Fluorescent Sensors for Zn2+Based on a Fluorescein Platform: Synthesis, Properties and Intracellular Distribution

Abstract
Two new fluorescent sensors for Zn2+ that utilize fluorescein as a reporting group, Zinpyr-1 and Zinpyr-2, have been synthesized and characterized. Zinpyr-1 is prepared in one step via a Mannich reaction, and Zinpyr-2 is obtained in a multistep synthesis that utilizes 4‘,5‘-fluorescein dicarboxaldehyde as a key intermediate. Both Zinpyr sensors have excitation and emission wavelengths in the visible range (∼500 nm), dissociation constants (Kd1) for Zn2+ of 2+ cation to the sensor, which inhibits a photoinduced electron transfer (PET) quenching pathway. The X-ray crystal structure of a 2:1 Zn2+:Zinpyr-1 complex has also been solved, and is the first structurally characterized example of a complex of fluorescein substituted with metal binding ligands.