Novel Tautomerisation Mechanisms of the Biologically Important Conformers of the Reverse Lowdin, Hoogsteen, and Reverse Hoogsteen G* . C* DNA Base Pairs via Proton Transfer: A Quantum-Mechanical Survey

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 Lowdin 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.C*(O2) (rWC), G*(N2).C(rWC), G*'(N2).C(rWC), G*(N7).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 TSG*.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.