We have used model tripeptides GXW (with X being one of the amino acid residues glycine (G), alanine (A), leucine (L), phenylalanine (F), glutamic acid (E), histidine (H), lysine (K), or arginine (R)) to study the effects of the basicity of the amino acid residue on the radical migrations and dissociations of odd-electron molecular peptide radical cations M.+ in the gas phase. Low-energy collision-induced dissociation (CID) experiments revealed that the interconvertibility of the isomers [G.XW]+ (radical centered on the N-terminal α-carbon atom) and [GXW].+ (radical centered on the π system of the indolyl ring) generally increased upon increasing the proton affinity of residue X. When X was arginine, the most basic amino acid, the two isomers were fully interconvertible and produced almost identical CID spectra despite the different locations of their initial radical sites. The presence of the very basic arginine residue allowed radical migrations to proceed readily among the [G.RW]+ and [GRW].+ isomers prior to their dissociations. Density functional theory calculations revealed that the energy barriers for isomerizations among the α-carbon-centered radical [G.RW]+, the π-centered radical [GRW].+, and the β-carbon-centered radical [GRWβ.]+ (ca. 32–36 kcal mol−1) were comparable with those for their dissociations (ca. 32–34 kcal mol−1). The arginine residue in these GRW radical cations tightly sequesters the proton, thereby resulting in minimal changes in the chemical environment during the radical migrations, in contrast to the situation for the analogous GGW system, in which the proton is inefficiently stabilized during the course of radical migration.
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