Diastereomeric Crowding Effects in the Competitive DNA Intercalation of Ru(phenanthroline)₂dipyridophenazine²⁺...

The biexponential excited-state emission decay characteristic of DNA intercalated tris-bidentate dppz-based ruthenium complexes of the general form Ru(L)₂dppz²⁺ has previously been explained by a binding model with two distinct geometry orientations of the bound ligands, with a distinct lifetime associated with each orientation. However, it has been found that upon DNA binding of Ru(phen)₂dppz²⁺ the fractions of short and long lifetimes are strongly dependent on environmental factors such as salt concentration and, in particular, temperature. Analyzing isothermal titration calorimetry for competitive binding of Ru(phen)₂dppz²⁺ enantiomers to poly(dAdT)₂, we find that a consistent binding model must assume that the short and long lifetimes states of intercalated complexes are in equilibrium and that this equilibrium is altered when neighboring bound ligands affect each other. The degree of intercomplex binding is found to be a subtle manifestation of several attractive and repulsive factors that are highly likely to directly reflect the strong diastereomeric difference in the binding enthalpy and entropy values. In addition, as the titration progresses and the binding sites on the DNA lattice become increasingly occupied, a general resistance for the saturation of the binding sites is observed, suggesting diastereomeric crowding of the neighboring bound ligands.