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

Abstract

Self-assembled fibrillar network hydrogels and organogels are commonly obtained through a crystallization process as fibers upon being induced by external stimuli such as temperature or pH. The gel-to-sol-to-gel transition is generally readily reversible, and the rate of change 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, and in situ rheology coupled with SAXS using synchrotron radiation shows that below 14 °C, twisted nanofibers are the thermodynamic phase. Between 14 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 for hours, even upon cooling, whichever the rate, to 4 °C. A combination of solution and solid-state nuclear magnetic resonance spectroscopy studies suggests two different conformations of the 1″, 1′, and 2′ 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, with each corresponding to a given sophorose conformation.