We report on a study of the interactions between holes and molecular vibrations on dry DNA using photoinduced infrared absorption spectroscopy. Laser photoexcited holes are found to have a room-temperature lifetime in excess of $\tau >1\mathrm{m}\mathrm{s}$, clearly indicating the presence of localization. However, from a quantitative model analysis of the frequency shifts of vibrational modes caused by the holes, we find the hole-vibrational coupling constant to be relatively small, $\lambda \approx 0.2$. This interaction leads to a change in the conformational energy of $\Delta E_0\sim 0.015\mathrm{e}\mathrm{V}$, which is too small to cause self-trapping at room temperature. We conclude that, at least in the dry $(A)$ form, DNA is best understood in terms of a double chain of coupled quantum dots arising from the pseudorandom chain sequence of base pairs, in which Anderson localization prevents the formation of a metallic state.

• Received 3 May 2004

DOI:https://doi.org/10.1103/PhysRevLett.93.218101