| Abstract
| - In this study, we employ a multistate generalized Mulliken−Hush approach for calculating electronic couplingsVda for charge transfer (CT) in DNA π-stacks consisting of three, four, and five base pairs. In these systemsthe guanine donor and acceptor sites are separated by several (AT) pairs. The Hartree−Fock calculations ofthe stacks are carried out with the standard 6-31G* basis sets. All possible superexchange pathways areaccounted for. We examine electronic couplings estimated using the two-state and multistate models. Althoughfor some systems the two-state scheme provides reasonable estimates of Vda, in general this simple modelfails to reproduce the electronic couplings calculated with the multistate approach. The two-state treatment ofπ-stacks with a tunneling gap less than 0.3 eV, for instance, GAAG and GAAAG, may lead to invalid estimatesof Vda. We consider the dependence of Vda on the length and composition of the bridge. The calculationsshow that Vda is less sensitive to the arrangement of nucleobases in the bridge, as can be predicted on thebasis of electronic couplings between adjacent base pairs.
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