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Ion-Pairing Molecular Recognition in Water:  Aggregation at Low Concentrations That Is Entropy-Driven

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journal contribution
posted on 20.11.2002, 00:00 by Mikhail Rekharsky, Yoshihisa Inoue, Suzanne Tobey, Axel Metzger, Eric Anslyn
Investigations into the thermodynamic parameters that characterize the binding of citrate to tris-guanidinium host 1 in water are reported. The parameters Ka, ΔH°, ΔS°, and ΔG° for the binding event were quantified using isothermal titration calorimetry (ITC) techniques. The 1:1 binding stoichiometry was verified by a Job plot derived from NMR data, and the microcalorimetry data was collected for solutions of 1 and citrate ranging from 1 to 100 mM using phosphate buffer concentrations of 5 and 103 mM. At low buffer concentrations (low ionic strength) complexes with greater than 1:1 stoichiometries were observed by ITC, and K1 was determined to range from 2.0 × 103 to 3.0 × 103 M-1. At higher buffer concentrations (high ionic strength) the higher-order complexes were not detected, and K1 was determined to be 409 M-1. The 1:1 association of host 1 and citrate is characterized by a large favorable entropy component and negative enthalpy. However, the complexes with higher-order stoichiometry arise from desolvation processes that result from the association of polyions in aqueous media and is entirely entropy driven. This leads to an unusual observation:  the dilution of one component of the host/guest complex leads to the formation of the higher-order complexes. The reason for this observation is discussed.