Intercalative stacking: A critical feature of DNA charge-transport electrochemistry

In electrochemistry experiments on DNA-modified electrodes, features of the redox probe that determine efficient charge transport through DNA-modified surfaces have been explored using methylene blue (MB⁺) and Ru(NH₃)₆³⁺ as DNA-binding redox probes. The electrochemistry of these molecules is studied as a function of ionic strength to determine the necessity of tight binding to DNA and the number of electrons involved in the redox reaction; on the DNA surface, MB⁺ displays 2e^-/1H⁺ electrochemistry (pH 7) and Ru(NH₃)₆³⁺ displays 1e^- electrochemistry. We examine also the effect of the electrode surface passivation and the effect of the mode (intercalation or electrostatic) of MB⁺ and Ru(NH₃)₆³⁺ binding DNA to highlight the importance of intercalation for reduction by a DNA-mediated charge-transport pathway. Furthermore, in experiments in which MB⁺ is covalently linked to the DNA through a σ-bonded tether and ionic strength is varied, it is demonstrated that intercalative stacking rather than covalent σ-bonding is essential for efficient reduction of MB⁺. The results presented here therefore establish that efficient charge transport to the DNA-binding moiety in DNA films requires intercalative stacking and is mediated by the DNA base pair array.