Determination of acidity constants of curcumin in aqueous solution and apparent rate constant of its decomposition

The stability of curcumin (H(3)Cur) in aqueous media is unproved when the systems in which it is present are at high pH values (higher than 11.7), fitting a model describable by a pseudo-zero order with a rate constant K for the disappearance of the Cur(3-) species of 1.39 (10(-9)) M min(-1). There were three acidity constants measured for the curcumin as follows: pK(A3) = 10.51 +/- 0.01 corresponding to the equilibrium HCur(2-) = Cur(3-) + H+, a pK(A2) = 9.88 +/- 0.02 corresponding to the equilibrium H(2)Cur(-) = HCur(-2) + H+. These PKA values were attributed to the hydrogen of the phenol part of the curcumin, while the pK(A1) = 8.38 +/- 0.04 corresponds to the equilibrium H(3)Cur = H(2)Cur(-) + H+ and is attributed the acetylacetone type group. Formation of quinoid structures play an important role in the tautomeric forms of the curcumin in aqueous media, which makes the experimental values differ from the theoretically calculated ones, depending on the conditions adopted in the study. (C) 2003 Elsevier B.V. All rights reserved.