Lambda-DNA is an excellent insulator
The question whether DNA is electrically conducting has attracted much interest lately. Experiments have turned up a distressingly large range of values for the equivalent resistivity r (from 10-4 Ohm cm to 106 Ohm-cm). Proximity induced superconductivity has also been reported.
We have approached this problem by forming direct chemical bonds between the ends of lambda-DNA to Au electrodes. The idea is to insert, at both ends of the DNA molecule, thyosines (T) that have been modified to include the thiol group -HS. The thiol group -HS has a high propensity to bind to Au. This provides the electrical contact between DNA and our Au electrodes. The top figure shows a cartoon of one of the original sticky ends of lambda-DNA which is comprised of 48,500 base pairs occupying a double helix 16 microns in length (see top right for color code).
In the middle figure, the DNA molecule is immersed in a solution of mostly modified thyosine (T') and lesser amounts of guanine (G) and adenine (A) [cytosine (C) is left out]. The DNA polymerase (large blue blob) proceeds to incorporate the modified T's, G and A nucleotides into both sticky ends. The absence of cytosines in solution means that the modified T's are paired with G's. Normally, such mismatches are rapidly deleted when the polymerase proof-reads its own actions. To suppress this undesired editing, we use a mutated polymerase that lacks this proof-reading capability. The bottom figure shows the sticky end filled-in with excess modified T's.
A drop of the solution of lambda-DNA (with ends modified) is placed on a substrate containing gold electrodes (light vertical stripes in figure). The DNA molecules are rendered visible by binding them to dye molecules. The 4 panels on the left (A1...B2) show DNA molecules 'flexing' in an alternating solution flow (flow indicated by arrows). The binding energy of the ends is sufficient to resist the strong shear forces in a strong flow. The right panel shows scores of DNA molecules with ends bound to different gold electrodes.
For electrical measurements, the solution is evaporated in a vacuum and a voltage is applied (up to 20 volts). These experiments show that lambda-DNA has a resistivity larger than 106 Ohm cm at room temperature. Our results contradict many reports of moderately high conductivity in lambda-DNA.
"Insulating behavior of lambda-DNA on the micron scale"
Y. Zhang, R. H. Austin, J. Kraeft, E. C. Cox, and N. P. Ong,
Phys. Rev. Lett. 89, 198102 (2002).