Early-Stage Incorporation Strategy for Regioselective Labeling of Peptides using the 2-Cyanobenzothiazole/1,2-Aminothiol Bioorthogonal Click Reaction

Abstract

Herein, we describe a synthetic strategy for the regioselective labeling of peptides by using a bioorthogonal click reaction between 2‐cyanobenzothiazole (CBT) and a 1,2‐aminothiol moiety. This methodology allows for the facile and site‐specific modification of peptides with various imaging agents, including fluorophores and radioisotope‐containing prosthetic groups. We investigated the feasibility of an early‐stage incorporation of dipeptide 1 into targeting vectors, such as c[RGDyK(C)] and HER2 pep, during solid‐phase peptide synthesis. Then, the utility of the click reaction to label bioactive peptides with a CBT‐modified imaging agent (FITC–CBT, 9) was assessed. The ligation reaction was found to be highly selective and efficient under various conditions. The fluorescently labeled peptides 2 and 3 were obtained in respective yields of 88 and 82 % under optimized conditions.