Regulation of Target Protein Knockdown and Labeling Using Ligand-Directed Ru(bpy)₃ Photocatalyst

Ligand-directed Ru­(bpy)₃ photocatalysts induce chromophore-assisted light inactivation (CALI) of target proteins under visible light irradiation in vitro and within cells. Here, histidine, methionine, and tryptophan residues were oxidized by the singlet oxygen (¹O₂) generated by Ru­(bpy)₃ with light. The addition of a tyrosyl radical trapper (TRT), such as <i>N</i>′-acyl-<i>N</i>,<i>N</i>-dimethyl phenylenediamine, inhibited peptide/protein oxidation and induced labeling on the tyrosine residue. This mechanistic study suggests that TRT scavenges ¹O₂, concomitant with the coupling reaction to the tyrosyl radical generated by Ru­(bpy)₃. Both CALI and labeling can be regulated by the Ru­(bpy)₃ photocatalysts in the absence or presence of TRT. Ligand-conjugated Ru­(bpy)₃ photocatalysts (local environmental single-electron transfer catalysts: LSCs) were used not only for target-selective protein labeling, but also for protein knockdown by CALI.