Proximity Histidine Labeling by Umpolung Strategy Using Singlet Oxygen

While electrophilic reagents for histidine labeling have been developed, we report an umpolung strategy for histidine functionalization. A nucleophilic small molecule, 1-methyl-4-arylurazole, selectively labeled histidine under singlet oxygen (O-1(2)) generation conditions. Rapid histidine labeling can be applied for instant protein labeling. Utilizing the short diffusion distance of O-1(2) and a technique to localize the O-1(2) generator, a photocatalyst in close proximity to the ligand-binding site, we demonstrated antibody Fc-selective labeling on magnetic beads functionalized with a ruthenium photocatalyst and Fc ligand, ApA. Three histidine residues located around the ApA binding site were identified as labeling sites by liquid chromatography-mass spectrometry analysis. This result suggests that O-1(2)-mediated histidine labeling can be applied to a proximity labeling reaction on the nanometer scale.