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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 solution, 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 medium. 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. Pluronic particle sizes with and without CUR were acquired by DLS. Interaction in the solid state was verified by DSC. The equilibrium rate that displaces to diketo form increased fivefold when the temperature rose from 21 to 41 ºC with activation energy of 61.2 kJ mol-1. 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 25 to 41 ºC. This process had a Gibbs free of energy of around -1 and -13 kJ mol-1 due to CUR solubilization into the inner core of pluronic micelles. Particle sizes of about 11 nm were obtained for both copolymers containing CUR in aqueous solution above critical micelle temperature. DSC measurements showed melting point depression of both CUR and F127. P123 presented no significant variation in melting point due to 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 safer and more efficient CUR administration in the bloodstream.