Ratiometric fluorescent probe based on symmetric peptidyl receptor with picomolar affinity for Zn²⁺ in aqueous solution

It is still a significant challenge to synthesize ratiometric fluorescent probes based on the new receptor with potent and tunable binding affinities for Zn²⁺. A fluorescent probe (1) based on the symmetric peptide receptor bearing two imidazole and two sulfonamide groups was synthesized for the ratiometric detection of Zn²⁺in aqueous solution. 1 selectively and sensitively detected Zn²⁺among biological relevant metal ions by a ratiometric response. Upon addition of Zn²⁺, monomer emissions of the pyrenes at 379 and 399 nm increased significantly and the excimer emission at 508 nm decreased with a blue shift. About 1 equiv of Zn²⁺ions completely induced the ratiometric response of 1. Job's plot analysis indicated that 1 formed a 1:1 complex with Zn²⁺. The dissociation constant for Zn²⁺was found to be 300 pM. The binding mode study revealed that two imidazole and two sulfonamide groups of the peptide receptor played an important role in the binding with Zn²⁺. The peptidyl probe successfully penetrated and detected intracellular Zn²⁺in live cells.