Figure 5.

Possible base-pairing interactions between isoG and G or 8H-imidazo-[1,2-a]-[1,3,5]-triazin-4-one (P) in duplex DNA (R = 2’-deoxyribose). (a) Proposal of Roberts et al.;70 (b) Involvement of the O2-H enol isoG tautomer proposed by Geyer et al.;77 (c) Interaction of the N1-H isoG tautomer with the non-natural purine analogue P as proposed by Seela et al.;80 (d) Model of isoG:G base pair in which size and hydrogen-bonding complementarity is maintained by employing the N1-H isoG tautomer in its syn conformation. The involvement of N3-H tautomer 2c in forming a purine:purine pair in duplex DNA has also been invoked elsewhere,⁷⁹ and an exceptionally strong pair involving 2-amino-8-(2-deoxy-D-erythro-pentofuranosyl)-8H-imidazo-[1,2-a]-[1,3,5]-triazin-4-one, which is hydrogen bond-complementary with isoG 2 in its N3-H tautomeric form, has been reported.⁸⁰