Probing the Role of the Bridging Nitrogen in the Signaling Mechanism of an Anthracene–Boronic Acid Sugar Sensor and a Different Version of the PET-Based Mechanism

The N-quaternized derivative 5 of the James–Shinkai
anthracene–boronic acid fluorescence sugar sensor 1 was prepared to probe the role of the bridging nitrogen in the signaling
mechanism of 1. Both 5 and 1 contain positively charged bridging groups NMe+ or NH+, respectively, but 5 lacks the ability to form
the intramolecular ammonium–boronate doubly ionic hydrogen
bond present in 1. Receptors 1 and 5 display opposite fluorescence vs pH profiles with a small
turn-on effect of the sugar binding to the zwitterion of 5 in contrast to a large effect observed with 1. It is
concluded that the ammonium–boronate hydrogen bond is essential
for the signaling mechanism of 1. Its possible function
is enabling the PET quenching effect by shifting the NH+ proton toward boronate anion inside the hydrogen bond, the degree
of which is modulated by the ester formation with diols affecting
the basicity of boronate anion. This mechanism agrees with observed
signaling selectivity of 1 toward a series of di- and
polyols of variable structures as well as with the behavior of 1 in buffered D2O and methanol solvents at controlled
pH and provides an addition to the established “loose bolt”
mechanism signaling mode essential for receptors with nonpolar fluorophores.