DNA bis‐intercalation: Application of theory to the binding of echinomycin to DNA

Abstract
The statistical mechanical model for the binding of bifunctional intercalating ligands to duplex DNA described in the preceding paper is applied to the example of echinomycin–DNA interactions. This is the only system for which binding curves have been obtained under conditions leading to binding by both bis‐intercalation and mono‐intercalation simultaneously. Binding parameters and Scatchard plots are calculated for a variety of conditions. A detailed comparison of these calculations with the results from the previous analysis of the same binding data in terms of the McGhee‐Von Hippel theory, assuming only one mode of binding, is presented. The results of our calculations are consistent with the model of bis‐intercalation requiring the two bound chromophores of a bifunctional ligand to be separated by two base pairs. It is not necessary to assume violation of the nearest‐neighbor exclusion principle, as occurred in the original analysis.