title: Structure of Bolaamphiphile Sophorolipid Micelles Characterized with SAXS, SANS, and MD Simulations.
abstract: The micellar structure of sophorolipids, a glycolipid bolaamphiphile, is analyzed using a combination of small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS), and molecular dynamics (MD) simulations. Numerical modeling of SAXS curves shows that micellar morphology in the noncharged system (pH< 5) is made of prolate ellipsoids of revolution with core-shell morphology. Opposed to most surfactant systems, the hydrophilic shell has a nonhomogeneous distribution of matter: the shell thickness in the axial direction of the ellipsoid is found to be practically zero, while it measures about 12 Å at its cross-section, thus forming a "coffee bean"-like shape. The use of a contrast-matching SANS experiment shows that the hydrophobic component of sophorolipids is actually distributed in a narrow spheroidal region in the micellar core. These data seem to indicate a complex distribution of sophorolipids within the micelle, divided into at least three domains: a pure hydrophobic core, a hydrophilic shell, and a region of less defined composition in the axial direction of the ellipsoid. To account for these results, we make the hypothesis that sophorolipid molecules acquire various configurations within the micelle including bent and linear, crossing the micellar core. These results are confirmed by MD simulations which do show the presence of multiple sophorolipid configurations when passing from spherical to ellipsoidal aggregates. Finally, we also used Rb(+) and Sr(2+) counterions in combination with anomalous SAXS experiments to probe the distribution of the COO(-) group of sophorolipids upon small pH increase (5 < pH < 7), where repulsive intermicellar interactions become important. The poor ASAXS signal shows that the COO(-) groups are rather diffused in the broad hydrophilic shell rather than at the outer micellar/water interface.
authors.affiliation: Sorbonne Universités , UPMC Univ Paris 06, CNRS, Collège de France, UMR 7574, Chimie de la Matière Condensée de Paris, F-75005 Paris, France.
authors.forename: Sabine
authors.initials: S
authors.lastname: Manet
authors.affiliation: Sorbonne Universités , UPMC Univ Paris 06, CNRS, Collège de France, UMR 7574, Chimie de la Matière Condensée de Paris, F-75005 Paris, France.
authors.forename: Anne-Sophie
authors.initials: AS
authors.lastname: Cuvier
authors.affiliation: Sorbonne Universités , UPMC Univ Paris 06, CNRS, Collège de France, UMR 7574, Chimie de la Matière Condensée de Paris, F-75005 Paris, France.
authors.forename: Claire
authors.initials: C
authors.lastname: Valotteau
authors.affiliation: Laboratoire Léon Brillouin, LLB, CEA Saclay, F-91191 CEDEX Gif-sur-Yvette, France.
authors.forename: Giulia C
authors.initials: GC
authors.lastname: Fadda
authors.affiliation: SWING, Synchrotron Soleil, BP 48, F-91192 Gif-sur-Yvette, France.
authors.forename: Javier
authors.initials: J
authors.lastname: Perez
authors.affiliation: Maison de la Simulation, USR 3441, CEA - CNRS - INRIA, Univ. Paris-Sud - Univ. de Versailles , 91191 CEDEX Gif-sur-Yvette, France.|Institute for Integrative Biology of the Cell (I2BC), Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA), Centre National de la Recherche Scientifique (CNRS), Université Paris-Sud, CEA-Saclay , F-91191 Gif-sur-Yvette, France.
authors.forename: Esra
authors.initials: E
authors.lastname: Karakas
authors.affiliation: Institute for Integrative Biology of the Cell (I2BC), Commissariat à l'Energie Atomique et aux Energies Alternatives (CEA), Centre National de la Recherche Scientifique (CNRS), Université Paris-Sud, CEA-Saclay , F-91191 Gif-sur-Yvette, France.
authors.forename: Stéphane
authors.initials: S
authors.lastname: Abel
authors.affiliation: Sorbonne Universités , UPMC Univ Paris 06, CNRS, Collège de France, UMR 7574, Chimie de la Matière Condensée de Paris, F-75005 Paris, France.
authors.forename: Niki
authors.initials: N
authors.lastname: Baccile