Complex formation of large-ring cyclodextrins with iodine in aqueous solution as revealed by isothermal titration calorimetry

Complex formation of large cyclodextrins(CDs) having DP 21–32 with iodine in aqueous KI solution was studied by isothermal titration calorimetry. The curves obtained for the titration of the CDs with iodine cannot be analyzed by a model based on a single set of identical sites, but, rather, by a model assuming 1 : 2 complex formation with identical interacting sites. For the two identical interacting sites, the binding constants K1 and K2 (K1 < K2), defined relative to the progress of saturation, lie in the range 0.7 to 7.3×103 M–1 and 3.0 to 62.6×103 M–1, respectively. The values of ΔH2 and T ΔS2 lie in the range –34.9 to –136.4 kJ·mol–1 and –15.5 to –112.8 kJ·mol–1, respectively. The largest values of –T ΔS2 obtained for a CD of DP 26 can, in part, be attributed to a large decrease in conformational flexibility of the CD which occurs during complex formation.