Supramolecular Assemblies of a Naturally Derived Sophorolipid

Abstract

Acidic sophorolipid (SL) molecules derived from yeasts represent a novel type of asymmetrical bolaamphiphiles due to their unique structural features that include an asymmetrical polar head size (disaccharide vs COOH), a kinked hydrophobic core (cis-9-octadecenoic chain), and a non-amide polar-nonpolar linkage. Light microscopy, small- and wide-angle X-ray scattering, FT-IR spectroscopy, and dynamic laser light scattering were used to investigate the supramolecular structures of the self-assembled aggregates of SL molecules at different pH values. In acidic conditions (pH < 5.5), giant twisted and helical ribbons of 5-11 microm width and several hundreds of micrometers length were observed for the first time. Increase in solution pH values slowed ribbon formation, decreased ribbon yield, and increased the helicity and entanglements of the giant ribbons. An interdigitated lamellar packing model of acidic SL-COOH molecules with a long period of 2.78 nm, stabilized by both the strong hydrophobic association between the cis-9-octadecenoic chains and strong disaccharide-disaccharide hydrogen bonding, is proposed. The neutralization of SL-COOH in water to SL-COONa produced clear solutions with the formation of short-range ordered aggregates. At concentrations below 1.0 mg/mL, the size of self-assembled aggregates increased as the concentration increased. At concentrations above 1.0 mg/mL, narrowly distributed micellar aggregates with a constant hydrodynamic radius (R(h)) of about 100 nm are formed. The large micelles show strong angular dependence with the fast mode appearing at scattering angle theta >/= 60 degrees.

pH-triggered formation of nanoribbons from yeast-derived glycolipid biosurfactants

“…Even if this system was reported to form giant ribbons (but never nanoscale ones), 50 we found them difficult to reproduce. The work of Prasad et al…”

“…24 In the case of sophorolipids, nanoscale chirality has never been reported even if giant ribbons seem to be formed from an elaidic acid (C18:1-trans) based compound while contradictory results exists (giant ribbons versus micelles) on the behavior of oleic acid derived (C18:1-cis) sophorolipids. 50,51,52,53 In the present paper, we present the pHtriggered formation of self-assembled fibers with nanoscale chirality using a saturated sophorolipid (C18:0 sophorolipids, Figure 1b) Preparation of the supramolecular assemblies: Different amounts of acidic C18:0 sophorolipids (c= 2 mg/mL and 5 mg/mL) were solubilized in deionized water. Due to the poor solubility of the C18:0 sophorolipids, pH was first increased to 11 using a 1 M NaOH solution and then decreased to pH= 2 and 6 with µmolar amounts of 0.5 and 0.05 M HCl solutions respectively.…”

Surface-induced assembly of sophorolipids

“…60 In some cases, acidic sophorolipids form twisted ribbons under acidic pH conditions. 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.…”

“…1), and in particular their pH-responsive self-assembly have been shown earlier. [55][56][57] It has also been established that the structure of the molecules greatly influenced the self-assembly. 58 Acidic monounsaturated sophorolipids form ellipsoidal micelles in water in a very broad pH range (from pH 11 to pH 3), 59 although at basic pH, formation of nanoplatelets is also observed.…”

“…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. 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.…”