Understanding Molecular Association and Isomers Recognition in Isomer−Cyclodextrin Multiple Complex Formation by Improved Liquid Chromatographic Studies

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Abstract
Equations are derived that allow the determination of guest-cyclodextrin primary (K(11)), secondary (K(12)), and higher order (K(1)(n)) binding constants as well as the degree of complexation n̄ (the percent of complexed guest), by monitoring the observed capacity factor k'(obs) for guests that can be structural, geometric, or optical isomers. The retention behavior (elution order) in a mixture of isomers is dependent on the cyclodextrin concentration in the mobile phase as well as on the stoichiometry and the binding constant of the guest-cyclodextrin (G-CD) complex. The simplification of higher order complexes (G-CD(n)) to that with 1:1 stoichiometry can lead to erroneous results; therefore, it is important for the stoichiometry to be determined accurately, prior to any binding constant calculations, following the continuous variation method. The proposed models are solved iteratively using nonlinear least-squares analysis and following specific algorithms.