In Situ Construction of Imidazopyridinium Fluorescent Labels for Bioconjugation

Simple, efficient transformations of fluorogenic nature that proceed under biocompatible conditions without the formation of byproducts are of high interest for in situ labeling and bioconjugation. Following these criteria, we describe in this work the discovery and optimization of imidazopyridinium dyes, obtained through in situ labeling of primary amines with pyridine, quinoline, and isoquinoline aldehydes. The so-generated dyes are excited with near-UV to violet light and emit in the orange region of the electromagnetic spectrum with Stokes shifts up to 12170 cm⁻¹. We employed the reaction to obtain fluorescently labeled amino acids, lipids, and sugars; furthermore, we expanded the scope to proteins and tags for bioimaging. The robustness of the chemistry also allowed us to on-resin staple peptides, cleanly generating fluorescent, cyclic analogs, which showcase the broad future impact of our transformation.