The physico-chemical "anatomy" of the tautomerization through the DPT of the biologically important pairs of hypoxanthine with DNA bases: QM and QTAIM perspectives.
The biologically important tautomerization of the Hyp·Cyt, Hyp*·Thy and Hyp·Hyp base pairs to the Hyp*·Cyt*, Hyp·Thy* and Hyp*·Hyp* base pairs, respectively, by the double proton transfer (DPT) was comprehensively studied in vacuo and in the continuum with a low dielectric constant (ε = 4) corresponding to hydrophobic interfaces of protein-nucleic acid interactions by combining theoretical investigations at the B3LYP/6-311++G(d,p) level of QM theory with QTAIM topological analysis. Based on the sweeps of the energetic, electron-topological, geometric and polar parameters, which describe the course of the tautomerization along the intrinsic reaction coordinate (IRC), it was proved that the tautomerization through the DPT is concerted and asynchronous process for the Hyp·Cyt and Hyp*·Thy base pairs, while concerted and synchronous for the Hyp·Hyp homodimer. The continuum with ε = 4 does not affect qualitatively the course of the tautomerization reaction for all studied complexes. The nine key points along the IRC of the Hyp·Cyt↔Hyp*·Cyt* and Hyp*·Thy↔Hyp·Thy* tautomerizations and the six key points of the Hyp·Hyp↔Hyp*·Hyp* tautomerization have been identified and fully characterized. These key points could be considered as electron-topological "fingerprints" of concerted asynchronous (for Hyp·Cyt and Hyp*·Thy) or synchronous (for Hyp·Hyp) tautomerization process via the DPT. It was found, that in the Hyp*·Cyt*, Hyp·Thy*, Hyp·Hyp and Hyp*·Hyp* base pairs all H-bonds are significantly cooperative and mutually reinforce each other, while the C2H…O2 H-bond in the Hyp·Cyt base pair and the O6H…O4 H-bond in the Hyp*·Thy base pair behave anti-cooperatively, i.e., they become weakened, while two others become strengthened.
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