Complex microwave conductivity of Na - DNA powders

We report the complex microwave conductivity, σ = σ 1 − iσ 2 , of Na-DNA powders, which was measured from 80 K to 300 K by using a microwave cavity perturbation technique. We found that the magnitude of σ 1 near room temperature was much larger than the contribution of the surrounding water molecules, and that the decrease of σ 1 with decreasing temperature was sufficiently stronger than that of the conduction of counterions. These results clearly suggest that the electrical conduction of Na-DNA is intrinsically semiconductive. The electrical conduction of DNA has attracted much recent interest from the viewpoints of the fundamental understanding of the charge transfer mechanism along a DNA double helix with π − π orbital stacking. 1 However, many experimental results have still been controversial, because of several experimental difficulties. 2–7 For instance, there are two serious difficulties in the two-probed dc measurements of DNA wires prepared by a nanometer-scale microfab-rication technique. 2–5 One is the difficulty in the direct physical contact between the metal electrode and the DNA sample, and the other is the influence of the ionic conduction due to a residual buffer. In addition, the dc measurements are easily influenced by the existence of disorder and defects in DNA wires. On the other hand, contactless measurements at the microwave frequencies are free from the contact problem, suggesting a new route toward the understanding of the electrical conduction of DNA. However, the previous studies at the microwave and quasi-optical frequencies were faced by another difficulty. 6, 7 That is, the dipole relaxation of water molecules surrounding DNA backbone seriously affects the ac conductivity in the frequency region above 100 GHz, depending on the humidity at which DNA samples are measured. 7 In this paper, by using a microwave cavity perturbation technique, we investigated the electrical conduction of DNA powders, which were sealed in a glass tube together with He gas in order to minimize the influence of dipole relaxation of water molecules. By removing the contribution of the glass tube to the microwave response very carefully, we succeeded in * obtaining the complex microwave conductivity, σ(= σ 1 − iσ 2) of DNA powders, showing a semiconductive behavior at least above 100 K. This semiconductive behavior was found to be very close to the intrinsic property of the charge transport of DNA molecules rather than the dipole relaxation of residual water molecules. 2. …