Diaza-18-crown-6-based saccharide receptor bearing two boronic acids. Possible communication between bound saccharides and metal cations

A novel diaza-18-crown-6-based saccharide receptor (4) bearing two boronic acid groups has been developed. In 4 boronic acid groups in the side arms interact as Lewis acids with amines and boron atoms are changed to sp³ hybridization through the B···N interaction. Because of this hybridization, the boronic acid groups can bind saccharides even in a neutral pH region. Judging from the distance between two boronic acid groups in 4 assuming a syn conformation, one can expect that 4 forms 1:1 cyclic complexes with monosaccharides. These are confirmed by circular dichroism spectroscopy of 4 in the presence of glucose and allose: UV photometric titration studies showed that the boronic acid groups and metal cation bound to the crown cavity competitively interact as Lewis acids with the crown amines. As a result, saccharides and metal cations 'communicate' with each other in an allosteric manner. It was shown that added metal cations can affect the saccharide binding ability of 4 through the competitive interaction with the crown amines and the conformational change of the crown ether ring.