UB3LYP/6-31+G* calculations were used to elucidate the structures of the complexes of Cu2+ with various ligands including aminoguanidine, pyridoxamine, Amadori compound and ascorbic acid. The solvent effect was simulated by using the CPCM method. The square-planar chelate containing two ligand molecules was found to be the most stable form in all complexes. The stability of the different chelates was consistent with the facts that aminoguanidine inhibits the oxidation of ascorbic acid but not that of Amadori compounds, whereas pyridoxamine inhibits both. (C) 2009 Elsevier B. V. All rights reserved.