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-N-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.