Studies of Arabinomannan Glycans in Carbohydrate-Protein Interactions and Thermotropic Liquid Crystal Behavior

Abstract

This thesis investigates the structural and functional roles of multivalency and self-assembly of arabinomannan-based glycoconjugates and glycolipids. Synthetic arabinomannan oligosaccharides were prepared, with defined glycosidic linkages and spatial arrangements to probe interactions with the mannose-binding lectin Concanavalin A (Con A). Among these, the trisaccharide with α(1→2)(1→3) linkages and the pentasaccharide displayed functional bivalency, engaging bivalent binding in a cross-linking (trans-) mode. In contrast, structurally similar oligosaccharides showed only monovalent binding, underscoring the critical role of linkage patterns over valency alone. Isothermal titration calorimetry (ITC), dynamic light scattering (DLS) and atomic force microscopy (AFM) ascertained the cross-linked aggregate formation, with binding driven by favorable enthalpic contributions and exhibiting enthalpy–entropy compensation. In order to explore the roles of glycan architecture in supramolecular behavior, a homologous series of arabinomannan glycolipids bearing mono- to tetrasaccharide headgroups and a uniform cetyl segment were synthesized. These glycolipids self-assembled in aqueous media into aggregates, exhibiting a morphological transition from sheet-like to compact spherical forms as glycan length increased. Critical aggregation concentration (CAC), DLS, AFM and TEM studies revealed that increasing carbohydrate size enhanced aggregate compactness and curvature. Multivalent lectin-binding was significantly amplified in the glycolipids, as shown by solution turbidity, DLS shifts and AFM images upon Con A complexation. Surface plasmon resonance (SPR) and ITC confirmed strong binding affinities for trisaccharide and tetrasaccharide glycolipids, driven by a combination of multivalent sugar presentation and hydrophobic interactions. These glycolipids exhibited distinct thermotropic liquid crystalline phases depending on the glycan segment. Polarized optical microscopy, differential scanning calorimetry and powder X-ray diffraction revealed that mono-, di- and trisaccharide glycolipids adopted smectic A lamellar phase, whereas the tetrasaccharide derivative formed a 2D hexagonal columnar mesophase. This mesophase transition is attributed to the increased hydrophilic volume from the oligosaccharide segment, altering amphiphilic packing and promoting varied organizations, depending only on the glycan segment. Overall, the thesis work demonstrates the roles precise glycosidic linkages and glycan lengths in arabinomannan conjugates in protein-binding structural and functional valencies and supramolecular assembly. These findings provide a framework for optimized structural and functional glycomaterials, with potential applications in lectin-targeted delivery, diagnostics and immunomodulations.