Journal of Molecular Pathology and Biochemistry
Open Access

Biological Chemistry Open Access Articles

Metal-specific fluorescence probes are of increasing importance in understanding the neurobiology and general cell biology of zinc. Several quinoline-based compounds such as TSQ and zinquin have been employed to detect zinc in fluorescence microscopy experiments in vivo; however, the aqueous solution chemistry remains equivocal. In some cases, this family of probes is said to reveal labile pools of Zn(II) inside the cell, yet in other cases, these probes are suggested to remove Zn(II) from tightly bound sites in proteins. Since the binding modes, coordination numbers, and thermodynamics of zinc−zinquin interactions in aqueous solution have not been established, these proposals are difficult to distinguish. Here we show that, under physiological conditions, the various forms of zinquin bind Zn(II) with a high degree of cooperativity forming 2:1 complexes. Potentiometric, UV−visible, and fluorescence methods all yield an overall binding constant of log K = 13.5 under physiological conditions. To put this number in perspective with other Zn chelators and biological ligands, we compare the calculated so-called pM values (−log[Zn(II)]free) for a series of compounds with different stoichiometries under a typical condition. The pZn value for zinquin (9.3) is similar to that of EGTA (9.5) but much smaller than the value for carbonic anhydrase (12.4) or EDTA (14.3) and, thus, serves as a useful gauge for comparing zinc affinities.