Electrons on the double helix: optical experiments on native DNA

Optical experiments on calf thymus DNA films subjected to different buffer environments are reported. The optical conductivity is that of a disordered or lightly doped semiconductor with a well-defined band-gap for charge excitations and low frequency transport determined by a small number of strongly localized electron states. The question whether electrons are delocalized along the DNA duplex has attracted substantial recent interest and controversy. Both short and long-range migration was found by studies that were chemical in nature [1]. DC conductivity measurements on single DNA strands led to equally controversial results-with metallic [2], semi-conducting [3] and insulating [4,5] behavior all observed. Recent experiments [6] are also suggestive of a proximity-induced superconductivity at low temperatures. DNA networks in contrast were found to be highly resistive [7]. Our earlier experiments [8] conducted on λ-DNA in the microwave spectral range gave evidence for a large resistance associated with the duplex and for a thermally driven transport process. These conflicting results may, to a large extent be due to different DNA varieties. A native DNA duplex with random (or nearly random) base pair sequences is expected to have electron states that are different than an oligomer with identical base pairs, such as a poly(C)-poly(G) track. In addition, the DNA configuration is sensitive to the buffer environment, which surrounds the duplex. In a dry form the duplex has non-parallel base pairs, and at higher water concentrations the duplex undergoes a structural change to a more ordered system [9]. This then may lead to widely different transport properties and also charge excitations of a different nature. In order to examine the nature of electron states in native DNA, we have conducted optical measurements spanning a wide spectral range, from microwave frequencies to the UV part of the electromagnetic spectrum, on oriented films, fabricated from DNA extracted from calf thymus. Our findings concerning the vibrational modes will be discussed elsewhere. Here we focus on the electronic excitations at high and at low energies. Our data gives evidence for well-defined charge excitations at high energies involving the p-orbitals of the base pairs, and at low energies charge excitations displaying all the characteristics of conduction due to a small number of localized electron states. Thus our experiments suggest that DNA is a wide bandgap semiconductor, with intrinsic disorder, counterion fluctuations, and possibly other sources leading to a small number of localized electronic states on the base pair sequence. Wet-spun, …