Energy Landscape of the Sugar Conformation Controls the Sol-to-Gel Transition in Self-Assembled Bola Glycolipid Hydrogels

Abstract

Self-assembled fibrillar network (SAFIN) hydrogels and organogels are commonly obtained by a crystallization process into fibers induced by external stimuli like temperature or pH. The gelto-sol-to-gel transition is generally readily reversible and the change rate of the stimulus determines the fiber homogeneity and eventual elastic properties of the gels. However, recent work shows that in some specific cases, fibrillation occurs for a given molecular conformation and the sol-to-gel transition depends on the relative energetic stability of one conformation over the other, and not on the rate of change of the stimuli. We observe such a phenomenon on a class of bolaform glycolipids, sophorosides, similar to the well-known sophorolipid biosurfactants, but composed of two symmetric sophorose units. A combination of oscillatory rheology, small-angle X-ray scattering (SAXS) cryogenic transmission electron microscopy (cryo-TEM) and in situ rheo-SAXS using synchrotron radiation shows that below 14°C, twisted nanofibers are the thermodynamic phase. Between 14°C and about 33°C, nanofibers coexist with micelles and a strong hydrogel forms, the sol-to-gel transition being readily reversible in this temperature range. However, above the annealing temperature of about 40°C, the micelle morphology becomes kinetically-trapped over hours, even upon cooling, whichever the rate, to 4°C. A combination of solution and solid-state nuclear magnetic resonance (NMR) suggests two different conformations of the 1ˈˈ, 1ˈ and 2ˈ carbon stereocenters of sophorose, precisely at the β(1,2) glycosidic bond, for which several combinations of the dihedral angles are known to provide at least three energetic minima of comparable magnitude and each corresponding to a given sophorose conformation.

Topological Connection between Vesicles and Nanotubes in Single-Molecule Lipid Membranes Driven by Head–Tail Interactions

“…The flip-flop interpretation now reasonably explains the strong differences in the sugar and fatty alcohol conformations, deduced by the multiple NMR signals attributed to chemically equivalent groups. The existence of various low-energy conformations, each having specific NMR signature, for sugars in solution − and solid state − , further supports our interpretation.…”

Shear recovery and temperature stability of Ca2+ and Ag+ glycolipid fibrillar metallogels with unusual β-sheet-like domains

“…Conformation of the sugar headgroup in glycolipids has also been reported to have a strong impact on the structure of their corresponding supramolecular aggregation. 70,71 The difference in terms of coordination geometry between silver and calcium, associated with a conformational effect of glucose could eventually explain the differences in terms of packing of the G-C18:1 dimers and the overall stability of the fibers and the corresponding hydrogels. The self-assembly of G-C18:1 with cations is unique, 19 even when compared to glycolipid amphiphiles with a similar structure, and which do not specifically react with mono and divalent cations.…”

Lyotropic Liquid-Crystalline Phases of Sophorolipid Biosurfactants

“…However, prolonged annealing at high temperature should be used here with great care because it can induce conformational changes of the biosurfactant molecules. 59 These changes can take months of aging at room temperature to be reversed, if ever. Also, pH is another important parameter because of the presence of acidic functional groups, and its influence on the mesomorphism would deserve additional investigations.…”

“…To solve this problem, resorting to high-temperature annealing is a common strategy. However, prolonged annealing at high temperature should be used here with great care because it can induce conformational changes of the biosurfactant molecules . These changes can take months of aging at room temperature to be reversed, if ever.…”