Intercalative Stacking: A Critical Feature of DNA Charge-Transport Electrochemistry

Abstract

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 electrode surface passivation and the effect of the mode (intercalation or electrostatic) of MB+ and Ru(NH3)63+ binding to 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 the 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.