Stoichiometric model of alpha-cyclodextrin complex formation.

The solubility, spectral, and kinetic methods were used to study complexing between alpha-cyclodextrin (ligand, L) and 3,5-dimethoxycinnamic acid, benzalacetone, and methyl cinnamate (substrates, S). In aqueous solution at 25 degrees and with an ionic strength of 0.01 M, the following stability constants were found (K11 for SL and K12 for SL2): 3,5-dimethoxycinnamic acid, K11=1965 M-1 and K12=0; benzalacetone, K11=105 M-1 and K12=15 M-1; and methyl cinnamate, K11=1200 M-1 and K12=50 M-1. A model of complex formation is proposed that can account for the observed stoichiometry and that yields stability estimates for two isomeric 1:1 complexes in the systems in which a 1:2 complex forms. For cinnamic acid, benzalacetone, and methyl cinnamate, stability constants are inversely correlated with the substrate dipole moment.