Capturing Elusive Polymorphs of Curcumin: A Structural Characterization and Computational Study

Curcumin, a compound derived from the herb turmeric, has gained considerable attention because of its purported pharmacological activity, but its poor water solubility and low bioavailability impedes its therapeutic potential. In addition to the frequently studied curcumin polymorph, referred to as form I, another polymorph with enhanced water solubility, referred to as form II, or red curcumin, has also been reported. We discuss experimental challenges in isolating the red curcumin polymorph. In the course of our studies, we were unable to obtain a third reported polymorph, form III. We redetermined crystal structures of forms I and II of curcumin and present 13C and 1H solid-state nuclear magnetic resonance (NMR) spectra for these two forms, as well as 13C–1H two-dimensional heteronuclear correlation (HETCOR) data. The experimental 1H and 13C nuclear magnetic resonance (NMR) chemical shifts are compared with GIPAW density functional theory values computed using CASTEP. Our research illustrates the utility of NMR spectroscopy in characterization of polymorphism in bulk samples.