Two water soluble isoflavone derivatives: sodium 4',7-bis(carboxymethoxy)isoflavone-3'-sulfonate (L1) and sodium 4'-hydroxy-7-carboxy-methoxyisoflavone-3'-sulfonate (L2) were synthesized and their structures were characterized by IR, UV, ¹H NMR and elemental analysis. The scavenging activities of L1, L2 and their parent daidzein to hydroxy radicals were detected by fluorescent spectrophotometry, while their scavenging activities on superoxide anion free radicals and 1,1-diphenyl-2-picrylhydrazyl free radical (DPPH•) were measured by ultraviolet spectrophotometry. The scavenging abilities of L1, L2 and daidzein to hydroxy radicals were also studied by quantum chemistry semiempirical calculation methods. The scavenging acitivity sites of L1, L2 and daidzein to hydroxyl radicals were analysed by the charge population of O—H and the difference of heat of formation between an antioxidant and its free radical (∆H_f), calculated by the semiempirical AM1 method. The experimental results showed that both L1 and L2 could scavenge hydroxy radicals, superoxide anion free radicals and DPPH• ef-fectively, and the scavenging activities of L1 and L2 were better than that of daidzein. The experimental re-sults were consistent with the calculational results.

Key words: isoflavone derivative, water soluble, antioxidant activity, free radical, theoretical calculation