Ion-pairing molecular recognition in water: Aggregation at low concentrations that is entropy-driven

被引:113
作者
Rekharsky, M
Inoue, Y
Tobey, S
Metzger, A
Anslyn, E
机构
[1] JST, ICORP, Entropy Control Project, Toyonaka, Osaka 5600085, Japan
[2] Univ Texas, Dept Chem & Biochem, Austin, TX 78712 USA
关键词
D O I
10.1021/ja020612e
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
Investigations into the thermodynamic parameters that characterize the binding of citrate to tris-guanidinium host 1 in water are reported. The parameters K(a), DeltaHdegrees, DeltaSdegrees, and DeltaGdegrees 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 K, was determined to range from 2.0 x 10(3) to 3.0 x 10(3) M(-1). At higher buffer concentrations (high ionic strength) the higher-order complexes were not detected, and K, 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.
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页码:14959 / 14967
页数:9
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