Influences of solvent on group transfer potentials and biochemical recognition of carbohydrates. Anomalous solvation of the anomeric hydroxyl group

When carbohydrates in aqueous solution combine with biological receptors, their interactions with the binding site take the place of previous interactions with solvent water. Free energies of binding can therefore be considered to depend on free energies of solvation of the interacting partners before and after complexation. Because carbohydrates contain so many polar substituents, their overall preferences for aqueous surroundings are overwhelming, so that they do not yield readily to analysis of their solvation properties. We therefore decided to eliminate most of the secondary hydroxyl groups and to compare the distribution properties of simple sugar analogs based on tetrahydropyran. The results suggest that compounds containing an anomeric hydroxyl group are less strongly solvated than expected from the properties of simple alcohols. Group transfer potentials of O- and N-glycosides, lactones, and acylals are substantially affected by differences in free energy of solvation between reactants and products. In contrast, equilibria of ring chain tautomerism and covalent hydration of aldoses are almost completely insensitive to the polarity of their surroundings.