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Hopping transport in alternating G DNA sequences. Holes from the left electrode migrate through each of the hopping sites and finally reach the right electrode, generating a current. Image Credit: Nature Chemistry

How electrical charges move through DNA

Electrical charges not only move through wires, they also travel along lengths of DNA, the molecule of life. This property is known as charge transport.

At Arizona State University in the Biodesign Institute, authors Limin Xiang, Julio Palma, and Christopher Bruot are part of a large team exploring the ways in which electrical charges move along DNA. Their work reveals a new mechanism of charge transport different from two previously recognized patterns in which an electrical charge either tunnels or hops along bases of DNA. Their results appear in the journal Nature Chemistry

Researchers predict that foundational work of this kind has important implications for the design of a new generation of functional DNA-based electronic devices. Such discoveries also provide new insights into health risks associated with transport-related damage to DNA.

Oxidative damage to DNA is thought to play a role in initiating cancer and its progression. It is also implicated in neurodegenerative disorders like Alzheimer's, Huntington's and Parkinson's diseases.

The transfer of electrons is a basic form of chemical reaction. It plays a critical role across a broad range of life-sustaining processes such as respiration and photosynthesis. But it can also produce negative effects on living systems. Oxidative stress in particular damages DNA and has been identified in a broad range of diseases.

"When DNA is exposed to UV light, there's a chance one of the bases — such as guanine — gets oxidized, meaning it loses an electron," according to Nongjian Tao PhD, Professor and Director of the Center for Biosensors and Bioelectronics, at the Biodesign Institute of Arizona State University. Guanine is easier to oxidize than the other three bases, cytosine, thymine, and adenine, making it the most important base for charge transport. In some cases, DNA damage is repaired when an electron migrates from a portion of the DNA strand to replace a missing electron.

DNA repair is an endless, ongoing process, though a gradual loss of repair efficiency over time is one factor of aging. Oxidation randomly damages RNA and DNA and can interfere with normal cell metabolism.

When the loss of an electron — or oxidation — occurs in a DNA base, a hole is left in place of the electron and this hole carries a positive charge. The hole travels along the length of DNA as an electrical or magnetic field.

Based on previous research, two primary mechanisms for charge transport have been identified. First, over short distances, an electron displays the properties of a wave — a quantum mechanical effect known as tunneling. Secondly, over longer distances the process is more like hopping. When a charge hops from point to point along a DNA segment, it loses its wavelike properties.

Electrical resistance increases exponentially during tunneling and linearly during hopping.

However, after attaching electrodes to the two ends of a DNA molecule, researchers were able to monitor the passage of a different charge through the molecule. Professor Tao: "What we found in this particular study is that there is an intermediate behavior. It's not exactly hopping because the electron still displays some of the wave properties." Instead, the holes observed in certain sequences of DNA are delocalized, spread over several base pairs, and the effect is neither a linear nor exponential increase in electrical resistance — but an oscillation.

A stack of base pairs of guanine-cytosine caused the oscillation. When G bases were alternated, rather than occurring in a sequential stack, linear resistance increased with the length of the molecule in conventional hopping behavior.

Altogether, DNA at room temperature is a molecule that is not like a wire in a conventional electronic device, but a highly dynamic structure that writhes and fluctuates.

Abstract
Charge transport in molecular systems, including DNA, is involved in many basic chemical and biological processes, and its understanding is critical if they are to be used in electronic devices. This important phenomenon is often described as either coherent tunnelling over a short distance or incoherent hopping over a long distance. Here, we show evidence of an intermediate regime where coherent and incoherent processes coexist in double-stranded DNA. We measure charge transport in single DNA molecules bridged to two electrodes as a function of DNA sequence and length. In general, the resistance of DNA increases linearly with length, as expected for incoherent hopping. However, for DNA sequences with stacked guanine–cytosine (GC) base pairs, a periodic oscillation is superimposed on the linear length dependence, indicating partial coherent transport. This result is supported by the finding of strong delocalization of the highest occupied molecular orbitals of GC by theoretical simulation and by modelling based on the Büttiker theory of partial coherent charge transport.

The work was carried out under the direction of Nongjian (NJ) Tao, who directs Biodesign's Center for Bioelectronics and Biosensors, in collaboration with Vladimiro Mujica at Arizona State University, and Mark Ratner at Northwestern University.

The research is part of a multi-institute project carried out under the Department of Defense's Multidisciplinary University Research Initiatives (MURI) Program--an initiative aimed at promoting "high priority topics and opportunities that intersect more than one traditional technical discipline."

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