The electronic structure of the purine-pyrimidine pairs of DNA

The study of the structures of the adenine-thymine (AT) and the guanine-cytosine (GC) pairs of DNA by the molecular-orbital method of quantum chemistry points to the existence of quantitative differences in a number of electronic properties associated with these 2 pairs. The GC pair should be both a better electron donor and a better electron acceptor than the AT pair. The resonance energy and the complementary stabilization due to hydrogen bonding should also be greater in the GC pair. From the point of view of chemical reactivity the purine components of the pairs should be more reactive than the pyrimidine ones. Finally, the nucleophilic reactivity of the ring nitrogens should be greater in the guanine nucleus of the GC pair than in the adenine nucleus of the AT pair. On the whole, the GC pair seems to constitute from the point of view of a number of electronic indices a more important component of DNA than the AT pair. The limited available experimental data seem to be in agreement with this point of view.