DFT calculations on the deprotonation site of the one-electron oxidised guanine-cytosine base pair.

As calculated by the density functional theory (DFT), the acidity of cytosine's exocyclic amine group (C-N(4)H2) in the base pair G-C is considerably increased upon its one-electron oxidation. The proton affinity (PA) of the amine moiety is lowered by ionisation of G-C (which yields G(*+)-C) from -348.1 to -269.1 kcal mol(-1). The PA is further decreased by 7.6 kcal mol(-1) as a result of the ensuing proton transfer from G(*+) to C to yield the spin-charge separated base pair G(-H)(*)-C(+H)(+). Under these conditions and taking the hydration energy of H(+) into account, the overall proton transfer from the C-N(4)H2 group to the aqueous phase in the major groove is exothermic by -2.4 kcal mol(-1). This proton transfer to water from the initially present DNA radical cation constitutes separation of charge from spin and thus reduces positive charge transfer in double stranded DNA.