We report a computational study of Ala–Lys (AK) and Lys–Ala (KA) dipeptide ions furnished with fixed-charged pyridinium groups that were attached by amide linkers to the N-terminal amino groups. Cation–radicals from one-electron reduction of the doubly charged AK and KA peptide conjugates showed various extents of unpaired electron density being delocalized between the pyridine and peptide moieties. The delocalization depended on the local recombination energies (RE_loc) of the charged groups. The RE_loc of the pyridine moieties were modified by introducing electron-donating substituents (CH₃, OCH₃, and N(CH₃)₂). The RE_loc of the peptide moieties were found to depend on the peptide conformation and internal solvation of the Lys ammonium groups. Substantial electron delocalization was found for combinations of pyridine substituents and peptide conformers with closely matched RE_loc, such as 4-dimethylamino­pyridine and internally solvated Lys ammonium or unsubstituted pyridine and free (unsolvated) Lys ammonium. The dissociation (ΔH_diss) and transition state energies (E_TS) for the loss of the pyridine ring from the conjugates were found to be ΔH_diss = 34–36 kJ mol^–1 and E_TS = 67–69 kJ mol^–1 for the unsubstituted pyridine moieties, but did not depend much on the peptide sequence.

Keywords: charge-tagged peptides, electron capture, peptide cation-radicals, electronic states, recombination energies