Superoxide Scavenging Effects of Some Novel Bis-Ligands and Their Solvated Metal Complexes Prepared by the Reaction of Ligands with Aluminum, Copper and Lanthanum Ions

Novel antioxidants have been synthesized and characterized by their chemical properties as antioxidants with high superoxide scavenging activity. (2R,3R)-diphenylethylenediamine is a spacer in antioxidants, and we synthesized targets 11a and 11b by conjugation with o-hydroquinone and p-hydroquinone at the two N-termini, respectively. Superoxide scavenging activities of the plant flavonoid-like 11a and 11b were compared with those of known antioxidants, and shown to increase in the following order: curcumin << ascorbic acid < Trolox < (+)-epicatechin < 11a < quercetin ° 11b. Compound 11a also formed a solvated 11a–metal complex with metal ions. The 11a–Cu(2+) complex was shown to have higher superoxide scavenging activity than that of 11a, 11b, Cu(2+), and the 11a–Al(3+) and 11a–La(3+) complexes, whereas the 11a–Al(3+) complex increased rather than decreased superoxide levels. The 11a–Al(3+) complex did not abstract one electron from the SOMO of O(2)(−.) since the HOMO and LUMO phases of Al(3+) do not exist in the center of the 11a–Al(3+) complex. However, the SOMO of the 11a–Cu(2+) complex distributed in the center of Cu(2+) may abstract one electron from the SOMO of O(2)(−.). These results suggest that 11a and 11b are powerful antioxidants.