Solution and Fluorescence Properties of Symmetric Dipicolylamine-Containing Dichlorofluorescein-Based Zn[superscript 2+] Sensors

The mechanism by which dipicolylamine (DPA) chelate-appended fluorophores respond to zinc was investigated by the synthesis and study of five new analogs of the 2′,7′-dichlorofluorescein-based Zn2+ sensor Zinpyr-1 (ZP1). With the use of absorption and emission spectroscopy in combination with potentiometric titrations, a detailed molecular picture has emerged of the Zn2+ and H+ binding properties of the ZP1 family of sensors. The two separate N3O donor atom sets on ZP1 each converge to form binding pockets in which all four heteroatoms participate in coordination to either Zn2+ or protons. The position of the pyridyl group nitrogen atom, 2-pyridyl or 4-pyridyl, has a large impact on the fluorescence response of the dyes to protons despite relatively small changes in pKa values. The fluorescence quenching effects of such multifunctional electron-donating units are often taken as a whole. Despite the structural complexity of ZP1, however, we provide evidence that the pyridyl arms of the DPA appendages participate in the quenching process, in addition to the contribution from the tertiary nitrogen amine atom. Potentiometric titrations reveal ZP1 dissociation constants (Kd) for Zn2+ of 0.04 pM and 1.2 nM for binding to the first and second binding pockets of the ligand, respectively, the second of which correlates with the value observed by fluorescence titration. This result demonstrates that both binding pockets of this symmetric, ditopic sensor need to be occupied in order for full fluorescence turn-on to be achieved. These results have significant implications for the design and implementation of fluorescent sensors for studies of mobile zinc ions in biology.