Abstract: | The optimal environment charge configurations are predicted for the tautomerization of complementary base pairs into their corresponding rare forms, and vice versa. Results indicate that cations approaching the N3 guanine site may induce tautomerization of the normal guanine—cytosine (G---C) base pair into its rare form. The reverse process requires that the cation approach the O2 thymine site of the rare adenine*—thymine* pair (A*---31T*) or the O6 guanine site of rare guanine*—cytosine* base pair (G*---C*). Possible mutagenic and antimutagenic roles of metal cations approaching base pairs are also discussed. |