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 i nteract as Lewis acids with amines and boron atoms are changed to sp(3) hyb ridization through the B ... N interaction. Because of this hybridization, the boronic acid groups can bind saccharides even in a neutral pH region. J udging from the distance between two boronic acid groups in 4 assuming a sy n conformation, one can expect that 4 forms 1:1 cyclic complexes with monos accharides. 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 co mpetitively interact as Lewis acids with the crown amines. As a result, sac charides and metal cations "communicate" with each other in an allosteric m anner. It was shown that added metal cations can affect the saccharide bind ing ability of 4 through the competitive interaction with the crown amines and the conformational change of the crown ether ring.