Geometric features of the Wiseman isotherm in isothermal titration calorimetry

Abstract

Isothermal titration calorimetry (ITC) has become a standard and preferred technique for studying intermolecular interactions. ITC is the only technique that allows determining the binding affinity, the binding enthalpy, and the stoichiometry in a single experiment. With a practical window of more than five orders of magnitude for reliable binding affinity determination (K _d range from millimolar to nanomolar) and its high sensitivity for measuring small heats of reaction (<1 µJ), it is especially suited for characterizing non-covalent interactions typical from biomacromolecular interactions. The Wiseman isotherm represents the usual way of presenting and analyzing the calorimetric binding data. A geometric analysis of the binding isotherm reveals important connections between certain geometric points in the isotherm and the binding parameters. Thus, although the binding parameters (in particular, affinity and enthalpy) must always be estimated through nonlinear analysis of the binding isotherm, it is also possible to easily obtain estimates of those binding parameters through the relationships presented in this work.