Dissociation of the N-Cα bond and competitive formation of the [zn−H]•+ and [cn+2H]+ product ions in radical peptide ions containing tyrosine and tryptophan: The influence of proton affinities on product formation

Dissociations at the N-C(alpha) 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 beta-carbon of the tryptophan or tyrosine residue to the carbonyl oxygen of the amide group, followed by cleavage of the N-C(alpha) 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.