Direct observation of hole transfer through DNA by hopping between adeninebases and by tunnelling

The function of DNA during oxidative stress(1) and its suitability as a pot ential building block for molecular devices(2-4) depend on long-distance tr ansfer of electrons and holes through the molecule, yet many conflicting me asurements of the efficiency of this process have been reported(5,6). It is accepted that charges are transported over long distances through a multis tep hopping reaction(7-11); this 'G-hopping'(8) involves positive charges m oving between guanines (Gs), the DNA bases with the lowest ionization poten tial. But the mechanism fails to explain the persistence of efficient charg e transfer when the guanine sites are distant(7,12), where transfer rates d o not, as expected, decrease rapidly with transfer distance. Here we show e xperimentally that the rate of charge transfer between two guanine bases de creases with increasing separation only if the guanines are separated by no more than three base pairs; if more bridging base pairs are present, the t ransfer rates exhibit only a weak distance dependence. We attribute this di stinct change in the distance dependence of the rate of charge transfer thr ough DNA to a shift from coherent superexchange charge transfer (tunnelling ) at short distances to a process mediated by thermally induced hopping of charges between adenine bases (A-hopping) at long distances. Our results co nfirm theoretical predictions of this behaviour(13-17), emphasizing that se emingly contradictory observations of a strong(8,9) as well as a weak(7,12) influence of distance on DNA charge transfer are readily explained by a ch ange in the transfer mechanism.