Cα–Cβ Bond Cleavage Occurs at the Side Chain of the N-Terminal Phenylalanine Residue During CID of Tyrosine-Containing Peptide Radical Cations

Abstract

Molecular radical cationic peptides (M•+) display rich and diverse gas-phase ion chemistry that can differ substantially from that of their protonated counterparts. Dissociation of M•+ radical cations can provide relevant structural information for protein sequencing, often complementary to that obtained using traditional approaches1-2. 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 Cα–Cβ 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 Cα–Cβ 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•+ moiety then induces β-scission at the side chain Cα–Cβ bond of the phenylalanine residue. This unique neutral radical loss (91 Da) from the N-terminal phenylalanine residue, observed here for the first time, should provide additional sequence information for biological tandem mass spectrometry. This discovery also demonstrates the role of the NH2 group in facilitating Cα–Cβ bond cleavage of the phenylalanine residue by the proximal tyrosine π-radical cation, probably involving a through-space electron transfer process.