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.

In this study, we observed unprecedented cleavages of the Cβ–Cγ bonds of tryptophan residue side chains in a series of hydrogen-deficient tryptophan-containing peptide radical cations (M•+) during ...

Ion–molecule reactions of Mg⁺(H₂O)_n (n≈20–60) with CH₃CN are studied by Fourier‐transform ion‐cyclotron resonance mass spectrometry. Collision with CH₃CN initiates the formation of MgOH⁺(H₂O)_n−1 together with CH₃CHN^. or CH₃CNH^., which is similar to the reaction of hydrated electrons (H₂O)_n⁻ with CH₃CN. In subsequent reaction steps, three more CH₃CN molecules are taken up by the clusters, to form MgOH⁺(CH₃CN)₃ after a reaction delay of 60 seconds. Density functional theory (DFT) calculations at the M06/6‐31++G(d,p) level of theory suggest that the bending motion of CH₃CN allows the unpaired electron that is solvated out from the Mg center to localize in a π*(CN)‐like orbital of the bent CH₃CN^.−, which undergoes spontaneous proton transfer to form CH₃CNH^. or CH₃CHN^., with the former being kinetically more favorable. The reaction energy for a cluster with the hexacoordinated Mg center is more exothermic than that with the pentacoordinated Mg. The CH₃CNH^. or CH₃CHN^. is preferentially solvated on the cluster surface rather than at the first solvation shell of the Mg center. By contrast, the three additional CH₃CN molecules taken up by the resulting MgOH⁺(H₂O)_n clusters coordinate directly to the first solvation shell of the MgOH⁺ core, as revealed by DFT calculations.