Investigation of the inclusion of the herbicide cycluron in native cyclodextrins by X-ray diffraction, nuclear magnetic resonance spectroscopy and isothermal titration calorimetry

摘要

The formation of inclusion complexes between the native cyclodextrins (CDs) and the urea herbicide cycluron has been investigated both in solution and in the solid state. Single-crystal X-ray structures of both the uncomplexed guest and the β-CDcycluron complex were determined while powder X-ray diffraction was used to confirm complexation between γ-CD and cycluron in the solid state. Solution-state complexation between the herbicide and α-, γ- and γ-CD was established using ~1H NMR spectroscopy and isothermal titration calorimetry (ITC). From the ~1H NMR spectroscopic studies 1:1 complex stoichiometry was indicated in all cases and association constant values (K) were determined as 228, 3254 and 155 for the complexes α-CDcycluron, β-CDcycluron and γ-CDcycluron, respectively. Assigning a 1:1 host-guest ratio, the ITC technique produced K values of the same order as those determined using the spectroscopic method. The thermodynamic parameters ΔH, ΔS and ΔG obtained using ITC provide insights into the driving forces involved during complex formation.