Mechanisms for DNA Charge Transport

DNA charge transport (CT) chemistry has received considerable attention by scientific researchers over the past 15 years since our first provocative publication on long range CT in a DNA assembly.1,2 This interest, shared by physicists, chemists and biologists, reflects the potential of DNA CT to provide a sensitive route for signaling, whether in the construction of nanoscale biosensors or as an enzymatic tool to detect damage in the genome. Research into DNA CT chemistry began as a quest to determine whether the DNA double helix, a macromolecular assembly in solution with π-stacked base pairs, might share conductive characteristics with π-stacked solids. Physicists carried out sophisticated experiments to measure the conductivity of DNA samples, but the means to connect discrete DNA assemblies into the devices to gauge conductivity varied, as did the conditions under which conductivities were determined. Chemists constructed DNA assemblies to measure hole and electron transport in solution using a variety of hole and electron donors. Here, too, DNA CT was seen to depend upon the connections, or coupling, between donors and the DNA base pair stack. Importantly, these experiments have resolved the debate over whether DNA CT is possible. Moreover these studies have shown that DNA CT, irrespective of the oxidant or reductant used to initiate the chemistry, can occur over long molecular distances but can be exquisitely sensitive to perturbations in the base pair stack. Here we review some of the critical characteristics of DNA charge transport chemistry, taking examples from a range of systems, and consider these characteristics in the context of their mechanistic implications. This review is not intended to be exhaustive but instead to be illustrative. For instance, we describe studies involving measurements in solution using pendant photooxidants to inject holes, conductivity studies with covalently modified assemblies, and electrochemical studies on DNA-modified electrodes. We do not focus in detail on the differences amongst these constructs but instead on their similarities. It is the similarity among these various systems that allows us to consider different mechanisms to describe DNA CT. Thus we review also the various mechanisms for DNA CT that have been put forth and attempt to reconcile these mechanistic proposals with the many disparate measurements of DNA CT. Certainly the debate among researchers has shifted from "is DNA CT possible?" to "how does it work?". This review intends to explore this latter question in detail