Selecting fixed-charge groups for electron-based peptide dissociations: A computational study of pyridinium tags

Fixed-charge groups based on pyridiniummethylcarboxamide moieties are analyzed by electronic structure theory calculations at combined B3LYP-PMP2 and CCSD(T) levels of theory to establish the ion recombination energies and thermodynamic and kinetic stability of radicals after electron capture. The fixed-charge pyridinium groups carry electron-donating and electron-withdrawing substituents and have higher recombination energies than protonated peptides, as modeled for (GK + H)+ and (GR + H)+. The pyridinium and peptide radicals formed by electron attachment have overlapping manifolds of electronic states that may allow for unidirectional or reversible intramolecular electron transfer in charge-tagged peptides. The pyridinium groups have moderate kinetic stabilities toward dissociation by N–CH2 bond cleavage after electron attachment. Alkylammonium ions coordinated to 18-crown-6-ether show extremely low recombination energies and may represent a special kind of charge tags. Electron attachment to [CH3NH3…18-crown-6-ether]+ complex forms a dipolar species resembling an organic electride, in which the ionic alkylammonium core is surrounded by a crown-solvated electron.