As with natural nucleic acids, pairing between artificial nucleotides can be influenced by tautomerism, with different placements of protons on the heterocyclic nucleobase changing patterns of hydrogen bonding that determine replication fidelity. For example, the major tautomer of isoguanine presents a hydrogen bonding donor donor acceptor pattern complementary to the acceptor acceptor donor pattern of 5-methylisocytosine. However, in its minor tautomer, isoguanine presents a hydrogen bond donor acceptor donor pattern complementary to thymine. Calculations, crystallography, and physical organic experiments suggest that this tautomeric ambiguity might be "fixed" by replacing the N-7 nitrogen of isoguanine by a CH unit. To test this hypothesis, we prepared the triphosphate of 2'-deoxy-7-deazaiso-guanosine and used it in PCR to estimate an effective tautomeric ratio "seen" by Taq DNA polymerase. With 7-deazaisoguanine, fidelity-per-round was similar to 92%. The analogous PCR with isoguanine gave a lower fidelity-per-round of similar to 86%. These results confirm the hypothesis with polymerases, and deepen our understanding of the role of minor groove hydrogen bonding and proton tautomerism in both natural and expanded genetic "alphabets", major targets in synthetic biology.