Fluorescence Enhancement of Curcumin upon Inclusion into Cucurbituril

The effect of the macrocyclic host compounds cucurbit[n]urils (Qn), with n = 5 -- 7, on the fluorescence of the biologically active compound curcumin has been studied. Curcumin, the main constituent of the Indian spice turmeric, is of growing interest because of its wide-ranging pharmaceutical properties. This compound forms strong 2:1 host–guest inclusion complexes with Q6 (the original cucurbituril), with an overall equilibrium constant of (1.9±0.8)×104 M−2. It is postulated that a Q6 host partially encapsulates each of the two phenyl groups at the ends of the curcumin molecule. The difference in magnitude of the equilibrium constants K 1 (72±2 M−1) and K 2 (260±120 M−1) for stepwise encapsulation of the two ends of the curcumin molecule indicates that encapsulation by the first Q6 significantly alters its entire electronic structure, resulting in a more favorable second encapsulation. A very large enhancement of the fluorescence of curcumin results from this complex formation, on the order of 5.0; this...