Cation–Cation Photosensitization for Protein Ligation and Intracellular Catalysis

We report here a photosensitized strategy for protein labeling in which N-substituted pyridinium salts are activated using a 2,4-diaryl-<i>N</i>-methyl quinolinium scaffold. Structure-reactivity relationships were performed to optimize the sensitizer structure and ultimately generated a system that gives protein labeling in minutes at micromolar reagent concentrations. Mechanistic studies suggest a photoinduced electron transfer-based sensitization mechanism. The mildness of this system enabled us to assay sensitization both on individual biomolecules and in complex proteomes and demonstrated excellent compatibility with lysate- and live-cell-based systems. Imaging of photolabeled HeLa cells was performed and revealed that catalysis occurs in multiple cellular compartments. Chemical proteomics performed at the lysate level resulted in the enrichment of 319 proteins with 93% selectivity to Tryptophan residues. Live cell labeling resulted in 101 enriched proteins, primarily from the nucleus.