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Issue 36, 2011
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Reactions of simple and peptidic alpha-carboxylate radical anions with dioxygen in the gas phase

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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 (˙CH2CO2) and 1-carboxylatobutyl (CH3CH2CH2˙CHCO2) 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)NH˙CHCO2) 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.

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

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Supplementary files

Article information


Submitted
16 Mar 2011
Accepted
22 Jun 2011
First published
11 Jul 2011

Phys. Chem. Chem. Phys., 2011,13, 16314-16323
Article type
Paper

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

T. Ly, B. B. Kirk, P. I. Hettiarachchi, B. L. J. Poad, A. J. Trevitt, G. da Silva and S. J. Blanksby, Phys. Chem. Chem. Phys., 2011, 13, 16314
DOI: 10.1039/C1CP20784A

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