Enhanced solubility of curcumin by complexation with fermented cyclic β-1,2-glucans

Curcumin (CUR) is a low-solubility polyphenolic compound with many physiological functions. Cyclic β-1,2-glucans (cyclosophoraoses [Cys]), which contain rings of different sizes with degrees of polymerization ranging from 17 to 23, were obtained from Rhizobium radiobacter ATCC 1333, a soil microorganism. The complexation ability and solubility enhancement of cyclic β-1,2-glucans with insoluble curcumin were investigated. Phase-solubility analysis revealed that the stoichiometric ratio of the inclusion complexes was 1:1. The stability constant of Cys was 930 M^-1, which was 7.68 times that of α-cyclodextrin (α-CD) and 2.09 times that of β-cyclodextrin (β-CD). The characteristics of the curcumin/Cys inclusion complexes were successfully determined by using Fourier transform infrared (FTIR) spectrometry, differential scanning calorimetry (DSC), nuclear magnetic resonance (¹H NMR) spectroscopy, and scanning electron microscopy (SEM). Moreover, a 1:1 molecular model of the curcumin/Cys inclusion complexes was established through molecular docking analysis. These findings indicated that cyclic β-1,2-glucans successfully formed complexes with curcumin, which suggested that they could be used as solubility-increasing agents. To the best of our knowledge, this is the first report in which curcumin has been embedded into cyclic β-1,2-glucans resulting in an increase in its aqueous solubility.