Investigation of Peptide Labeling with ortho-Phthalaldehyde and 2‑Acylbenzaldehyde

ortho-Phthalaldehyde (OPA) with highreactivityto the amine group has been widely used to modify proteins. We discoverednew modifications of OPA and 2-acylbenzaldehyde and proposed the reactionmechanism. Using isotope labeling mass spectrometry-based experiment,we identified new cross-linking properties of OPA and 2-acylbenzaldehyde.This reactivity revealed that OPA has the potential to probe proximalamino acids in biological systems.