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

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Abstract
The function of DNA during oxidative stress1 and its suitability as a potential building block for molecular devices2,3,4 depend on long-distance transfer of electrons and holes through the molecule, yet many conflicting measurements of the efficiency of this process have been reported5,6. It is accepted that charges are transported over long distances through a multistep hopping reaction7,8,9,10,11; this ‘G-hopping’8 involves positive charges moving between guanines (Gs), the DNA bases with the lowest ionization potential. But the mechanism fails to explain the persistence of efficient charge transfer when the guanine sites are distant7,12, where transfer rates do not, as expected, decrease rapidly with transfer distance. Here we show experimentally that the rate of charge transfer between two guanine bases decreases with increasing separation only if the guanines are separated by no more than three base pairs; if more bridging base pairs are present, the transfer rates exhibit only a weak distance dependence. We attribute this distinct change in the distance dependence of the rate of charge transfer through 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 confirm theoretical predictions of this behaviour13,14,15,16,17, emphasizing that seemingly contradictory observations of a strong8,9 as well as a weak7,12 influence of distance on DNA charge transfer are readily explained by a change in the transfer mechanism.