Objective   To synthesize three kinds of metal complexes of aloe-emodin and compare the antioxidant activities of the ligands and the complexes.

  Methods  Three kinds of aloe emodin metal complex, the aloe-emodin-iron (Ⅱ), the aloe-emodin-copper (Ⅱ) and the aloe-emodin-magnesium (Ⅱ) complexes, were synthesized by dissolving and stirring in anhydrous ethanol solvent, and their structures were characterized. The Fe²⁺-H₂O₂-methylene blue method, the diphenyl bitter hydrazine radical method (DPPH method) and other assays were used to determine the clearance effect of ligands and complexes on superoxide radicals (O₂^－•), hydroxyl radicals (•OH) and phenyl bitter hydrazine radical (DPPH•).

  Results  Three kinds of aloe emodin metal complex, the aloe-emodin-iron (Ⅱ), the aloe-emodin-copper (Ⅱ) and the aloe-emodin-magnesium (Ⅱ) complexes, were successfully synthesized. According to the results of structural characterization, we speculated that the aloe-emodin metal complexes were formed at the site between the two molecules of aloe-emodin and one molecule of metal ions (Fe²⁺, Mg²⁺, Cu²⁺) via the 9^th carbonyl and 8^th hydroxyl groups of the aloe-emodin molecules. Both the complex and the ligand have clearance effects on three kinds of free radicals, and the complex showed stronger effects than its ligand (P<0.05).

  Conclusion  Coordination of aloe-emodin with metal ions, such as Fe²⁺, Cu²⁺, and Mg²⁺, could enhance the antioxidant activity of the ligand itself.