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Keywords:

  • cooperative effects;
  • host–guest systems;
  • kinetics;
  • linear free energy relationships;
  • noncovalent interactions

Abstract

Cooperativity is one of the most relevant features displayed by biomolecules. Thus, one of the challenges in the field of supramolecular chemistry is to understand the mechanisms underlying cooperative binding effects. Traditionally, cooperativity has been related to multivalent receptors, but Williams et al. have proposed a different interpretation based on the strengthening of noncovalent interactions within receptors upon binding. According to such an interpretation, positive cooperative binding operates through structural tightening. Hence, a quite counterintuitive kinetic behavior for positively cooperative bound complexes may be postulated: the more stable the complex, the slower it is formed. Such a hypothesis was tested in a synthetic system in which positive cooperative binding was previously confirmed by calorimetric experiments. Indeed, a linear correlation between the thermodynamics (ΔG°) and the kinetics (ΔG) of guest binding confirmed the expected behavior. These distinctive kinetics provide solid evidence of positive cooperative guest binding, which is particularly useful bearing in mind that kinetic experiments are frequently and accurately carried out in both synthetic and biological systems.