pH-Driven Self-Assembly of Acidic Microbial Glycolipids

Abstract

Microbial glycolipids are a class of well-known compounds, but their self-assembly behavior is still not well understood. While the free carboxylic acid end group makes some of them interesting stimuli-responsive compounds, the sugar hydrophilic group and the nature of the fatty acid chain make the understanding of their self-assembly behavior in water not easy and highly unpredictable. Using cryo-transmission electron microscopy (cryo-TEM) and both pH-dependent in situ and ex situ small angle X-ray scattering (SAXS), we demonstrate that the aqueous self-assembly at room temperature (RT) of a family of β-d-glucose microbial glycolipids bearing a saturated and monounsaturated C18 fatty acid chain cannot be explained on the simple basis of the well-known packing parameter. Using the “pH-jump” process, we find that the molecules bearing a monosaturated fatty acid forms vesicles below pH 6.2, as expected, but the derivative with a saturated fatty acid forms infinite bilayer sheets below pH 7.8, instead of vesicles. We show that this behavior can be explained on the different bilayer membrane elasticity as a function of temperature. Membranes are either flexible or stiff for experiments performed at a temperature respectively above or below the typical melting point, T M, of the lipidic part of each compound. Finally, we also show that the disaccharide-containing acidic cellobioselipid forms a majority of chiral fibers, instead of the expected micelles.

Self-Assembly Mechanism of pH-Responsive Glycolipids: Micelles, Fibers, Vesicles, and Bilayers

“…At basic pH, the system is formed by micelles and some polymorph giant structures. 39 The slope at low q then gently decreases to zero at pH 7, while a sharp transition takes place below pH 7. Interestingly, q min decreases from about 1.85 to 1.16 nm −1 between pH 7 and 8.…”

“…(c) The G-C18:0 system in Figure 3 evolves from a micellar system at basic pH toward a bilayer sheet at acidic pH. 39 As for the G-C18:1, the form factor oscillation at q > 1 nm −1 (micelle) transforms into a broad oscillation attributable to a bilayer morphology. This is shown in the G-C18:0 contour plot in Figure S3, where one can see that the micelle-to-bilayer transition region is concentrated in a narrow pH range below 8.5.…”

“…Below pH 4, a white precipitate forms, characterized by a diffraction pattern typical of a lamellar phase. 39 Countour plots of G-C18:1 in Figure S3 visually show the phase transition in the 7.5 < pH < 6 region between the micelles and the vesicles by following the minimum of the form factor (arrow 1) and the appearance of the broad oscillation, attributable to the forming vesicles (arrow 2). (c) The G-C18:0 system in Figure 3 evolves from a micellar system at basic pH toward a bilayer sheet at acidic pH.…”

“…All C18:1 compounds contain a monounsaturation at position C9,10 of the fatty acid, while C18:0 compounds are characterized by a fully saturated chain. Table 1 gives the exact composition of each sample used in this work determined by the peak area ratio in HPLC-ELSD measurements 42,39 as well as the previous studies in which these same batches have been used. Upon use, all compounds are in their acidic (COOH) form and fully deacetylated.…”

“…38,39 At acidic pH, SL-C18:1 and SL:C18:0 sophorolipids respectively assemble into micelles 40,41 and twisted ribbons; 42 in addition, G-C18:1 glucolipids form vesicles while G-C18:0 forms infinite bilayer sheets. 39 The aim of this study is to find insights into the structure−property relationships and pHdriven assembly mechanism for a set of structurally related microbial glycolipids using pH-resolved in situ small angle X-ray scattering (SAXS). In particular, we will show the tight relationship between the COOH/COO − ratio, the carbohydrate hydration, the lipid melting temperature, T m , membrane flexibility, and the domain of stability for micelles, ribbons, vesicles, and bilayers.…”