Electron Transfer and Transport in Nucleic Acids


Group Members: Edward Beall, Emil Wierzbinski
Collaborators: Catalina Achim (Carnegie Mellon), David Beratan (Duke)

The ability to design and build artificial supramolecular nanomachines that perform complex chemical transformations with high efficiency and selectivity is an important goal of modern chemistry. Toward this goal, this project is exploring the use of synthetic nucleic acids (NAs) to program the self-assembly of electroactive structures that separate charge on the tens of nanometers length scale.  Single molecule conductance measurements are performed using a modified version of the pioneering scanning tunneling microscope break junction (STM-BJ) method in which the voltage applied across the molecular junction has a triangular waveform. A characteristic solvent response can be distinguished from the molecular current response and the current responses can be separated through a fit to a devised model circuit.

The left image shows a schematic diagram illustrating the measurement of a molecule’s conductance.  The right image shows a time trajectory for a single octanedithiol molecule in a junction, which evolves from a low resistance state (2x108 ohm) to a higher resistance state (1.5x109 ohm) before the junction breaks and gives only a capacitive response (yellow region of time trajectory).

We are using this new modulated bias protocol to determine the single molecule conductance of nucleic acid duplexes as a function of backbone and nucleobase sequence. PNA homoduplexes are found to exhibit a larger conductance than DNA homoduplexes of the same sequence, with DNA/PNA heteroduplexes having an intermediate conductance. The findings support a conformationally-gated charge transfer mechanism in which the increased flexibility of the PNA homoduplexes facilitate electronic coupling.

DNA/PNA heteroduplexes will be the focus of future studies with the long-term goal of studying redox-active moieties in precise 3D arrangements on nucleic acid scaffolds.