Proper and improper aminoketyl radicals in electron-based peptide dissociations

Abstract

Two types of aminoketyl radicals and cation-radicals are distinguished by each's structure and reactivity. As proper aminoketyl radicals we denote the presumed intermediates of N C α bond dissociations induced by electron attachment to protonated peptides. These radicals have pyramidized aminoketyl groups, C αC (OH)NH , high spin density on the central carbon atom, and undergo facile N C α bond dissociations. The critical energies for N C α bond cleavages in proper aminoketyl radicals are summarized here and typically do not exceed 60 kJ mol −1 in peptide cation-radicals. In contrast, a different type of intermediates, which we call improper aminoketyl radicals, is formed by collisional dissociation of peptide cation-radicals, such as decarboxylation of z n fragments. Improper aminoketyl radicals have near planar C αC(OH)NH groups, and the spin density is delocalized over several atoms adjacent to the aminoketyl moiety. Improper aminoketyl radicals show higher transition energies for N C α bond cleavage and undergo H-atom transfers resulting in side-chain losses.