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 of the James-Shinkai anthracene-boronic acid fluorescence sugar sensor was prepared to probe the role of the bridging nitrogen in the signaling mechanism of . Both and contain positively charged bridging groups NMe or NH, respectively, but lacks the ability to form the intramolecular ammonium-boronate doubly ionic hydrogen bond present in . Receptors and display opposite fluorescence vs pH profiles with a small turn-on effect of the sugar binding to the zwitterion of in contrast to a large effect observed with . It is concluded that the ammonium-boronate hydrogen bond is essential for the signaling mechanism of . 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 toward a series of di- and polyols of variable structures as well as with the behavior of in buffered DO and methanol solvents at controlled pH and provides an addition to the established "loose bolt" mechanism signaling mode essential for receptors with nonpolar fluorophores.