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) X 10(4) 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-1 (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 is a significant fraction of the polarity sensitivity factor (PSF) of 39 measured for curcumin, that is the ratio of fluorescence intensity in ethanol vs. water. Surprisingly, no such enhancement could be observed in the case of Q7, indicating that the interactions between the guest and the host cavity are not favorable in this case, contrary to expectations. Similarly, no enhancement was observed in the case of Q5, which is not unexpected, because of the extremely small size of the host cavity and portal in this case.