Luminescent Rhenium(I) Perfluorobiphenyl Complexes as Site-specific Labels for Peptides to Afford Photofunctional Bioconjugates

A four amino-acid sequence (FCPF, termed as “π-clamp”) has been developed for the site-specific cysteine labeling of peptides and proteins in aqueous media. This short sequence undergoes selective and rapid cysteine arylation with perfluorobiphenyl (PFBP) derivatives without the need for catalysts. Given the rich photophysical and photochemical properties of rhenium(I) polypyridine complexes, for example, their long-lived triplet emission and efficient singlet oxygen generation, these complexes have been widely exploited for biological applications. Herein, we report rhenium(I) PFBP complexes that specifically label the cysteine residue of the π-clamp sequence. The complexes were conjugated to π-clamp-modified peptides to afford novel conjugates that selectively stained the lysosomes and mitochondria of KYSE-510 cells. Also, the photocytotoxicity activity of the conjugates toward the cells was investigated. Additionally, a phosphorogenic substrate for caspase-3/7 was developed to monitor the activity of these enzymes in live cells.