Cation-Induced Fibrillation of Microbial Glycolipid Biosurfactant Probed by Ion-Resolved In Situ SAXS.

Biological amphiphiles are molecules with a rich phase behavior. Micellar, vesicular, and even fibrillar phases can be found for the same molecule by applying a change in pH or by selecting the appropriate metal ion. The rich phase behavior paves the way toward a broad class of soft materials, from carriers to hydrogels. The present work contributes to understanding the fibrillation of a microbial glycolipid, glucolipid G-C18:1, produced by Starmerella bombicola ΔugtB1 and characterized by a micellar phase at alkaline pH and a vesicular phase at acidic pH. Fibrillation and prompt hydrogelation is triggered by adding either alkaline earth, Ca 2+ , or transition metal, Ag + , Fe 2+ , Al 3+ , ions to a G-C18:1 micellar solution. A specifically designed apparatus coupled to a synchrotron SAXS beamline allows the performing of simultaneous cation- and pH-resolved in situ monitoring of the morphological evolution from spheroidal micelles to crystalline fibers, when Ca 2+ is employed, or to wormlike aggregates, when Fe 2+ or Al 3+ solutions are employed. The fast reactivity of Ag + and the crystallinity of Ca 2+ -induced fibers suggest that fibrillation is driven by direct metal-ligand interactions, while the shape transition from spheroidal to elongated micelles with Fe 2+ or Al 3+ rather suggest charge screening between the lipid and the hydroxylated cation species.