Arginine-Facilitated Isomerization: Radical-Induced Dissociation of Aliphatic Radical Cationic Glycylarginyl(iso)leucine Tripeptides

The gas phase fragmentations of aliphatic radical cationic glycylglycyl(iso)leucine tripeptides ([G•G(L/I)]+), with well-defined initial locations of the radical centers at their N-terminal α-carbon atoms, are significantly different from those of their basic glycylarginyl(iso)leucine ([G•R(L/I)]+) counterparts; the former lead predominantly to [b2 – H]•+ fragment ions, whereas the latter result in the formation of characteristic product ions via the losses of •CH(CH3)2 from [G•RL]+ and •CH2CH3 from [G•RI]+ through Cβ–Cγ side-chain cleavages of the (iso)leucine residues, making these two peptides distinguishable. The α-carbon-centered radical at the leucine residue is the key intermediate that triggers the subsequent Cβ–Cγ bond cleavage, as supported by the absence of •CH(CH3)2 loss from the collision-induced dissociation of [G•RLα‑Me]+, a radical cation for which the α-hydrogen atom of the leucine residue had been substituted by a methyl group. Density functional theory calculations at the B3LYP 6-31++G(d,p) level of theory supported the notion that the highly basic arginine residue could not only increase the energy barriers against charge-induced dissociation pathways but also decrease the energy barriers against hydrogen atom transfers in the GR(L/I) radical cations by ∼10 kcal mol–1, thereby allowing the intermediate precursors containing α- and γ-carbon-centered radicals at the (iso)leucine residues to be formed more readily prior to promoting subsequent Cβ–Cγ and Cα–Cβ bond cleavages. The hydrogen atom transfer barriers for the α- and γ-carbon-centered GR(L/I) radical cations (roughly in the range 29–34 kcal mol–1) are comparable with those of the competitive side-chain cleavage processes. The transition structures for the elimination of •CH(CH3)2 and •CH2CH3 from the (iso)leucine side chains possess similar structures, but slightly different dissociation barriers of 31.9 and 34.0 kcal mol–1, respectively; the energy barriers for the elimination of the alkenes CH2CH(CH3)2 and CH3CHCHCH3 through Cα–Cβ bond cleavages of γ-carbon-centered radicals at the (iso)leucine side chains are 29.1 and 32.8 kcal mol–1, respectively.