title: Determination of acidity constants of curcumin in aqueous solution and apparent rate constant of its decomposition.
abstract: The stability of curcumin (H3Cur) in aqueous media is improved 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 Cur3- species of 1.39 (10(-9)) Mmin(-1). There were three acidity constants measured for the curcumin as follows: pKA3 = 10.51 +/- 0.01 corresponding to the equilibrium HCur2- = Cur3- + H+, a pKA2 = 9.88 +/- 0.02 corresponding to the equilibrium H2Cur- = HCur-(2) + H+. These pKA values were attributed to the hydrogen of the phenol part of the curcumin, while the pKA1 = 8.38 +/- 0.04 corresponds to the equilibrium H3Cur = H2Cur- + 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.
authors.affiliation: Departamento de Química, Universidad Autónoma Metropolitana-Iztapalapa, Area de Química Analítica, Apdo. Postal 55-534, C.P. 09340, México, D.F. México.
authors.forename: Margarita
authors.initials: M
authors.lastname: Bernabé-Pineda
authors.forename: María Teresa
authors.initials: MT
authors.lastname: Ramírez-Silva
authors.forename: Mario
authors.initials: M
authors.lastname: Romero-Romo
authors.forename: Enrique
authors.initials: E
authors.lastname: González-Vergara
authors.forename: Alberto
authors.initials: A
authors.lastname: Rojas-Hernández
mesh_terms: D000042:Absorption; D003474:Curcumin / Q000737:chemistry*; D006859:Hydrogen / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008956:Models, Chemical; D008968:Molecular Conformation; D011522:Protons; D012995:Solubility; D012996:Solutions / Q000737:chemistry*; D013053:Spectrophotometry / Q000379:methods*; D013816:Thermodynamics; D013997:Time Factors; D014466:Ultraviolet Rays
chemical_list: D011522:Protons; D012996:Solutions; D006859:Hydrogen; D003474:Curcumin