Distribution of metal complexes bound to DNA determined by normal pulse voltammetry

The effects of DNA binding on the normal pulse voltammetry of metal complexes have been investigated. Studies were performed both for oxidation of OsL(3)(2+/3+) and for reduction of CoL(3)(3+/2+) (L is bpy = 2,2'-bipyridine or phen = 1,10-phenanthroline). The diffusive current obtained from voltammograms at potentials well past E(1/2) gives an accurate measure of the extent to which the complexes codiffuse with DNA or are free in solution, and this response is not affected by kinetic factors resulting from slow heterogeneous electron transfer. Analysis of the diffusion-limited current using the appropriate binding isotherm provides binding constants in good agreement with those measured by other methods. For the bpy complexes, the ionic strength dependence, the relative binding constants for the 2+ and 3+ forms, and the associated change in E(1/2) upon DNA binding are in good agreement with the predictions of polyelectrolyte theory where the 3+ ion binds more strongly. For the phen complexes, the reverse trend is observed and is consistent among the absolute binding constants, ionic strength dependence, and E(1/2) shift; this behavior is ascribed to a hydrophobic interaction. The technique is also applied to two-electron couples based on [(tpy)(L)RuOH2] (2+)/[(tpy)(L)RuO](2+) that exhibit slow heterogeneous electron transfer; however, these kinetic complications do not prohibit accurate determination of the binding energetics using normal pulse voltammetry. Taken together, the data provide a comprehensive picture of the effects of partial DNA binding on voltammetry, which provides a basis for determining homogeneous kinetic rate constants for electrocatalytic DNA oxidation from voltammograms.