Reactivity of Tyr–Leu and Leu–Tyr dipeptides: identification of oxidation products by liquid chromatography–tandem mass spectrometry

The exposure of peptides and proteins to reactive hydroxyl radicals results in covalent modifications of amino acid side‐chains and protein backbone. In this study we have investigated the oxidation the isomeric peptides tyrosine–leucine (YL) and leucine–tyrosine (LY), by the hydroxyl radical formed under Fenton reaction (Fe²⁺/H₂O₂). Through mass spectrometry (MS), high‐performance liquid chromatography (HPLC‐MS) and electrospray tandem mass spectrometry (HPLC‐MSⁿ) measurements, we have identified and characterized the oxidation products of these two dipeptides. This approach allowed observing and identifying a wide variety of oxidation products, including isomeric forms of the oxidized dipeptides. We detected oxidation products with 1, 2, 3 and 4 oxygen atoms for both peptides; however, oxidation products with 5 oxygen atoms were only present in LY. LY dipeptide oxidation leads to more isomers with 1 and 2 oxygen atoms than YL (3 vs 5 and 4 vs 5, respectively). Formation of the peroxy group occurred preferentially in the C‐terminal residue. We have also detected oxidation products with double bonds or keto groups, dimers (YL–YL and LY–LY) and other products as a result of cross‐linking. Both amino acids in the dipeptides were oxidized although the peptides showed different oxidation products. Also, amino acid residues have shown different oxidation products depending on the relative position on the dipeptide. Results suggest that amino acids in the C‐terminal position are more prone to oxidation. Copyright © 2009 John Wiley & Sons, Ltd.