Toward a Traceless Tag for the Thiol‐Mediated Uptake of Proteins

Abstract The emergence of thiol‐mediated uptake (TMU) as a powerful strategy to penetrate cells calls for the development of practical small‐molecule TMU tools for traceless delivery. Toward this goal, esters are explored here as bioreversible linkers between dynamic covalent cascade exchangers accounting for TMU and the protein of interest (POI). The method relies on α‐aryl‐α‐diazoamides that react with carboxylic acids of the POI to form esters that can be enzymatically hydrolyzed inside cells to release the native POI. A two‐step protocol is established for bioreversible conjugation of TMU tags to the POI. Despite the small number of tags attached to POIs to prevent isoelectric precipitation, POIs with traceless TMU tags are shown to efficiently enter cells not only in 2D culture but also in 3D spheroids mimicking deep tissue, confirming a key advantage of TMU. Uptake inhibition by various thiol‐reactive agents confirms the participation of cell‐surface thiols in cell penetration, i. e., the occurrence of TMU.