Proper and improper aminoketyl radicals in electron-based peptide dissociations

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 NCα bond dissociations induced by electron attachment to protonated peptides. These radicals have pyramidized aminoketyl groups, CαC(OH)NH, high spin density on the central carbon atom, and undergo facile NCα bond dissociations. The critical energies for NCα 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 zn fragments. Improper aminoketyl radicals have near planar CαC(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 NCα bond cleavage and undergo H-atom transfers resulting in side-chain losses.

Graphical abstractProper and improper aminoketyl radicals and cation-radicals are two types of intermediates that are distinguished by structure and reactivity.Figure optionsDownload full-size imageDownload high-quality image (214 K)Download as PowerPoint slide