A phenazine-based near-infrared (NIR) chemodosimeter for cysteine obtained via a carbonyl-assisted cycloaddition process

A phenazine derivative with a succinimide endcap (PHS) was designed and synthesized for cysteine (Cys) with near-infrared (NIR) emission. Upon the addition of Cys, the color of the solution changed clearly from greenish black to orange, which was attributed to the disappeared absorbance at 616 nm. An enhanced NIR emission band at 670 nm was found in the fluorescence spectra and a corresponding excitation peak at 466 nm arose in the excitation spectra. Verified by the dynamic reaction, the cyanoacetic acid moiety should be susceptible to be attacked by the –SH group in a fast rate through Michael addition reaction. Being highly reactive to the amino group (–NH2), the NHS-active carbonyl site could target the amino acid and accelerate the process of nucleophilic attack from the –SH group. This cycloaddition mechanism of the α,β-unsaturated carbonyl NHS-ester was proved by 1H NMR titration. In addition to its good biocompatibility, PHS was successfully applied to the detection of Cys in Hela cell lines with NIR fluorescence signals.