Tautomerization and Dissociation of Molecular Peptide Radical Cations

Radical-mediated dissociations of peptide radical cations have intriguing unimolecular gas phase chemistry, with cleavages of almost every bond of the peptide backbone and amino acid side chains in a competitive and apparently “stochastic” manner. Challenges of unraveling mechanistic details are related to complex tautomerizations prior to dissociations. Recent conjunctions of experimental and theoretical investigations have revealed the existence of non-interconvertible isobaric tautomers with a variety of radical-site-specific initial structures, generated from dissociative electron transfer of ternary metal-ligand-peptide complexes. Their reactivity is influenced by the tautomerization barriers, perturbing the nature, location, or number of radical and charge site(s), which also determine the energetics and dynamics of the subsequent radical-mediated dissociatons. The competitive radical- and charge-induced dissociations are extremely dependent on charge density. Charge sequesting can reduce the charge densities on the peptide backbone and hence enhance the flexibility of structural rearrangement. Analysing the structures of precursors, intermediates and products has led to the discovery of many novel radical migration prior to peptide backbone and/or side chain fragmentations. Upon these successes, scientists will be able to build peptide cationic analogues/tautomers having a variety of well-defined radical sites.