In this chapter we start our discussion of binding processes. Here we deal explicitly with processes that involve the system as a whole. The properties of the macromolecule in this global description can be derived in a straightforward manner from the principles outlined in the previous sections.

The reference system

The simplest system of interest in the discussion of binding processes is that composed of a macromolecule M containing a single site for a ligand X. The term ‘ligand’ indicates a molecule that interacts specifically with M according to the law of mass action as discussed in Section 1.6. In the previous chapter we have considered water as ligand. Here we are interested in the interaction of M with a ‘specific’ ligand and regard the solvent as an ‘inert’ component. A treatment of linked hydration effects is given in Section 2.6. For the sake of simplicity in the following we consider temperature and pressure constant. We also assume that the macromolecule does not change its aggregation state. A rather extensive and elegant treatment of aggregation effects linked to binding processes has been developed by Hermans and Premilat (1975). The macromolecule is modeled as a generalized ensemble for which P and T are fixed, while the number of ligands bound is subject to fluctuations under the effect of the driving force μ = kBT In x, where x is the ligand activity.