Micelles versus Ribbons: How Congeners Drive the Self‐Assembly of Acidic Sophorolipid Biosurfactants

Abstract

Sophorolipids (SLs), a class of microbially derived biosurfactants, are reported by different research groups to have different self-assembled structures (either micelles or giant ribbons) under the same conditions. Here we explore the reasons behind these contradictory results and attribute these differences to the role of specific congeners that are present in minute quantities. We show that a sample composed of a majority of oleic acid (C18:1) sophorolipid in the presence of only 0.5 % (or more) of congeners with stearic acid (C18:0) or linoleic acid (C18:2) results in the formation of micelles that are stable over long periods of time. Conversely, the presence of only 10 to 15 % of congeners with a stearic acid chain gives fibrillar structures instead of micelles. To study the mechanisms responsible, oleic acid SLs devoid of any other congeners were prepared. Very interestingly, this sample can self-assemble into either micelles or fibers depending on minute modifications to the self-assembly conditions. The findings are supported by light scattering, small-angle X-ray scattering, transmission electron microscopy under cryogenic conditions, high-pressure liquid chromatography, and NMR spectroscopy.

Adjuvant Antibiotic Activity of Acidic Sophorolipids with Potential for Facilitating Wound Healing

“…Other congeners present include a mixture of sophorolipid structures with chain lengths of C 16 to C 18 containing 0 to 2 double bonds. The sophorolipids used in these studies are micelles forming, as recently described (23). Sophorolipid solutions of various concentrations (by mass) were prepared in the relevant culture media for in vitro experiments or sterile PBS for in vivo studies.…”

Surface-induced assembly of sophorolipids

“…55,58 We have recently shown that the micelle-ribbon duality depends on purity; a standard non-acetylated, monounsaturated, acidic sophorolipid batch forms micelles, unless it is contaminated with 15 wt% of the saturated congener, which induces the ribbon formation. 63 In terms of surface-adsorption properties of the sophorolipids, only alkyl ester derivatives have been explored, which nonetheless have shown adsorption on alumina being two orders of magnitude higher than on silica beyond a solution concentration of 10 À4 M. 64 The surface self-assembly and patterning ability of an aqueous micellar solution of non-acetylated, monounsaturated, acidic sophorolipids have been shown in the present investigation. We have investigated both the pH influence and the crucial role of the underlying surface (silica, gold, titanium dioxide-anatase structure).…”

“…55,58 However, this was not the case for the compound used in the present study, which only formed a stable micellar solution. In a recent study, 66 we have given the conditions for a micellar vs. fibrillar stability of sophorolipids in water at acidic pH. We were able to show that a standard batch of acidic non acetylated monounsaturated sophorolipid generally formed micelles in water, unless 15 wt% of the saturated form contaminated the batch.…”

“…They have been recovered using method 1 as reported in a previous study by Baccile et al 65 Their purity has been evaluated at about 90% of both terminal and subterminal C18:1 congener and their equilibrium state in water is micellar as described earlier. 56,62,66 Sodium dodecyl sulfate (SDS) has been purchased from Aldrich and used as received. C10-Alkylpolyglycoside (APG, commercial name: Oramix) was provided by SEPPIC (France).…”

“…The formation of fibers, which may seem quite odd if one considers the stable micellar phase in solution, is actually not surprising as acidic sophorolipids have shown to form both micelles 56,62 and fibers in water under acidic pH conditions. 55,58 At the moment, the conditions that favor one phase over the other were only partially known 66 and a tentative explanation at the molecular level in terms of surface-sophorolipid interactions favouring fiber formation would be highly speculative. However, we must consider that similar effects have been described for amyloid-b peptides on mica and can undergo a micelle-to-fibril transition as a function of number density of the peptides on the surface, which depends on the deposition and diffusion rates of the peptides.…”