Studies on pH and temperature dependence of inclusion complexes of bisdemethoxycurcumin with β-cyclodextrin derivatives

The effect of pH and temperature on the inclusion complexes of bisdemethoxycurcumin (BDMC) with sulfobutylether-β-cyclodextrin (SBE-β-CD) and methylated-β-cyclodextrin (M-β-CD) were investigated by phase solubility and fluorescence spectroscopy methods. pH and temperature variations are in the range of pH 6.0-8.5 and 298.2-313.2 K, respectively. The phase solubility profiles of BDMC by UV-Vis spectrophotometry in the presence of SBE-β-CD and M-β-CD were classified as AL-type, indicating the formation of 1:1 inclusion complexes under experimental conditions, in agreement with the conclusion obtained by fluorescence analysis. Based on the complexation constants KC, the values of enthalpy and entropy changes for the complex formation were obtained from the Van't Hoff equation. Dissolution enthalpy and entropy were calculated from the intrinsic solubilities of BDMC. Different influence of temperature on the total solubility and intrinsic solubility was discussed based on the difference between dissolution enthalpy and complexation enthalpy. The effect of pH and cyclodextrin (CD) structure on the solubility and aqueous stability of BDMC were also discussed. These experimental results were confirmed by molecular modeling which showed that the aromatic ring of BDMC was included inside the hydrophobic cavity of CD and M-β-CD was more closely integrated with BDMC.