Structure and fragmentation chemistry of the peptide radical cations of glycylphenylalanylglycine (GFG)

• Published in: PloS one Vol. 19; no. 8; p. e0308164
• Main Authors: Li, Yinan, Lau, Justin Kai-Chi, van Wieringen, Teun, Martens, Jonathan, Berden, Giel, Oomens, Jos, Hopkinson, Alan C, Siu, K W Michael, Chu, Ivan K
• Format: Journal Article
• Language: English
• Published: United States Public Library of Science 13.08.2024; Public Library of Science (PLoS)
• Subjects: Absorption spectra; Amino acids; Biology and Life Sciences; Carbon; Cations; Cations - chemistry; Chemistry; Copper; Copper - chemistry; Copper compounds; Density functional theory; Density functionals; Dissociation; Electron transport; Energy; Engineering and Technology; Enthalpy; Experiments; Free Radicals - chemistry; Isotopic labeling; Labeling; Lasers; Nitrogen; Oligopeptides - chemistry; Peptides; Peptides - chemistry; Physical Sciences; Radioactive labeling; Spectroscopy; Thermodynamics; United States > US
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0308164

Herein, we explore the generation and characterization of the radical cations of glycylphenylalanylglycine, or [GFG]•+, formed via dissociative electron-transfer reaction from the tripeptide to copper(II) within a ternary complex. A comprehensive investigation employing isotopic labeling, infrared multiple-photon dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations elucidated the details and energetics in formation of the peptide radical cations as well as their dissociation products. Unlike conventional aromatic-containing peptide radical cations that primarily form canonical π-radicals, our findings reveal that 75% of the population of the experimentally produced [GFG]•+ precursors are [GFα•G]+, where the radical resides on the middle α-carbon of the phenylalanyl residue. This unexpected isomeric ion has an enthalpy of 6.8 kcal/mol above the global minimum, which has an N-terminal captodative structure, [Gα•FG]+, comprising 25% of the population. The [b₂-H]•+ product ions are also present in a ratio of 75/25 from [GFα•G]+/ [Gα•FG]+, the results of which are obtained from matches between the IRMPD action spectrum and predicted IR absorption spectra of the [b₂-H]•+ candidate structures, as well as from IRMPD isomer population analyses.