Antioxidant capacity of curcumin-directed analogues: Structure–activity relationship and influence of microenvironment

Curcumin is the active ingredient of turmeric powder with a variety of biological activities including antioxidative activity. In order to find more active antioxidants with curcumin as the lead compound we synthesised a series of enone analogues of curcumin. The present work studied and compared the capacity of curcumin-directed analogues to scavenge 2,2-diphenyl-1-picrylhydrazyl (DPPH) and protect human red blood cells (RBCs) from oxidative haemolysis. It was found that these compounds which bear o-diphenoxyl and o-dimethoxyphenoxyl groups exhibited significantly higher DPPH-scavenging and anti-haemolysis activities than those which bear no such groups. In contrast to curcumin analogues that retained the 7-carbon spacer, the compounds with a 5-carbon linker had lower activity. In the case of the latter, the introduction of a ring further decreased DPPH-scavenging activity. However, the introduction of a ring did increase anti-haemolysis activity, suggesting that the lipophilicity of these compounds might play an important role in the antioxidant activity.