Evaluation of stability against oxidation and acid dissociation properties for tetrahydrocurcumin in aqueous solution

Tetrahydrocurcumin has been of interest, the antioxidative activity of which has been reported to be higher than that of curcumin. In an attempt to investigate the function, we studied the stability of tetrahydrocurcumin in aqueous solutions and the acid dissociation properties, and compared them with those of curcumin. In aqueous solutions of pH 3.0-10.0 under air - saturated conditions, curcumin showed complete decomposition within l h. At pH 8.0, curcumin decomposed rapidly with a half-life time of 1.0 min. On the other hand, tetrahydrocurcumin did not show any detectable decomposition within 2 h, showing higher stability in aqueous solutions under air. The acid-dissociation constants of tetrahydrocurcumin were estimated to be pK_a1 = 8.59 ± 0.06, pK_a2 = 9.53 ± 0.01, pK_a3 = 10.74 ± 0.02, which are slightly larger than those of curcumin (pK_a1 = 8.47 ± 0.032, pK_a2 = 9.43 ± 0.025, pK_a3 = 10.59 ± 0.03) by 0.1 pK_a unit.