| Summary: | 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–Cu2+ complex was shown to have higher superoxide scavenging activity than that of 11a, 11b, Cu2+, and the 11a–Al3+ and 11a–La3+ complexes, whereas the 11a–Al3+ complex increased rather than decreased superoxide levels. The 11a–Al3+ complex did not abstract one electron from the SOMO of O2−. since the HOMO and LUMO phases of Al3+ do not exist in the center of the 11a–Al3+ complex. However, the SOMO of the 11a–Cu2+ complex distributed in the center of Cu2+ may abstract one electron from the SOMO of O2−.. These results suggest that 11a and 11b are powerful antioxidants.
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