∆E – the relative electronic energy of the tautomerized complex; ∆∆ETS – the activation barrier of tautomerisation in terms of electronic energy; ΔΔE=ΔΔETS-ΔE – the reverse barrier of tautomerisation in terms of electronic energy; ν – the frequency of the vibrational mode of the tautomerized complex which becomes imaginary in the transition state of tautomerisation

Table 7. Energetic characteristics of DNA bases tautomerisation in studied base pairs obtained at the MP2/6-311++G(2df,pd)//B3LYP/6-311++G(d,p) level of theory in vacuum#

## **6. Conclusions**

In this study, we made an attempt to answer some actual questions related to physicochemical nature of spontaneous point mutations in DNA induced by prototropic tautomerism of its bases.

It was shown that the lifetime of mutagenic tautomers of all four canonical DNA bases exceeds by many orders not only the time required for replication machinery to enzymatically incorporate one incoming nucleotide into structure of DNA double helix (~4·10-4 s), and even a typical time of DNA replication in cell (~103 s). The high stability of mutagenic tautomers of DNA bases is mainly determined by the absence of intramolecular H-bonds in their canonical and mutagenic forms.

This finding substantially supports the tautomeric hypothesis of the origin of spontaneous point mutations, for instance replication errors, removing all doubts on instability of mutagenic tautomers of isolated DNA bases, which are sometimes expressed by biologists.

Notwithstanding a tremendous heuristic and methodological role of the classical Löwdin's mechanism of the origin of spontaneous point mutations during DNA replication, it was demonstrated that this mechanism probably has substantial limitations. From the physicochemical point of view, the advantage of Löwdin's mechanism lies in the fact that the tautomerisation of base pairs does not disturb standard Watson-Crick base-pairing geometry. Its main disadvantage is the instability of Ade\*·Thy\* base pair and metastability of Gua\*・Cyt\* base pair. The lifetime of tautomerized (Löwdin's) Ade\*Thy\* and Gua\*Cyt\* base pairs is less by orders than a characteristic time required for replication machinery to separate any Watson-Crick base pair (~10-9 s). Figuratively speaking, the Löwdin's base pairs "slip away" from replication apparatus: they transform to canonical base pairs and then dissociate without losing their canonical coding properties, as they haven't enough time to dissociate to mutagenic tautomers. These facts put the possibility of such mispairs involving mutagenic tautomers formation under a doubt, not to mention their complicated dissociation into mutagenic tautomers.

Gua·Cyt↔Gua\*·Cyt\* 7.87 13.02 5.15 1800.8 2951.8 Ade·Thy↔Ade\*·Thy\* 12.26 12.37 0.11 37.7 3349.2 Ade·Cyt\*↔Ade\*·Cyt 3.67 10.11 6.44 2253.5 3024.9 Gua\*・Thy↔Gua・Thy\* 1.13 5.29 4.16 1455.4 3155.7
