Thioredoxin reductase-triggered fluorogenic donor of hydrogen sulfide: a model study with a symmetrical organopolysulfide probe with turn-on near-infrared fluorescent emission.

We describe herein the rational development of an organopolysulfide-based fluorogenic donor of hydrogen sulfide (H 2 S) DCI-PS, which can be activated by the antioxidant selenoenzyme thioredoxin reductase (TrxR) with concomitant release of the dicyanoisophorone-based near-infrared (NIR) fluorophore. Along with the polysulfide probe DCI-PS capable of releasing the NIR fluorophore and H 2 S, the corresponding disulfide-probe DCI-DS was also rationally designed and synthesized, which releases the fluorophore without donating H 2 S. Detailed spectroscopic and kinetic studies in an aqueous medium revealed significantly higher reactivity of the probes towards DTT (for TrxR activity) over the well-known cellular abundant biothiol GSH. Mechanistically, the nucleophilic attack at the disulfide/polysulfide linkage by the thiol/selenol group of the bio-analytes leads to the self-immolative cyclization process with the release of the turn-on fluorophore with/without H 2 S. Considering the overexpression of mammalian TrxR in cancer cells, the turn-on fluorogenic H 2 S donation process from the cellular non-toxic DCI-PS was validated in a representative breast cancer cell line (MDA-MB-231) for the sustained donation of H 2 S with concomitant release of the red-emitting NIR fluorophore. The TrxR-triggered fluorescence turn-on process in DCI-PS was further supported by the significant inhibition of the fluorogenic process in the presence of TrxR-selective small-molecule inhibitors and by the significant binding affinity predicted by the protein-ligand docking study. Results with the antioxidant enzyme-triggered intracellular sustained donation of H 2 S with concomitant fluorescence turn-on will certainly find wider biomedical applications in the near future, particularly in H 2 S-mediated therapeutics in disease states.