Fluorescent Amino Acid Initiated de novo Cyclic Peptides for the Label‐Free Assessment of Cell Permeability

The major obstacle in applying peptides to intracellular targets is their low inherent cell permeability. Standard approaches to attach a fluorophore (e. g. FITC, TAMRA) can change the physicochemical properties of the parent peptide and influence their ability to penetrate and localize in cells. We report a label‐free strategy for evaluating the cell permeability of cyclic peptide leads. Fluorescent tryptophan analogues 4‐cyanotryptophan (4CNW) and β‐(1‐azulenyl)‐L‐alanine (AzAla) were incorporated into in vitro translated macrocyclic peptides by initiator reprogramming. We then demonstrate these efficient blue fluorescent emitters are good tools for monitoring peptide penetration into cells.