Expanding the Utility of Sulfonated Fluorophores in cellulo

Abstract

Many of the best and brightest small molecule fluorophores are sulfonated, which greatly increases their water solubility, but precludes cellular permeability. Thus, these dyes are limited to in vitro and extracellular applications. To increase the utility of sulfonated fluorophores in cellulo, we have developed a modular delivery approach for sulfonated molecules. First, the negative charge on the free sulfonate is neutralized by forming a chemically stable sulfonyl ester, allowing membrane permeation. Then, upon cellular entry, a trigger that is labile to the intracellular environment causes the release of the free sulfonate. Using an esterase‐labile scaffold, we have delivered sulfonated fluorophores into mammalian cells. Here, we describe work to design reductively‐labile scaffolds that rely on the reducing intracellular environment to release the free sulfonate. These reductively‐labile scaffolds are complementary to the esterase‐labile approach, as they do not rely on enzyme activity or recognition. Both types of scaffolds can deliver otherwise impermeable sulfonated fluorophores into live mammalian cells. We envision that these strategies will be broadly applicable to the cellular delivery of a vast array of sulfonated molecules.