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

Abstract

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