Comparison of the Self-Assembly and Conformations of Glucose- and Galactose-Based Glycopyranosides in Dilute Aqueous Solution.

A series of glycolipids (glycopyranosides) was prepared by coupling glucosamine or galactosamine (at the C2 position) with myristic (tetradecanoic) or palmitic (hexadecanoic) acid. The conformation and self-assembly were examined in aqueous solutions containing 10% methanol. Circular dichroism and FTIR spectroscopy reveal notable differences in the chiral ordering and conformation comparing analogues with glucose and galactose "headgroups". A glucose derivative (to palmitic acid) was also prepared with a different (C1-) substitution position of the lipid chain, and this was found to significantly influence chiral ordering and conformation. The self-assembly of the glycolipids was examined using cryogenic-transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS), which reveals lamellar structures, unilamellar for the glucose-based glycolipids, but multilamellar for the galactose-based analogues. Thus, the conformation and self-assembly of the molecules are very distinct, even though the glucose and galactose homologues have very similar structures, differing only in the orientation of a single hydroxyl group as epimers. These findings were rationalized with information from atomistic molecular dynamics (MD) simulations, which showed large differences in hydrogen-bonding density for glucose and galactose derivatives. The number of hydrogen bonds within interdigitated bilayers was much higher for the glucose variants, leading to stabilized unilamellar structures. The unexpectedly large differences in conformation and self-assembly of glycolipids bearing epimer monosaccharides may influence their properties and bioactivities.