title: Ion-pairing molecular recognition in water: aggregation at low concentrations that is entropy-driven.
abstract: Investigations into the thermodynamic parameters that characterize the binding of citrate to tris-guanidinium host 1 in water are reported. The parameters K(a), DeltaH degrees, DeltaS degrees, and DeltaG degrees 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(1) 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(1) 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.
authors.affiliation: Entropy Control Project, ICORP, JST, 4-6-3 Kammishinden, Toyonaka 560-0085, Japan.
authors.forename: Mikhail
authors.initials: M
authors.lastname: Rekharsky
authors.forename: Yoshihisa
authors.initials: Y
authors.lastname: Inoue
authors.forename: Suzanne
authors.initials: S
authors.lastname: Tobey
authors.forename: Axel
authors.initials: A
authors.lastname: Metzger
authors.forename: Eric
authors.initials: E
authors.lastname: Anslyn
mesh_terms: D002151:Calorimetry; D019343:Citric Acid / Q000737:chemistry*; D019277:Entropy; D019791:Guanidine / Q000737:chemistry*; D007700:Kinetics; D009994:Osmolar Concentration; D012996:Solutions; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D012996:Solutions; D014867:Water; D019343:Citric Acid; D019791:Guanidine