This Perspective focuses on thiol-mediated uptake, that is, the entry of substrates into cells enabled by oligochalcogenides or mimics, often disulfides, and inhibited by thiol-reactive agents. A short chronology from the initial observations in 1990 until today is followed by a summary of cell-penetrating poly(disulfide)s (CPDs) and cyclic oligochalcogenides (COCs) as privileged scaffolds in thiol-mediated uptake and inhibitors of thiol-mediated uptake as potential antivirals. In the spirit of a Perspective, the main part brings together topics that possibly could help to explain how thiol-mediated uptake really works. Extreme sulfur chemistry mostly related to COCs and their mimics, cyclic disulfides, thiosulfinates/-onates, diselenolanes, benzopolysulfanes, but also arsenics and Michael acceptors, is viewed in the context of acidity, ring tension, exchange cascades, adaptive networks, exchange affinity columns, molecular walkers, ring-opening polymerizations, and templated polymerizations. Micellar pores (or lipid ion channels) are considered, from cell-penetrating peptides and natural antibiotics to voltage sensors, and a concise gallery of membrane proteins, as possible targets of thiol-mediated uptake, is provided, including CLIC1, a thiol-reactive chloride channel; TMEM16F, a Ca-activated scramblase; EGFR, the epithelial growth factor receptor; and protein-disulfide isomerase, known from HIV entry or the transferrin receptor, a top hit in proteomics and recently identified in the cellular entry of SARS-CoV-2.

Oligonucleotide phosphorothioates (OPS) are DNA or RNA mimics where one phosphate oxygen is replaced by a sulfur atom. They have been shown to enter mammalian cells much more efficiently than non‐modified DNA. Thus, solving one of the key challenges with oligonucleotide technology, OPS became very useful in practice, with several FDA‐approved drugs on the market or in late clinical trials. However, the mechanism accounting for this facile cellular uptake is unknown. Here, we show that OPS enter cells by thiol‐mediated uptake. The transient adaptive network produced by dynamic covalent pseudo‐disulfide exchange is characterized in action. Inhibitors with nanomolar efficiency are provided, together with activators that reduce endosomal capture for efficient delivery of OPS into the cytosol, the site of action.

Cyclic oligochalcogenides are emerging as powerful tools to penetrate cells. With disulfide ring tension maximized, selenium chemistry had to be explored next to enhance speed and selectivity of dynamic covalent exchange on the way into the cytosol. We show that diseleno lipoic acid (DiSeL) delivers a variety of relevant substrates. DiSeL‐driven uptake of artificial metalloenzymes enables bioorthogonal fluorophore uncaging within cells. Binding of a bicyclic peptide, phalloidin, to actin fibers evinces targeted delivery to the cytosol. Automated tracking of diffusive compared to directed motility and immobility localizes 79 % of protein‐coated quantum dots (QDs) in the cytosol, with little endosomal capture (0.06 %). These results suggest that diselenolanes might act as molecular walkers along disulfide tracks in locally denatured membrane proteins, surrounded by adaptive micellar membrane defects. Miniscule and versatile, DiSeL tags are also readily available, stable, soluble, and non‐toxic.