Dissociation of the N-C_α Bond and Competitive Formation of the [z_n - H]^•+ and [c_n + 2H]⁺ Product Ions in Radical Peptide Ions Containing Tyrosine and Tryptophan : The Influence of Proton Affinities on Product Formation

Dissociations at the N-C_α bond of tryptophan and tyrosine residues are the prevalent pathways in the fragmentations of radical cations of tripeptides that contain such as residues. This process involves a proton transfer from the β-carbon of the tryptophan or tyrosine residue to the carbonyl oxygen of the amide group, followed by cleavage of the N-C_α bond, generating low-lying proton-bound dimers that dissociate to give each an ionic and a neutral product. Formation of the [z_n - H]^•+ or [c_n + 2H]⁺ ion is a competition between the two incipient fragments for the proton in a dissociating proton-bound dimer. © 2008 American Society for Mass Spectrometry.