Dark nπ* states were shown to have substantial contribution to the destructive photochemistry of pyrimidine nucleobases. Based on quantum-chemical calculations, we demonstrate that the characteristic hydrogen bonding pattern of the GC base pair could facilitate the formation of a wobble excited-state charge-transfer complex. This entails a barrierless electron-driven proton transfer (EDPT) process which enables damageless photodeactivation of the base pair. These photostabilizing properties are retained even when guanine is exchanged to hypoxanthine. The inaccessibility of this process in the AT base pair sheds further light on the reasons why cytosine is less susceptible to the formation of photodimers in double-stranded DNA.
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