Calorimetric investigation of guanidinium-carboxylate interactions

Isothermal titration calorimetry was utilized to study the association between a series of guanidinium derivatives and tetrabutylammonium acetate. This technique provides a measure of association strength, stoichiometry of binding, as well as thermodynamic parameters of association from a single experiment. Guanidinium derivatives which can form bidentate linear hydrogen bonds with acetate show significant, exothermic binding in DMSO (Ka = 5600 M^-1; ΔH = -3.6 kcal/mol), while derivatives which lack this bidentate linear hydrogen bonding interaction result in complexes where association is weaker (Ka ≃ 100 M^-1) and enthalpically neutral or endothermic. Additionally calorimetry permits the complete assessment of the multiple binding equilibria when derivatives complex two equivalents of guest.