Dissociation of the N-C(alpha) 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.