Products of Cu(II)-catalyzed oxidation of the N-terminal fragments of α-synuclein in the presence of hydrogen peroxide

Reactive oxygen species (ROS) may provide the covalent modifications of amino acid residues in proteins, formation of protein–protein cross-linkages, and oxidation of the protein backbone resulting in protein fragmentation. In an attempt to elucidate the products of the copper(II)-catalyzed oxidation of the (1–17), (1–28), (1–39) and (1–39)(A30P) fragments of α-synuclein, the high performance liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI–TOF MS) methods and Cu(II) /hydrogen peroxide as a model oxidizing system were employed. Peptide solution (0.50 mM) was incubated at 37 °C for 24 h with metal:peptide:hydrogen peroxide molar ratio 1:1:4 in phosphate buffer, pH 7.4. Oxidation targets for all peptide studied are the methionine residues (M1, M5). Incubation 24 h of the (1–28), (1–39) and (1–39)(A30P) fragments in aerobic conditions lead to the oxidation of one methionine residue to methionine sulfoxide. Reaction of hydrogen peroxide with all fragments of α-synuclein resulted in oxidation of two methionine residues (M1, M5) to methionine sulfoxides. For the Cu(II):peptide:hydrogen peroxide 1:1:4 molar ratio systems the further oxidation of methionine residues to sulfone was observed. The cleavage of the peptide bond M1–D2 for all peptides studied was observed as metal binding residues. For the (1–39) and (1–39)(A30P) fragments of α-synuclein the molecular ions with lower molecular masses (A11–Y39, E13–Y39) were also detected.