Abstract
For the first time, in this study with the use of QM/QTAIM methods we have exhaustively investigated the tautomerization of the biologically-important conformers of the G(*)·C(*) DNA base pair-reverse Löwdin G(*)·C(*)(rWC), Hoogsteen G(*)'·C(*)(H), and reverse Hoogsteen G(*)'·C(*)(rH) DNA base pairs-via the single (SPT) or double (DPT) proton transfer along the neighboring intermolecular H-bonds. These tautomeric reactions finally lead to the formation of the novel G· CO2* (rWC), GN2* · C(rWC), G*'(N2)·C(rWC), GN7* · C(H), and G*'(N7)·C(rH) DNA base mispairs. Gibbs free energies of activation for these reactions are within the range 3.64-31.65 kcal·mol(-1) in vacuum under normal conditions. All TSs are planar structures (C(s) symmetry) with a single exception-the essentially non-planar transition state TS(G*·C*(rWC)↔G(+)·C(-)(rWC)) (C(1) symmetry). Analysis of the kinetic parameters of the considered tautomerization reactions indicates that in reality only the reverse Hoogsteen G(*)'·C(*)(rH) base pair undergoes tautomerization. However, the population of its tautomerised state G*'(N7)·C(rH) amounts to an insignificant value-2.3·10(-17). So, the G(*)·C(*)(rWC), G(*)'·C(*)(H), and G(*)'·C(*)(rH) base pairs possess a permanent tautomeric status, which does not depend on proton mobility along the neighboring H-bonds. The investigated tautomerization processes were analyzed in details by applying the author's unique methodology-sweeps of the main physical and chemical parameters along the intrinsic reaction coordinate (IRC). In general, the obtained data demonstrate the tautomeric mobility and diversity of the G(*)·C(*) DNA base pair.