Long-Range Electronic Couplings Observed in DNA By SR-EPR

Abstract

DNA can self-assemble into a wide variety of pre-programmed 2-dimensional and 3-dimensional structures. This property, combined with the ability to conduct charge, makes DNA an attractive material for the fabrication of nanoscale circuitry, sensors, and catalysts. Fermi's Golden Rule, an equation governing the rate of charge transfer, describes the ease with which DNA conducts electrons or “holes” (positive charges). However, the predictive power of Fermi's Golden Rule is limited by the difficulty of independently measuring the terms that appear in this equation. One of these terms, the square of the electronic coupling matrix element, is proportional to the scalar exchange coupling between the charge donor and acceptor. Using a nitroxide radical and the paramagnetic Dy(III) ion as surrogates for donor and acceptor, we have measured scalar exchange couplings and their distance dependence in a family of DNA duplexes via saturation-recovery electron paramagnetic resonance (SR-EPR). Scalar exchange couplings are observed at distances as great as 5.6 nm. The decay in the scalar exchange coupling parallels the decay in electron transfer rates recently measured in DNA. The SR-EPR methodology is general and provides a new tool for determining the electronic coupling matrix element in Fermi's Golden Rule. The knowledge gained from these measurements may prove useful in designing DNA-based electronic devices.