On the analysis of linear binding effects associated with curved Scatchard plots

First the question is examined as to which binding data, especially if given as Scatchard plots, can be described in terms of a basic model mechanism. This referes to a linear lattice of equivalent binding sites (as for example located on a linear biopolymer) which can exert cooperative interaction between nearest neighbors. It is shown that the effect of overlapping of potential site (so-called "multiple-contact"binding), as may occur with larger ligands, will largely be compensated by a higher degree of cooperativity. Therefore, in practice such properties can scarcely be separated by means of oridnary binding experiments. A pronounced inflection point in the Scatchard plot turns out to be clearly indicating a more complex mechanism involving at least two rather antagonistic cooperative interactions which may, however, occur even between equivalent binding sites. Finally some consequences of different classes of bindings sites are considered. In particular a simple approach is introduced by which the binding to mutually exclusive classes of sites may be described. Such a model is of interest for multiple-mode binding of ionic ligands to oppositely charged polymers.