Dimerization and α-cyclodextrin inclusion of propantheline bromide as studied by NMR and molecular modeling

The proton chemical shifts of propantheline bromide (PB) and alpha-cyclodextrin (alpha-CyD) are determined as a function of the PB concentration in the absence and presence of 5 mmol dm(-3) alpha-CyD. The dimerization constant and the critical micelle concentration of PB are determined to be 20 dm(3) mol(-1) and 18 mmol dm(-3) The chemical shift variations of PB protons induced with the dimerization of PB agree very well with the values calculated on the basis of the antiparallel stacking of two xanthene rings. The 1:1 binding constant and the chemical shifts of PB and alpha-CyD protons for their complex are evaluated from the concentration dependence of the chemical shifts. On the basis of both these shift data and molecular mechanical calculations, it is estimated that one of the benzene rings of PB in the complex is included shallowly from the wider side into the alpha-CyD cavity. The chemical shifts of the alpha-CyD protons, calculated on the basis of the effect of the xanthene ring current for this structure, agree excellently with the observed ones.