Non-Standard base pairing of rare tautomers
base pairing is determined by the number of hydrogen bonds available.
Recall that he standard forms of C
and G pair with three H-bonds
and the standard
forms of A and T pair with two.
The rare form of C'
has two H-bonds, and pairs with the
purine, A. Likewise, the rare
form of T'
has three H-bonds, and pairs with the alternative pyrimidine, G. By a slightly different
mechanism, the rare forms of G and A force
non-standard numbers of H-bonds on T
and C, respectively.
Thus, the rare tautomers
pair with the
alternative ("wrong") purine
or pyrmidine: C' with
with G, G'
with T, and A'