Stabilization of Sugar-Boronic Esters of Indolylboronic Acid in Water via Sugar–Indole Interaction: A Notable Selectivity in Oligosaccharides

Abstract

Abstract 5-Indolylboronic acid (1) reversibly forms boronic esters with reducing sugars in water. The resulting boronates are stronger Lewis acids than the starting boronic acid in the deprotonation of water; pKa = 10 for 1 and < 7 for its sugar-boronates. Thus, at pH 9.0, there exists an experimentally-evaluable sugar-binding equilibrium between neutral trivalent acid 1 and anionic tetravalent ester thereof, which can be conveniently monitored by fluorescence or 11B NMR spectroscopy. A characteristic aspect of the present sugar-binding process is the notable selectivity of host 1 for oligosaccharides. The binding constants for maltodextrins (α-1,4-linked glucose oligomers) increase with increasing chain lengths or repetition numbers of the glucose unit. Such a chain length selectivity and those among stereoisomers and linkage isomers of disaccharides are discussed in terms of intracomplex oligosaccharide–indole interaction.