C_α–C_β bond cleavage occurs at the side chain of the N-terminal phenylalanine residue during CID of tyrosine-containing peptide radical cations

Molecular radical cationic peptides (M^{[rad] +}) display rich and diverse gas-phase ion chemistry that can differ substantially from that of their protonated counterparts. Dissociation of M^{[rad] +} radical cations can provide relevant structural information for protein sequencing, often complementary to that obtained using traditional approaches. In this study, low-energy collision-induced dissociation (CID) of a series of tyrosine-containing peptide molecular radical cations having an N-terminal phenylalanine residue led to new products resulting from radical-mediated C_α–C_β bond cleavage at the side chain of the phenylalanine residue. The chemical identity of the species formed through side chain loss (91 Da) occurring via N-terminal C_α–C_β bond cleavage was examined using a combination of low-energy CID experiments, isotopic labeling mass spectrometric experiments, and density functional theory calculations. This unusual dissociation process occurs with fascinating unimolecular gas phase ion chemistry and requires a π-radical cationic tyrosine residue with significant spin density delocalized into the N-terminal amino group. The radical character at the resulting –NH₂ ^{[rad] +} moiety then induces β-scission at the side chain C_α–C_β bond of the phenylalanine residue. This discovery also demonstrates the role of the NH₂ group in facilitating C_α–C_β bond cleavage of the phenylalanine residue by the proximal tyrosine π-radical cation, probably involving a through-space electron transfer process.