Analogues of Zinpyr-1 provide insight into the mechanism of zinc sensing

Three compounds structurally related to the fluorescent zinc sensor Zinpyr-1 (ZP1) have been synthesized and characterized. In each of these ZinAlkylPyr (ZAP) analogues, an alkyl group (methyl, benzyl) replaces one of the metal-binding picolyl moieties in ZP1. The methyl-for-picolyl substitutions in ZAP1 and ZAP2 have a negligible effect on the optical spectrum of the fluorophore but elevate the quantum yields (Phi = 0.82 (ZAP1), 0.74 (ZAP2)) to values near that of Zn2+-saturated ZP1 (Phi = 0.92). The benzyl-for-picolyl substitution in ZAP3 similarly enhances the quantum yield (Phi = 0.52) relative to that of metal-free ZP1 (Phi = 0.38). As previously observed for methylated ZP1 sensors, methylation of the 6-position of the pyridyl ring diminishes the emission by lowering both the molar extinction coefficient and the quantum yield. Although these new ZAP compounds cannot detect Zn2+ fluorimetrically at neutral pH, complexation of Zn2+ does occur, as evidenced by sizable changes in the optical spectra. The ZAP1-3 probes can detect Zn2+ fluorimetrically at pH 9, indicating that proton- induced background emission obscures any Zn2+-induced fluorescence at pH 7. The tertiary amine groups in ZAP1-3 are less basic than those in ZP1, which implies that the additional pyridine rings are responsible for the emissive response to Zn2+ at pH 7.0.