For the development of new receptor molecules that can precisely recognize sugar molecules, we synthesized a number of diboronic acids. Since one boronic acid can react with two OH groups (one diol group) to form a boronate ester, one diboronic acid can immobilize two diol units to form a sugar-containing macrocycle. The selectivity can be tuned by the relative spatial position of the two boronic acids and the complexation event can be read out by circular dichroism spectroscopy. When a boronic acid group is combined intramolecularly with an aminomethylfluorophore, the complexation event can be conveniently read out by fluorescence spectroscopy. This is a novel application of a PET (photoinduced electron transfer) sensor: the sugar-binding changes the strength of the B···N interaction, which eventually changes the fluorescence quenching efficiency of the amine. We have demonstrated using a chiral 1,1'-binaphthyl group as a fluorophore that even chiral recognition of sugars is possible. These abundant examples support the superiority of boronic-acid-based covalent bond recognition over hydrogen-bond-based non-covalent bond recognition for sugars in water.
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