Oxidation Products of S‐Adenosyl Methionine Probed by Infrared Multiple Photon Dissociation Spectroscopy

S‐adenosyl methionine (SAM) is a key biomolecule in cellular processes, acting as a primary methyl donor, having different roles in enzymatic reactions and being used as a scavenger of hydroxyl radicals. Due to the role played by SAM and its antioxidant properties, the one electron oxidation products of SAM produced by gamma radiolysis, which can mimic the oxidation effect produced by hydroxyl radical in the oxidative stress process, have been investigated in this study by collision‐induced dissociation tandem mass spectrometry (CID‐MS ² ) and infrared multiple photon dissociation spectroscopy (IRMPD). We have revealed the modified sites in the molecule and characterized the 3D structure of the oxo‐forms of SAM in the gas phase. Both CID‐MS ² and IRMPD experiments, when coupled with quantum chemistry calculations have shown that only the adenine moiety of the molecule is oxidized, protecting the methionine from oxidation.