nanometer Scale Organization of Mixed Surfactin/Phosphatidylcholine Monolayers

Abstract

Mixed monolayers of the surface-active lipopeptide surfactin-C(15) 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.

Physicochemical studies of the interaction of the lipoheptapeptide surfactin with lipid bilayers of l-α-dimyristoyl phosphatidylcholine

“…Presumably surfactin integrates into the lamellar membrane fragments by growth of the biosurfactant clusters rather than by direct interaction with the lipid molecules. There is a coexistence between DMPC-rich and surfactin-rich domains, as demonstrated by Grau et al [25] and Deleu et al [26,27]. Finally, at surfactin concentrations higher than 15 mol% conversion of the membrane fragments into smaller DMPC-surfactin mixed micelles of ellipsoidal conformation occured which were characterized by small angle neutron scattering experiments.…”

“…It seems obvious that its amphiphilic nature and its specific surface-and membrane-active character play an important role in the expression of its biological activities. Particularly relevant are studies on the interaction of surfactin with artificial model membranes which have been performed by several authors [9,[25][26][27][28][29]. In previous work it has been demonstrated by electron microscopic analysis that surfactin inactivates mycoplasma [15] and enveloped viruses [18,19] by purely physicochemical mechanisms.…”

Probing peptide–membrane interactions using AFM

“…Molecular modeling data showed that mixing surfactin with DPPC has a destabilizing effect on DPPC monolayers while it has a stabilizing effect toward DPPE and DPPS molecular interactions. In summary, these data emphasize the important role that phospholipid chain length (14,16, and 18 carbon atoms) and polar head groups (large and zwitterionic PC, small and zwitterionic PE, negatively charged PS) play in modulating surfactin-phospholipid interactions.…”

“…A variety of approaches are available to probe the structure, composition and properties of lipid films, including fluorescence [7] and Brewster angle microscopy [8] ; X-ray reflection [9] and diffraction [10] methods; neutron reflectivity [11] and fluorescence recovery after photobleaching techniques. [12] Transferring lipid films onto solid substrata offers the possibility to apply a range of surface analytical techniques that could not be used to study real biological membranes, such as ellipsometry, [13] X-ray photoelectron spectrometry, [14] and time-of-flight secondary ion mass spectrometry. [15] However, little was known until recently about the structure and properties of lipid films at the nanometer level, due to a lack of high-resolution surface imaging techniques.…”

Diesel degradation and biosurfactant production by Gram-positive isolates

“…Biosurfactants are directly involved in the process of hydrocarbon removal from the environment through increased bioavailability and subsequent biodegradation of the hydrocarbons by direct cell contact (Hommel, 1990;Leahy and Colwell, 1990;Deleu et al, 1999;Banat et al, 2000).…”