How Symmetry Influences the Dissociation of Protonated Cyclic Peptides

Abstract

Protonated cyclic dipeptides undergo collision-induced dissociation, and this reaction mechanism strongly depends on the symmetry and the nature of the residues. We review the main dissociation mechanism for a series of cyclic dipeptides, obtained through chemical dynamics simulations. The systems range from the symmetrical cyclo-(glycyl-glycyl), with two possible symmetrical protonation sites located on the peptide ring, to cyclo-(tyrosyl-prolyl), where the symmetry of protonation sites on the peptide ring is broken by the dissimilar nature of the different residues. Finally, cyclo-(phenylalanyl-histidyl) shows a completely asymmetric situation, with the proton located on one of the dipeptide side chains, which explains the peculiar fragmentation mechanism induced by shuttling the proton, whose efficiency is strongly dependent on the relative chirality of the residues.