| Abstract
| - Electron hole transfer mediated by short DNA fragments containing the Watson−Crick pairs (AT) and (GC)and their 7-deaza purine counterparts (zAT) and (zGC) has been modeled. On the basis of quantum-chemicalresults for the energetics of hole transfer and the electronic coupling of π stacks, the effective electroniccoupling of hole donor and acceptor connected via short bridges has been estimated using an approximateGreen function approach. The distance decay parameters βel of the rate constant derived from the effectivecoupling has been determined at βel = 0.79 Å-1 for (T)n and (A)n bridges, while βel = 0.68 Å-1 resulted for(AT)n/2 π stacks. We also considered the effects of bridging bases B on the hole transfer mediated by theduplexes TTBTT which recently have been studied experimentally. Incorporation of zG into π stacks suppressesthe hole transfer. If one replaces adenine by 7-deazaadenine in TAT and TTATT, then the effective couplingincreases by a factor of 2.3. A much more pronounced effect, namely, an increase by a factor of ∼70, wasfound for the substitution AzAA → AAA. A comparison of the effective couplings of donors and acceptorsmediated by π stacks TBT and ABA (B = A, zA, G, T, C) shows that the effects of B strongly depends onthe neighboring nucleobases.
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