Electron Hole Transport in DNA

Electron hole transport along DNA chains is studied theoretically. Couplings between the DNA bases adenine, cytosine, guanine, and thymine, placed as in DNA in a single strand or diagonally in a double strand, are calculated using ab initio methods. Coupling as well as reorganization energies, which are also calculated, show a great variation between bases. To decide whether a periodic DNA strand has localized or delocalized electrons, a novel theoretical model that uses reorganization energy and coupling as input, is used. The model suggests that electron holes on infinite one-dimensional chains of periodic DNA localize if the ratio of reorganization energy to coupling is larger than about four. The hole is weakly trapped on an infinite guanine strand (G) x whereas it is delocalized in thymine (T) x and cytosine (C) x strands. Some mixed DNA strands also show a delocalized behavior, but most are localized to a single G in agreement with EPR measurements. Rates of stepwise hole transfer between G sites in mixed chains are calculated and show agreement with experimental data. In electron-transfer steps between localized sites, the tunneling rate decreases exponentially with distance with a quite large β factor in agreement with earlier theoretical work.