The results of geometry optimisation of possible Watson-Crick-like pairs of
2,6-diamino-4-oxy-5-formamidopyrimidine (fapy-adenine) or 4,6-diamino-5-fo
rmamidopyrimidine (fapy-guanine) were presented. In the absence of the exte
rnal field the fapy-adenine is able to form pairs with all four canonical n
ucleic acid bases. However, pairs with guanine, cytosine and thymine the mo
st stable are. Thus, the potential miscoding abilities may be observed. In
contrast, in the presence of the external field the mispairing abilities of
fapy-adenine become insignificant since the most stable dimers are formed
with thymine.
The pairing properties of fapy-guanine are complex and depend on its tatome
ric form. In the absence of an external field the 4-enol-6-keto-diamino tau
tomer of fapyG is able to form stable dimers with thymine and cytosine, whi
le the 4,6-diketo-diamino tautomer forms the most stable pairs with cytosin
e and guanine. The presence of the water solvent does not significantly alt
er the pairing abilities of fapy-guanine. However, pairs with thymine are a
t least as stable as the Watson-Crick GC pair. Thus, in polar conditions th
e mispairing potential of fapyG will be extended and may be enriched by pot
ential GC --> AT transition.