Effect of the N-terminal basic residue on facile Cα–C bond cleavages of aromatic-containing peptide radical cations

Abstract

Fragmentation of radical cationic peptides [R(G)_n−2X(G)_7−n]˙⁺ and [R(G)_m−2XG]˙⁺ (X = Phe or Tyr; m = 2–5; n = 2–7) leads selectively to a_n⁺ product ions through in situ C_α–C peptide backbone cleavage at the aromatic amino acid residues. In contrast, substituting the arginine residue with a less-basic lysine residue, forming [K(G)_n−2X(G)_7−n]˙⁺ (X = Phe or Tyr; n = 2–7) analogs, generates abundant b–y product ions; no site-selective C_α–C peptide bond cleavage was observed. Studying the prototypical radical cationic tripeptides [RFG]˙⁺ and [KFG]˙⁺ using low-energy collision-induced dissociation and density functional theory, we have examined the influence of the basicity of the N-terminal amino acid residue on the competition between the isomerization and dissociation channels, particularly the selective C_α–C bond cleavage via β-hydrogen atom migration. The dissociation barriers for the formation of a₂⁺ ions from [RFG]˙⁺ and [KFG]˙⁺via their β-radical isomers are comparable (33.1 and 35.0 kcal mol⁻¹, respectively); the dissociation barrier for the charge-induced formation of the [b₂ − H]˙⁺ radical cation from [RFG]˙⁺via its α-radical isomer (39.8 kcal mol⁻¹) was considerably higher than that from [KFG]˙⁺ (27.2 kcal mol⁻¹). Thus, the basic arginine residue sequesters the mobile proton to promote the charge-remote selective C_α–C bond cleavage by energetically hindering the competing charge-induced pathways.