Skip to main content

Reactions of simple and peptidic alpha-carboxylate radical anions with dioxygen in the gas phase

Journal Article


Download full-text (Open Access)

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 (CH2CO 2-) and 1-carboxylatobutyl (CH3CH 2CH2CHCO2-) radical anions with dioxygen yield carbonate (CO3-) 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 Cα-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 Cα-radical anions: the acetylglycinate radical anion (CH3C(O)NHCHCO2-) 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 Cα 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. © 2011 the Owner Societies

Authors


  •   Ly, Tony (external author)
  •   Kirk, Benjamin B. (external author)
  •   Hettiarachchi, Pramesh I. (external author)
  •   Poad, Berwyck L. J.
  •   Trevitt, Adam J.
  •   da Silva, Gabriel (external author)
  •   Blanksby, Stephen J. (external author)

Publication Date


  • 2011

Citation


  • Ly, T., Kirk, B. B., Hettiarachchi, P. I., Poad, B. L., Trevitt, A. J., da Silva, G. & Blanksby, S. J. (2011). Reactions of simple and peptidic alpha-carboxylate radical anions with dioxygen in the gas phase. Physical Chemistry Chemical Physics, 13 (36), 16314-16323.

Scopus Eid


  • 2-s2.0-80053284644

Ro Full-text Url


  • http://ro.uow.edu.au/cgi/viewcontent.cgi?article=2162&context=scipapers

Ro Metadata Url


  • http://ro.uow.edu.au/scipapers/1096

Has Global Citation Frequency


Number Of Pages


  • 9

Start Page


  • 16314

End Page


  • 16323

Volume


  • 13

Issue


  • 36

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 (CH2CO 2-) and 1-carboxylatobutyl (CH3CH 2CH2CHCO2-) radical anions with dioxygen yield carbonate (CO3-) 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 Cα-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 Cα-radical anions: the acetylglycinate radical anion (CH3C(O)NHCHCO2-) 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 Cα 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. © 2011 the Owner Societies

Authors


  •   Ly, Tony (external author)
  •   Kirk, Benjamin B. (external author)
  •   Hettiarachchi, Pramesh I. (external author)
  •   Poad, Berwyck L. J.
  •   Trevitt, Adam J.
  •   da Silva, Gabriel (external author)
  •   Blanksby, Stephen J. (external author)

Publication Date


  • 2011

Citation


  • Ly, T., Kirk, B. B., Hettiarachchi, P. I., Poad, B. L., Trevitt, A. J., da Silva, G. & Blanksby, S. J. (2011). Reactions of simple and peptidic alpha-carboxylate radical anions with dioxygen in the gas phase. Physical Chemistry Chemical Physics, 13 (36), 16314-16323.

Scopus Eid


  • 2-s2.0-80053284644

Ro Full-text Url


  • http://ro.uow.edu.au/cgi/viewcontent.cgi?article=2162&context=scipapers

Ro Metadata Url


  • http://ro.uow.edu.au/scipapers/1096

Has Global Citation Frequency


Number Of Pages


  • 9

Start Page


  • 16314

End Page


  • 16323

Volume


  • 13

Issue


  • 36