Abstract

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.