Temperature Evaluation of Curcumin Keto-Enolic Kinetics and Its Interaction with Two Pluronic Copolymers

Curcumin (CUR), a natural hydrophobic polyphenol isolated from Curcuma longa, has been reported to possess two main equilibria in aqueous solutions, diketo/ keto-enolic tautomerism and self-aggregation. The thermodynamics of tautomeric equilibrium is well established; however, its kinetic parameters have been sparsely studied. Various efforts have been made to improve CUR solubility in aqueous media. We evaluated how the kinetics of tautomerism and the interaction of CUR with pluronic P123 and F127 copolymers in solution were affected by temperature, using UV-vis and fluorescence spectroscopies. Pluronic particle sizes with and without CUR were acquired by dynamic light scattering. The interaction in the solid state was verified by differential scanning calorimetry (DSC). The equilibrium rate that displaces to the diketo form increased fivefold when the temperature rose from 294 to 314 K with an activation energy of 61.2 kJ mol(-1). The CUR solubility increased from 2.58 to 6.77 mg g(-1) when incorporated in P123 and from 0.05 to 3.54 mg g(-1) when incorporated in F127 with a change in the temperature from 298 to 314 K. This process had a Gibbs free energy of around -1 and -13 kJ mol(-1) because of CUR solubilization into the inner core of pluronic micelles. Particle sizes of about 11 nm were obtained for both copolymers containing CUR in an aqueous solution above the critical micelle temperature. DSC measurements showed melting point depression of both CUR and F127. P123 presented no significant variation in the melting point because of its low melting enthalpy. The results indicate that temperature significantly influences CUR kinetic tautomerism and its interaction with both P123 and F127 copolymers. P123 presents a higher interaction in aqueous solution with CUR than F127. Both pluronics could contribute to a safer and more efficient CUR administration in the bloodstream.