dibenzocyclooctendiones (DBCDOs) are efficient chemical reagents for the site-specific labeling of arginine-containing biomolecules. Unlike the commonly used probes, DBCDOs undergo an irreversible ring-contracted rearrangement with the guanidinium group on arginineresidues under mild reaction conditions. The regioselective dual-labeled arginineresidues were obtained in a one-pot reaction with our tested