The interactions of proteins with reactive oxygen species (ROS) may result in 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 metal-catalyzed oxidation of the human (H) and mouse (M) (1-10H), (1-10M), (1-16H) and (1-16M) fragments of beta-amyloid peptide, the high performance liquid chromatography (HPLC) and matrix-assisted laser desorption/ionization mass spectrometry (MALDI-TOF MS) methods and Cu(II)/H(2)O(2) as a model oxidizing system were employed. Peptide solution (0.50 mM) was incubated at 37 degrees C for 24 h with metal:peptide:H(2)O(2) molar ratio 1:1:1 for the (1-16H), (1-16M) fragments, and 1:1:2 for the (1-10H), (1-10M) peptides in phosphate buffer, pH 7.4. Oxidation targets for all peptide studied are the histidine residues coordinated to the metal ions. For the (1-16H) peptide are likely His(13) and/or His(14), and for the (1-16M) fragment His(6) and/or His(14), which are converted to 2-oxo-His. Metal-binding residue, the aspartic acid (D(1)) undergoes the oxidative decarboxylation and deamination to pyruvate. The cleavages of the peptide bonds by either the diamide or alpha-amidation pathways were also observed.