Theoretical Estimations of the 298 K Gas-Phase Acidities of the Purine-Based Nucleobases Adenine and Guanine

Abstract

Theoretical estimations at both the B3LYP/6-31+G(d)//B3LYP/6-31+G(d) and B3LYP/aug−cc−pVTZ//B3LYP/6-31+G(d) levels for the 298 K gas-phase acidities (GPA) of all the possible deprotonation sites of the most stable tautomers of adenine and guanine are reported in terms of the enthalpy change of deprotonation. The gas-phase acidities of the nucleobases estimated by direct calculations and derived from isodesmic reactions are very similar. At the B3LYP/aug−cc−pVTZ//B3LYP/6-31+G(d) level of theory, for the most stable adenine tautomer, 9H-adenine, the GPAs of N9, N4−Ha, N4−Hb, C8, and C2 sites are 337.8 ± 2.1, 355.6 ± 2.1, 355.3 ± 2.1, 375.7 ± 0.5, and 401.2 ± 0.5 kcal/mol, respectively. For the most stable guanine tautomer, 7H-keto-guanine, the GPAs of N3, N7, N2−Ha, N2−Hb, and C8 sites are 336.4 ± 2.2, 338.0 ± 2.1, 347.7 ± 2.1, 341.9 ± 2.1, and 376.0 ± 0.5 kcal/mol, respectively. For the second most stable tautomer, 9H-keto-guanine, the GPAs of N3, N9, N2−Ha, N2−Hb, and C8 are 338.6 ± 2.2, 337.4 ± 2.1, 343....