pairing of rare tautomers
of base pairing between two-ring purines and one-ring
pyrimidines is determined by the number of hydrogen
(H) bonds available. Recall that the standard forms of C and G pair with three H-bonds,
and the standard forms of A
and T pair with two.
The rare tautomeric form C*
has only two H-bonds,
and pairs with the alternative purine, A. Likewise, the rare tautomeric
form T* has three H-bonds,
and pairs with the alternative pyrimidine, G.
contrast, the rare
tautomeric form G* retains the correct
number of H-bonds, but the tautomeric arrangement of
bonds forces these three bonds on standard T. The rare
tautomeric form A* retains two bonds
but forces them on standard C.
The net result is that the rare tautomers
pair with the alternative ("wrong")
purine or pyrimidine: C*
with A, T* with G, G* with T, and A* with C,