Figure 7.

Mechanism of G-to-A substitutions. (a) Previously proposed mechanism of diaminopurine formation, which is slightly modified based on this study¹⁹. (b) Optimized structure of the model of intermediates. (c) Positions of unnatural nucleosides. (d) Observed error rates related to deoxyguanosine. The synthetic conditions for Pac₂O (n = 2) are 5-benzylthio-1H-tetrazole in anhydrous acetonitrile as an activator, phenoxyacetic anhydride in THF as a capping reagent A, 10% 1-methylimidazole in 10% pyridine-THF as a capping reagent B, 0.02 M I₂ in THF/pyridine/H₂O (90:<1:10, v/v/v) as an oxidation reagent, and 3% trichloroacetic acid in dichloromethane (TCA) as a deblocking reagent. The synthetic conditions for Ac₂O (n = 1) are 1H-tetrazole in anhydrous acetonitrile as an activator, acetic anhydride in THF as a capping reagent A, 10% 1-methylimidazole in 10% pyridine-THF as a capping reagent B, 0.02 M I₂ in THF/pyridine/H₂O (90:<1:10, v/v/v) as an oxidation reagent, and 3% trichloroacetic acid in dichloromethane (TCA) as a deblocking reagent. Q5 High-Fidelity DNA polymerase was used for assembling reaction.