Mechanism for the deamination of ammeline, guanine, and their analogues

Mechanisms for the deamination of ammeline (AMN), guanine (Gua), and their analogues with nH2O (n = 1–3) have been investigated using B3LYP and G3MP2B3. The deamination reactions of AMN and Gua with 3H2O/OH−, and protonated AMN and Gua with 3H2O, were investigated using DFT. The rationale behind this work was to compare the deamination reactions of AMN and Gua analogues. Optimized geometries of the reactants, transition states, intermediates, and products were determined at B3LYP/6-31G(d,p) level of theory, and solvent calculations were performed using the solvation model on density (SMD). Thermodynamic properties (ΔH, and ΔG), proton affinities (PAs), gas-phase basicities (GBs), deprotonation enthalpies (ΔacidH), and gas-phase acidities (ΔacidG) were also calculated. For the SN2 mechanism, deamination can proceed via two possible pathways involving either two- or three-stepwise mechanisms producing a tetrahedral intermediate via four-, six-, or eight-membered transition states. The lowest overall activation energies of 128 (131) and 131 (133) kJ mol−1 at B3LYP/6-31G(d,p) in the gas phase (SMD) were obtained for the deamination of AMN and Gua with 3H2O, whereas the values of 140 (115) and 139 (117) kJ mol−1 were obtained for AMN and Gua with 3H2O/OH−, respectively. For protonated AMN and Gua with 3H2O in the gas phase (SMD), the overall activation energies are 233 (155) and 240 (162) kJ mol−1, respectively.