Terahertz spectra of curcumin and catechol co-crystals

Abstract

Curcumin (CUR) is a commonly used pharmaceutical with anti-inflammatory, antioxidant and anti-cancer effects, but its solubility in water is relatively low. In recent years, pharmaceutical co-crystal has been an effective method of enhancing the solubility of limited water-soluble pharmaceuticals. Based on this, terahertz time-domain spectroscopy (THz-TDS) is used to study the THz spectra of curcumin-catechol co-crystal. Firstly, the experimental spectra of curcumin, catechol (CTL), their physical mixture and their co-crystal are measured in a range of 0.5–3.5 THz, respectively. The experimental data show that CUR obtains six THz absorption peaks, while CTL possess three THz absorption peaks, the physical mixture obtains four absorption peaks, and their CUR-CTL co-crystal obtains three absorption peaks. These results indicate that THz-TDS can effectively identify curcumin, catechol and their co-crystals. The fact that the absorption peak at 3.31 THz obtained in co-crystal is entirely different from those of raw materials, implying that new weak interactional forces are generated between CUR molecule and CTL molecule, the co-crystal forms a new three-dimensional structure compared with their raw materials. These results are also verified by X-ray diffraction spectra of raw material and their Co-crystal. Moreover, four possible theoretical forms of curcumin-catechol co-crystal are optimized and simulated by using density functional theory (DFT). The calculated results indicate that the data of co-crystal form III are in good agreement with the experimental spectrum, and the simulation effectively reconstructs the experimental spectrum. So it can be inferred that the co-crystal is formed through the hydrogen bond between the carbonyl C10=O3 of CUR and the hydroxy O61-H55 of CTL. In addition, depending on the good match between experimental data and theoretical results, it is found that the three absorption peaks in the co-crystal do not origin from the action of a single molecule, but the joint action of the functional groups of the two molecules under the driving by the hydrogen bond. The existence of weak interaction forces, such as the hydrogen bond, not only changes the structural parameters of the two molecules, but also reestablishes a new intermolecular force, which then affects the interactional motions of the co-crystal. This fact directly leads the CUR-CTL co-crystal to exhibit THz absorption peaks different from those of raw materials in the THz band.