4-Azidophthalimide, previously reported as a photoaffinity labeling (PAL) tag, was repurposed as a post-conjugation warhead. Using the phthalimide–amine reaction, cyanine (fluorescence) amines were successfully conjugated to target proteins after PAL under biological-relevant conditions. Additionally, we showed that the conjugated amine can be replaced with other amines, demonstrating its “switchable” nature. These findings highlight phthalimide's unique characteristics as a novel chemical tool for bioorthogonal conjugations.
This result was published in the Royal Society of Chemistry’s journal “Chemical Communications” on August 28, 2025 (issue 61).
This work was supported the Japan Society for the Promotion of Science (JSPS), the Japan Agency for Medical Research and Development (AMED), Institute for Open and Transdisciplinary Research Initiatives (OTRI), and Hoansha Foundation.

Journal: Chemical Communications
Title： Phthalimide: a potential warhead for switchable and bioorthogonal conjugation
Author： Kosuke Chiba, Takumi Yoshida, Kana Okada, Minoru Ishikawa, Yuichi Hashimoto, Satoshi Obika, and Takao Yamaguchi