Charge transfer in DNA: The role of thermal fluctuations and of symmetry

The DNA double helix is a linear one-dimensional molecule, and charge transfer occurs along the base-pairs stacked along its longitudinal axis. DNA, however, is highly subject to disruptions and modifications in its configurational stacking due, for instance, to thermal fluctuations. These departures from a rigid, crystal-like structure must be taken in account for a correct description of the charge transfer process, so that the usual solid-state tight-binding pictures of charge transfer along organic one-dimensional crystals, such as the Bechgaard salts, cannot be used.; We propose a model Hamiltonian for charge transfer between the DNA base-pairs with temperature driven fluctuations in the base-pair positions acting as the rate limiting factor. The underlying idea is that charge tunneling between base-pairs that fluctuate significantly from their nominal configuration can occur only when an optimal base-pair relative configuration is reached. We focus on this aspect of the process by modeling two adjacent base pairs in terms of a classical damped oscillator subject to thermal fluctuations and charge transfer to the acceptor. The Fokker-Planck equation for the system yields an unusual two-stage process, with distinct initial and late-time charge transfer rates. This result is in agreement with experimental findings and is not contemplated by other charge transfer paradigms.; Another known consequence of charge transfer between DNA base-pairs is the geometrical modification of the base-pairs after the addition or removal of the migrating charge. This structural deformation breaks the mirror symmetry of the original DNA base-pair, leading to two alternate, symmetry related, ‘left’ and ‘right’ ionic configurations. We study charge transfer between donor-acceptor molecules subject to a mirror symmetry constraint in the presence of a dissipative environment. The symmetry requirement leads to the breakdown of the standard single reaction-coordinate paradigm of charge transfer and to a new description based on two coordinates of equal relevance. Two regimes of adiabatic and nonadiabatic charge transfer persist in the description offered by the new paradigm. The adiabaticity parameter however is now inverse temperature dependent, so that at the moderately high DNA temperatures the non-adiabatic character of the charge transfer is greatly enhanced by the symmetry requirement. (Abstract shortened by UMI.)...