Comparative antioxidant activities of curcumin and its demethoxy and hydrogenated derivatives

The antioxidant activities of curcumin, its natural demethoxy derivatives (demethoxycurcumin, Dine and bisdemethoxycurcumin, Bdmc) and metabolite hydrogenated derivatives (tetrahydrocurcumin, THC; hexahydrocurcumin, HHC; octahydrocurcumin; OHC) were comparatively studied using 2,2-diphenyl-1-picrylhydrazyl (DDPH) radical, 2,2'-azobis(2-amidinopropane)dihydrochloride (AAPH) induced linoleic oxidation and AAPH induced red blood cell hemolysis assays. Hydrogenated derivatives of curcumin exhibited stronger DPPH scavenging activity compared to curcumin and a reference antioxidant, trolox. The scavenging activity significantly decreased in the order THC > HHC=OHC > trolox > curcumin > Dmc >>> Bdmc. Stronger antioxidant activities toward lipid peroxidation and red blood cell hemolysis were also demonstrated in the hydrogenated derivatives. By the model of AAPH induced linoleic oxidation, the stoichiometric number of peroxyl radical that can be trapped per molecule (n) of hydrogenated derivatives were 3.4, 3.8 and 3.1 for THC, HHC and OHC, respectively. The number (n) of curcumin and Dine were 2.7 and 2.0, respectively, which are comparable to trolox, while it was 1.4 for Bdmc. The inhibition of AAPH induced red blood cell hemolysis significantly decreased in the order OHC > THC=HHC > trolox > curcumin=Dmc. Results in all models demonstrated the lower antioxidant activity of the demethoxy derivatives, suggesting the ortho-methoxyphenolic groups of curcumin are involved in antioxidant activities. On the other hand, hydrogenation at conjugated double bonds of the central seven carbon chain and beta diketone of curcumin to THC, HHC and OHC remarkably enhance antioxidant activity.