As calculated by the density functional theory (DFT), the acidity of cytosine's exocyclic amine group (C–N⁴H₂) 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–N⁴H₂ 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.