Quantum chemical modeling of electron hole transfer through π stacks of normal and modified pairs of nucleobases

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-chemical results for the energetics of hole transfer and the electronic coupling of π stacks, the effective electronic coupling of hole donor and acceptor connected via short bridges has been estimated using an approximate Green function approach. The distance decay parameters β_el of the rate constant derived from the effective coupling has been determined at β_el = 0.79 Å^-1 for (T)_n and (A)_n bridges, while β_el = 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 the duplexes TTBTT which recently have been studied experimentally. Incorporation of ^zG into π stacks suppresses the hole transfer. If one replaces adenine by 7-deazaadenine in TAT and TTATT, then the effective coupling increases by a factor of 2.3. A much more pronounced effect, namely, an increase by a factor of ∼70, was found for the substitution A^zAA → AAA. A comparison of the effective couplings of donors and acceptors mediated by π stacks TBT and ABA (B = A, ^zA, G, T, C) shows that the effects of B strongly depends on the neighboring nucleobases.