Excited States Decay of the A-T DNA: A PCM/TD-DFT Study in Aqueous Solution of the (9-Methyl-adenine)2•(1-methyl-thymine)2 Stacked Tetramer

By exploiting some of the most recent advances in the quantum mechanical methods, we have been able to analyze the behavior of the lowest energy excited states of A-T B-DNA using a realistic model, namely a double strand tetramer formed by two thymine-adenine stacked pairs in aqueous solution. The equilibrium structure of the lowest energy bright and dark excited states has been determined and their main properties disclosed. On this ground, our study provides a detailed atomistic picture of the excited state decay and of the emission process, and it highlights the specific roles of base stacking and pairing. While absorption involves excited states delocalized over different stacked bases, emission mainly takes place from individual monomers and it is dominated by thymine bases. We show that fast "monomer-like" excited state decay routes are operative also in the double strand. On the other hand, the long living components of the excited state population of (dA).(dT) oligomers correspond to a dark excimer produced by intermonomer charge transfer between two stacked adenine bases, whereas adenine-thymine proton transfer plays a minor role in the excited state decay.