Construction of Photofunctional Peptide Conjugates through Selective Modification of N-terminal Cysteine with Cyclometalated Iridium(III) 2-Formylphenylboronic Acid Complexes for Organelle-Specific Imaging, Enzyme Activity Sensing, and Photodynamic Therapy

2-Formylphenylboronic acids (2-FPBAs) can react selectively and rapidly with N-terminal cysteine (N-Cys) to form a highly stable thiazolidino boronate derivative. In this study, three luminescent iridium(III) complexes bearing a 2-FPBA moiety were designed and synthesized. Upon photoexcitation, all the complexes displayed intense and long-lived greenish-yellow to red emission in solutions. One of the complexes was utilized to modify N-Cys-containing organelle-targeting peptides to afford conjugates with different localization properties. Also, a peptide conjugate containing a furin-cleavable peptide sequence between the complex and the emission quencher QSY7 was prepared to examine furin activity. Notably, the conjugate exhibited phosphorogenic response toward furin in live cells, enabling the specific targeting of cancer cells based on furin activity.