The interaction between macromolecules such as proteins or macromolecular systems with proteins, ligands, ions, lipids or detergents is of great importance for many biological functions. In contrast to optical spectroscopy, thermoanalytical techniques are especially suitable for the investigation of nanoparticulate systems. Based on a given model, microcalorimetric titrations can be carried out to determine binding enthalpies (ΔH) together with binding affinities and stoichiometries, provided the enthalpy change of the system under consideration is not equal to zero. For equilibria, the binding enthalpy (ΔS) can be calculated according to

where K_th is the thermodynamic association constant, which is identical with the reciprocal dissociation constant. As a major advantage, the resulting enthalpic and enthropic contributions to the binding affinity can be obtained and interpreted in more detailed molecular and structural terms than if only the association, respectively dissociation constant is known.

In the case of a one-step binding process (stoichiometric coefficient equal 1) between a protein P and a ligand L, titration calorimetry under conditions of constant ionic strength and temperature allows to determine an apparent association constant K_app according to

                                     K_app = K_th / K_γ = c_PL / (c_P c_L) _,

if the standard concentration is expressed in molar terms and where Kγ is the term related to activity coefficients and c denotes concentrations (for details cf.[1]). Kapp represents a suitable parameter to compare affinities of equilibria determined under identical experimental conditions.