title: Reactions of simple and peptidic alpha-carboxylate radical anions with dioxygen in the gas phase.
abstract: α-Carboxylate radical anions are potential reactive intermediates in the free radical oxidation of biological molecules (e.g., fatty acids, peptides and proteins). We have synthesised well-defined α-carboxylate radical anions in the gas phase by UV laser photolysis of halogenated precursors in an ion-trap mass spectrometer. Reactions of isolated acetate (˙CH(2)CO(2)(-)) and 1-carboxylatobutyl (CH(3)CH(2)CH(2)˙CHCO(2)(-)) radical anions with dioxygen yield carbonate (CO(3)˙(-)) radical anions and this chemistry is shown to be a hallmark of oxidation in simple and alkyl-substituted cross-conjugated species. Previous solution phase studies have shown that C(α)-radicals in peptides, formed from free radical damage, combine with dioxygen to form peroxyl radicals that subsequently decompose into imine and keto acid products. Here, we demonstrate that a novel alternative pathway exists for two α-carboxylate C(α)-radical anions: the acetylglycinate radical anion (CH(3)C(O)NH˙CHCO(2)(-)) and the model peptide radical anion, YGGFG˙(-). Reaction of these radical anions with dioxygen results in concerted loss of carbon dioxide and hydroxyl radical. The reaction of the acetylglycinate radical anion with dioxygen reveals a two-stage process involving a slow, followed by a fast kinetic regime. Computational modelling suggests the reversible formation of the C(α) peroxyl radical facilitates proton transfer from the amide to the carboxylate group, a process reminiscent of, but distinctive from, classical proton-transfer catalysis. Interestingly, inclusion of this isomerization step in the RRKM/ME modelling of a G3SX level potential energy surface enables recapitulation of the experimentally observed two-stage kinetics.
authors: School of Chemistry, University of Wollongong, NSW 2522, Australia.
authors: Tony
authors: T
authors: Ly
authors: Benjamin B
authors: BB
authors: Kirk
authors: Pramesh I
authors: PI
authors: Hettiarachchi
authors: Berwyck L J
authors: BL
authors: Poad
authors: Adam J
authors: AJ
authors: Trevitt
authors: Gabriel
authors: G
authors: da Silva
authors: Stephen J
authors: SJ
authors: Blanksby
mesh_terms: D000595:Amino Acid Sequence; D000838:Anions / Q000737:chemistry; D002384:Catalysis; D005609:Free Radicals / Q000737:chemistry*; D005740:Gases / Q000737:chemistry*; D017665:Hydroxyl Radical / Q000737:chemistry; D007536:Isomerism; D007700:Kinetics; D008958:Models, Molecular; D010100:Oxygen / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry*
chemical_list: D000838:Anions; D005609:Free Radicals; D005740:Gases; D010455:Peptides; D017665:Hydroxyl Radical; D010100:Oxygen