see designations in Table 2

into mutagenic tautomers (Table 6).

theory in vacuum #

Ade·Thy→Ade\*·Thy\* 11.05 5.49104 1.8210-5 1.2610-5

Ade\*·Thy\*→Ade·Thy -1.01 3.881013 2.5710-14 1.7810-14 Gua·Cyt→Gua\*·Cyt\* 9.70 9.02105 1.1110-6 7.6810-7

Gua\*·Cyt\*→Gua·Cyt 1.49 9.581011 1.0410-12 7.2410-13 Ade·Cyt\*→Ade\*·Cyt 7.76 2.26107 4.4210-8 3.0610-8

Ade\*·Cyt→Ade·Cyt\* 3.75 1.991010 5.0310-11 3.4910-11 Gua·Thy\*→Gua\*·Thy 2.33 2.741011 3.6510-12 2.5310-12

Gua\*·Thy→Gua·Thy\* 1.17 1.931012 5.1810-13 3.5910-13

microwave spectroscopy (López et al., 2010). Investigation of the structure of the adducts from the rotational constants of the different isotopologues shows that the observed conformers of bases correspond to the most stable forms in which water closes a cycle with the nucleic acid bases through H-bonds (López et al., 2010).

In this work we for the first time present a complete study of the proton transfer kinetic of intramolecular water-assisted tautomerisation mechanism for all DNA bases (Fig. 4) by computing the rate constants with the conventional transition state theory (Atkins, 1998), including the Wigner's tunnelling correction (Wigner, 1932).

Fig. 4. Water-assisted tautomerisation of the DNA bases. The dotted lines indicate H-bonds AH…B (their lengths H…B are presented in angstroms), while continuous lines show covalent bonds. Relative Gibbs free energies (T=298.15 K, in vacuum) are obtained at the MP2/6-311++G(2df,pd)//B3LYP/6-311++G(d,p) level of theory and reported near each structure in kcal/mol

We found that the interaction of the canonical tautomers of the DNA bases with a water molecule at the Watson-Crick edge changes the gas-phase stability: the relative Gibbs free energies of the Ade and Thy decrease, while those of the Cyt and Gua – increase (Table 4). So, it means that in the case of complexes with water, the order of stability of Ade and Thy mutagenic tautomers remains the same as for isolated bases; moreover, they are stabilized in these complexes. On the contrary, the order of stability of Cyt and Gua mutagenic tautomers

Elementary Molecular Mechanisms of the Spontaneous Point

water to the periphery of the interaction interface.

**4.5 Tautomerisation of the DNA bases in dimers** 

tautomerisation

level of theory in vacuum#

Mutations in DNA: A Novel Quantum-Chemical Insight into the Classical Understanding 77

The time necessary to reach 99.9% of the equilibrium concentration of mutagenic tautomer in the system (τ99.9%) for these barriers falls within the range 3.8410-8 ÷ 2.1310-4 s, which is by orders smaller, except Cyt, than the time of an elementary act of one base pair replication (*ca.* 4·10-4 s). The barriers for the reverse reactions lead to a half-lifetime of about 10-9 s, and tunneling effects will further facilitate the reverse process. So, complexes "mutagenic tautomer-water" produced in the DPT process represent unstable intermediates, which quickly converted back into the complexes "canonical tautomer-water" in the time scale of the nucleotide-water interaction. However, if the dissociation of the water from the tautomerized complex occurs, the mutagenic tautomer would be a long-lived species, as the barrier for the reverse conversion to canonical tautomer is more than *ca.* 27 kcal/mol (see Table 2). It should be noted that electronic energy of the dissociation of the Ade\*·H2O and Thy\*·H2O complexes (Table 5) are lower than the corresponding reverse barriers. So, it can mean that these complexes more probably decay to the mutagenic tautomers and water molecule. To the contrary, in the case of Gua and Cyt – the Gua\*·H2O and Cyt\*·H2O transition to the complexes involving canonical tautomers will be more probable than the decay of the tautomerized complexes. Following the electronic energies of the interaction between bases and molecules of water, we could conclude that transition to the complexes containing mutagenic tautomers of Ade and Thy isn't preferential as they have larger electronic energies of the interaction that complicates their dissociation into mutagenic tautomers (Table 5). Interaction energy of the DNA bases with water is less than the energy of interaction with the complementary bases. So, the nucleotide bases competing with water for binding will displace

Complex -ΔEint ΔΔE ΔΔG Ade·H2O 9.60 - - Ade\*·H2O 12.72 11.56 8.56 Thy·H2O 8.74 - - Thy\*·H2O 12.48 9.55 6.69 Сyt·H2O 11.26 - - Сyt\*·H2O 10.16 14.93 11.75 Gua·H2O 11.52 - - Gua\*·H2O 9.72 12.40 9.36 # ΔEint – the counterpoise-corrected electronic energy of interaction; ΔΔE – the reverse barrier (difference in electronic energy) of tautomerisation; ΔΔG – the reverse barrier (difference in Gibbs free energy) of

Table 5. Electronic and Gibbs free energies (in kcal/mol) (T= 298.15 K) of complexes of DNA bases with water molecule obtained at the MP2/6-311++G(2df,pd)//B3LYP/6-311++G(d,p)

Theoretical and experimental studies also explored agents other than water, which can enhance the stability of rare tautomers of DNA bases in the gas phase. Of particular interest were their interactions with amino acids (Fan et al., 2010; Samijlenko et al., 2001, 2004;

changes in their complexes with water. So, equilibrium constants of tautomerisation for the AdeH2O and ThyH2O complexes (4.8910-8 and 3.3910-7, respectively) fall into the mutationally significant range, while for the CytH2O and GuaH2O complexes (4.1610-3 and 2.1610-2, respectively)these values are considerably higher (Table 4).

For comparison, computation results reported by Gorb and Leszczynski (Gorb & Leszczynski, 1998a, 1998b) are of a special interest. As part of their comprehensive study of water-mediated proton transfer between canonical and mutagenic tautomers of Cyt and Gua, the authors have shown that the interaction with water changes the order of relative energies of cytosine tautomers.

