The understanding of the relationship between molecular structure and the thermodynamics of host-guest binding is essential for the rational design of the applications of inclusion complexes. To obtain insight into the factors governing the driving force of complex formation in aqueous solutions, the encapsulation of five pharmaceutically important protoberberine alkaloids was studied in sulfobutylether-β-cyclodextrin having on average 6.4 degrees of substitution (SBE6.4βCD). Spectrophotometric, fluorescence spectroscopic, and isothermal calorimetric measurements showed 1:1 complexation in dilute solutions. From 1.92 × 104 M−1, about an eight-fold decrease of the association constant was observed in the series of berberine ≈ coptisine >> palmatine > epiberberine > dehydrocorydaline. The embedment of these alkaloids in the SBE6.4βCD cavity was entropy-controlled with mildly negative enthalpy contributions. These findings suggest that the stabilization of the examined complexes arises primarily from the hydrophobic interaction between the constituents. The more than three orders of magnitude smaller association constants of protoberberine alkaloids with SBE6.4βCD than with cucurbit[7]uril, a host having similar cavity size, originates from the much smaller exothermicity of the confinement in the former macrocycle.
Keywords: alkaloid; calorimetry; drug delivery; fluorescence; host-guest binding; inclusion complex; macrocycle; self-assembly; thermodynamics.