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.

pH-Driven Self-Assembly of Acidic Microbial Glycolipids

“…16 The self-assembly of saturated acidic sophorolipids, a family of microbial glycolipids, can be strongly influenced by its physico-chemical history and should be treated as a system out of equilibrium: if the compound is directly dissolved in water at neutral pH, 24 flat sheets are formed; however, if the compound is initially treated at basic pH to increase its water solubility before being stabilized at neutral pH, stable twisted ribbons are formed. 25 In a series of recent works, we have shown how two pH-responsive structurallysimilar sophorolipids, 26 only differing in the presence or absence of a double bond, show a drastically different behaviour in water at pH values below 7: the monounsaturated acidic sophorolipid forms micelles 27,28 while the saturated congener forms twisted ribbons. 25 Interestingly, under basic pH conditions these two compounds show exactly the same behaviour, forming a mixture of micelles and aggregated nanoscale platelets.…”

Surface-induced assembly of sophorolipids

“…60 We also reported the micellar structure and their surface charge induced by pH. 61,62 The same molecules were also reported to form giant ribbons. 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.…”

“…56,[60][61][62] For pH o 5, sophorolipids form neutral spheroidal micelles, 61 while for 5 o pH o 7, micelles interact with one another due to negative charge coming from the partial deprotonation of the carboxylic acid. 61,62 Finally, for pH 4 8, small spheroidal micelles and platelets to some extent are detected using combined SAXS and cryo-TEM. 60 In some cases, acidic sophorolipids form twisted ribbons under acidic pH conditions.…”

Development of a Cradle-to-Grave Approach for Acetylated Acidic Sophorolipid Biosurfactants

“…The non-acetylated C18:1cis sophorose headgroup area was estimated to be 104 Å 2 at the air-water interface based on SANS data, 30 while we have measured between 70 and 80 Å 2 at the dry core/hydrated shell interface of a non-acetylated acidic C18:1 sophorolipid micelle in water. 31 Since these values do not take into account the presence two acetyl groups, a0 is in reality probably slightly larger than 80 Å 2 , probably closer to 100 Å 2 . The calculation of p using average values for v (430 Å 3 ), lc (20 Å) and a0 (100 Å 2 ) gives p= 0.2, that is significantly lower than p < 0.3, the upper limit value for spherical micelles to be stable.…”

“…These assumptions are confirmed by fitting the SAXS data in the q> 0.01 Å -1 region using the form factor of a core-shell ellipsoid of revolution, 32 previously used to successfully model the self-assembly behaviour of non-acetylated acidic C18:1 sophorolipid micelles. 31 The hydrophobic core is assumed to be constituted by the fatty acid tail while we assume the hydrophilic shell to be constituted by the acetylated sophorose and possibly the carboxylic acid. One should keep in mind that the presence of the acetyl groups most likely decreases the hydrophilic character of the sophorose headgroup and a core-single shell model is probably an approximation.…”