Mechanistic Examinations for Facile Cleavage of the N-terminal Cα―C Bond in Tyrosine-containing Peptide Radical Cations in the Gas Phase

Gas-phase dissociations of radical peptide ions in a mass spectrometer form the theoretical basis of the top-down micro-sequencing approaches for protein identifications in bioanalytical applications. Selective bond cleavage at the N―C_α bond along the peptide backbone is the characteristic feature of the dissociation of peptide radicals. Recently, an efficient cleavage at the N-terminal C_α―C bond in molecular radical cations (M^•+) of tyrosine-containing peptides, yielding an uncommon x-type radical fragment, was discovered. Mechanism of this unusual dissociation channels has been investigated using collision-induced dissociation (CID) experiments and density function theory (DFT). Different dissociations channels were compared using Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. In this talk, our established mechanism, involving a direct N-terminal C_α―C bond cleavage from a π-radical-cation tautomer of tyrosine, will be discussed.