Reactive oxygen species (ROS) such as hydrogen peroxide
(H2O2) and the hydroxyl radical (•OH) have specific functions in biological processes, while their
uncontrolled production and reactivity are known to be determining
factors in pathophysiology. Methionine (Met) residues act as endogenous
antioxidants, when they are oxidized into methionine sulfoxide (MetSO),
thus depleting ROS and protecting the protein. We employed tandem
mass spectrometry combined with IR multiple photon dissociation spectroscopy
to study the oxidation induced by OH radicals produced by γ
radiolysis on model cyclic dipeptides c(LMetLMet), c(LMetDMet), and
c(GlyMet). Our aim was to characterize the geometries of the oxidized
peptides in the gas phase and to understand the relationship between
the structure of the 2-center 3-electron (2c–3e) free radical
formed in the first step of the oxidation process and the final compound.
Density functional theory calculations were performed to characterize
the lowest energy structures of the final product of oxidation and
to interpret the IR spectra. Collision-induced dissociation tandem
mass spectrometry (CID-MS2) experiments of oxidized c(LMetLMet)H+ and c(LMetDMet)H+ led to the loss of one or two
oxidized sulfenic acid molecules, indicating that the addition of
one or two oxygen atoms occurs on the sulfur atom of both methionine
side chains and no sulfone formation was observed. The CID-MS2 fragmentation mass spectrum of oxidized c(GlyMet)H+ showed only the loss of one oxidized sulfenic acid molecule. Thus,
the final products of oxidation are the same regardless of the structure
of the precursor sulfur-centered free radical.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.