Isothermal Calorimeter (ITC)

Isothermal Titration Calorimetry (ITC) measures heat of interaction between two molecules. In ITC a syringe containing a "ligand" is titrated into a cell containing a solution of the "macromolecule". As the two elements interact, heat is released or absorbed. When the macromolecule in the cell becomes saturated with added ligand, the heat signal diminishes until only the background heat of dilution is observed. An example of ITC data collected in the Biophysical Resource. A good review for ITC application for protein studies is given by Pierce et al.

ITC performs the measurement on native molecules in solution; no modifications are needed and the measurement does not require any consumable.

ITC is an universal tool for monitoring binding reactions because it measures binding from the universal heat/enthalpy change associated with all binding interactions. The analysis is performed at equilibrium, in solution phase and without any labeling or need for a fluorescent or other probe. An ITC experiment provides direct information about the thermodynamics of binding. The enthalpy of binding is measured directly and when combined with the equilibrium binding constant (also measured directly) yields the entropy change. The temperature dependence of the enthalpy changes yields the heat capacity which provides insight into the surface area buried during binding. ITC can also be used to determine the mass ratio between interacting species; however, the absolute stoichiometry of the final complex needs to be verified by other methods.