Theoretical investigation of hydrogen atom transfer in the adenine-thymine base pair and its coupling with the electronic rearrangement. Concerted vs. stepwise mechanism.

Two different mechanisms to obtain the imino-enol tautomer of the adenine-thymine base pair, a concerted hydrogen atom transfer and a stepwise process, have been studied and compared. The first mechanism includes both the concerted movement of two hydrogen atoms, in the bridges that bond the two bases, and an electronic reorganisation of the bonds. The stepwise mechanism is the simplest one where there is a correlation between the movement of the hydrogen atoms, but two or more steps can be identified. In this study, a different behaviour has been found when the first atom to move is the hydrogen in the N-N bridge or that in the N-O one.