Characterization of glycylglycyltyrosine radical cations: Insights into β-radical formation and N–Cα peptide bond dissociation at the tyrosine residue

We investigated the dissociation and characterization of radical cations of glycylglycyltyrosine [GGY]^•+, focusing on β-radical-induced N–C_α peptide bond cleavage reactions at the tyrosyl residue. By combining density functional theory (DFT) calculations with experimental studies utilizing low-energy collision-induced dissociation (CID) mass spectrometry and deuterium labeling on the two β-hydrogen atoms of the tyrosyl residue in [GGY]^•+, we elucidated the intricacies of the β-radical structure and its origin. Unlike tryptophan-containing [GGW]^•+, which forms canonical π-radical precursors, infrared multiphoton dissociation (IRMPD) spectroscopy results reveal that the β-radical [GGY_β^•]⁺ isomerizes from the phenoxy-radical [GGY_o^•]⁺, with the radical localized on the β-carbon of the tyrosyl residue and the phenolic oxygen atom, respectively. The isomerization barriers from [GGY_o^•]⁺ to [GGY_β^•]⁺ are <109 kJ mol⁻¹.