Interaction of Curcumin with Phosphatidylcholine: A Spectrofluorometric Study

Curcumin [1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione], the main constituent of the rhizomes of the plant Curcume longa L. (turmeric), is a powerful antioxidant in both enzymatic and nonenzymatic systems. The interactions of curcumin with egg and soy phosphatidylcholine were followed by fluorescence spectroscopy. Curcumin had very weak fluorescence in aqueous system, which was enhanced in apolar environments. Curcumin emitted at 490 nm after being excited at 451 nm in phosphatidylcholine micelles. The equilibrium constants for the interaction of curcumin with egg and soy phosphatidylcholine were (3.26 ± 0.2) × 105 and (2.64 ± 0.2) × 105 M-1, respectively. From the Scatchard plot of the fluorometric data, it was inferred that one molecule of curcumin could bind six molecules of phosphatidylcholine. The equilibrium constant for the phosphatidylcholine−curcumin interaction decreased with temperature, indicating the amphiphilic nature of curcumin. The ΔG, ΔH, and ΔS values obtained for the interaction of egg phosphatidylcholine−curcumin were −7.8 ± 0.3 kcal/mol, −9.6 ± 0.4 kcal/mol, and −6.8 ± 0.2 cal/mol/K, respectively. The fluorescence anisotropy measurements of curcumin with phosphatidylcholine suggested that the anisotropy of the curcumin molecule did not change in phosphatidylcholine. The interaction of divalent metal ions with phosphatidylcholine−curcumin in comparison with phosphatidylcholine−1-anilino-8-naphathalenesulfonic acid complex suggested the strong binding of curcumin to metal ions. Keywords: Phosphatidylcholine; micelles; fluorescence; curcumin