NMR study of the inclusion complex formed between γ‐cyclodextrin and TmDOTP⁵⁻

The host–guest complex formed in aqueous solution between γ‐cyclodextrin (γ‐CD) and the macrocyclic paramagnetic shift reagent TmDOTP⁵⁻ was examined by NMR spectroscopy. Paramagnetic lanthanide‐induced shifts (LIS) and spin‐lattice relaxation rate enhancements in the proton resonances of γ‐CD on binding of TmDOTP⁵⁻ were used to define the position of the guest anion within the host cavity. A proportionality constant which relates pseudo‐contact shifts induced by TmDOTP⁵⁻ to the geometrical position of each ligand nucleus (previously determined from a LIS study of the TmDOTP⁵⁻ complex) was used to determine a fully bound LIS value for each γ‐CD proton. These values, in combination with experimental LIS data from TmDOTP⁵⁻ titrations at constant γ‐CD concentration, were then used to obtain the association constant for the host‐guest complex (3.9–4.5 mol l⁻¹). These constants were indicative of a weak host‐guest interaction and consistent with the location of the TmDOTP⁵⁻ binding site reported by the LIS values well above the center of the γ‐CD cavity.