Molecular Recognition of Sugars by Monolayers of Resorcinol-Dodecanal Cyclotetramer

Resorcinol-dodecanal cyclotetramer (1) forms a stable monolayer at the air-water interface. Molecular interactions of this monolayer with sugars and related water-soluble substances have been studied by combinations of Potentiometric responses of monolayer-modified SnO₂ electrodes, UV-visible, FT-IR, and XPS spectroscopies of LB films, and surface pressure-area isotherms. Sugars and water-soluble polymers bearing hydrogen-bonding groups bind to this monolayer selectively, resulting in anodic Potentiometric responses of the 1-modified electrode. The affinity of sugars to monolayer 1 increases in the order: glucose < fucose ∼ galactose ∼ arabinose < xylose < ribose. This selectivity is different from that of sugar extraction into CCl4 from the aqueous phase: ribose is bound to 1 effectively in both systems, while fucose, which is easily extracted into CCl4, is less effective in the monolayer system. Galactose, which is complexed weakly in CCl₄, shows significant binding to the monolayer. Formation of sugar complexes which expose more hydroxyl groups and less lipophilic parts to water is preferred at the interface. Binding to the monolayer host is efficient owing to the high densities of host molecules aligned at the interface. Spectroscopic techniques are applied to study binding of riboflavin and poly(vinylpyrrolidone) to the monolayer of 1. Binding of sugars to monolayers of octadecyl alcohol, N, N-didodecyl(gluconoamido)hexamide, and octadecanoic acid have also been studied. The present results strongly indicate that the specificity of sugar binding is brought out by hydrogen bonding; the presence of multiple hydroxyl groups and their fixed mutual arrangements in resorcinol cyclotetramer is required to obtain efficient sugar binding.