Fluorescence enhancement of curcumin upon inclusion into parent and modified cyclodextrins

The effect of α-, β-, and γ-cyclodextrin (CD) and their hydroxypropylated (HP) derivatives on the solubility and fluorescence of the compound curcumin has been studied. Curcumin, the main constituent of the Indian spice turmeric, is of growing interest due to its wide-ranging pharmaceutical properties. All of these six cyclodextrins significantly increase the aqueous solubility of curcumin, with the greatest solubility observed in HP-γ-CD. Curcumin forms 2:1 host–guest inclusion complexes with these cyclodextrins, with the strongest complexes formed in the case of HP-β-CD. These 2:1 complexes are postulated to form when a cyclodextrin host encapsulates each of the two phenyl rings at the ends of the curcumin molecule. The equilibrium constant for encapsulation by the second cyclodextrin host is significantly smaller than that for the first in each case, probably a result of steric bulk of the first cyclodextrin, with deep inclusion of the curcumin molecule, hindering encapsulation by the second cyclodextrin host. In fact, in the case of β-CD, a 1:1 host:guest model fit the data just as well, or better than, the 2:1 model. In the case of γ-CD and HP-γ-CD, the initial 1:1 complex formed was found to be less fluorescent than the free curcumin; this is postulated to be a result of inclusion of a folded curcumin molecule, which breaks the conjugation along the length of the molecule and hence greatly reduces its fluorescence quantum yield.