Lateral distribution of charged species along a polyelectrolyte probed with a fluorescence blob model.

The distribution of metal counterions binding onto the oppositely charged surface of a model polyelectrolyte, namely, DNA, was characterized by conducting fluorescence quenching experiments. In these experiments, DNA was used as a molecular ruler to measure the average distance (d(blob)) over which electron transfer takes place between DNA-intercalated ethidium bromide (DNA-EB) and the electrostatically bound divalent metal cations Ni(2+) and Cu(2+). Analysis of the fluorescence decays of DNA-EB quenched by Cu(2+) and Ni(2+) with the fluorescence blob model showed that d(blob) was equal to the Debye length (κ(-1)). This surprisingly simple result considering the overall complexity of the system under study led to the straightforward proposal that counterions bind to a polyelectrolyte by distributing themselves randomly into an array of self-defined subdomains of dimension κ(-1). In turn, this insight can be utilized to rationalize the complex behavior of polyelectrolytes in aqueous solution.