Polaron transport mechanism in DNA.

A theory of polaron-hopping transport is studied in DNA in the presence of an electric field. The helix structure of DNA consists of a molecule of phosphoric acid, a molecule of sugar and a molecule of a nitrogen compound called a nitrogen base. The charge carriers are localized near the bases. Phonons are created due to internal motions such as changes in winding or the inclination angle of the helix. It is considered that, due to the interaction between a charge carrier and a phonon, a localized polaron is formed in the helix near a base. These internal motions also promote hopping of the localized polarons. By interacting with a phonon, the polaron undergoes a hopping process in the helix structure. We consider that the localized polaron sites are distributed randomly in both space and energy coordinates. A polaron hops from one site to another site in this space. Conduction is a result of many series of hops through this hopping space. This approach differs from the percolation method and others in the calculation of the conductivity. The present theory is used to explain the electric-field- and temperature-dependent conductivity experiments of DNA. A good agreement is found between theory and experiments.