Convenient Access to Fluorescent Probes by Chemoselective Acylation of Hydrazinopeptides: Application to the Synthesis of the First Far‐Red Ligand for Apelin Receptor Imaging

Herein, we develop a convenient method to facilitate the solution-phase fluorescent labelling of peptides based on the chemoselective acylation of α-hydrazinopeptides. This approach combines the advantages of using commercially available amine-reactive dyes and very mild conditions, which are fully compatible with the chemical sensitivity of the dyes. The usefulness of this approach was demonstrated by the labelling of apelin-13 peptide. Various fluorescent probes were readily synthesized, enabling the rapid optimization of their affinities for the apelin receptor. Thus, the first far-red fluorescent ligand with sub-nanomolar affinity for the apelin receptor was characterized and shown to track the receptor efficiently in living cells by fluorescence confocal microscopy.