Conformational dependence of the electronic coupling for hole transfer between adenine and tryptophan

Oxidation of DNA may lead to mutagenic lesions generated far away from the initial oxidized site because of migration of radical cation states through the π stack. In DNA-protein complexes, transfer of the excess positive charge from nucleobases to aromatic amino acid residues protects DNA from possible mutations. In the present paper, we explore how the probability of the hole transfer (HT) process between adenine (A) and tryptophan (Trp) depends on the mutual position of these sites. To accomplish this, we carry out DFT calculations of HT electronic coupling in different conformations of the A-Trp complex. Stacked and T-shaped structures are considered. The HT rate in the system is shown to be very sensitive to the mutual position of the nucleobase and amino acid residue. Interestingly, the strongest coupling is obtained in stacked structures where only one of two rings in each molecule are involved in the π-π interaction and a surprisingly weak coupling is found in the eclipsed conformation of the A-Trp complex with the perfect overlap of the aromatic systems. Although the HT rate derived for T-shaped conformations is in most cases slower than in π stacks, several T-shaped conformations are found where the HT process should be quite efficient.