Simulating electron transfer attachment to a positively charged model peptide

Ab initio electronic structure methods, including stabilization method tools for handling electronically metastable states, are used to treat a model system designed to probe the electron-transfer event characterizing electron-transfer dissociation (ETD) mass spectroscopic studies of peptides. The model system consists of a cation H3C-(C=O)NH-CH2-CH2-NH3+, containing a protonated amine site and an amide site, that undergoes collisions with a CH3- anion. Cross-sections for electron transfer from CH3- to the protonated amine site are shown to exceed those for transfer to the Coulomb-stabilized amide site by 2 orders of magnitude. Moreover, it is shown that the fates of the amine-attached and amide-attached species are similar in that both eventually lead to the same carbon-centered radical species H3C-(C-center dot-OH)NH-CH2-CH2-NH2, although the reaction pathways by which the two species produce this radical are somewhat different. The implications for understanding peptide fragmentation patterns under ETD conditions are also discussed in light of this work's findings.