On the Value of c: Can Low Affinity Systems Be Studied by Isothermal Titration Calorimetry?

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Abstract
Isothermal titration calorimetry (ITC) allows the determination of ΔG°, ΔH°, and ΔS° from a single experiment and is thus widely used for studying binding thermodynamics in both biological and synthetic supramolecular systems. However, it is widely believed that it is not possible to derive accurate thermodynamic information from ITC experiments in which the Wiseman “c” parameter (which is the product of the receptor concentration and the binding constant, Ka) is less than ca. 10, constraining its use to high affinity systems. Herein, experimental titrations and simulated data are used to demonstrate that this dogma is false, especially for low affinity systems, assuming that (1) a sufficient portion of the binding isotherm is used for analysis, (2) the binding stoichiometry is known, (3) the concentrations of both ligand and receptor are known with accuracy, and (4) there is an adequate level of signal-to-noise in the data. This study supports the validity of ITC for determining the value of Ka and, hence, ΔG° from experiments conducted under low c conditions but advocates greater caution in the interpretation of values for ΔH°. Therefore, isothermal titration calorimetry is a valid and useful technique for studying biologically and synthetically important low affinity systems.